Effects of systemic and intracranial inhibition of angiotensin-converting enzyme on isoproterenol-induced drinking in the rat

Interactions of the systemic and brain renin-angiotensin systems in the control of drinking and the central mediation of pressor responses

Most of the biological actions of the circulating (a.k.a., the systemic or blood-borne) renin-angiotensin system require the generation of the octapeptide angiotensin (ANG) II from the decapeptide ANG I. In the case of circulating ANG I, the lungs are generally considered the major site for this conversion. The present experiments explored the possibility that under conditions of marked elevations of blood-borne ANG I, the generation of ANG II takes place within brain-associated target tissues, most notably circumventricular organs (CVOs) that lack a blood-brain barrier. The first important result of these experiments demonstrates that intracerebroventricular (i.c.v.) infusion of the converting enzyme inhibitor, captopril, completely blocks the drinking response and significantly attenuates the pressor response produced by systemically infused ANG I. This result indicates that under physiological/pathophysiological conditions associated with large elevations of circulating ANG I, an important part of the biological responses derived from blood-borne ANG may result from local conversion of ANG I to ANG II within specific brain target tissues which have high concentrations of converting enzyme. This local conversion process provides an important mechanism that would act to reinforce the "classic" conversion process which takes place in the lungs thereby delivering more ANG II immediately to central target receptors. The second important finding from these studies showed that drinking produced by systemically infused ANG II was not attenuated by an i.c.v. dose of captopril which was effective in blocking a comparable dipsogenic response induced by i.v. ANG I. This observation suggests that drinking induced by systemic ANG II does not require an intact metabolic cascade within the brain for the formation of ANG II (or ANG II-like effector peptide) from ANG I.

The neuroendocrinology of thirst and salt appetite: visceral sensory signals and mechanisms of central integration

This review examines recent advances in the study of the behavioral responses to deficits of body water and body sodium that in humans are accompanied by the sensations of thirst and salt appetite. Thirst and salt appetite are satisfied by ingesting water and salty substances. These behavioral responses to losses of body fluids, together with reflex endocrine and neural responses, are critical for reestablishing homeostasis. Like their endocrine and neural counterparts, these behaviors are under the control of both excitatory and inhibitory influences arising from changes in osmolality, endocrine factors such as angiotensin and aldosterone, and neural signals from low and high pressure baroreceptors. The excitatory and inhibitory influences reaching the brain require the integrative capacity of a neural network which includes the structures of the lamina terminalis, the amygdala, the perifornical area, and the paraventricular nucleus in the forebrain, and the lateral parabrachial nucleus (LPBN), the nucleus tractus solitarius (NTS), and the area postrema in the hindbrain. These regions are discussed in terms of their roles in receiving afferent sensory input and in processing information related to hydromineral balance. Osmoreceptors controlling thirst are located in systemic viscera and in central structures that lack the blood-brain barrier. Angiotensin and aldosterone act on and through structures of the lamina terminalis and the amygdala to stimulate thirst and sodium appetite under conditions of hypovolemia. The NTS and LPBN receive neural signals from baroreceptors and are responsible for inhibiting the ingestion of fluids under conditions of increased volume and pressure and for stimulating thirst under conditions of hypovolemia and hypotension. The interplay of multiple facilitory influences within the brain may take the form of interactions between descending angiotensinergic systems originating in the forebrain and ascending adrenergic systems emanating from the hindbrain. Oxytocin and serotonin are additional candidate neurochemicals with postulated inhibitory central actions and with essential roles in the overall integration of sensory input within the neural network devoted to maintaining hydromineral balance.

Regulation of alcohol consumption by the renin-angiotensin system: a review of recent findings and a possible mechanism of action

The renin-angiotensin system has traditionally been associated with the regulation of fluid and electrolyte balance. In this review we summarize the data which ascribes a completely new function to this system, i.e., the regulation of alcohol consumption. In addition, we suggest a possible mechanism for this effect based on the concept of a satiety or stop process. The approach taken was to examine the effect on alcohol intake of a wide variety of drug, genetic, dietary, surgical and neurosurgical manipulations, each of which has a range of biological effects characteristic of that manipulation, but all of which share the common property of altering activity in the renin-angiotensin system. The effect of these manipulations on alcohol intake was most parsimoniously explained by reference to their ability to raise or lower activity in the renin-angiotensin system. Any intervention which modulates activity in this system, either directly or indirectly, is likely to have consequences for alcohol consumption.

Pregastric food-contingent stimulation elicits drinking in the absence of systemic dehydration in the rat

Adult male Sprague-Dawley rats surgically fitted with a stainless steel gastric cannula were prepared following 24-h food deprivation for sham feeding liquid diet with open gastric fistula. Sham feeding (pregastric food-contingent stimulation) of 5, 10, 20 or 40 ml of liquid diet elicited water intake that was not in proportion to volume of liquid diet sham fed. Rats sham feeding between 1 and 68 ml of sweetened milk showed no evidence of cellular or extracellular dehydration as measured by plasma osmolality and hematocrit, respectively. Subcutaneous injection of 100 mg/kg captopril, a dose sufficient to block conversion of angiotensin I to angiotensin II in brain and periphery, inhibited drinking elicited by sham feeding without effect on sham feeding. These results demonstrate that pregastric food-contingent stimulation elicits water intake that is not in proportion to amount of liquid diet sham fed and that occurs in the apparent absence of systemic dehydration. That such drinking could depend upon endogenous angiotensin II provokes consideration of a role for angiotensin II in the mediation of a pregastric (perhaps histaminergic) mechanism that initiates drinking in advance of postprandial dehydration.

Roles of peripheral and central angiotensin-converting enzyme (ACE) in hypovolemic thirst induced by compound 48/80 in rats

Subcutaneous (s.c.) injection of Hoe 498, an angiotensin converting enzyme (ACE) inhibitor, at the doses of 0.1, 0.5, 1.0 and 4.0 mg/kg produced a dose-related inhibition of compound 48/80-induced hypovolemic thirst in rats. A significant time-response relationship was observed between the pretreatment time of Hoe 498 at a dose of 4.0 mg/kg and the inhibition of compound 48/80-induced water intake. Nearly 90% of plasma ACE activity was inhibited by Hoe 498 at all doses used, and this inhibition at the dose of 4.0 mg/kg of Hoe 498 continued for more than 4 hr. Intracerebroventricular (i.c.v.) or s.c. injection of Hoe 498 in doses ranging from 0.5 to 20 micrograms comparably inhibited plasma ACE activity in a dose-dependent manner. The compound 48/80-induced water intake was significantly reduced by i.c.v. injection of Hoe 498 (20 micrograms) 30 min after compound 48/80 administration, but not reduced when the drug was given 15 min prior to injection of dipsogen. The inhibition of water intake by Hoe 498 seems to be dependent on the dose and time between administration of Hoe 498 and compound 48/80. The present data suggest that brain ACE is more involved in compound 48/80-induced water intake than peripheral systemic ACE.

The beta adrenergic agonist isoproterenol suppress voluntary alcohol intake in rats

The effects of isoproterenol on alcohol consumption were examined to investigate whether beta adrenergic stimulation can reduce voluntary alcohol intake. Two and one-half, 5 and 10 micrograms/kg isoproterenol administered subcutaneously (SC) just prior to alcohol availability produced a dose-dependent reduction in alcohol intake and elevation in water intake. Blood alcohol levels measured subsequent to a SC injection of 5 micrograms/kg isoproterenol or vehicle followed by an intraperitoneal injection of 2.5 g/kg alcohol showed that the adrenergic agonist did not alter the distribution or metabolism of alcohol. Since beta adrenergic agonists such as isoproterenol are potent releasers of renin, these findings support previous work showing that different kinds of interventions which share the common property of elevating activity in the renin-angiotensin system (beta adrenergic stimulation in the present case) consistently result in the reduction of voluntary alcohol intake.

Cerebral renin-angiotensin mediation of isoproterenol-induced thirst in the dog

Pretreatment of dogs with s.c. isoproterenol (10 micrograms/kg) caused a significant increase in drinking when 100 ng renin substrate was administered 3 min later to the lateral cerebral ventricles or subfornical organ. Isoproterenol itself was a potent peripheral (10 micrograms/kg), but unreliable central (0.01-1 microgram) dipsogen. The increased drinking after combined s.c. isoproterenol and intracerebroventricular (i.c.v.) renin substrate injections was significantly attenuated by i.c.v. captopril (20 micrograms), but was not influenced by s.c. captopril (500 micrograms/kg). However, combined i.c.v./s.c. pretreatment with captopril nearly abolished drinking to peripheral isoproterenol, or the combination of s.c. isoproterenol and i.c.v. renin substrate. Finally, single intracranial injections of the components of the renin-angiotensin system elicited dose-dependent and site-specific drinking. Renin substrate, angiotensin I and angiotensin III produced greater intakes at forebrain tissue sites than after i.c.v. or subfornical organ injections. Renin, on the other hand, was more potent i.c.v. than at forebrain loci. These results suggest that the cerebral renin-angiotensin system may participate in beta-adrenergic thirst mechanisms by increasing local angiotensin II biosynthesis in specific areas of the brain.

Captopril-induced hyponatremia with irreversible neurologic damage

A 61-year-old Chinese-American man with a history of congestive heart failure and hypertension was admitted to the San Francisco Veterans Administration Hospital with confusion, cortical blindness, and generalized flaccidity. Serum sodium level on admission was 114 meq/liter. Administration of captopril had been begun for afterload reduction two weeks before admission with a concomitant fall in serum sodium level from 137 meq/liter to 126 meq/liter in one week. A history of marked thirst with consumption of large volumes of water was reported for over one week prior to hospitalization. Despite correction of the hyponatremia within 24 hours at a rate of 0.9 meq/liter per hour, the patient remained semi-comatose and died four days later with a gastrointestinal bleed. It is suggested that the thirst phenomenon and hyponatremia were caused by the introduction of captopril. This lead to irreversible neurologic damage and death, despite the correction of the serum sodium level.

The short-term effect of captopril on salt and water intake in the rat is not taste-specific

We have investigated the extent to which captopril's short-term (1 h) effects on salt and water intake in the rat are caused by effects on taste. In single-bottle tests a low dose of captopril (0.5 mg/kg s.c.), which blocks the synthesis of angiotensin II in the blood but not the brain, increased equally the intakes of water, 0.05, 0.15, 0.30 and 0.45 M NaCl, 0.3 M KCl, 10 mM HCl, 0.14 mM quinine hydrochloride and 0.1 mM saccharin solutions without changing the animals' preference for or aversion to each with respect to water. In two-choice tests this dose increased water but not 0.15 or 0.45 M NaCl intake. A large dose of captopril (100 mg/kg s.c.), to block the synthesis of angiotensin II also in the brain, did not enhance water or NaCl intake. Neither dose affected NaCl or water intake by rats drinking in response to 2 M NaCl, 5 ml/kg i.p. We conclude that during the first hour following injection captopril has no major effect on taste perception or preference in the rat and does not stimulate sodium appetite in the sodium-replete rat. Our results support the hypothesis that low doses of captopril increase fluid intake by enhancing the synthesis of angiotensin II in the brain.

Increased or decreased thirst caused by inhibition of angiotensin-converting enzyme in the rat

We have investigated the effects on water intake of subcutaneous (S.C.) injections of low (0.5 mg/kg) and high (100 mg/kg) doses of captopril, an inhibitor of angiotensin-converting enzyme (CE). Low doses block the synthesis of angiotensin II only in the circulation whereas high doses block CE in both the blood and the brain. The low dose of captopril enhanced drinking in response to three hypotensive drugs, isoprenaline (0.1 mg/kg, S.C.), phentolamine (5 mg/kg, S.C.) and serotonin (2 mg/kg, S.C.), whereas the high dose of captopril abolished drinking in response to these stimuli. The low dose of captopril also enhanced drinking in response to histamine (0.25-5.0 mg/kg, intraperitoneal, I.P.), but in this case the high dose of captopril only partially reduced the drinking response. The low dose of captopril enhanced drinking after 24 h water deprivation but high doses had no significant effect on deprivation-induced thirst. Hypovolaemia was produced either by injecting polyethylene glycol (30% w/v, 10 ml/kg) S.C. or by replenishing the cellular deficit in water-deprived rats with 10 ml water (by gavage). The low dose of captopril enhanced the drinking response to hypovolaemia but the high dose had no significant effect. Neither the high nor the low dose of captopril significantly affected drinking in response to cellular dehydration caused by injecting 2 M-NaCl (2 ml) I.P. or by replenishing the extracellular deficit in water-deprived rats (10 ml balanced salt solution by gavage). Nephrectomy (but not ligation of the ureters) or injections of propranolol (5 mg/kg, S.C.) to prevent renin secretion prevented the enhancement of deprivation-or serotonin-induced thirst by the low dose of captopril. The low dose of captopril did not enhance drinking in response to I.V. injections of renin (1 Goldblatt unit), or intracerebroventricular (I.C.V.) injections of angiotensin I or II. The high dose of captopril blocked drinking in response to I.V. injections of renin or I.C.V. injections of angiotensin I but did not reduce drinking in response to angiotensin II, I.C.V. These results are consistent with the hypothesis that blocking CE only in the circulation enhances drinking in response to hypotension or hypovolaemia because angiotensin I, accumulating in high concentration in the blood, enters the brain and is converted intracerebrally to angiotensin II. These findings suggest that the enhancement of drinking caused by low doses of captopril s.c. is a sensitive indicator of whether the renin- angiotensin system participates at all in the regulatory response to a particular stimulus to drink.(ABSTRACT TRUNCATED AT 400 WORDS)