Atrial receptors, vasopressin and blood volume in the dog

Elevated arginine vasopressin and lowered atrial natriuretic factor associated with hypertension in coarctation of the aorta

Impairment of humoral and neural regulation of blood pressure may contribute to preoperative and postoperative hypertension in coarctation of the aorta and may also affect the release of vasopressin and atrial natriuretic factor. Because vasopressin and atrial natriuretic factor have potent vasoactive effects, we measured plasma vasopressin and atrial natriuretic factor levels by radioimmunoassay before operation and for 5 days after operation in 11 patients aged 9 months to 12 years undergoing coarctation repair and in 12 control patients undergoing other cardiovascular operations. Six patients in the coarctation group required minimal antihypertensive therapy (group I) and five required prolonged intravenous antihypertensive therapy (group II). Before operation, vasopressin levels correlated with systolic blood pressure for all patients in the coarctation group (r = 0.83, p < 0.01) whereas atrial natriuretic factor levels did not. Before operation, atrial natriuretic factor levels were lower (28 +/- 5 vs 41 +/- 7 and 50 +/- 8 pg/ml, p < 0.05) and vasopressin levels were higher (28 +/- 6 vs 5.4 +/- 0.9 and 7 +/- 3 pg/ml, p < 0.05) in group II than in group I or control patients. Vasopressin levels were higher (p < 0.05) on the day of operation and on postoperative days 2 through 5 in group II than in group I and in control patients. Atrial natriuretic factor levels were lower during the day of operation in group II than in group I or in control patients (26 +/- 7 vs 51 +/- 16 and 50 +/- 7 pg/ml, p < 0.05) and remained lower than control values on postoperative days 1 and 3 through 5. Elevated vasopressin and lowered atrial natriuretic factor levels may contribute to preoperative and postoperative hypertension in coarctation.

Plasma vasopressin and atrial natriuretic factor during haemorrhage: influence of cardiac and aortic receptors

1. The changes in plasma concentrations of immunoreactive vasopressin (iVP) and atrial natriuretic factor (iANF) in response to haemorrhage (10-30% blood volume) were measured in 10 anaesthetized rabbits before and after cardiac receptor denervation (vagal nerve section). Carotid sinus pressure was maintained constant (60 mmHg) to eliminate any changing input from carotid baroreceptors. 2. Haemorrhage increased iVP before and after vagal nerve section indicating that withdrawal of input from aortic baroreceptors may have contributed to the increase in iVP. 3. Section of the vagus nerves attenuated the iVP response to haemorrhage. 4. There was no correlation between release of iVP and iANF. 5. Haemorrhage decreased iAF before and after vagal nerve section. Section of the vagus nerves increased iANF. Plasma iANF was highly correlated with atrial pressure and mean arterial pressure suggesting iANF release was secondary to changes in cardiac haemodynamics.

Control of blood and extracellular volume

Blood and extracellular fluid volume are maintained within narrow limits despite considerable daily variations in the intake in salt and water. As summarized schematically in Figure 15, the urinary excretion of salt and water responds to changes in blood volume and arterial pressure. Volume-sensitive receptors located predominantly in the cardiac atria and arterial tree sense acute changes in the filling of the blood volume compartment, and urinary sodium excretion is adjusted in response to these detector mechanisms by virtue of alterations in both glomerular filtration rate and tubular sodium reabsorption. The reabsorption of sodium by the tubule responds to changes in extracellular fluid volume as well as to changes in filtered sodium load. Glomerular filtration rate and tubular reabsorption of sodium are influenced importantly by physical properties of the plasma in glomerular and peritubular capillaries and by the composition of the tubular fluid. The renal arterial perfusion pressure is a major factor regulating tubular reabsorption of sodium and water as signalled via changes in renal interstitial hydrostatic fluid pressure. Renal nerves and a variety of systemic and local hormones also influence tubular reabsorption of sodium and water directly by effects on transepithelial sodium transport and/or indirectly by altering renal medullary haemodynamics and the pressure-natriuresis-diuresis relationships. Thus, utilizing a variety of overlapping effector mechanisms that influence renal sodium and water excretion, mammalian organisms have achieved a high degree of stability of body fluid volumes. The fundamental relationship between arterial pressure and renal excretion appears to be the major mechanism which provides for the long-term control of body fluid volume. The sensitivity of the pressure-natriuresis-diuresis relationship is modified by the efferent pathways of the rapid-acting reflex and mechanoreceptor detectors of volume. Working together, these mechanisms provide a remarkable degree of rapid and long-term extracellular and blood volume stability.

Control of neurosecretory vasopressin cells by noradrenergic projections of the caudal ventrolateral medulla

Activation of noradrenergic afferents arising from the A1 cell group of the caudal VLM excites neurosecretory AVP cells of both the supraoptic and paraventricular nuclei, thus stimulating the release of this potent vasoconstrictor into the circulation. Although this effect is mimicked by application of alpha 1-adrenoreceptor agonists to AVP cells, the excitatory effects of A1 afferents may not be mediated by activation of post-synaptic alpha 1-receptors. Evidence has also been obtained that the actions of A1 afferents are not dependent upon the release of excitatory amino acids or NPY, although the latter is co-stored with NA in A1 cells and potentiates the actions of low concentrations of NA on AVP cells. Although a projection to AVP and OXY neurosecretory cells from the A2 NA cell group of the NTS has been established, this projection does not appear to contribute directly to the control of SON AVP cell activity. Rather, NTS stimulation excites SON AVP cells via a relay projection through the A1 cell group. This pathway is likely to correspond to that involved in the stimulatory effects of haemorrhage and caval constriction on AVP secretion, although it is uncertain whether the effects of these particular stimuli are contingent upon unloading of arterial baroreceptors and atrial stretch receptors, as commonly presumed, or upon the activation of other receptors such as ventricular mechanoreceptors or chemoreceptors. On balance, current evidence suggests that the A1 projection is unlikely to be critically involved in mediating the effects of arterial baroreceptor, arterial chemoreceptor, or atrial stretch receptor activation on AVP cells.

Variable arginine vasopressin levels in neonatal congestive heart failure

Arginine vasopressin levels in 17 neonates with cardiac disease were compared with control levels in 10 healthy newborn infants. Infants with congestive heart failure who were free of left ventricular outflow tract obstruction had a mean level of 80 +/- 18 pg/ml, which was significantly greater than the mean control level (p less than 0.001). Infants with congestive heart failure and left ventricular outflow tract obstruction had a mean vasopressin level of 3 +/- 0.7 pg/ml, which was lower than the mean control level of 6 +/- 0.7 pg/ml (p less than 0.05). The data suggest that impaired forward flow to high pressure sinoaortic and ventricular baroreceptors is necessary for vasopressin release in congestive heart failure. In left ventricular outflow tract obstruction with heart failure these receptors may be impaired or absent, leading to decreased vasopressin release. Low plasma arginine vasopressin may adversely affect circulatory homeostasis.

Effect of peripherally administered atriopeptin III on water intake in rats

1. Extracellular fluid deficits of 33% were produced in male Long-Evans rats by peritoneal dialysis. The conscious, unrestrained animals were then infused I.V. for 30 min with atriopeptin III at doses of 0.01, 0.1, 0.5 and 1.0 micrograms/min. At 5 min into the infusion, the rats were offered water and subsequent intakes were monitored. Since atrial natriuretic peptide (ANP) causes hypotension, one group of control animals was given an injection of diazoxide sufficient to match this fall in blood pressure. 2. A similar group of rats was prepared for measurement of plasma ANP achieved by infusion. 3. Relative to the saline-infused controls, atriopeptin III did not reduce water intake. Indeed, intake was increased at the highest dose of 1.0 micrograms/min. 4. Relative to the diazoxide controls, water intake was influenced by atriopeptin III in a dose-dependent manner, the greatest attenuation being observed at infusions of 0.1 microgram/min. 5. Infusion of atriopeptin III at 0.1 microgram/min caused plasma ANP levels to rise from 252 +/- 21 to 532 +/- 136 pg/ml (n = 9, P less than 0.05) at 15 min. The lowest dose (0.01 microgram/min) caused no detectable increase in plasma levels. 6. It is concluded that, in groups of hypovolaemic rats matched for blood pressure, atriopeptin III caused a dose-related reduction in water intake.

Effects of adrenalectomy and aldosterone on the action of centrally administered rat atrial natriuretic peptide-(99-126)

Intracerebroventricular (i.v.t.) administration of rat atrial natriuretic peptide-(99-126) (rANP) increases urinary volume and sodium excretion, but the mechanism is undefined. A diminished mineralocorticoid effect on the kidneys may explain the natriuretic phenomenon. This hypothesis was tested by i.v.t. rANP injection (1.25 micrograms/5 microliters) in conscious, hydrated rats pretreated beforehand with d-aldosterone (20 micrograms/kg, i.p.). Although the absolute amount of sodium excreted was reduced, aldosterone did not affect rANP-induced sodium output at 1 and 3 h. Rats which were sham-operated or bilaterally adrenalectomized (ADX) after four days were pretreated with aldosterone and given an oral water load followed by i.v.t. rANP or saline. In ADX rats natriuresis and diuresis after rANP were still evident. Our results indicate that the natriuretic effect of i.v.t. rANP is unrelated to plasma levels of mineralocorticoids. Likewise, diuresis and natriuresis can occur in the absence of the adrenal glands.

Inhibition of muscle sympathetic nerve activity in humans by arginine vasopressin

Arginine vasopressin, a potent vasoconstrictor, does not raise arterial pressure in normal humans even at pathophysiological plasma levels. To examine whether the pressor effect of vasopressin in humans is buffered by baroreceptor reflex inhibition of sympathetic nerve activity, we recorded postganglionic muscle sympathetic nerve activity directly from the peroneal nerve in 12 normal men before, during, and after a 20-minute intravenous infusion of vasopressin, 4 ng/kg/min, that increased mean plasma concentrations from 6.2 +/- 0.6 to 320 +/- 68 (SE) pg/ml. During the first 5 minutes (n = 8), mean arterial pressure increased from 91 +/- 3 to 97 +/- 4 mm Hg (p less than 0.05) and integrated sympathetic nerve activity decreased from 271 +/- 45 to 156 +/- 33 units (p less than 0.05). At 15 minutes (n = 12), arterial pressure did not differ from control values whereas forearm vascular resistance fell (p less than 0.05) and central venous pressure and heart rate increased (p less than 0.05). Sympathetic nerve activity remained below control levels throughout the infusion (202 +/- 31 vs 254 +/- 40 units before infusion; p less than 0.05). An effect of vasopressin on ganglionic transmission was excluded, since the sympathoexcitatory response to apnea was not attenuated during vasopressin. Thus, pathophysiologic levels of vasopressin in humans cause inhibition of muscle sympathetic nerve activity that is not due to a ganglionic blocking action. The sympathoinhibition may be caused in part by the modest increases in mean arterial and central venous pressures and attendant stimulation of arterial and cardiac baroreceptors. The reflex decrease in sympathetic nerve activity would be expected to buffer the direct vasoconstrictor effects of vasopressin.

The effects of stress on salt and water balance

The effect of 'stress' on salt and water balance is largely determined by intrinsic renal factors supported by a series of inter-related neuro-humoral mechanisms which serve to enhance salt and water reabsorption and to maintain arterial pressure. As shown by acute hypovolaemic stress, the hormone response is characterized by increased renin-angiotensin production and associated hyperaldosteronism, which is further augmented by states of Na+ depletion. As well as increasing aldosterone secretion, angiotensin II has a direct renal effect to increase Na+ retention, and may stimulate both thirst and arginine vasopressin secretion. Additional support to volume homeostasis is provided by increased secretion of hypothalamic hormones (arginine vasopressin and corticotrophin releasing factor) and activation of the sympathetic nervous system, which further enhances renin secretion. These mechanisms, in restoring extracellular fluid volume and renal perfusion pressure, eventually diminish the stimuli to renin once normovolaemia is achieved. A variety of other acute 'stressors', not associated with major changes in ECF volume or hypotension, also stimulate adrenocortical hormones (cortisol and aldosterone), renin and arginine vasopressin. The biological significance of these changes is uncertain, but their brief duration make it unlikely that the responses contribute significantly to changes in salt or water balance. Atrial peptide hormones could also be important in ECF volume regulation. However, recent studies show little if any change in basal plasma atrial natriuretic peptide levels during acute hypovolaemic stress, whereas severe treadmill exercise (but not surgical stress or acute hypoglycaemia) stimulates venous levels in man. The relevance of these findings to the regulation of salt and water balance during stress clearly requires further study.

Plasma alpha natriuretic peptide in cardiac impairment

Regional plasma alpha human atrial natriuretic peptide concentrations were measured, and their relation to intracardiac pressures assessed, in an unselected series of 45 patients undergoing diagnostic cardiac catheterisation. Arteriovenous gradients in plasma concentrations of alpha human atrial natriuretic peptide were consistent with its cardiac secretion and its clearance by the liver and kidneys. Plasma concentrations of the peptide in the pulmonary artery, aorta, and superior vena cava correlated closely with the mean right atrial and pulmonary arterial pressures, and similar, though weaker, positive relations were seen with the left ventricular end diastolic and pulmonary artery wedge pressures. Concentrations of both atrial natriuretic peptide and renin showed significant inverse relations with serum sodium concentrations. Plasma concentrations of alpha human atrial natriuretic peptide are an additional objective indicator of the severity of haemodynamic compromise in patients with cardiac impairment.

Increase in atrial pressure releases atrial natriuretic peptide from isolated perfused rat hearts

To elucidate the mechanism involved in the release of atrial natriuretic peptide, we modified the isolated perfused rat heart preparation to permit a step-wise increase in right atrial tension. Perfusate was introduced into the right atrium through the superior vena cava and was collected via the pulmonary artery. Right atrial pressure was manipulated by changing the perfusion rate. Perfusate from the pulmonary artery was collected in 1-min-fractions, extracted, and assayed for atrial natriuretic peptide like immunoreactivity (ANP-li). The basal rate of ANP-li release at an atrial pressure of 1.41 +/- 0.31 mm Hg was 964 +/- 144 pg/min (n = 11). As right atrial pressure was increased (range 0.4-4.5 mm Hg), a linear correlation (r = 0.85, P less than 0.001) was observed between the change in ANP-li release and the change in atrial pressure. High pressure liquid chromatography revealed that the major fraction in the perfusate had the same elution time than alpha-rANP. This peak fraction, as well as synthetic atriopeptin III, caused a dose-dependent relaxation in rat aortic strips that had been subjected to contraction with norepinephrine. Further, it corresponded exactly to the material we previously identified in rat plasma. These results suggest that atrial distension is involved in the release of ANP. In addition, ANP is released per se, as the active peptide.

Circulating atrial natriuretic peptides in conscious rats: regulation of release by multiple factors

Cardiocytes in the atria contain a prohormone that gives rise to atrial natriuretic peptides (ANP's), which have intrinsic hemodynamic regulatory activity. The distribution of ANP's in the brain suggests the involvement of these peptides in central cardiovascular regulation. In conscious rats with chronic indwelling catheters, volume loading with isotonic saline or glucose increased the amount of circulating immunoreactive ANP's by a factor of 4 to 5, as determined by radioimmunoassay. Hyperosmotic challenge with a hypertonic NaCl solution or anesthesia with halothane caused similar increases in plasma ANP's. Results obtained with the denervated-heart preparation indicate that neuronal influences are important in the release of ANP's induced by volume loading. As judged from reversed-phase high-performance liquid chromatography of extracted plasma and radioimmunoassay of collected fractions, the circulating physiologically important ANP's in the conscious rodent appear to be alpha-rANP(5-28) (atriopeptin III) and either alpha-rANP(3-28) [ANF(8-33)] or alpha-rANP(1-28) (ANF).