242 Salt Supplementation Does Not Alter the Pressor Effect of Blocking Atrial Natriuretic Peptide in Nucleus Tractus Solitarii Renhui Yang, Hongkui Jin, James Michael Wyss, Yiu-Fai Chen, and Suzanne Oparil We have previously shown that microinjection of monoclonal antibody to atrial natriuretic peptide (ANP) into the caudal nucleus tractus solitarii causes a pressor response in salt-sensitive spontaneously hypertensive rats (SHR) fed a basal (1%) salt diet, suggesting that endogenous ANP in this region may be involved in the centrally mediated regulation of blood pressure in this model. The present study tested the hypothesis that the pressor effect of blocking endogenous ANP in caudal nucleus tractus solitarii is enhanced by dietary salt supplementation in salt-sensitive SHR. Monoclonal antibody to ANP (0.55 /ig) in 50 nl artificial cerebrospinal fluid or control immunoglobulin G was microinjected into the caudal nucleus tractus solitarii of conscious salt-sensitive SHR, salt-resistant SHR, and Wistar-Kyoto rats fed 1% or 8% salt diets for 3 weeks. Microinjection of the monoclonal antibody into the caudal nucleus tractus solitarii evoked similar increases in mean arterial pressure hi salt-sensitive SHR on both 1% and 8% salt diets and in salt-resistant SHR on a 1% salt diet but had no effect in Wistar-Kyoto rats. In contrast, microinjection of control immunoglobulin G into this brain area did not alter mean arterial pressure or heart rate in any experimental group. Thus, endogenous ANP in caudal nucleus tractus solitarii mediates tonic control of blood pressure in both salt-sensitive and sail-resistant SHR hut not in Wistar-Kyoto rats, and this effect is independent of the salt sensitivity of hypertension and of dietary salt intake. (Hypertension 1992^0:242-246) KEY WORDS • antibodies, monoclonal • natriuretic peptides, atrial • brain • microinjections • sodium, dietary • blood pressure O ur previous studies have demonstrated that salt-sensitive spontaneously hypertensive rats (SHR-S) from Taconic Farms (IBU3 colony, Germantown, N.Y.) exhibit significant increases in blood pressure and sympathetic outflow when fed a high-NaCl diet. 1 " 3 In contrast, salt-resistant SHR (SHR-R) from Charles River Breeding Laboratories (Kingston, N.Y.) and nonnotensive control Wistar- Kyoto (WKY) rats do not develop these changes in blood pressure and sympathetic nervous system activity in response to dietary NaCl supplementation. We re- cently demonstrated that microinjection of monoclonal antibody (MAb) to atrial natriuretic peptide (ANP) into the caudal nucleus tractus solitarii (C-NTS) produces significant increases in blood pressure in SHR-S fed basal (1%) NaCl diets but not in WKY rats. 4 Control injection of an equal volume of immunoglobulin G (IgG) into the C-NTS had no effect on blood pressure in SHR-S. Furthermore, control microinjection of the From the Vascular Biology and Hypertension Program, Division of Cardiovascular Disease, Departments of Medicine and Cell Biology, University of Alabama at Birmingham. Supported in part by research grants HL-22544, HL-07457, HL-35051, HL-37722, HL-44195, and HL-47081 from the Na- tional Heart, Lung, and Blood Institute, National Institutes of Health, and by the Council for Tobacco Research, U.S.A., Inc. Address for correspondence: Suzanne Oparil, MD, 1034 Zeigler Research Building, UAB Station, Birmingham, AL 35294. Received December 30, 1991; accepted in revised form March 30, 1992. MAb into the hypoglossal nucleus, spinal trigeminal nucleus, or cuneate nucleus did not significantly alter blood pressure in either strain. These data suggest that endogenous ANP in C-NTS may be involved in the centrally mediated regulation of blood pressure in SHR-S. Our laboratory has previously shown that arterial and cardiopulmonary baroreceptor reflex-mediated control of lumbar sympathetic nerve activity in SHR-S main- tained on a basal NaCl diet is impaired. SHR-S main- tained on a 1% NaCl diet have blunted baroreceptor reflex control of lumbar sympathetic nerve activity during acute increases or decreases in mean arterial pressure (MAP) and during acute plasma volume ex- pansion compared with SHR-R, WKY, and Sprague-Dawley rats. 5 - 7 Furthermore, we have ob- served that high dietary NaCl exposure enhances arte- rial baroreceptor reflex control of lumbar sympathetic nerve activity in SHR-S during phenylephrine-induced increases in MAP. 7 In contrast, high dietary NaCl exposure exacerbates the impairment in lumbar sympa- thetic nerve activity responses to acute volume expan- sion in SHR-S (unpublished observation). The finding of further blunting of an already impaired cardiopul- monary baroreceptor reflex in SHR-S during dietary NaCl supplementation is consistent with the observed inability of NaCl-sensitive hypertensive subjects to re- duce sympathetic nervous system activity appropriately in response to volume expansion. by guest on April 9, 2018 http://hyper.ahajournals.org/ Downloaded from
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242
Salt Supplementation Does Not Alter thePressor Effect of Blocking Atrial Natriuretic
Peptide in Nucleus Tractus Solitarii
Renhui Yang, Hongkui Jin, James Michael Wyss, Yiu-Fai Chen, and Suzanne Oparil
We have previously shown that microinjection of monoclonal antibody to atrial natriuretic peptide (ANP)into the caudal nucleus tractus solitarii causes a pressor response in salt-sensitive spontaneouslyhypertensive rats (SHR) fed a basal (1%) salt diet, suggesting that endogenous ANP in this region may beinvolved in the centrally mediated regulation of blood pressure in this model. The present study tested thehypothesis that the pressor effect of blocking endogenous ANP in caudal nucleus tractus solitarii isenhanced by dietary salt supplementation in salt-sensitive SHR. Monoclonal antibody to ANP (0.55 /ig)in 50 nl artificial cerebrospinal fluid or control immunoglobulin G was microinjected into the caudalnucleus tractus solitarii of conscious salt-sensitive SHR, salt-resistant SHR, and Wistar-Kyoto rats fed 1%or 8% salt diets for 3 weeks. Microinjection of the monoclonal antibody into the caudal nucleus tractussolitarii evoked similar increases in mean arterial pressure hi salt-sensitive SHR on both 1% and 8% saltdiets and in salt-resistant SHR on a 1% salt diet but had no effect in Wistar-Kyoto rats. In contrast,microinjection of control immunoglobulin G into this brain area did not alter mean arterial pressure orheart rate in any experimental group. Thus, endogenous ANP in caudal nucleus tractus solitarii mediatestonic control of blood pressure in both salt-sensitive and sail-resistant SHR hut not in Wistar-Kyoto rats,and this effect is independent of the salt sensitivity of hypertension and of dietary salt intake.(Hypertension 1992^0:242-246)
Our previous studies have demonstrated thatsalt-sensitive spontaneously hypertensive rats(SHR-S) from Taconic Farms (IBU3 colony,
Germantown, N.Y.) exhibit significant increases inblood pressure and sympathetic outflow when fed ahigh-NaCl diet.1"3 In contrast, salt-resistant SHR(SHR-R) from Charles River Breeding Laboratories(Kingston, N.Y.) and nonnotensive control Wistar-Kyoto (WKY) rats do not develop these changes inblood pressure and sympathetic nervous system activityin response to dietary NaCl supplementation. We re-cently demonstrated that microinjection of monoclonalantibody (MAb) to atrial natriuretic peptide (ANP) intothe caudal nucleus tractus solitarii (C-NTS) producessignificant increases in blood pressure in SHR-S fedbasal (1%) NaCl diets but not in WKY rats.4 Controlinjection of an equal volume of immunoglobulin G(IgG) into the C-NTS had no effect on blood pressure inSHR-S. Furthermore, control microinjection of the
From the Vascular Biology and Hypertension Program, Divisionof Cardiovascular Disease, Departments of Medicine and CellBiology, University of Alabama at Birmingham.
Supported in part by research grants HL-22544, HL-07457,HL-35051, HL-37722, HL-44195, and HL-47081 from the Na-tional Heart, Lung, and Blood Institute, National Institutes ofHealth, and by the Council for Tobacco Research, U.S.A., Inc.
Address for correspondence: Suzanne Oparil, MD, 1034 ZeiglerResearch Building, UAB Station, Birmingham, AL 35294.
Received December 30, 1991; accepted in revised form March30, 1992.
MAb into the hypoglossal nucleus, spinal trigeminalnucleus, or cuneate nucleus did not significantly alterblood pressure in either strain. These data suggest thatendogenous ANP in C-NTS may be involved in thecentrally mediated regulation of blood pressure inSHR-S.
Our laboratory has previously shown that arterial andcardiopulmonary baroreceptor reflex-mediated controlof lumbar sympathetic nerve activity in SHR-S main-tained on a basal NaCl diet is impaired. SHR-S main-tained on a 1% NaCl diet have blunted baroreceptorreflex control of lumbar sympathetic nerve activityduring acute increases or decreases in mean arterialpressure (MAP) and during acute plasma volume ex-pansion compared with SHR-R, WKY, andSprague-Dawley rats.5-7 Furthermore, we have ob-served that high dietary NaCl exposure enhances arte-rial baroreceptor reflex control of lumbar sympatheticnerve activity in SHR-S during phenylephrine-inducedincreases in MAP.7 In contrast, high dietary NaClexposure exacerbates the impairment in lumbar sympa-thetic nerve activity responses to acute volume expan-sion in SHR-S (unpublished observation). The findingof further blunting of an already impaired cardiopul-monary baroreceptor reflex in SHR-S during dietaryNaCl supplementation is consistent with the observedinability of NaCl-sensitive hypertensive subjects to re-duce sympathetic nervous system activity appropriatelyin response to volume expansion.
Because the C-NTS plays an important role in barore-ceptor reflex-mediated control of blood pressure andsympathetic outflow, both of which are sensitive todietary NaCl intake in the SHR model, the presentstudy was designed to test whether the pressor effect ofblocking endogenous ANP in the C-NTS with MAb isaffected by dietary NaCl supplementation in SHR-S.We studied SHR-S on 1% and 8% NaCl diets, SHR-Ron a 1% NaCl diet, and WKY rats on 1% and 8% NaCldiets. Microinjection of IgG purified from mouse ascitesfluid into the C-NTS served as a vehicle control. Wefound that blockade of endogenous ANP in the C-NTSincreased blood pressure to the same extent in SHR-Sfed 1% and 8% NaCl diets and in SHR-R fed the 1%NaCl diet but had no effect in WKY rats on either diet.Thus, endogenous ANP in the C-NTS mediates toniccontrol of blood pressure in both SHR-S and SHR-Rbut not in WKY rats, and this effect is independent ofthe NaCl sensitivity of hypertension and of dietary NaClintake.
MethodsMale SHR-S and normotensive WKY control rats
were obtained from Taconic Farms (IBU-3 colony,Germantown, N.Y.) at 7 weeks of age. Male SHR-Rwere obtained from Charles River Breeding Laborato-ries (Kingston, N.Y.) at the same age. All rats weremaintained four per cage at constant humidity(65±5%), temperature (24±1°C), and light cycle (6AM-6 PM). Three days after arrival, one half of theSHR-S and WKY rats were placed at random on an 8%NaCl diet (ICN Biochemicals Purina Chow with 8%NaCl, Costa Mesa, Calif.), and the other half remainedon the 1% NaCl diet (Ralston-Purina Diet 5001, St.Louis, Mo.). SHR-R were placed on the 1% NaCl diet.Food and water were available ad libitum throughoutthe study.
Nineteen days after initiation of the special diets,each rat was anesthetized with sodium pentobarbital (50mg/kg i.p.), and a cannula (polyethylene PE-10 fusedwith PE-50) was implanted into the abdominal aortathrough the right femoral artery. The rat was thenplaced into a stereotaxic apparatus, the skin overlyingthe middle of the skull was incised, and a small hole wasdrilled through the appropriate portion of the skull. Aguide cannula (26-gauge stainless steel tubing) waslowered to a position 2.0 mm dorsal to the C-NTS(anterior/posterior, 4.8-5.1 mm from the interauralline; medial/lateral, 0.5 mm; dorsal/ventral, 8.5 mm;incisor bar, 4.5 mm). The guide cannula was fixed to theskull with stainless steel screws and fast polymerizedcannula cement. A 32-gauge obturator (stainless steelwire) was inserted into the guide cannula afterimplantation.
Forty-eight hours after surgery, the arterial catheterwas connected to a model CP-01 pressure transducer(Century Technology Co., Inglewood, Calif.) coupled toa polygraph (Model 7, Grass Instruments, Quincy,Mass.). MAP and heart rate (HR) were measuredsimultaneously. After a 45-minute stabilization period,the obturator was removed from the guide cannula andreplaced with an inner cannula (32-gauge stainless steeltubing) filled with the agent to be administered. The tipof the inner cannula extended 2 mm beyond the guidecannula. The inner cannula was attached to a 0.5 -/xl
Hamilton syringe through tubing (PE-20) filled withartificial cerebrospinal fluid. A small air bubble wasmade between the artificial cerebrospinal fluid and theinjection solution. After insertion of the inner cannulaand the return of vital signs to baseline, each rat wasmicroinjected with either MAb to ANP (MAb KY-ANP-II) (0.55 ^g) purified by the procedure outlinedbelow or mouse IgG (0.55 fig) purified from ascites fluidas a control. All injections were made in 50 nl artificialcerebrospinal fluid. Each rat received only a singleinjection. All microinjection experiments were carriedout in conscious, free-moving rats.
At the conclusion of each experiment, 1% methyleneblue solution (50 nl) was injected through the cannula.The rat was anesthetized with sodium pentobarbital (60mg/kg i.p.) and decapitated, and the cannula was re-moved from the brain. The brain was removed from theskull and sectioned at 30 t̂m on a freezing microtome(Slee Medical Equipment Ltd., London). Sections weremounted and stained with 1% thionine for verificationof the microinjection site and for measurement of extentof spread of the dye.
The MAb used in these studies was the high-affinityantibody against rat a-ANP, the 28-amino acid form ofANP, produced by Mukoyama et al8 and named MAbKY-ANP-II. MAb KY-ANP-II recognizes human ANP(a-h ANP) and rat ANP (o-r ANP) equally and blocksthe ability of both exogenous and endogenous ANP toelevate plasma cyclic GMP (cGMP) levels.9 Further-more, elevated plasma cGMP levels in stroke-proneSHR (SHRSP) and deoxycorticosterone acetate-salt-hypertensive rats were significantly reduced by intrave-nous administration of MAb KY-ANP-II, indicatingthat the antibody can block the activity of a-x ANP inthe intact rat. We purified IgG containing MAb KY-ANP-II from mouse ascites fluid (1 ml) using a proteinA agarose column.10 Retained IgG with MAb KY-ANP-II was eluted from the protein A column with 3 MMgCl and dialyzed against 0.9% saline overnight. Wedemonstrated that the purified IgG (1.1 mg/ml) withMAb KY-ANP-II bound 50% of l2I-ANP (17,000 cpm)at 1:100,000 final dilution in a total volume of 500 /tl.11
In addition, we observed that intravenous injection of a100-Mg dose of purified MAb KY-ANP-II inhibited theincrease in plasma cGMP induced by exogenous ANP(20 Mg/kg i.v.) in the intact rat,12 confirming the previ-ous characterization of Itoh et al.9 The dose of MAbKY-ANP-II (0.55 fig) administered in the present studyis equivalent to the amount of anti-ANP antibodycontained in 0.55 ,̂1 of mouse ascites fluid, 0.5% of theperipheral intravenous dose (100 fil of ascites fluid) ofthis MAb used in previous studies by Itoh et al.9
Statistical AnalysisResults are expressed as mean±SEM. Analysis of
variance (ANOVA) was performed to assess the differ-ences in MAP and HR responses to MAb KY-ANP-IIamong the five experimental groups and to comparedifferences over time in each group. Significant differ-ences were then subjected to Neuman-Keuls post hocanalysis. A value of p<0.05 was considered significant.
ResultsTwenty-five SHR-S, 20 WKY rats, and nine SHR-R
were studied. Histological examination confirmed that
lightly counterstained, coronal sectionfrom an 8% NaCl-fed salt-sensitivespontaneously hypertensive rat inwhich a 50-nl injection of the monoclo-nal antibody to atrial natriwetic pep-tide elicited a pressor response. Thedarkly stained area in the caudal nu-cleus tractus solitarii marks the meth-ylene blue-labeled injection site. NTS,nucleus tractus solitarii; MNV, dorsalmotor nucleus of the vagus; AP, areapostrema; HN, hypoglossal nucleus.
-•v
cannulas were properly placed in the C-NTS in 21SHR-S (10 on the 1% NaCl diet and 11 on the 8% NaCldiet), 17 WKY rats (10 on the 1% NaCl diet and sevenon the 8% NaCl diet), and eight SHR-R on the 1%NaCl diet. In one SHR-S and one WKY rat, the cannulaentered the cerebellum; in two SHR-S and one WKYrat, the cannula was placed in the hypoglossal nucleus.In one SHR-S and one WKY rat, the cannula pene-trated the superior cerebellar vessels; in one SHR-R,the cannula damaged NTS tissue. These four SHR-S,three WKY rats, and one SHR-R were excluded fromthe analysis of experimental results. Examination of 1%thionine-stained sections revealed that the extent ofspread of the injectate was <200 fim (Figure 1). Neu-rons near the injection tip had normal morphology inNissl-stained sections, indicating little damage at thissite.
SHR-S and SHR-R on the 1% NaCl diet had signif-icantly higher pretreatment MAP than WKY rats oneither diet at the time of study (Table 1). High NaClintake caused significant increases in MAP in SHR-Sbut not in WKY rats. There was no significant differencein MAP between SHR-S and SHR-R on the 1% NaCl
TABLE 1. Pretreatment Levels of Mean Arterial Pressure, HeartRate, and Body Weight
MAP, mean arterial pressure: HR, heart rate: BW. body weight;bpm. beats per minute: SHR-S, salt-sensitive spontaneously hyper-tensive rats: SHR-R, salt-resistant SHR; WKY. Wistar-Kyoto rats.
*p<0.01 compared with WKY on either diet.tp<0.05 compared with 1% SHR-S or SHR-R.
diet. There was no difference in pretreatment HRamong the five experimental groups (Table 1).
Microinjection of MAb KY-ANP-II into the C-NTSresulted in significant increases in MAP in SHR-S onboth diets and in SHR-R on the 1% NaCl diet but notin WKY rats on either diet (Figure 2, panel A). InSHR-S and SHR-R, the pressor responses to MAbKY-ANP-II began almost immediately after injection,reached maximal levels at 5 minutes, and returned tobaseline by 40 minutes after injection. High NaCl intakedid not significantly alter the pressor response to themicroinjected MAb in SHR-S. There was no significantdifference in the pressor response to MAb KY-ANP-IIbetween SHR-S on either diet and SHR-R on the 1%NaCl diet. Microinjection of MAb KY-ANP-II into theC-NTS did not alter HR significantly in any diet/straingroup (Figure 2, panel B). Microinjection of control IgGinto the C-NTS did not alter MAP or HR significantly inany diet/strain group (w = 3 in each group). All controlIgG injections were histologically verified as beingwithin the C-NTS.
DiscussionThe present study demonstrated that microinjection
of MAb KY-ANP-II into the C-NTS produced signifi-cant increases in MAP in SHR-S on either diet and inSHR-R on the 1% NaCl diet but not in WKY rats oneither diet. There was no significant difference in thepressor response to the MAb among the three SHRgroups. Control injection of an equal volume of IgG intothe caudal NTS had no effect on MAP in any diet/straingroup. These data suggest that endogenous ANP in thecaudal NTS may be involved in the centrally mediatedregulation of blood pressure in SHR-S and SHR-R butnot in WKY rats and that this effect is independent ofthe NaCl sensitivity of hypertension and of dietary NaClintake.
Previous studies have shown that ANP and its recep-tors are localized on cell bodies and nerve terminals inthe NTS'3 1 5 and that microinjection of ANP into NTS
o — o IX Hod, WKT/Wb (n-7)• — • 1X Nod SHR-S/U& (n-7)A—A W Mod «Cr/U«) (n-4)A A 8X NO
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FIGURE 2. Graphs showing effects of microinjection ofmonoclonal antibody (MAb) to at rial natriuretic peptide intocaudal nucleus tractus solitarii on mean arterial pressure(MAP) (panel A) and heart rate (HR) (panel B) in NaCl-sensitive spontaneously hypertensive rats (SHR-S), NaCl-resistant SHR (SHR-R), and Wistar-Kyoto (WKY) rats on1% NaCl and 8% NaCl diets. **p<0.01, comparison be-tween SHR and WKY groups over first 30 minutes (two-wayanalysis of variance), bpm, Beats per minute.
produces significant increases in the firing rate of NTSneurons associated with reductions in arterial pressurein anesthetized Wistar rats.16-17 In the latter studies, themajority of ANP-responsive sites (85%) were locatedbetween 0.55 mm rostral and 1.5 mm caudal to the obex,corresponding to the site of termination of baroreceptorand chemoreceptor afferents.17 It has also been shownthat the single units excited by microinjection of ANPinto NTS are excited by activation of arterial barorecep-tors and inhibited by baroreceptor unloading.17 Further-more, studies from our own laboratory have demon-strated that administration of exogenous ANP into theC-NTS blunts the bradycardiac response to systemicadministration of phenylephrine.18 Together, these find-ings suggest that ANP in the NTS participates inbaroreceptor reflex activation. Our observation thatblockade of endogenous ANP in C-NTS with MAbKY-ANP-II caused rapid-onset pressor responses inSHR-S fed a basal NaCl diet supports this interpreta-tion.4 The pressor response to microinjection of theanti-ANP antibody decreased progressively in magni-tude with increasing distance rostral in the NTS, sug-gesting that the neuronal population involved was in thebaroreceptor reflex pathway.
The finding that injection of MAb KY-ANP-II intothe C-NTS has a pressor effect in SHR-S and SHR-R
but not in WKY rats is consistent with previous evi-dence that ANP stores and receptor numbers are al-tered in SHR compared with WKY control rats. Studiesfrom a number of laboratories have demonstrated thatthe ANP content of the hypothalamus, pons, and sep-tum is significantly elevated in SHR compared withage-matched WKY controls.19-23 Furthermore, intrave-nous injection of ANP into the intact rat or applicationof ANP to brain slices in vitro causes greater increasesin cGMP levels in hypothalamus and brainstem of SHRthan of WKY rats.22-24 Thus, levels of ANP-sensitiveparticulate guanylate cyclase activity in the hypothala-mus and brain stem appear to be greater in SHR than inWKY rats. The NTS contains a particularly high densityof ANP binding sites,25-26 and NTS neurons display asignificantly greater increase in cGMP after administra-tion of ANP to slice preparations than do most otherneurons in the central nervous system.27 Although thefunctional significance of these alterations in endoge-nous brain ANP and its second messenger with respectto cardiovascular regulation has only begun to be stud-ied, the current results lend further support to thehypothesis that the central ANP system is altered inhypertensive rats.
The location of the neurons that innervate the C-NTSremains incompletely defined. Although in the rat thenodase ganglion contains ANP,28-29 and ANP-positiveneurons have been identified in dorsal root sensoryganglia,30 it has yet to be demonstrated that the vagalafferents to the NTS use ANP as a neurotransmitter.Several areas that project to the NTS contain ANPimmunoreactive-positive neurons, as do some NTS neu-rons themselves.25-31 Any of these could be responsiblefor the effects observed in response to microinjectionsof ANP or MAb KY-ANP-II into the C-NTS of SHR-S,but further studies are needed to define the neuronalpathways involved.
Combined with the results of previous immunocyto-chemical and electrophysiological studies of ANP andits receptors in NTS, the present finding that blockadeof endogenous ANP in C-NTS elicits a pressor responsein the SHR is consistent with the hypothesis that NTSneurons are tonically activated by endogenous ANP inthis model of hypertension. This would tend to bufferthe hypertension in SHR, and the hypertension wouldbecome more severe when the ANP was removed, as byadministration of an anti-ANP antibody. The absence ofa pressor response to blockade of endogenous ANP inNTS of WKY rats suggests that NTS neurons are nottonically activated by ANP in the normotensive WKYrat. Tonic activation of the central baroreceptor reflexarc by ANP in the SHR could lead to blunting ofbaroreceptor reflex responsiveness to stimulation byvolume expansion and phenylephrine infusion in SHRcompared with WKY rats,5 contributing to the centraldefect in the baroreceptor reflex pathway previouslydescribed in SHR.32 The observation that the pressorresponsiveness to blockade of endogenous ANP in theNTS was not altered by dietary NaCl supplementationin SHR-S is consistent with previous findings thatsensitivities of the arterial and cardiopulmonary re-flexes are shifted in opposite directions by dietary NaClsupplementation in this model, likely leading to no netchange in overall baroreceptor reflex sensitivity. This,plus the finding that the magnitude of the pressor
response to injection of anti-ANP antibody into theNTS was the same in SHR-S on both diets as in SHR-R,suggests that tonic control of blood pressure by endog-enous ANP in NTS of SHR is independent of the NaClsensitivity of hypertension and of dietary NaCl intake.
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