Journal of Physiology (1991), 434, pp. 453-467 453Writh 9 fignres

Printed in Great Britain

THE EFFECT OF STATIC EXERCISE ON RENAL SYMPATHETIC NERVEACTIVITY IN CONSCIOUS CATS

BY KANJI MATSUKAWA, JERE H. MITCHELL, P. TIM WALLAND L. BRITT WILSON

From the Departments of Physiology and Internal Medicine and theHarry S. Moss Heart Center, The University of Texas Southwestern Medical Center at

Dallas, TX 75235, USA

(Received 27 June 1990)

SUMMARY

1. Renal sympathetic nerve activity (RNA), heart rate (HR), arterial bloodpressure (AP), and force development were measured simultaneously duringvoluntary static (isometric) exercise performed by conscious cats. The cats wereoperantly trained to press a bar with one forelimb. When the force applied to the barexceeded a predetermined value (threshold), a sound was emitted by a buzzer foraudio-feedback. If the cat continued to produce the appropriate force for a period of26-55 s, food was given as a reward.

2. A total of eighty-nine exercise trials were performed by seven cats. The peakforce applied to the bar was 468 + 28 g (mean + S.E.M.). RNA, HR, and AP increasedsignificantly from the control value during static exercise by 102 + 14%, 23 + 2beats/min, and 11+ 1 mmHg, respectively.

3. The increase in RNA had both an initial and a late component. The initialcomponent occurred at or immediately before the onset of force development andlasted for 10 s, while the late component gradually increased 14 s after the onset ofstatic exercise and was sustained until the exercise was terminated.

4. HR also increased at the beginning of static exercise with a similar time courseas RNA. Then, HR returned to the control value and remained at that level duringthe remainder of exercise. The increase in AP was delayed by 10 s from the initialincrease in RNA and then continued to rise throughout the period of exercise.

5. The sound of the buzzer was emitted during rest to determine any influence ofanticipation or conditioning on the response. RNA and AP increased slightly, butHR did not change. The increases in RNA and AP were much smaller than theincreases obtained during static exercise. Thus, the increases in RNA, HR and APduring static exercise appeared to be associated with the exercise itself and not dueto anticipation and/or conditioning.

6. When AP was elevated by a bolus injection of noradrenaline, RNA during restwas almost abolished and the increase of RNA during static exercise was markedlyinhibited. Thus the arterial baroreflex significantly influences RNA both during restand during static exercise.

7. This study suggests that the initial increases in RNA and HR at the beginningMS 8604

K. MATSUKA WA ANAD OTHERS

of static exercise in conscious cats are caused by descending input from higher braincentres and not by afferent feedback signals from muscle receptors or by arterialbaroreceptors. In contrast, the late increases in RNA and AP during static exercisetend to be related to muscle metabolism and a feedback signal from the exercisingmuscle may play a role.

INTRODUCTION

Static (isometric) exercise produces significant increases in arterial blood pressure,left ventricular systolic pressure, and heart rate using a conscious animal model(Diepstra, Gonyea & Mitchell, 1980; Gonyea, Diepstra, Muntz & Mitchell, 1981).Also, it is known that blood flow to the kidneys and spleen decreases during staticexercise while blood flow to the exercising muscles increases (Diepstra, Gonyea &Mitchell, 1982). The autonomic nervous system is thought to control thecardiovascular adjustments during static exercise because phentolamine attenuatesthe increase in left ventricular systolic pressure (Waldrop, Bielecki, Gonyea &Mitchell, 1986) and atropine attenuates the increase in heart rate (Diepstra et al.1980). In particular, sympathetic nerve activity to the kidney may play a role in thedistribution of cardiac output during exercise by causing renal vasoconstriction,since renal blood flow did not decrease during exercise after denervating the kidney(Diepstra et al. 1982). The above indirect evidence suggests that the increases inarterial blood pressure, left ventricular systolic pressure and renal vascular resistanceare caused primarily by activation of the sympathetic nervous system, while theincrease in heart rate is caused by withdrawal of cardiac parasympathetic nerveactivity.Renal sympathetic efferent nerve activity during static exercise in conscious

animals has not been studied. Therefore, the purpose of this study was (1) todetermine directly the time courses of changes in renal sympathetic nerve activity,heart rate and arterial blood pressure during voluntary static exercise in consciouscats, and (2) to examine the neural mechanisms responsible for these cardiovascularadjustments during static exercise. A preliminary report has been published(Matsukawa, Wilson, Wall & Mitchell, 1989).

METHODS

Experimental animalsThe experiments were conducted on seven cats (body weight, 3 4-5 4 kg). There were four males

and three females. The experimental protocols were approved by the Institutional Review Boardfor Animal Research.

Training of catsCats were operantly conditioned to perform static exercise. A clear plastic enclosure with a small

tunnel (35 x 3-5 x 3-5 cm) was used in the training procedure. The training procedure andapparatus were modified from that described previously (Diepstra et al. 1980; Gonyea et al. 1981).The cats were trained to sit quietly in the box, extend one forelimb through the tunnel, and pressa bar, while maintaining a sitting posture. If the force applied to the bar exceeded a predeterminedthreshold value (100-400 g), a sound was emitted by a buzzer to give an audio-feedback signal tothe cat as shown in Fig. 1. When the cat maintained the predetermined force for a given period oftime, a food reward was given automatically by a feeding apparatus. The training was conductedon 5 consecutive days per week, and the cats were fed only during the training period. On the other2 days they were fed in their cages ad libitum. A training period of 3 or 4 months was required.

454

SYMPATHETIC NERKVE ACTIVITY AiND STATIC EXERCISE 455

Threshold 1

Force ]Qkg

Buzzer

I Food

10 sFig. 1. A scheme of the training protocol. If the force that the cat applied to the barexceeded a predetermined threshold value (100-400 g), a sound was emitted by a buzzerto give an audio-feedback signal to the cat. When the cat maintained the predeterminedforce for a given period of time, a food reward was given automatically by a feedingapparatus.

A

RNAN 0 level

AP \R\Nr\NT !N

J 240

mmHg

40

B

- 0 level240

AP ~~~~~~~~~~mmHg

40

is

Fig. 2. Discharge pattern of renal sympathetic nerve activity (RNA) at rest and duringadministration of noradrenaline. In A, at rest RNA showed grouped dischargessynchronized with the cardiac cycle and respiration. In B, on administration ofnoradrenaline (2-5 ,tg/kg i.v.) the grouped discharges of RNA disappeared and RNA was

reduced to a noise level. AP, arterial blood pressure.

Preparation of animalsThe cats were anaesthetized with 2-5% halothane for surgical implantation of recording

electrodes and catheters. Each cat was intubated with an endotracheal tube and ventilated witha mixture of halothane and 02-enriched room air. During surgery, heart rate and respiration were

continuously monitored to maintain an appropriate level of surgical anaesthesia.

RNA .4.404h,lo k.A Foe-

4K. IA TSUTKA WIA AND OTHERS

Under aseptic conditions either the right or left kidney was exposed retroperitoneally. A branchof the renal nerve was separated from the renal plexus and surrounding connective tissue near therenal artery and vein using a dissecting microscope (Zeiss). A pair of silver-wire electrodes werecarefullv wound around the renal nerve branch, anid the wires and the branch were covered withsilicone gel as previously described in detail (Matsukawa & Ninomiya, 1987). The renal nervebranch was left intact in all cats. A stainless-steel wire was placed as a ground electrode under theskin of the back.

Polyethylene catheters (PE-60) were inserted into an external jugular vein for administeringdrugs and into a carotid artery for measuring arterial blood pressure. The lead wires of therecording electrodes and catheters were brought to the exterior in the intrascapular region. Duringthe exercise experiments, the wires were connected to a recording instrument by light lead cables.

Antibiotics (penicillin G procaine and dihydrostreptomycin, 20000 units/kg I.M.) and nalbuphinehydrochloride (2 mg/kg I.M.) were given to the cats for 3-5 post-operative days.

Recording of dataThe original renal nerve activity was amplified by a differential preamplifier (Grass, P511K) with

a bandpass filter of 30-3000 Hz. The amplified output was rectified by a full-wave rectifier circuitand then integrated by an R-C integrator with a time constant of 50 ms. The integrated signal wasused to monitor renal sympathetic nerve activity (RNA). Mean renal sympathetic nerve activity(MR'_A) was obtained by integrating the rectified signal with a time constant of 1 s. In all cats,RNA showed grouped discharges synchronized with the cardiac cycle and respiration as shown inFig. 2A. At the end of each experiment, noradrenaline (2-5 ,ug/kg) was intravenously administeredto identify the background noise level. On administration of noradrenaline the grouped dischargesof RNA disappeared and the RNA and MRNA were reduced to noise levels as shown in Fig. 2B.The noise level was subtracted from the integrated signals (RNA and MRNA).

Arterial blood pressure (AP) was measured through the carotid artery catheter connected to apressure transducer (Spectramed, TDN-R). Mean arterial blood pressure (MAP) was obtained byintegrating the AP signal with a time constant of 1 s. Heart rate (HR) was derived from thearterial pressure pulse (Gould, Biotach). The actual force that the cats applied to the bar wasmeasured by a calibrated force cell (Statham, UC-2). The onset of the increase in force was definedas the onset of exercise.RNA, MRNA, AP, HR and force were simultaneouslv recorded on an 8-channel recorder

(Hewlett Packard, 7758A) and stored on an FM magnetic tape-recorder (Ampex). The data wereanalysed off-line with a minicomputer (DEC, PDP-1 1/23).

Experimental protocolThree days after surgery the cats had recovered and were able to perform static exercise. The

experiments were then conducted over the next 5-7 days. A total of eighty-nine exercise trials wereperformed by seven cats, and the trials were divided into three groups according to the durationof the exercise period: (1) thirty-nine trials with a duration of 26-35 s; (2) thirty-three trials of36-45 s duration; (3) seventeen trials of 46-55 s duration.To separate any influence of food reward and/or eating behaviour on the changes in MRNA, HR

and MAP that occurred following static exercise, no food reward was given in twenty-one exercisetrials in six cats. To examine whether or not there was an influence of anticipation and/orconditioning of renal nerve and cardiovascular responses during exercise, the sound of the buzzerwas emitted alone without static exercise in eleven trials in five cats.To examine any influence of the arterial baroreceptor reflex on the renal nerve response during

static exercise, nine static exercise trials were performed in three cats when resting aortic pressurewas significantly elevated by an injection of noradrenaline (2-5 ,ug/kg i.v.).

During naturally occurring body movements such as walking, standing-up, and groomingbehaviours, MRNA, HR and MAP were also measured in five cats.

Collection of data and statistical analysisMRNA, HR, MAP and force were simultaneously sampled for 5-10 min with an analog-to-digital

convertor (sampling frequency, 64 Hz). Their average values over a period of 1 s were calculatedsequentially. The absolute voltage (,uV) of MRNA varied between animals. Therefore, the MRNAmeasured during the control period of 30-60 s before the onset of static exercise (cMRNA) was

456

SYMIPATHETIC NERVE ACTIVITY AND STATIC EXERCISE 457

defined as 100% control value in an individual trial. Then, the percentage change in AIRNA(AMRNA, %) from the cMRNA before, during, and after static exercise was calculated as follows:

AMIRNA (%) - IMRNA (pV)-cMRNA (XV)'cMRNA (flY)

The changes in MRNA, MAP and HR during static exercise were compared with their controlvalues before exercise by the analysis of variance and the Dunnett's method (Snedecor & Cochran,1980). The level of the statistical significanice was defined as P < 0 05. The data in the results andthe figures are expressed as the means+S.E. of mean with sampled numbers.

0 25

Force J , kgO

RNA- 0 level

MRNA- 0 level200

AP j ~ |mmHg

________O level

IZIZi [Exercise |ExerciseI 1( sExercise

Fig. 3. Changes in force, renal sympathetic nerve activity (RNA), mean renal sympatheticnerve activity (MRNA), and arterial blood pressure (AP) during three successive trials ofvoluntary static exercise in an awake cat. RNA, MRNA and AP increased markedly eachtime that static exercise was performed. The increase in RNA tended to occurimmediately before the onset of static exercise whereas the rise in AP was delayed fromthe beginning of exercise.

RESULTS

During rest, HR and MAP were 220 + 7 beats/min and 110 + 4 mmHg in sevencats, respectively. A total of eighty-nine static exercise trials were performed bythese cats. MRNA, HR and MAP increased significantly during static exercise by102+14 %, 23+2 beats/min and 11+1 mmHg, respectively.

Time course of the changes in renal sympathetic nerve activity, heart rate and arterialblood pressure during static exerciseAn example of changes in force, RNA, MRNA and AP during three successive

trials of voluntary static exercise is shown in Fig. 3. RNA, MRNA and AP increasedmarkedly each time that voluntary static exercise was performed. The increase inRNA tended to occur immediately before the onset of static exercise whereas the risein AP was delayed from the beginning of exercise.

4K. MA4TSUKA WA AND OTHERS

The time courses of the average changes in force, MRNA, HR and MAP before,during and after static exercise are summarized in Fig. 4. In seven cats a total ofthirty-nine exercise trials were performed and the duration ranged from 26 to 35 s.The average duration of static exercise was 30+0 5 s. In these trials a food reward

A I B t500400 -

Force (9) 300 [100

0125100751-

Change in 50[MRNA (%) 2,5

-25

Change in HR(beats/min)

Change inMAP (mmHg)

.+i~~~~~~+~* * +*i+*'+

**~~~+4+f +-i+

30 -

20 -

100

-10

105015

-5

lI Exercise II 10 s

Fig. 4. The time courses of the average changes in force. mean renal sympathetic nerve

activity (MRNA), heart rate (HR), and mean arterial blood pressure (AIAP) before,during and after static exercise. A total of thirty-nine exercise trials were performed byseven cats and the duration ranged from 26 to 35 s. The average duration of static exercisewas 30 + 05 s. In these trials a food reward was given after exercise. In A, the changes inforce. AIRNA, HR and NIAP from the pre-exercise values in an individual trial were

aligned at the onset of exercise (a downward arrow) and then averaged. In B, the same

data were also aligned at the cessation of exercise (an upward arrow) and then averaged.The means+S.E. of mean of each variable are indicated by dots and vertical bars.Significant changes from the control are indicated by stars (P < 0 05).

was given after exercise. The data for force, MRNA, HR and MAP in an individualtrial were aligned at the onset of exercise and then averaged (Fig. 4A). The same datawere also aligned at the cessation of exercise and then averaged (Fig. 4B).The force applied to the bar was developed quickly and reached a peak of

536+46 g at 2 s after the onset of exercise. Thereafter, the force development was

maintained at a near constant value with a small range of 400-470 g during the entireperiod of static exercise (Fig. 4A and B).MRNA increased significantly (P < 0-05) during static exercise. The increase in

MRNA had an initial and a late component. The initial increase in MRNA (P < 0 05)began immediately before the onset of exercise and reached a peak value of 92 + 13 %at 2 s after the onset of exercise (Fig. 4A). The initial increase in MRNA lasted fora short period of 10 s and then decreased to near the control value. Thereafter, a late

**:*j*

**..***

458

SYMPATHETIC NERV"E ACTIVITY AND STATIC EXERCISE 459

increase in MRNA gradually developed during the later (14-26 s) period of staticexercise. The peak value of the late increase was 44+9 % at 26 s.HR increased significantly (P < 005) during static exercise (Fig. 4A). The

significant increase in HR occurred 1 s after the onset of exercise and reached a peak

A B

500 r400 -

Force (9) 200 -100log125100 r

Change in 75MRNA (%) 25 F

01:-25.30Q

Change in HR 20 [10 1-(beats/min) 0[

-10

Change inMAP (mmHg)

, iIAA AAA4

AA NAAAAIn,a 8 A A A A A A

HESANASEp '4gStAvy~~

OP,.c. ,cPo°°Ooooo-°°ax~p:O0M%-oo"%bF MJ°

o°t0°SLvRoO6H koR ~~~~~~p

105 -

-5

ZIEise Exercise |

10 s

Fig. 5. Av-erage changes in force, mean renal sympathetic nerve activity (MRNA), heartrate (HR) and mean arterial blood pressure (MAP) during a 36 s period of static exercise(n = 33 trials, A) and a 46 s period (n = 17 trials, B). Even when static exercise wasprolonged for a period of 36 or 46 s, MRNA and MAP remained elevated above the controllevel and tended to increase further throughout the late period of exercise. In contrast,HR tended to remain at the control level during the late period of static exercise.

of 22+2 beats/min at 3 s. The initial increase in HR lasted for 11 s and thereafterHR returned to control and remained at that level throughout the later period ofstatic exercise (Fig. 4A).MAP increased slowly during static exercise (Fig. 4A). The increase was significant

(P < 0 05) at 8 s after the onset of exercise and it was delayed by 10 s from the initialincrease in MRNA. Thereafter, MAP continued to increase throughout the period ofstatic exercise. The peak increase in MAP was 11 + 1 mmHg at 26 s from the onset ofstatic exercise.

It should be noted that there was a similar increase in MRNA and HR without anychange in MIAP at the beginning of static exercise; whereas, during the late exerciseperiod (14-26 s), MRNA and MAP increased but HR returned to the control value.

K. MATSUKA WA AND OTHERS

Exercise duration and the renal sympathetic nerve responseDuring the late (14-26 s) period of exercise, MRNA increased again but HR

remained within the control level. The next question was whether or not there stillexisted such dissociation between the changes in MRNA and HR if static exerciselasted for a longer period.

0 5kg

Force 0

MRNA \ kAv\ - O level

300

HR beats/min1 180

160

AP mmHg60

Buzzer Exercise + buzzer

~ls10 s

Fig. 6. Changes in mean renal sympathetic nerve activity (MRNA), heart rate (HR), andarterial blood pressure (AP) in response to a sound of the buzzer. MRNA and AP wereunchanged in response to the sound of the buzzer alone, although HR tended to increaseslightly. In contrast, MRNA, HR and AP increase markedly in the succeeding trial ofstatic exercise.

The averages changes in force, MRNA, HR and MAP during a 36 s period of staticexercise (n = 33 trials) and a 46 s period (n = 17 trials) are shown in Fig. 5. Evenwhen static exercise was prolonged for a period of 36 or 46 s, MRNA and MAPremained elevated above the control value and tended to increase further throughoutthe late period of exercise. Thus, these late increases in MRNA and MAP were nottransient but were sustained as long as static exercise was continued. In contrast,HR tended to remain at the control value during the late period of static exercise.From these data, it appears that RNA and HR are controlled by differentmechanisms during the late period of static exercise.

Food reward and the renal sympathetic nerve responseWhen a food reward was given immediately after static exercise, MRNA, HR and

MAP increased markedly by 112+19%, 18+3 beats/min and 17+3 mmHg,respectively (Fig. 4B). To examine whether or not the marked increases afterexercise were related to eating behaviour triggered by the food reward, no foodreward was given in twenty-one exercise trials in six cats. The duration of exerciseranged from 14 to 45 s. The average changes in MRNA, HR and MAP following

460

SYMPATHETIC, NERVE ACTIVITY AND STATIC EXERCISE 461

exercise were examined in these trials. In exercise trials without food reward, MRNAand MAP returned to the control levels within 4 s after the cessation of exercise butHR showed a slight and delayed increase of 8-9 beats/min at 7-8 s after the end ofexercise. However, this increase in HR was much smaller (P < 005) than thatobserved after exercise with a food reward.

10075 *

Change in 25 1-MRNA(%) 0 [

20

Change in HR 10

(beats/min) 0-10

10Change in 5 **** ***** *

MAP (mmHg) 0 [ _ _-5

Buzzerl

10 s

Fig. 7. Average changes in mean renal sympathetic nerve activity (MIRNA), heart rate(HR), and mean arterial blood pressure (MAP) in response to the sound of the buzzer ineleven trials in five cats. When the sound of the buzzer was given during rest, HR did notchange significantly while MRNA and MAP increased slightly. The increases in MRNAand MAP was much smaller (P < 0 05) than their increases observed during static exercise(Figs 4 and 5). The means+S.E. of mean of each variable are indicated by dots andvertical bars. Significant changes from the control are indicated by stars (P < 0 05).

Anticipation and/or conditioning and the renal sympathetic nerve responseTo study the possibility that the increases in MRNA, HR and MAP observed

during static exercise were caused by an influence of anticipation and/orconditioning, only the sound emitted by the buzzer was given to a cat withoutstatic exercise.An example of changes in MRNA, HR and AP in response to the sound of the

buzzer is shown in Fig. 6. MRNA and AP were unchanged in response to the soundof the buzzer alone, although HR tended to increase slightly. In contrast, MRNA,HR and AP increased markedly in the succeeding trial of static exercise (Fig. 6). Theaverage changes in MRNA, HR and MAP in response to the sound of the buzzer ineleven trials in five cats are summarized in Fig. 7. When the sound of the buzzer wasgiven during rest, HR did not change significantly while MRNA and MAP increased(P < 005) by 42+17 % and by 4+1 mmHg respectively (Fig. 7). However, the

K. MATSUKAWA A-ND OTHERS

significant increase in MRNA was observed only at 4 s after the onset of the soundof the buzzer and the increases in MRNA and MAP was much smaller (P < 0 05) thantheir increases observed during static exercise. Therefore, the increases in MRNA,HR and MAP observed during static exercise do not appear to be caused byanticipation and/or conditioning but are related to the static exercise per se.

Force 1kg

RNA LLL __ u uJ~L~JLJkili -0O levelMRNA

-0 level

MRNA 0VJ ~ - level

300

HR ~ beats/min

100260

AP tk0YaSyltil|llllSXtS#litE19Sl1lllll1mMllll\vlt.i9! | mmHg60

t Exercise 10 s

Fig. 8. Changes in force, renal sympathetic nerve activity (RNA), mean renal sympatheticnerve activity (MRNA), heart rate (HR) and arterial blood pressure (AP) during staticexercise on intravenous administration of noradrenaline (2 ,ag/kg) in an awake cat. Staticexercise was performed 30 s after a bolus injection of noradrenaline (an upward arrow).When resting AP was elevated by noradrenaline, resting RNA and HR were decreasedand the increases in RNA and HR during static exercise were also inhibited. However,there was a tendency for RNA to increase at the beginning of exercise and during the lateperiod of exercise, as shown by the triangles (V), in spite of the increased arterial bloodpressure due to the injection of noradrenaline.

Static exercise during administration of noradrenaline and the renal sympatheticnerve responseTo examine the influence of the arterial baroreflex on the changes in MRNA, nine

static exercise trials were performed by three cats when resting AP was elevated asshown in Fig. 8. When resting AP was elevated to 146 + 3 mmHg by a bolus injectionof noradrenaline (2-5,tg/kg i.v.), resting RNA, MRNA and HR were decreased.Static exercise was performed 52+15 s after the bolus injection of noradrenaline.The increases in RNA and HR during static exercise were also inhibited as shownin Fig. 8. However, there was a tendency for RNA to increase at the beginning ofexercise and during the late period of exercise, in spite of the increased arterial bloodpressure due to the injection of noradrenaline.

462

SYMPATHETIC NERVE ACTIVITY AND STATIC EXERCISE

Spontaneous body movement and the renal sympathetic nerve responseSpontaneous body movements such as walking, standing up and grooming

behaviour were observed. Changes in MRNA, HR and AP were measured in a totalof eighteen spontaneous body movements in five cats as shown in Fig. 9. MRNA, HR

MRNA _ -0 level

J 60

300

HR ] beats/minGo160

r ~~~~Grooming

10 s

Fig. 9. Changes in mean renal sympathetic nerve activity (MRNA), arterial blood pressure(AP) and heart rate (HR) during grooming behaviour in an awake cat. Spontaneous bodymovement occurred with grooming behaviour. The magnitude and the time course of theincreases in MRNA, HR and AP during spontaneous body movements appeared to besimilar to the increases observed during static exercise.

and MAP increased by 116 + 15 %, 28 + 3 beats/min, and 26+ 4 mmHg during bodymovements. The magnitude of the increases in MRNA and HR during bodymovements were the same as the increases observed during static exercise. Also, theincreases in MRNA and HR tended to occur almost simultaneously with the onsetof body movement. Therefore, the magnitude and the time course of the increases inMRNA, HR and AP during spontaneous body movements appear to be similar to theincreases observed during static exercise.

DISCUSSION

The present study has shown for the first time that renal sympathetic efferentnerve activity (RNA) increases during voluntary static (isometric) exerciseperformed by conscious cats. This finding indicates that activation of sympatheticefferents to the kidney is involved in the cardiovascular adjustments during staticexercise. The analysis of the time course of RNA during exercise showed that theincrease in RNA consisted of an initial and a late component. The initial increase inRNA was abruptly induced just before the onset of static exercise, stronglysuggesting that it is probably caused by descending input from higher brain centresand not from a reflex mechanism originating in the exercising muscle. In contrast,the late increase in RNA began 14 s following the beginning of exercise and wassustained as long as static exercise was continued. Thus, the late increase in RNA

463

K. MATSUKAWA AND OTHERS

during static exercise tends to be related to muscle metabolism and a feedback signalfrom the exercising muscle may play a role.

In this study, the sound of a buzzer was used as an audio-feedback signal fortraining the animal to maintain a force above a pre-set value. Thus, the animals mayhave anticipated a food reward from the sound of the buzzer and such anticipationcould evoke increases in RNA, heart rate (HR) and arterial blood pressure (AP). Toexamine this possibility, only the sound of the buzzer was given to the cats withoutstatic exercise being performed. Even though RNA and AP increased in response tothe sound, the increase in RNA lasted for only a brief period of 1 s and the increasesin RNA and AP were much smaller than those observed during static exercise (Fig.7). Further, the cats voluntarily pressed the bar which in turn produced the soundof the buzzer. The initial increase in RNA preceded the sound of the buzzer,suggesting that the increase in RNA occurred independently of the sound. Inaddition RNA, HR and AP increased during spontaneous body movements, and themagnitude of the responses was similar to those seen during static exercise. Theseresults suggest that the major changes in RNA, HR and AP are associated withvoluntary static exercise and are not caused by any influence of anticipation and/orconditioning.During the food reward following static exercise, RNA increased markedly (Fig 4).

This increase in RNA following static exercise was associated with eating behaviourtriggered by a food reward, as has been reported previously (Matsukawa &Ninomiya, 1987), because RNA returned to control immediately after the cessationof exercise without a food reward. It is of interest that the initial increase in RNAat the beginning of the eating behaviour (Fig. 4B) seemed to have the same timecourse and response magnitude as the initial increase at the beginning of staticexercise (Fig. 4A).From the time course of the changes that occur in RNA, HR and AP during static

exercise, the neural mechanisms that are responsible for the cardiovascularadjustments can be explained. The time course of the increases in RNA and HR atthe beginning of exercise was the same (Figs 4 and 5). RNA and HR began toincrease at or immediately before the onset of static exercise and the initial increaseslasted for about 10 s. The rapid onset of the increases in RNA and HR suggests thatthey are not due to a reflex induced by stimulation of mechanosensitive and/ormetabolism-sensitive receptors in the contracting muscle. Also, since the increases inRNA and HR at the beginning of exercise appeared without any significant changein AP, they are unlikely to be a reflex initiated by arterial baroreceptors. Therefore,the initial increases in RNA and HR seem to be evoked directly by descending inputfrom higher brain centres (central command) (Mitchell & Schmidt, 1983; Mitchell,1985, 1990).Regarding the pure reflex response of RNA arising from the contracting skeletal

muscle, it has been recently reported that RNA increases significantly during evokedisometric muscle contraction in anaesthetized cats (Victor, Rotto, Pryor & Kaufman,1989; Matsukawa, Wall, Wilson & Mitchell, 1990). This reflex increase in RNAreached a peak value of about 50% at 10-20 s after the onset of contraction and wassustained throughout the period of contraction (Matsukawa et al. 1990). The timecourse and magnitude of the reflex increase in RNA is quite different from that of the

464

SYMPATHETIC NERVE ACTIVITY AND STATIC EXERCISE 465

increase in RNA during voluntary static exercise in awake cats examined in thisstudy. This difference between the responses of RNA in the anaesthetized conditionand the awake condition supports our hypothesis that the initial increase in RNA atthe beginning of static exercise in conscious cats is caused by descending input fromhigher brain centres and not by afferent feedback signals from the exercising muscle.

In contrast to the initial phase of static exercise, there was a close relationshipbetween the increases in RNA and AP during the late period of exercise (14-46 s fromthe onset of exercise). AP tended to increase in proportion to the late increase inRNA, suggesting that activation of renal sympathetic nerve efferents actually maycause renal vasoconstriction. In fact, Diepstra et al. (1982) reported that renal tissueblood flow measured with the radioactive microsphere technique decreased duringstatic exercise using the same preparations as the present study. Thus, the increasein RNA during static exercise should induce a decrease in renal blood flow and anincrease in renal vascular resistance. This increase in renal vascular resistance maycontribute to the rise in AP, in concert with increases in resistance in other vascularbeds.

There was a dissociation, however, between the changes in RNA and HR duringthe late period of static exercise. RNA increased again whereas HR remained nearthe control value (Figs 4 and 5). This dissociation between RNA and HR during thelate period of exercise suggests that RNA and HR are controlled during that periodby different neural mechanisms. In humans, it has been demonstrated that there isa dissociation between the changes in HR and sympathetic nerve activity to non-exercising skeletal muscle during static and rhythmic hand grip exercise (Mark,Victor, Nerhed & Wallin, 1985; Victor, Seals & Mark, 1987). HR increases at thebeginning of static exercise whereas muscle sympathetic nerve activity does notincrease during the first 1 min period of static exercise but increases progressivelythereafter (Mark et al. 1985). Furthermore, muscle ischaemia induced by vascularocclusion at the termination of static or rhythmic exercise, which is assumed tostimulate muscle metabolism receptors, augments muscle sympathetic nerve activitybut HR returns to the control value (Mark et al. 1985; Victor et al. 1987). Theseresults suggest that sympathetic or parasympathetic efferent nerve activity to theheart may have a response pattern different from sympathetic nerve activity toother peripheral vascular beds during static exercise in conscious animals andhumans. Thus, individual vascular beds may be separately controlled by theautonomic nervous system during static exercise.The dissociation between the changes in RNA and HR during the late period of

exercise might be explained by a difference of the arterial baroreflex influences onRNA and HR during this time. Since AP continued to increase during the late periodof exercise, the arterial baroreflex initiated by the rise in blood pressure shouldinfluence RNA and HR. To examine any influence of the arterial baroreflex, staticexercise was performed while AP was elevated by a bolus injection of noradrenaline.RNA and HR were decreased at rest by the arterial baroreflex and remained atreduced levels even during static exercise (Fig. 8), suggesting that most of theincreases in RNA and HR associated with static exercise were not able to overcomethe baroreceptor-dependent inhibition of RNA and HR. Thus, it appears that thearterial baroreflex is active during static exercise, as well as at rest, and inhibits not

K. MA TSUKT4A WTA AND OTHERS

only HR but also RNA. Also, it is unlikely that stimulation of the arterialbaroreceptors initiated by the rise in AP during the late period of exercise hasqualitatively different influences on RNA and HR.

If the gain of the arterial baroreceptor-RNA reflex during static exercise isassumed to be the same as the gain at rest (1-7-1-9%/mmHg) (Ninomiya,Matsukawa, Honda, Nishiura & Nabuchi, 1988; Matsukawa & Ninomiya, 1989), thebaroreflex due to the late increase in AP (11 mmHg) would cause a decrease in RNAby about 20% during the late period of exercise. Instead, RNA was slowly butprogressively increased by 40-50% during the late period of exercise, in spite of therise in AP. Thus, this finding suggests that neural control to the kidney may receivean excitatory input during the late period of exercise and the excitatory input mayovercome an inhibitory input from the arterial baroreceptors activated by the rise inAP. In the case of HR, however, both the excitatory and inhibitory inputs may bebalanced during the late period of exercise. The difference of the excitatory inputs forthe kidney and for the heart may explain the dissociation between the changes inRNA and HR during the late period of exercise.

It has been reported that the increase in muscle sympathetic nerve activity to non-exercising muscle during static hand grip exercise in humans is related to aprogressive increase in the EMG of the exercising muscle (Seals & Enoka, 1989) andto fatigue sensation (Saito, Mano & Iwase, 1989), suggesting that the increase inmuscle sympathetic nerve activity is related to the development of muscle fatigueduring static exercise in humans. Further, ischaemia of the exercising musclefollowing static exercise enhances muscle sympathetic nerve activity (Mark et al.1985). Taken together, these results suggest that a feedback signal from musclemetabolism receptors in the fatiguing muscle may cause increases in musclesympathetic nerve activity during static exercise in humans.

In this study, it is possible that muscle fatigue tends to occur during the late phaseof static exercise in conscious cats, since force development was kept almost constantfor a period of 46 s. The fact that RNA and AP increased gradually and progressivelyas long as static exercise was continued is in favour of the assumption that the lateincreases in RNA and AP may be related to the development of muscle fatigueduring the late period of exercise in conscious cats. However, it remains unknownwhether a feedback signal from metabolism receptors in the fatiguing muscle elicitsa reflex increase in sympathetic nerve activity (McCloskey & Mitchell, 1972; Mitchell& Schmidt, 1983), or central command from higher brain centres augmentssympathetic nerve activity when muscle fatigue occurs (Schibye, Mitchell, Payne &Saltin, 1981).

It is interesting that some burst discharges ofRNA still appeared at the beginningof static exercise and during the late period of static exercise even though resting APwas elevated by noradrenaline (Fig. 8). From this result, it can be speculated that aportion of the increase in RNA during static exercise may be induced through aneural pathway independent of the arterial baroreflex pathway.

In conclusion, the initial increases in RNA and HR at the onset of static exerciseare probably caused by direct descending input from higher brain centres and not bya reflex arising from the exercising muscle or from arterial baroreceptors. In contrast,the increase in RNA during the late period of static exercise may be related to musclemetabolism and a feedback signal from the exercising muscle.

466

SYMPATHETIC, NERVE ACTIVITY AND STATIC EXERCISE 467

WVe greatly thank Juan Baez, Pamela MIaass and Cindy Lawson. This research was supported bya NHLBI Program Project Grant No. HL06296, NHLBI Training Grant No. HL07360 (Drs Walland WVilson), the Lawson and Rogers Lacy Research Fund in Cardiovascular Diseases, and theFrank M. Ryburn Jr Chair in Heart Research.

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