The effects of intracerebroventricular administration of renin on drinking and blood pressure.

The aim of the present investigation was to (a) determine if renin-substrate (angiotensinogen) is present in cerebrospinal fluid; (b) investigate the effects of intracerebroventricular administration of renin on drinking and blood pressure; and (c) determine if such effects are mediated via the formation of angiotensin II. Angiotensinogen concentration in cerebrospinal fluid was measured in 15 dogs and averaged 205 +/- 34 ng/ml. This value was approximately 1/5th of the corresponding plasma angiotensinogen concentration but the ratio of angiotensinogen:total protein in cerebrospinal fluid was approximately 15 times greater than in plasma. Intraventricular injection of hog renin (0.1 Goldblatt units) stimulated drinking in each of 8 dogs; the mean volume drunk in the 15 min period following the injection was 485 +/- 84 ml. When the renin was preceded by intraventricular saralasin acetate, a specific antagonist of angiotensin II, the drinking response was reduced to 8 +/- 6 ml. In eight pentobarbital anesthetized dogs, intraventricular dog or hog renin (0.05-0.25 Goldblatt units) increased systolic pressure from 152 +/- 10 to 168 +/- 10 mm Hg (P less than 0.001) and diastolic pressure from 101 +/- 8 to 116 +/- 7 mm Hg (P less than 0.001). This response, which lasted from 30 min to more than 3 h, was also abolished by saralasin acetate. These data indicate that centrally administered renin increases drinking and blood pressure and that these effects are mediated via the formation of angiotensin II.

Mechanism of the dipsogenic action of tetradecapeptide renin substrate in dogs.

Dogs with chronically implanted third ventricular cannulae showed significant drinking responses to central injections of angiotensin II and tetradecapeptide renin substrate (TDP). The threshold dose for angiotensin II was 1 pmol and for TDP was 70 pmol. Although central injections of TDP led to drinking and appearance of angiotensin II in cerebrospinal fluid, renin substrate prepared from dog cerebrospinal fluid had no effect. The dipsogenic action of TDP was blocked by prior administration of converting enzyme inhibitor SQ20881 (P less than 0.01) but was not affected by either pepstatin or N-acetyl-pepstatin. Thus, converting enzyme acts directly on TDP to produce angiotensin I and then angiotensin II. The results of the present study do not provide evidence for the presence of an enzyme in the brain with renin-like activity.

Effect of tetradecapeptide renin substrate on blood pressure, plasma renin activity, and vasopressin and adrenocorticotropin concentrations.

The effects of third ventricular injection of tetradecapeptide renin substrate (TDP) and natural renin substrate prepared from dog cerebrospinal fluid were compared in anesthetized dogs. Central injection of 350 pmol TDP caused a long lasting increase in arterial blood pressure, a reduction in PRA, and increases in plasma levels of vasopressin, and ACTH. In marked contrast, central administration of equimolar doses of natural renin substrate had no effect on these variables. Intracranial administration of the converting enzyme inhibitor SQ 20881 prevented the effects of central injection of TDP. Thus, TDP exerts its effects via conversion to angiotensin II and does not necessitate the postulation of the action of an enzyme with renin-like activity in the brain.

Lesions of the organum vasculosum of the lamina terminalis (OVLT) attenuate osmotically-induced drinking and vasopressin secretion in the dog.

Drinking and secretion of arginine vasopressin (AVP) in response to intravenous infusion of hypertonic NaCl were studied in dogs before and after destruction of the OVLT. The functional relationship between plasma osmolality and plasma AVP was eliminated following destruction of the OVLT. Furthermore, osmotically-induced drinking was significantly reduced in dogs with OVLT lesions. These data are compatible with the hypothesis that the OVLT contains osmoreceptors in the dog.

Evidence that the effects of isoproterenol on water intake and vasopressin secretion are mediated by angiotensin.

The effect of isoproterenol (6 microgram/kg sc) on drinking, urine flow, and vasopressin secretion was examined in a group of trained dogs with chronically implanted third ventricular cannulae. Isoproterenol stimulated drinking in association with a reduction in urine flow and an increase in urine to plasma osmolality ratio. Plasma renin activity increased from 3.1 +/- 0.8 to 13.0 +/- 2.7 ng/ml/3 h and plasma vasopressin concentration increased from 11.3 +/- 1.3 to 40.3 +/- 12.5 pg/ml. The effect of isoproterenol was reexamined during an intracerebroventricular infusion of the angiotensin II antagonist, saralasin (0.02 microgram/kg/min). This treatment did not affect the isoproterenol-induced increase in plasma renin activity, but inhibited the drinking, antidiuresis, and increase in plasma vasopressin concentration. These data indicate that the effects of isoproterenol on drinking, urine flow, and vasopressin secretion are mediated via the renin-angiotensin system.

Increased right or left atrial pressure stimulates release of atrial natriuretic peptides in conscious dogs.

To compare release of immunoreactive atrial natriuretic peptides (iANP) caused by distention of the right and left atria, dogs were prepared with occluding cuffs around either the ascending aorta (n = 5) or the pulmonary artery (n = 4). Graded inflation of the ascending aortic cuff for 60 min caused increments in left atrial pressure (LAP) but no change or a decrease in right atrial pressure (RAP). Plasma iANP increased significantly (P less than 0.01) in response to increases in LAP as small as 2.9 +/- 0.4 mmHg. There was a significant correlation between the increment in LAP and the rise in plasma iANP (r = 0.64, n = 25, P less than 0.01). Graded inflation of the pulmonary artery cuff caused increments in RAP and a fall in LAP. Plasma iANP increased significantly (P less than 0.01) in response to increases in RAP as small as 2.8 +/- 1.1 mmHg. Also, there was a significant correlation between the increments in RAP and the rise in plasma iANP (r = 0.69, n = 20, P less than 0.01). These results indicate that physiologic increases in either RAP or LAP is sufficient to cause increased plasma levels of iANP in conscious dogs.

Apparent dissociation of adrenocorticotropin and corticosteroid responses to 15 ml/kg hemorrhage in conscious dogs.

The blood pressure, heart rate, ACTH, corticosteroid, vasopressin, and renin responses to rapid 15 ml/kg hemorrhage were measured in six conscious healthy dogs with chronically maintained femoral arterial catheters. The hemorrhage decreased the mean arterial blood pressure slightly (P less than 0.001), increased the heart rate (P less than 0.001), and increased arterial plasma levels of ACTH (P less than 0.01), corticosteroids (P less than 0.001), vasopressin (P less than 0.001), and renin activity (P less than 0.001). Overall and in the individual experiments, there appeared to be little correspondence between the ACTH and corticosteroid responses. In none of the experiments was there a clear rise in ACTH above control levels before the first rise in corticosteroids. To ascertain that adrenal secretion of corticosteroids was increased during 15 ml/kg hypovolemia, changes in the clearance and distribution volume of cortisol were estimated by counting tritium extracted from plasma of five dogs infused with [1,2-3H] cortisol to steady state levels before and during hypovolemia. The stimulus caused a 30% reduction from steady state levels of dichloromethane-extractable tritium counts (P less than 0.001). Combined with the observed increase in plasma corticosteroid levels, these results show that the increase in adrenal secretion of corticosteroids after hemorrhage was underestimated by measurement of changes in peripheral plasma levels. The hypothesis that hemorrhage results in an increase in adrenal sensitivity to ACTH is tested in the following paper.

Effects of increases in plasma vasopressin concentration on plasma renin activity, blood pressure, heart rate, and plasma corticosteroid concentration in conscious dogs.

It is known that vasopressin decreases PRA and heart rate and increases blood pressure and plasma corticosteroid concentration. The purpose of this study was to determine the plasma concentration of vasopressin required to produce these effects. Arginine vasopressin was administered iv to five normal conscious dogs as priming injections of 0.1, 0.5, 1.0, 2.5, 5.0, and 10.0 ng/kg, followed by infusions of 0.01, 0.05, 0.1, 0.25, 0.5, and 1.0 ng/kg x min, respectively, for 30 min. These doses produced increases in the plasma vasopressin concentration (+/- SE) of 1.0 +/- 0.8, 2.1 +/- 4.3, 4.3 +/- 1.8, 11.4 +/- 1.0, 19.7 +/- 6.4, and 30.8 +/- 7.8 pg/ml, respectively, from a basal level of 2.7 +/- 0.2 pg/ml. An increase in the plasma vasopressin concentration of 2.1 +/- 0.3 pg/ml suppressed PRA by 19 +/- 5% (P < 0.02); increases of 4.2 +/- 1.8 pg/ml or more suppressed PRA by 34 +/- 12% (P < 0.005). Only the highest dose of vasopressin produced a significant pressor effect (9 +/- 3 mm Hg; P < 0.05) or lowered the heart rate (18 +/- 4 beats/min; P < 0.005). An increase in plasma vasopressin concentration of 19.7 +/- 6.4 pg/ml was required to increase the plasma corticosteroid concentration (1.2 +/- 0.2 to 2.2 +/- 0.4 microgram/dl; P < 0.01); the largest dose of vasopressin increased the plasma corticosteroid concentration from 1.5 +/- 0.1 to 2.4 +/- 0.6 microgram/dl (P < 0.02). Twenty-four-hour water deprivation in the same dogs increased the plasma vasopressin concentration from 2.5 +/- 0.2 to 7.4 +/- 0.6 pg/ml (P < 0.01). Nonhypotensive hemorrhage in another group of dogs increased the plasma vasopressin concentration from 2.5 +/- 0.2 to 47.4 +/- 16.8 pg/ml (P < 0.05). These data indicate that elevations in the plasma vasopressin concentration within the range observed during 24 h of water deprivation and nonhypotensive hemorrhage produced significant decreases in renin secretion and heart rate and elevations in blood pressure and corticosteroid secretion.

Mechanism of the dipsogenic action of tetradecapeptide renin substrate.

The mechanism of the dipsogenic action of synthetic tetradecapeptide renin substrate (TDP) was studied in rats with chronically implanted lateral ventricular cannulae. All hormones and drugs were injected via the ventricular cannulae. The dipsogenic action of TDP was unaffected by the renin inhibitor pepstatin but was markedly reduced by the angiotensin converting enzyme inhibitor SQ 20881. Homogenates of rat brain readily formed angiotensin II from TDP in vitro and this was likewise unaffected by pepstatin but was reduced or abolished by SQ 20881 or by chelating agents. Natural renin substrate did not cause drinking and did not generate angiotensin II when incubated with brain homogenates. These results demonstrate that rat brain converting enzyme can generate angiotensin II from TDP and that this effect is responsible for the dipsogenic action of TDP.

Regulation of fluid intake in dogs following water deprivation.

Whereas water loss in land living animals occurs continuously, water intake takes place discontinuously. At the normal operating set point of plasma osmolality, urine is more concentrated than plasma due to secretion of vasopressin. Thus animals operate around a state of mild dehydration. As water loss occurs, the severity of dehydration and thirst increase in intensity and at some point water intake occurs. Sufficient water is consumed to return plasma osmolality to the normal operating set point. Food intake and water balance are interdependent as food provides the osmoles which determine obligatory renal solute excretion. When dry food with the same osmotic content was substituted for canned food (water content 74%), dogs increased water intake from 24.2 +/- 4.3 to 62.2 +/- 8.8 ml/kg. Urine output and urine osmolality were unchanged, as under conditions of normal hydration, near maximal urine concentration is achieved. Changing water intake is the only available variable to maintain water balance. During water deprivation, the major renal mechanism appears to be natriuresis. In rehydration, satiety mechanisms ensure appropriate water intake and renal sodium conservation restores sodium balance.

Physiological responses to low dose infusions of atrial peptide in conscious dogs.

Mongrel dogs prepared with chronic catheters in their femoral artery and vein and urinary bladder received 60 minute infusions of atrial peptide ranging from 5 to 100 ng/kg/min. Infusion of atrial peptides caused dose dependent increases in plasma atrial peptide concentration with doses of 25 ng/kg/min or less increasing plasma concentrations to levels observed in normal animals during stimulation of endogenous atrial peptide secretion. Atrial peptide infusion at doses of 10 ng/kg/min and above caused significant decreases in mean arterial pressure which were not accompanied by statistically significant changes in heart rate. Atrial peptide infusion at doses of 25 ng/kg/min and above increased urinary sodium excretion and urine flow rate. Atrial peptide infusion was without effect on plasma vasopressin, ACTH and corticosterone concentrations. However, atrial peptide infusion resulted in dose dependent decreases in plasma aldosterone concentration and plasma renin activity, but the decreases were only significant with the high physiologic (25 ng/kg/min) and pharmacologic doses (50 & 100 ng/kg/min). These data show that atrial peptide infusions in conscious dogs have minimal effects when infused in small doses that mimic endogenous atrial peptide release. At higher doses, significant effects on the cardiovascular, renal and endocrine systems can be observed but their physiological significance is unclear.

Hill plots of muscles bathed in hypertonic solutions.

The velocity of muscle contraction is found to decrease as the osmotic pressure increases for constant load. Vmax evaluated from computer fitted load/velocity curves shows two distinct activation processes when plotted against the reciprocal of absolute temperature. Hill's equation proves to be an accurate descriptor of the experimental P/V curves for each osmotic pressure and temperature. The area under the P/V curves was found to provide a more reliable parameter representing the overall muscle response than Vmax. When the bathing solution is made sufficiently hypertonic only one activation process remains. These effects are found to be reversible. The results are explained in terms of the inhibition of ATP splitting during the crossbridge power stroke.

The importance of thirst in maintenance of fluid balance.

Plasma osmolality is maintained within very narrow limits by the control of water intake via thirst and water output via secretion of vasopressin. Osmoreceptors are situated in the brain, but on the blood side of the blood-brain barrier in a circumventricular organ. These regions are stimulated by an increase in plasma osmolality and form the most important input to cause thirst and drinking. Cardiopulmonary and arterial baroreceptors sensitive to blood volume and blood pressure also can be important, so hypovolaemic events such as haemorrhage can stimulate thirst. Both raised plasma osmolality and reduced blood volume contribute to thirst and vasopressin secretion following water deprivation. The importance of the nucleus medianus in the neural circuitary involved in integrating thirst should be emphasized. Mechanisms which stop drinking are different from those which initiate it, and oropharyngeal metering of the volume of fluid consumed provides the important input. There are a number of situations in humans where thirst thresholds and sensitivities are altered. The elderly have higher thirst thresholds and this can cause symptoms of dehydration. Increased drinking is seen in congestive heart failure, renal hypertension and certain cerebral lesions. Thirst thresholds are set at lower levels in pregnancy and in the luteal phase of the menstrual cycle and may contribute to fluid retention in these situations.