Osmotic control of plasma vasopressin in anesthetized dogs

The relation between plasma osmolality (pOsm) and the concentration of immunoreactive vasopressin in plasma from an external jugular vein (pAVP) was studied in dogs prepared with carotid loops and anesthetized with chloralose-pentobarbital. Control mean pAVP was 0.6-1.0 microM/ml, after 24 h of dehydration pAVP was tripled. Isosmotic volume expansion for 10 min elicited a decrease in pAVP in all cases not associated with low control values of pAVP. I.v. hyperosmotic infusions (delta pOsm: 9 or 18 mOsm/kg in 10 min) increased pAVP. No significant alterations in pAVP occurred in relation to infusions during which the head or the remainder of the body was selectively supplied by hyperosmotic blood. Statistically, the distribution of the pAVP values suggests the existence of two populations, in euhydrated animals high concentrations (greater than 8 microU/ml) were found in a small fraction of the samples. It is concluded that (i) in chloralose-anesthetized, non-traumatized dogs pAVP is very similar to values found in venous plasma from conscious dogs, (ii) the pAVP measurements support the concept of an episodic secretion of vasopressin, and (iii) under the present circumstances, an ubiquitous increase in pOsm is a more effective stimulus for vasopressin release than a similar but selective increase in the osmolality of the blood flowing towards the head.

Microvascular pressure distribution and responses of pulmonary allografts and cheek pouch arterioles in the hamster to oxygen

Despite extensive investigations of the pulmonary circulation using both in vitro and in vitro preparations, few direct microcirculatory studies have been made. Consequently, the mechanisms involved in the response of the pulmonary microvasculature to changes in oxygen tension remain unclear. The present study represents the first direct observation of the responses in pulmonary microvessels to alterations in oxygen tension. Neonatal lung tissue was transplanted into the hamster cheek pouch using a chamber technique. Both tissues were characterized with respect to their microvascular pressure profile and vascular response to hypoxia. The results showed the two tissues to be remarkably different. Small pulmonary and cheek pouch arterioles exhibited opposite responses to changes in oxygen environment; hypoxia elicited a constriction of pulmonary arterioles, but a dilation of cheek pouch arterioles. Pulmonary capillary pressure, although comparable to that measured in the intact lung (13 mm Hg), was substantially lower than cheek pouch capillary pressure, which was within the range of that described for several systemic vascular beds. The microcirculatory effects of oxygen on both tissues were confined to the arteriolar segments. The characteristics of this pulmonary microcirculation are such that it is a unique model for further physiological and pharmacological studies.

Responses of pulmonary allograft and cheek pouch arterioles in the hamster to alterations in extravascular pressure in different oxygen environments

The responses to changes in transmural pressure were investigated in pulmonary allograft and cheek pouch arterioles in two oxygen environments. Neonatal hamster lung tissue was transplanted into adult hamster cheek pouches. After vascularization (8-10 days), pulmonary and cheek pouch vessels were observed by intravital microscopy in hamsters anesthetized with pentobarbital. By gassing the suffusion solution (bicarbonate-buffered Ringer's) (pH 7.4 at 35-37 degrees C) with either low oxygen (95% N2/5% CO2) or high oxygen (75% N2/5% CO2/20% O2) and after sealing the top of the chamber, extravascular pressure was altered by varying the fluid volume of the closed chamber. Changes in arteriolar diameters in response to positive and negative square-wave pressure pulses were quantified using a video micrometer and close-circuit TV system. Pulmonary arterioles showed a passive dilation or constriction in response to increases or decreases in transmural pressure (+/-20 mm Hg). These responses were not altered either by changes in PO2 or nitroprusside. In contrast, cheek pouch arterioles showed myogenic responses by constricting when transmural pressure was increased and vice versa. These responses were potentiated at high PO2 and abolished with nitroprusside. It is concluded that a myogenic response is dominant in cheek pouch arterioles but not in pulmonary arterioles under these conditions. These latter observations are consistent with results obtained from isolated, intact lung.

Angiotensin I conversion by the microcirculation of the nonhuman primate

Previous studies have shown that angiotensin I (AI) is converted to angiotensin II (AII) by converting enzyme (CE) or by tonin. Since all work on tonin has been done in the rodent, its relevance to the primate is yet to be shown. This study examines the conversion of AI to AII by the cheek pouch microcirculation of the monkey (macaca fascicularis). In 9 of the 11 monkeys studied, the conversion of AI to AII was blocked by converting enzyme inhibitor (CEI). However, in 2 older monkeys, AI conversion was not altered by CEI although the vascular response to AI was attenuated by both the AII antagonist (saralasin) and the antibody to AII (Ab-AII). In these 2 older animals, renin substrate caused vasoconstriction that was not altered by CEI, but was reduced by saralasin and Ab-AII. It is concluded that the tonin-angiotensin system is not active in the young macaque, but may be present in the older animals.

Direct evidence for myogenic autoregulation of the renal microcirculation in the hamster

We transplanted neonatal hamster renal tissue into a hamster check pouch chamber and subjected the renal tissue to increases and decreases in extravascular pressure. A decrease in extra-vascular pressure decreased, and an increase in extravascular pressure increased, the diameter of preglomerular arterioles. Thus, the change in preglomerular arteriolar diameter was directly related to alterations in extravascular pressure. Neither saralasin nor indomethacin affected these changes, whereas papaverin prevented them. The efferen arterioles responded passively to changes in extravascular pressure; i.e., the changes in their diameter were inversely related to changes in extravascular myogenic autoregulation of preglomerular vessels.

Morphological changes accompanying physiological changes in atrial stretch receptors

A shunt connecting the descending aorta and posterior vena cava was established surgically in six mongrel dogs. All animals subsequently developed hemodynamic and clinical signs of congestive heart failure at which time the shunt was closed. Six to seven weeks after ligation of the shunt, activity of the left atrial stretch receptors was recorded and found to be similar to that of sham-operated dogs. Methylene blue-stained sections of left atrial endocardium indicated that approximately one-third of the receptors were normal in histological appearance. The remaining two-thirds were fragmented and degenerate. Evidence is presented which indicates that structural changes in these receptors parallel their changes in physiological activity.

Intravascular mechanoreceptor modulation of renal sympathetic nerve activity in the cat

Experiments were performed in chloralose-anesthetized cats to characterize intravascular mechanoreceptro input to renal nerve activity in the intact and vagotomized sinoaortic-denervated states. High-pressure intravascular mechanoreceptors were stimulated by rises in arterial pressure caused by norepinephrine. Low-pressure intravascular mechanoreceptors were stimulated by progressive blood volume expansion (14-23%) at a rate of 4.4 or 17.6 ml/min. In addition, veratrine was used to stimulate directly both high- and low-pressure receptors. In the intact animal the administration of norepinephrine or blood volume expansion was associated with substantial decreases in renal nerve activity. Veratrine also caused a large dose-related decrease in renal nerve activity. However, in the vagotomized sinoaortic-denervated animal there was no change in renal nerve activity with norepinephrine, volume expansion, or veratrine administration. These experiments demonstrate that the major afferent pathways for renal sympathetic circulatory reflexes are confined to the carotid sinus and aortic and vagus nerves. No evidence was found for a significant contribution from sympathetic afferent nerves.

Characterization of high and low pressure baroreceptor influences on renal nerve activity in the primate Macaca fascicularis

We characterized the influence of high pressure and low pressure intravascular receptors on renal nerve activity in the pentobarbital sodium-anesthetized nonhuman primate Macaca fascicularis. Epinephrine-induced increases in arterial pressure were used to stimulate high pressure receptors, and intravascular volume expansion was used to stimulate both high and low pressure receptors. In addition, the intravascular mechanoreceptors were stimulated directly by intravenous veratrine administration. All interventions produced large decreases in renal nerve activity in the intact state. Denervation of the carotid sinus or bilateral cervical vagal section dimished, whereas sinoaortic denervation with vagotomy completely abolished all responses of renal activity to these interventions. We conclude that the nonhuman primate possesses very sensitive renal nerve sympathetic reflexes that are modulated by intravascular mechanoreceptors whose afferents traverse the carotid sinus nerves and the vago-aortic trunks. The carotid sinus nerves and the vago-aortic trunks appear to be equally effective in inhibiting renal nerve activity in response to increases in arterial pressure. In addition, there are no afferent pathways mediating intravascular mechanoreceptor modulation of renal nerve activity outside the carotid sinus nerves and the vago-aortic trunks.

Failure of acute intravascular volume expansion to alter plasma vasopressin in the nonhuman primate, Macaca fascicularis

Experiments were carried out in the anesthetized monkey to determine the influence of acute hypervolemia on plasma vasopressin (ADH) concentration. The administration of an isotonic-isooncotic high molecular weight dextran infusion in two steps, each in an amount equal to 15% of the estimated blood volume, produced substantial elevations of left ventricular end diastolic pressure (17.1 cm H2O); however, no consistent change in mean plasma ADH concentration or mean arterial pressure occurred. The results support the position that in the primate, arterial blood pressure rather than blood volume is the major modulator of ADH secretion when blood volume is increased isoosmotically.

Initial renal responses of nonhuman primate to immersion and intravascular volume expansion

Experiments were performed in anesthetized Macaca fascicularis monkeys to determine if the initial renal responses of these animals to head-out vertical water immersion and isoncotic, isotonic volume expansion are similar, especially with regard to the onset of any changes in solute or water excretion. Significant increases in urine flow, sodium excretion, and osmolar clearance occurred after 10 min of immersion but not until 30 min after volume expansion. Potassium excretion increased during immersion but decreased after volume expansion. Mean arterial blood pressure increased after 30 min of immersion but was unchanged after volume expansion. Indices of vascular filling, central venous pressure in the immersed animals and left ventricular end-diastolic pressure in the volume-expanded animals, increased immediately after the intervention. Effective renal plasma flow increased in both groups but glomerular filtration rate was not consistently elevated. These results suggest that, in the nonhuman primate, immersion and volume expansion exert their renal effects through different afferent and/or efferent mechanisms and should not be considered as similar volume stimuli.

The contribution of neural pathways to blood volume homeostasis in the subhuman primate

Studies are presented which indicate that the neural components of an atrio-renal reflex appear to be present in the primate and thus presumably in man. However, this reflex does not appear to contribute importantly to blood volume homeostasis in the primate. It is our hypothesis that it is the high-pressure baroreceptors, i.e., those in the carotid sinus and those in the aortic arch which play the major role in the neural control of blood volume in the primate and thus in man. This apparent evolutionary change in the importance of high pressure vs. low pressure receptors in the neural modulation of blood volume may be related to the assumption of an upright or semi-upright posture.

Changes in the sensitivity of left atrial receptors following reversal of heart failure

Recordings were made from left atrial type B receptors in six mongrel dogs after recovery from the cardiac effects of a chronic AV fistula. All animals showed hemodynamic and clinical signs of congestive heart failure after 44.5 +/- 3.6 days with a patent Dacron shunt between the aorta and inferior vena cava below the level of the renal arteries. The stimulus-response curves of the left atrial stretch receptors (change in spikes/cardiac cycle or in spikes/minute vs. change in left atrial pressure) after 45.2 +/- 7.2 days of shunt closure were similar to those seen in sham-operated dogs from a previous study. However, the slope of the stimulus-response curve of the dogs in which the AV fistula was closed was significantly greater than the slope of the curve from the AV fistula dogs with heart failure. Radiographs indicated that after shunt closure, cardiac dilatation had regressed. This study incidates that a decrease in the sensitivity of left atrial receptors in dogs with congestive heart failure is a reversible phenomenon and that the initial depression is most likely related to the concomitant cardiac dilatation that accompanies the failure state.

Hematocrit as an index of changes in plasma volume in conscious dogs

Hematocrit (HCT) measurements were made in intact and splenectomized conscious dogs to determine if observed decreases in HCT were produced by plasma volume expansion or splenic sequestration of erythrocytes. We found that in conscious dogs HCT is a poor indicator of changes in plasma volume.

Neither dorsal root nor baroreceptor afferents are necessary for eliciting the renal responses to acute intravascular volume expansion in the primate Macaca fascicularis

We determined the contribution of the dorsal roots, vagi, and sino-aortic nerves to the renal responses to acute isotonic, isooncotic intravascular volume expansion in the nonhuman primate, Macaca fascicularis. Expansion of the estimated blood volume by 15% produced a significant natriuresis and diuresis. There was no significant difference between the time to peak response for either. Neither dorsal rhizotomy (C6-T7) nor vagotomy and sino-aortic denervation had a significant effect on these responses. We conclude that these pathways are not necessary for eliciting the renal responses to hypervolemia in the nonhuman primate.

Renal effects of left atrial distension in dogs with chronic congestive heart failure

The renal response to left atrial balloon inflation in normal dogs was compared with that in dogs with chronic congestive heart failure (CHF). CHF was induced by the production of an aortocaval fistula below the level of the renal arteries. CHF dogs showed elevated left ventricular end-diastolic pressure, enlarged hearts, a depression of myocardial contractility, pulmonary edema, ascites, and peripheral edema. They also showed significant decreases in urine flow, creatinine clearance, para-aminohippurate clearance, sodium and potassium excretion, fractional sodium excretion, osmolar clearance, arterial blood pressure, and heart rate. Balloon distension of the left atrium evoked a significant increase in urine flow and free-water clearance in the normal group. The reflex nature of this response was indicated by its blockade after bilateral cervical vagotomy. In contrast, the CHF group did not exhibit significant changes in urine flow or free-water clearance during balloon inflation. Plasma antidiuretic hormone (ADH) was significantly elevated in the CHF group; however, balloon distension reduced plasma ADH in both groups of dogs. Plasma renin activity was significantly elevated in the CHF dogs and was not changed by balloon distension in either group of dogs. It is concluded that animals with high-output CHF do not exhibit the atrial-diuretic reflex in spite of their ability to reduce ADH levels by atrial distension.

Direct evidence for the presence of a different converting enzyme in the hamster cheek pouch

Kininase II (angiotensin I-converting enzyme) is generally accepted to be the enzyme responsible for the conversion of angiotensin I (A I) to angiotensin II (A II). This study examined the response of the microvasculature of the hamster cheek pouch to the local application of A I, A II, and the renin substrate, tetradecapeptide (TDP). A I and TDP caused a localized vasoconstriction that was not blocked by converting enzyme inhibitors (CEI: BPF5a for A I and BPF5a and the nonapeptide inhibitor for TDP). However, both the A II antagonist [Sar1, Ala8]angiotensin II and the antiserum to A II blocked completely the A I- and TDP-induced vasoconstriction. Sixty-eight percent of the applied A I was converted to A II in the presence of CEI as well as in its absence. It is concluded that the vasculature of the hamster cheek pouch converts significant amounts of A I to A II by a route that does not involve kininase II.

Reactivity of gomerular afferent and efferent arterioles in renal hypertension

Renal tissue from neonatal hamsters was grafted into the cheek pouch of adult hamsters. Renal hypertension was induced in 28 hamsters, and sham operation was performed in 27. When blood pressure became elevated in hypertensive hamsters (10 to 12 days), the renal microcirculation of both groups was evaluated by direct microscopy. Wall and luminal diameter and vascular responses to norepinephrine (NE) and angiotensin II (AII) were determined on glomerular afferent and efferent arterioles and on the corresponding cheek pouch arterioles. In the hypertensive hamsters, the luminal diameter of the afferent arteriole increased, as did the wall/lumen ratio of the efferent arteriole. Although all vessels responded to direct application of NE and AII in both groups, the response of the afferent to AII and the response of the efferent to both NE and AII was greater in the hypertensive hamsters as compared to the sham-operated hamsters. These results indicate that during the development of renal hypertension structural alterations of glomerular vessels and the selective vascular responses to vasoactive agents would lead to an increased glomerular capillary pressure.

Evidence for the conversion of angiotensin I to angiotensin II by the coronary microcirculation

Cardiac tissue from hamster neonates was transplanted into the hamster cheek pouch. After vascularization of the transplanted tissue the vessels were tested for their ability to convert angiotensin I (AI) to angiotensin II (AII). AI caused vasoconstriction of the coronary vessels which was reduced significantly by converting enzyme inhibitor and the AII antagonist (Sar1-Ala8) angiotensin II. It was concluded that the vessels of transplanted cardiac allografts convert AI to AII in a manner similar to that observed in the whole heart indicating that this procedure provides a good model for studying the microcirculation of the heart.

Contribution of peripheral pooling to the renal response to immersion in the dog

The present investigation evaluated the renal and hemodynamic responses to head-out water immersion in dogs. Dogs were immersed in the vertical (seated) position in a 34 degrees C bath. Urine flow (V), osmolar clearance (Cosm), free water clearance (CH2O), sodium excretion (UNa+V), potassium excretion (UK+V), GFR, effective renal plasma flow (ERPF), central venous pressure (CVP), and cardiac output (CO) all increased significantly during immersion. This response was unchanged by bilateral cervical vagotomy or by deoxycorticosterone acetate and antidiuretic hormone administration. The control values of these dogs were low and indicated a state of peripheral vascular pooling which was readjusted to normal by the immersion maneuver. The renal and hemodynamic values during the period of immersion were similar to values of a group of dogs which were recumbent in air. Furthermore, when the latter group of dogs were tilted head down 19 degrees, there was no further increase in any of the measured parameters. These data are consistent with the view that water immersion in the upright dog simply redistributes blood volume back to that level seen in the recumbent dog, a position which is more natural for this species.

Contribution of vagal pathways to the renal responses to head-out immersion in the nonhuman primate

Studies were carried out to determine the contribution of cardiopulmonary receptors to the renal responses to head-out water immersion in the nonhuman primate. Immersion to the suprasternal notch was associated with significant increases in central venous pressure, urine flow, and sodium excretion. The increased sodium excretion was due primarily to a significant increase in the percent of the filtered sodium excreted. Deoxycorticosterone acetate (DOCA) and antiduretic hormone (ADH) had no substantial effects on these responses. The finding of a vasopressin-resistant hyposthenuria is consistent with the natriuresis of immersion being due, at least in part, to a decrease in sodium reabsorption proximal to the diluting segment, possibly the proximal tubule. Bilateral cervical vagotomy had no substantial influence on the renal responses to immersion, demonstrating that cardiopulmonary receptors whose axons traverse the vagus nerves are not necessary for the homeostatic adjustments to central hypervolemia in the primate. Since the renal and cardiovascular responses of the primate to immersion are essentially the same as those seen in man, it is probable that vagal pathways also are not necessary in man. However, it is possible that sympathetic afferents are involved in the natriuresis observed in the primate during immersion.

Failure of left atrial distension to alter renal function in the nonhuman primate

Experiments were undertaken to determine the influence of increasing left atrial pressure on renal function in the nonhuman primate. Significant elevations of left atrial pressure, produced by using an intra-atrial balloon, had no effect on salt or water excretion, renal plasma flow, or glomerular filtration rate. There were no significant changes in heart rate or blood pressure. We conclude that, unlike those in the dog, atrial receptors in the nonhuman primate play little or no role in modulating salt and water excretion.

The mechanism of adaptation of left atrial stretch receptors in dogs with chronic congestive heart failure

Chronic congestive heart failure (CHF) was induced in dogs by the construction of an aorto-caval fistula below the level of the renal arteries. Aorto-caval fistula dogs showed signs of CHF which included ascites, hind limb edema, and pulmonary congestion. Ventricular catheterization indicated a significantly higher left ventricular end diastolic pressure and lower maximum velocity of left ventricular pressure development/left ventricular end diastolic pressure in CHF dogs when compared to sham-operated controls. Heart weight/body weight ratios were significantly higher in CHF dogs. Electrophysiological recordings from medullated left atrial type B receptors from the cervical vagus indicated a depressed sensitivity of these receptors in CHF dogs when compared to sham-operated control dogs. For any given change in left atrial pressure, the discharge of left atrial receptors was significantly reduced in CHF dogs compared with sham-operated controls. The mechanism for this depressed sensitivity was investigated. Sonomicrometry of the left atrial appendage indicated a decreased compliance of the left atrial appendage in the dogs with chronic CHF. In addition, microscope examination of the complex unencapsulated receptor endings taken from the left atrial endocardium indicated a marked alteration in receptor morphology. A loss of the end arborization was the most typical finding. It is concluded that chronic CHF brought about by an aorto-caval fistula results in a depressed left atrial stretch receptor response and that both decreased left atrial compliance and structural alterations in the receptor endings may account for this depressed sensitivity.

Cardiopulmonary vagal afferents in the monkey: a survey of receptor activity

A survey was made of vagal afferents whose endings originated in cardiopulmonary areas of the Rhesus monkey. Recordings of action potentials from single fiber preparations of the left cervical vagus were made in both open and closed chest monkeys. A total of 425 receptors were identified in sixteen animals. These consisted of 347 pulmonary stretch receptors (one of which increased its discharge during expiration), 42 aortic baroreceptors, 4 ventricular pressure receptors, 1 epicardial ventricular receptor, 7 type A atrial receptors and 24 type B atrial receptors. The response of each cardiovascular receptor was tested by altering the stimulus for their discharge. Aortic and ventricular baroreceptors increased their discharge in response to an increase in blood pressure induced by intravenous norepinephrine. Type A atrial receptors did not increase their discharge in response to an increase in atrial pressure during intravenous administration of isotonic saline, while type B atrial receptors did. The discharge of the latter became continuous following the intravenous administration of veratridine sulphate (20 microgram). It is concluded that the basic types of cardiopulmonary afferents exist in the non-human primate and that they respond similarly to those which have been demonstrated in lower species.

Effect of volume expansion and veratrine on salt gland secretion in the goose

The influence of acute intravascular volume expansion on salt gland secretion of conscious, adult geese was investigated. The intravenous administration of 5% dextran in Krebs-bicarbonate-Ringer solution in an amount equivalent to 30% of the estimated blood volume caused a transient but highly significant increase in salt gland secretion independent of changes in plasma osmolality or sodium concentration. Intravenous veratrine (60 microng) caused a similar increase in salt gland secretion only when administered after the volume load. Intravenous 5% NaCl always caused a prolonged and significant increase in salt gland secretion which was not potentiated by veratrine. Volume expansion and hypertonic saline caused a significant tachycardia while veratrine caused a significant bradycardia. It is concluded that a volume component may contribute to the initiation of salt gland secretion in the goose and that the peripheral receptor involved is most likely vascular in origin.

The response of atrial stretch receptors to increases in heart rate in dogs

The discharge characteristics of type B left atrial receptors were analyzed during alterations in heart rate. Recordings were made from single-fiber preparations of the left cervical vagus of pentobarbital-anesthetized, open-chest dogs. The heart was paced following a sinoatrial crush at frequencies ranging from 60 to 240 beats/min. Left atrial transmural pressure was varied at each heart rate by the intravenous infusion of warm isotonic NaCl. As heart rate was increased there was a progressive decrease in the level of peak "v" wave left atrial pressure. Concomitantly with the decrease in left atrial pressure, the number of spikes per cardiac cycle decreased as did the maximal instantaneous frequency of discharge. A significant positive relationship could be demonstrated with either the discharge per minute [(spikes per cycle) X heart rate] or discharge per cycle vs. the peak "v" wave of the left atrial pressure, regardless of heart rate. The number of impulses that entered the central nervous system per unit of time remained relatively constant at heart rates between 90 and 240/min. It is concluded from these data that the reflex effects which have been attributed in the past to atrial stretch receptor stimulation during clinical episodes of atrial tachyarrhythmias may be better correlated with some aspect of receptor discharge other than frequency or the number of discharges per cycle.

Myocardial responses to acetylstrophanthidin during veratrum alkaloid refractoriness

Studies were undertaken in the isolated blood perfused dog heart to determine the nature of the myocardial refractoriness induced by veratrum alkaloids. The initial intracoronary injection of cryptenamine acetates (Unitensin) was always associated with an increase in myocardial contractility and a loss of myocardial potassium. Subsequent doses produced refractoriness with respect to both the contractile and potassium responses. Acetylstrophanthidin still produced an inotropic and potassium losing effect duct of the glycoside was potentiated whereas the contractile response was not.

Influence of volume expansion on sodium excretion during parabiotic dialysis

Dialysis of a saline-infused against a noninfused dog was associated with a significant increase in Na excretion in both animals without a significant change in creatinine clearance or filtered Na. This indicates that there was movement by diffusion of a humoral agent between the two animals. The observed increase in Na excretion in the noninfused animal resulted from gain of a natriuretic hormone, since in a second series of experiments a natriuretic rather than antinatriuretic hormone appeared to be involved.

Lack of a direct effect of efferent cardiac vagal nerve activity on atrial receptor activity

Experiments were carried out to determine the extent to which the efferent cardiac vagus may modulate the discharge from atrial receptors when heart rate was not controlled. Stimulation of the vagus increased atrial pressure and the discharge from type-B atrial receptors. In order to determine the extent to which the increase in atrial receptor discharge was the result of the associated hemodynamic changes, studies were done in which the vagus was stimulated while heart rate was maintained constant using atrioventricular pacing, as atrial pressure was varied over a wide range by volume expansion. In paced hearts vagal stimulation depressed atrial contractile force but did not alter atrial receptor discharge when compared to paced hearts without vagal stimulation. These results indicate that the effect of vagal stimulation on atrial receptor discharge is an indirect one mediated through the rise in atrial pressure due to cardiac slowing rather than a direct effect on the atrial stretch receptor itself.

The contribution of atrial stretch receptors to salt and water homeostasis in the human

Studies carried out in both the human and experimental animals provide reasonably strong evidence that receptors contained within the atria are involved in the control of body water. When atrial pressure is increased (and the atria distended) atrial receptor discharge is increased leading to an increase in urine flow and, depending upon the experimental condition, a less consistent increase in sodium excretion. At least two mechanisms appear to contribute to the renal response; inhibition of the secretion of antidiuretic hormone and inhibition of renal nerve discharge. None of the factors presently known to alter sodium excretion have been shown to account completely for the increase in sodium excretion. The failure of patients with chronically distended atria to experience a chronic diuresis appears to be partly the result of a resetting of atrial receptor sensitivity. Recent evidence suggests that the sensitivity of atrial volume receptors has a species variation.

Influence of altering total body sodium on angiotensin I systemic converting activity (38573)

Studies were carried out in the dog to determine the extent to which alterations in total body sodium influence the systemic conversion of angiotensin 1. When the animals were maintained on a high salt diet and DOCA, conversion of angiotensin I increased signifcantly and returned to control levels when a normal salt diet was provided. When another group of animals were provided a low salt diet and mercuhydrin, angiotensin I conversion decreased significantly, although returning the animals to a normal salt diet had no further influence upon conversion. These experiments indicate that there is a direct relationship between total body sodium and the systemic conversion angiotensin 1.

Discharge of type B atrial receptors during changes in vascular volume and depression of atrial contractility

1. Single unit recordings from type B atrial receptors were obtained from slips of the left cervical vagus nerve during acute increases and decreases in atrial pressure produced by infusion of isotonic saline and withdrawal of blood.2. Atrial receptor discharge reached a peak frequency at the peak of the ;v' wave of the atrial pulse, a time when the rate of change of atrial pressure was zero.3. The receptor discharge exhibited hysteresis when increases in atrial pressure were compared to decreases in atrial pressure during circulatory volume expansion and haemorrhage, respectively. This hysteresis was not due to a change in atrial compliance.4. The hysteresis was still observed when the pulse pressure of the ;v' wave was plotted against receptor discharge.5. Type B atrial receptors did not show adaptation after 15 min of a sustained increase in left atrial pressure.6. Acute depression of atrial contractility with propranolol increased resting fibre discharge as a result of the associated increase in atrial pressure.

Evidence for an indirect sympathetic control of atrial stretch receptor discharge in the dog

Experiments were performed to determine the influence of cardiac sympathetic efferent nerve stimulation on the discharge rate of atrial type B stretch receptors in the anesthetized, open-chest dog. In all experiments, the left stellate ganglion was stimulated following volume expansion. To determine the effects of stellate stimulation, the responses of atrial receptors were observed during the withdrawal of blood in steps following intravascular volume expansion. Stimulation of the stellate ganglion decreased receptor discharge and left atrial pressure in all experiments. A change from a control left atrial pressure of 2.5 cm H 2 O (in peak left atrial v -wave pressure) resulted in a mean increase in receptor discharge of 6.6 ± 1.4 spikes/cardiac cycle during stellate stimulation; however, a change from a control pressure of 0.0 cm H 2 O resulted in a mean decrease in receptor discharge of 1.5 ± 1.5 spikes/cardiac cycle. The curves relating the change in atrial receptor discharge to the change in left atrial pressure during hemorrhage and the curve for these parameters during stellate stimulation were not significantly different from each other. Apparently, the decreased discharge during sympathetic stimulation was the result of a decline in left atrial pressure rather than a result of any direct effect on the receptor per se.

Left atrial receptor discharge during atrial arrhythmias in the dog

Afferent receptor discharge originating in the atria was recorded from slips of the cervical vagus in the open-chest dog. The activity of two types of atrial receptor endings was identified. Type A receptors exhibited a discharge pattern that was synchronous with the a wave of the atrial pressure pulse in the control recordings, and type B receptors discharged only during the v wave of the atrial pressure pulse in the control recordings. Atrial arrhythmias (flutter or fibrillation) were induced by mechanical stimulation of the area around the sinoatrial node. During atrial arrhythmias, there was a moderate rise in mean left atrial pressure (7.5 ± 0.7 cm H 2 O to 8.6 ± 0.6 cm H 2 O during arrhythmias in which type B receptors were studied, 6.1 ± 0.1 cm H 2 O to 7.8 ± 0.4 cm H 2 O during arrhythmias in which type A receptors were studied), a fall in aortic blood pressure, and a decrease in right atrial force. Atrial contractions were asynchronous and rapid. Type A atrial receptors showed a relatively greater increase in discharge (184.1%) during fibrillation than did type B atrial receptors (27.5%). These experiments demonstrated that both types of atrial receptors increased their discharge rate during atrial arrhythmias, indicating that they might be involved in a reflex diuresis which occurs during these arrhythmias in humans.