Acta physiol. scand. 1971. 82. 375-381 From the Institute of Physiology, University of Oslo, Norway

Hyperosmolarity and Pulmonary Vascular Capacitance BY

G. BQ, A. HAUGE and G. NICOLAYSEN

Received 15 January 1971

B0, G., A. HAUGE and G. NICOLAYSEN. Hyperosmolarity and Pulmonary Vascular Capacitance. Acta physiol. scand. 1971. 82. 375-381.

Since plasma osmolarity rises during general muscle exercise and the lung is thought to have a function as a blood depot we have investigated the effect of plasma hyperosmolarity on pul- monary vascular capacitance. Hyperosmolar solutions of sodium chloride, urea and ethylene- glycol (listed in order of potency) were infused into the pulmonary artery of an isolated rabbit lung preparation perfused with plasma at constant volume pulsatile inflow and ventilated by positive pressure. All the test-substances caused dose-dependent reductions in the weight of the preparations, which was followed continuously by the use of a force transducer. The effect was not related to change in bronchomotor tone or pulmonary vascular pressures. Nor could the weight reductions be explained solely as a result of loss of water from the lungs. The capacitance vessels of the lung appear to constrict when exposed to increased plasma osmolarity, a response which is in direct contrast to the effect of this stimulus on the resistance vessels of the lung. The finding strengthen the concept of the lung as a blood depot.

This work has been motivated by considerations related to 3 sets of experimental findings:

1) The lung appears to have a function as a blood depot which can be drawn upon during various forms of circulatory stress, e.g. acute hemorrhage (Aarseth 1970).

2) Elevation of extracellular osmolarity in a rat portal vein preparation will induce a fall in smooth muscle cell volume and a concomitant volume-related inhibition of electrical and mechanical activity of the smooth muscles (Johansson and Jonsson 1968). Furthermore, resistance vessels of the lung relax when exposed to moderate elevation of blood osmolarity (Hauge and B0 197 1 ) .

3) The osmolarity of the effluent blood of exercising muscles is increased. Systemic arterial osmolarity has been found to be elevated by 25 mosm/l after a few min of genera1 exercise (Lundvall et al. 1970). The osmolarity of mixed venous blood must then be elevated even more.

The two last observations first led us to wonder whether the capacitance vessels of the lung would also relax during general exercise. Such a response seemed, however, to be teleologically disadvantageous as it would give a tendency towards pooling of blood in the lung. If the lung has a function as a’blood depot we would rather expect a reduction of pulmonax$ blood volume during exercise. We therefore decided to

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3 76 0. B0, A. HAUGE AND G . NICOLAYSEN

investigate the response pattern of the pulmonary capacitance vessels during slow, graded rises in plasma osmolarity.

A preliminary report on some of the data has been given elsewhere (B0, Hauge and Nicolaysen 1970).

Methods T h e lung preparations were taken from albino rabbits of either sex weighing 2870 f 230 g. The rabbits were anesthetized with pentobarbitone (NembutalQO, Abbott) 30-50 mg/kg i.v. Heparin -750 I.U./kg-was also given i.v. The procedure for removal of the heart and lungs and for connection of the lungs to the perfusion circuit was as described by Hauge, Lunde and Waaler (1965).

Perfusion of the pulmonary vascular bed was carried out with a Dale gZ Schuster pump at constant volume pulsatile inflow. The perfusate was heparinized (3000 1.U.l 100 ml) horse plasma which was kept frozen until 1 hr before the start of an experiment. I t was then thawed at 38” C and filtered through one layer of filter paper. In all the experiments a flow of 250 ( . f 5 ) ml/min was selected, and the perfusion was started 10 to 12 min after the rabbit’s own circulation had been stopped. The first 15 ml of effluent perfusate were discarded in order to bring the hematocrit down to well below 1 per cent. The circulating perfusate volume was 210 ( + 2 ) ml at the outset of each experiment. The outflow pressure ( = left atrial pressure, PLA) was kept constant in each experiment and at 0.5-1.5 cm of water. The inflow pressure, Pp.4, was recorded with a Statham P23Db pressure transducer connected to a Sanborn, two- channel, model 320 recorder. Perfusate temperature was kept at 38” C.

Ventilation was carried out using a Starling “Ideal” pump (C. F. Palmer, London) and positive pressure technique. End tidal pressures were kept at 10 and 1.5 cm H 2 0 respectively, by the use of water seals. Ventilation gas was 5 % Con in air. Ventilation overflow was measured by the method of Konzett and Rossler (1940).

Weighing. The preparation was suspended under a Sanborn force transducer (FTA-100-1) by the use of a string which was fastened to a string around the atrioventricular groove. Since the axis of the transducer had a very low displacement on variations in load, only diminutive changes in tension of the perfusion- and ventilation-tubings could occur when changes in preparation weight developed. Calibration of the system was done at the end of every experi- ment by placing different known weight loads on the preparation. Weight changes down to 50 mg could be detected and recorded on the Sanborn recorder.

Test-solutions were infused into the pulmonary arterial tubing from syringes mounted in a constant flow infusion pump (Harvard model 947). Different stepwise elevations of plasma osmolarity were obtained by changing the speed of the infusion pump. Three test-substances of different chemical nature and with different membrane permeability characteristics were chosen : sodium chloride, urea and ethyleneglycol. The osmolarity of the test-solutions were measured in 1 : 20 dilution with distilled water by the use of a Knaur osmometer. The osmolari- ty of the undiluted test solutions are listed below. Sodium chloride solution 7000 mosmll Urea solution 5680 ” E thyleneglycol 8600 ”

Results

The start of the perfusion and the ventilation caused an immediate rise in the weight of the preparation. This increase in weight was interpreted as being due to an in- crease in the intravascular volume of the lungs. After such an initial adjustment the weight of an undisturbed preparation remains stable or with small and slow adjust- ments for at least two hr thus allowing tests with hyperosmolar solutions.

Fig. 1 demonstrates an example of a response to a step increase in plasma osmolari- ty. The test solution of sodium chloride was infused for a period of 45 sec. A period of this length was chosen in order to avoid recirculation of test-solution during the infusion. The hyperosmolar solution caused a reduction of the inflow pressure and,

PULMONARY VASCULAR CAPACITANCE 377

25r

Fig. 1. Effects of an elevation of plasma osmolarity of

infusion rate: 1.53 mllmin; infusion period: 45 sec. PPA: pulmonary arterial pressure. 3min

43 mosmll. Isolated perfused rabbit lungs. Flow: 250 ml/min; test-solution : sodium chloride, 7000 mosmll; ( g )

rn

at the same time, a fall in the weight of the preparation of about 1 g. The total weight of the lung preparation is about 18 g, of which at least 11 g will be intra- vascular fluid (Lunde 1967). Thus, if the total weight reduction was due to a shift in perfusate volume from lungs to the extrapulmonary part of the perfusion circuit, a ten per cent reduction in vascular capacitance would have taken place. Reduction in inflow pressure was regularly seen as a response to the first infusion but, in most cases, could not be obtained upon repeated infusions in one and the same pair of lungs. The weight responses were, however, always transient and reproducible. No change in ventilation overflow was seen as response to this or other infusions of hyperosmolar solutions.

Our first object was then to obtain knowledge about the dose/response relationship betwen stepwise rises in plasma osmolarity and weight reductions of the lungs. Nine lung perfusions were carried out in order to examine the effect of each of the 3 test-substances, i.e. 3 perfusions per test substance were performed. An infusion period of 45 sec was selected for all the 27 infusions. The individual dose/response curves were constructed on the basis of 3 stepwise elevations of plasma osmolarity. Before the start of the first infusion a plasma sample was collected and plasma osmo- laity determined. For the stepwise elevations of plasma osmolanty we used calcu- lated values, knowing the osmolarity of the test-solutions, the infusion rate and the plasma flow rate.

The results are listed in Table I and a graphical representation is demonstrated in Fig. 2. Weight reductions obtained after 45 sec of infusion are plotted against the induced osmolarity gradients, defined as the increase in perfusate osmolarity during

TABLE I. Reductions in lung weight in g caused by rises in plasma osmolarity. Each figure repre- sents the mean values from three expts.

Increase in osmolarity NaCl Urea Ethyleneglycol Osmolarity at start of infusion

0.083 0.23 0.27 0.07 0.166 0.56 0.48 0.26 0.25 0.91 0.73 0.39

- cn 01 0 t

- 0.5

e c F s

1.0

infusion divided by the perfusate osmolarity at the start of that same infusion. The weight reductions increased with the size of the osmolarity gradient induced. The responses to ethyleneglycol infusions were much smaller than those to urea and sodium chloride when related to the osmolarity gradients. There was furthermore a tendency towards greater responses to sodium chloride than to urea infusions.

A critical question to answer is the extent to which the weight changes observed represented reductions in vascular capacitance. Loss of lung tissue water due to transcapillary flux of fluid in response to increased intravascular osmolarity could also be expected to cause weight reductions of the preparations. In order to answer this question we infused the test-solution of sodium chloride until a maximal weight reduction was obtained. The one experiment of this type giving the largest weight response is depicted in Fig. 3 and 4. The first of these two figures demonstrates the actual polygraph tracing, the second is a drawn out graph of the down-slope from the weight recording. To simplify our calculations we will assume that the lung tissue behaved as a perfect osmometer and that the pulmonary vessels were permeable to water only. The wet weight of the lung preparation without any perfusate is about 7 g (Lunde 1967), and the percentage of water in bloodless rabbit lungs is certainly not more than 70 (Mountcastle 1968). Consequently 5 g of extravascular water must be a maximal figure for this preparation.

3 test substances. Weight reduc- tions of the lungs reached after 45 sec of infusion are plotted

lenglycol against the induced osmolarity gradients, defined as the increase in perfusate osmolarity during in- fusion divided by the perfusate osmolarity at the start of that same infusion. The osmolarity of the perfusate before any additions

-

, -

pPA (cmH,O 1

Weight change

( 9 )

. ..

- 3min

Fig. 3. Effects of a cuniulative rise of plasma osmolarity. Isolated perfused rabbit lungs. Flow : 250 ml/min; test- solution : sodium chloride, 7000 mosm/l; infusion rate 4.7 m1/45 sec. Infusion period is marked by horisontal bar. The two vertical dotted lines indicate the first 45 sec of the response time.

PULMONARY VASCULAR CAPACITANCE 379

Fig. 4. The figure shows a drawn out graph of the down-slope of the weight recording demonstrated 0 30 60 90 120 150 in Fig. 3.

-4

Time after start of infusion ( s e c )

During the first 45 sec of infusion, i.e. before any recirculation took place, 187.5 ml of plasma passed through the pulmonary vascular bed. With the infusion-pump setting selected for this particular experiment 4.7 ml of a sodium chloride solution of 7000 mosm/l were infused during that same period. The increment in milliosmoles must then have been 32.9 mosm in a total volume of 192.2 ml (187.5+4.7), that is an increment of 171 mosm/l. Because of previous additions of ethyleneglycol plasma osmolarity before the start of perfusion was found to be 500 mosm/l. I n spite of this the lung preparation had a normal weight, indicating that no pulmonary edema was present. Assuming that water only crosses the vessel wall we will have the following relationship :

X - _ - 500 500+171 5

x is the extravascular water volume after a new osmotic equilibrium has been reached. In this experiment x was 3.7, consequently the maximal transvascular waterflux was 1.3 ml during the first 45 sec of infusion. If we turn to Fig. 4 we will see that at this point the weight reduction of the preparation was 1.6 g and, furthermore, the fall in weight was still rapidly proceeding despite the fact that recirculation had not yet started.

Since the vascular walls are not permeable to water only, the osmotic gradient certainly must have been smaller than assumed in our calculations. Furthermore, it is unlikely that the total lung water content is available for osmotic equilibration within such a short period of time. These considerations will further tend to reduce the importance of transvascular water flux in causing the weight reductions observed. I t should also be stressed that if the total weight reduction obtained was due solely to dehydration of the lungs more than half of the total lung water content would have been lost. We consider this to be highly improbable. Thus, at least a part of the weight reduction must have been caused by a fall in intravascular volume.

380 G . BO, A. HAUGE A N D 0. NICOLAYSEN

Discussion The present experiments confirm the previous observations from cat that small and moderate elevations of plasma osmolarity are able to reduce pulmonary vascular resistance. This effect appears to be mediated by way of a reduction in vascular smooth muscle tone (Hauge and BPI 1971). I n the present preparation this response was in most cases a relatively long-lasting one and would therefore, as a rule, only be obtained a t the first infusion of a hyperosmolar solution. In contrast, the effect of plasma hyperosmolarity of lung weight was present throughout the experiment, thus making it reasonable to conclude that the weight reductions observed were in- dependent of changes in pulmonary vascular resistance. I t is, furthermore, unlikely that the weight responses were secondary to airway changes since bronchomotor reactions were not observed. Although a part of the weight reductions observed might be due to dehydration of the lung, we believe that the experiment illustrated in Fig. 3 and 4 proves that this cannot be the sole explanation of our finding. Some reduction of pulmonary vascular capacitance must have occurred as a response to plasma hyperosmolari ty.

When the 3 test-substances are compared with regard to their ability to reduce the weight of the preparation, it is evident from Fig. 2 that ethyleneglycol is the least effective one and that sodium chloride is the most potent one of the three. Urea takes an intermediate position not much different from sodium chloride. I t is known that ethyleneglycol has the highest rate of penetration into smooth muscle cells of the 3 substances tested (Johansson 1969), and also that urea comes as no. 2 in this respect. The capillary permeability to sodium chloride and urea is, however, about equal (Pappenheimer 1953). The differences observed may therefore reflect differences in the ability of the test-substances to change the smooth muscle tone of capacitance vessels in the lung. By increasing extracellular osmolarity in a rat portal vein Johans- son and Jonsson (1968) showed that a fall in the smoth muscle cell volume was ac- companied by inhibition of its electrical and mechanical activity. Their observation is compatible with our finding that hyperosmolarity reduces PVR but it fails to explain the reduction in intravascular volume. In this connection it is of interest to note that another stimulus, namely an elevation of plasma catecholamine levels, elicits the same response pattern in the pulmonary vascular bed as did infusion of hyperosmolar solutions in the present study (Hauge, Lunde and Waaler 1967). Both stimuli may be operative under various forms of circulatory stress and contribute to mobilization of blood from the lungs.

The lung vasculature is known to react differently from systemic vascular beds in the response to various stimuli, e.g. hypoxia, ATP and acidosis (Waaler, Hauge and Lunde 1966). I t may therefore be of interest to compare the effects of hyperosmolar solutions on capacitance vessels in the lung with the effect of this stimulus on the same functional type of vessels in muscle. Although hypemsmolarity is a potent vasodilator stimulus in skeletal muscle (Mellander et al. 1967), no effect on capacitance vessels in skeletal muscles has been detected (Mellander et al. 1967). Apparently there is large segmental differences in the two vascular beds in the responses to a rise in intravascular osmolarity.

PULMONARY VASCULAR CAPACITANCE

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