Biomedical subjects

M D Evered

Publications and source records attributed to M D Evered.

At least 19 recordsLinked to original sources

PubMed2000

Renal Na excretion in dehydrated and rehydrated adrenalectomized sheep maintained with aldosterone.

The effect of water deprivation for 19 h on renal Na excretion of conscious adrenalectomized (ADX) sheep maintained on a constant intravenous infusion of aldosterone and cortisol (ADX-constant steroid sheep) was investigated. Both ADX and normal sheep showed large increases in renal Na excretion when they were deprived of water. ADX-constant steroid sheep also exhibited a normal postprandial natriuresis 3-6 h after feeding, whether or not water was available to drink. In another experiment, sheep deprived of water for 41 h were then allowed to drink water. Both normal and ADX-constant steroid sheep exhibited a large reduction of renal Na excretion in the 6 h after rehydration. Changes in plasma Na and K concentration and osmolality were similar in normal and ADX-constant steroid sheep during periods of dehydration and rehydration. These results show that change in aldosterone secretion is not a major factor in causing either dehydration-induced or postprandial natriuresis. Neither is it a major cause of rehydration-induced renal Na retention.

Adrenalectomy↗

PubMed1998

Central losartan blocks natriuretic, vasopressin, and pressor responses to central hypertonic NaCl in sheep.

This study investigated the effect of intracerebroventricular administration of the angiotensin AT1 receptor antagonist losartan on the natriuresis, pressor effect, and arginine vasopressin (AVP) secretion caused by intracerebroventricular infusion of either ANG II, hypertonic saline, or carbachol. Losartan (1 mg/h) or artificial cerebrospinal fluid (CSF) was infused into the lateral ventricle before, during, and after infusions of either ANG II at 10 microg/h for 1 h, 0.75 mol/l NaCl at 50 microl/min for 20 min, or carbachol at 1.66 microg/min for 15 min. Intracerebroventricular infusions of ANG II, 0.75 mol/l NaCl, or carbachol caused increases in renal Na+ and K+ excretion, arterial pressure, and plasma AVP levels. Increases in arterial pressure, Na+ excretion, and plasma AVP concentration ([AVP]) in response to intracerebroventricular ANG II or intracerebroventricular 0.75 mol/l NaCl were either abolished or attenuated by intracerebroventricular infusion of losartan but not by intracerebroventricular infusion of artificial CSF or intravenous losartan. Intracerebroventricular losartan did not reduce the increase in plasma [AVP] or arterial pressure in response to intracerebroventricular carbachol, but it did attenuate the natriuretic response to intracerebroventricular carbachol. We conclude that an intracerebroventricular dose of losartan (1 mg/h) that inhibits responses to intracerebroventricular ANG II also inhibits vasopressin secretion, natriuresis, and the pressor response to intracerebroventricular hypertonic saline. These results suggest that common neural pathways are involved in the responses induced by intracerebroventricular administration of ANG II and intracerebroventricular hypertonic NaCl. We propose that intracerebroventricular infusion of hypertonic saline activates angiotensinergic pathways in the central nervous system subserving the regulation of fluid and electrolyte balance and arterial pressure in sheep.

Angiotensin II↗

PubMed1998

The role of angiotensin in arterial blood pressure regulation in the toad Bufo marinus.

Little is known about the role of the renin-angiotensin system in anuran amphibians, although they appear to possess the functional components of such a system. We investigated the role of angiotensin (ANG) in arterial blood pressure regulation in the conscious toad Bufo marinus using the angiotensin-converting enzyme blocker captopril. We found that conversion of endogenous ANG I to ANG II made a significant contribution to mean arterial pressure in undisturbed animals. The vascular tone contributed by ANG II was not mediated via &agr ; adrenergic mechanisms because increases in pressure in response to ANG infusion were unaffected by the presence of the &agr ; antagonist phentolamine. Angiotensin-induced vasoconstriction was shown to be an important mechanism in arterial blood pressure regulation in the face of an acute hypotensive perturbation of pressure brought about by sodium nitroprusside. Blockade of the conversion of ANG I to ANG II significantly delayed the recovery of mean arterial pressure after sodium nitroprusside-induced hypotension. This suggests that the renin-angiotensin system may play an important role in the initial responses to hypotension in anurans, whether brought about by haemorrhage or dehydration.

Angiotensin II↗

PubMed1997

Intracerebroventricular losartan inhibits postprandial drinking in sheep.

We investigated the contribution of brain angiotensinergic mechanisms to postprandial drinking in sheep. Sheep in fluid balance were given 0.8 kg chaff for 30 min, and water intake was measured for the next hour. Intracerebroventricular infusion of the AT1 type angiotensin II (ANG II) receptor blocker losartan (1 mg/h) reduced postprandial drinking by approximately 70% (n = 7, P < 0.01) but did not affect food intake. The same losartan dose given intravenously had little or no effect on prandial drinking. Feeding increased Na+ concentrations in plasma and cerebrospinal fluid (CSF; n = 5, P < 0.05). Intracerebroventricular losartan (1 mg/h) inhibited the drinking responses to intracarotid infusion of ANG II (0.8 microg/min for 30 min, n = 4, P < 0.01) and to intracerebroventricular infusion of 0.5 M NaCl (1 ml/h for 1 h, n = 5, P < 0.05) but had no effect on drinking responses to intravenous infusion of 4 M NaCl (1.3 ml/min for 30 min). These findings indicate that a brain ANG II-dependent mechanism is involved in postprandial drinking in sheep. They suggest also that the mechanism by which increasing CSF Na+ causes thirst involves brain ANG II and is different from the mechanism subserving the drinking response to changes in blood Na+.

Angiotensin II↗

PubMed1993

Stimulation of drinking by bacterial endotoxins in the rat.

We investigated the effects of endotoxins on water balance, rectal temperature, and food intake in male Long-Evans rats with femoral venous catheters. Extracts of Escherichia coli or Salmonella minnesota, in doses ranging from 125 to 500 micrograms/kg IV, stimulated drinking and reduced urinary water loss for several hours. The net gain of 5 ml water 2 h after the lowest dose of E. coli endotoxin was sufficient to reduce plasma osmolality and sodium concentration 2 to 3%. Drinking occurred during the period of hypothermia that frequently precedes the onset of endotoxin-induced fever, so cannot be attributed to increased body temperature. Doses of endotoxin causing drinking inhibited both spontaneous and deprivation-induced feeding. The cause of the drinking is not known, but may involve mechanisms other than the known dehydrational signals controlling thirst.

Animals↗

PubMed1993

Schedule-induced polydipsia in rats with gastric fistulas.

We have investigated the development and maintenance of schedule-induced polydipsia (SIP) when ingested water (and food) was allowed to drain from the stomach. Fourteen male Long-Evans rats were prepared with permanent gastric cannulas and, after recovery, their body weight was reduced to 80%. Water intake was measured, with cannulas open or closed, during 42 daily 1-h sessions in which 45-mg food pellets were delivered one per minute. Allowing ingested material to drain from the stomach impaired the development of SIP and reduced the polydipsia in rats in which SIP had already been established. In contrast, opening the gastric fistulas increased drinking in these rats when all the food pellets were provided at once or after water deprivation. The opposite effects of gastric drainage on dehydration and schedule-induced drinking is consistent with the view that SIP is not a fluid-regulating phenomenon. It is not clear, however, how these unexpected findings fit current hypotheses to explain SIP that are based on oral, neural excitatory, or emotional mechanisms.

Animals↗

PubMed1993

Endotoxin stimulates drinking in rats without changing dehydrational signals controlling thirst.

Intravenous injections of endotoxins from Escherichia coli or Salmonella minnesota stimulate drinking and reduce urinary excretion of water and solutes in rats. E. coli endotoxin (0.15 or 0.45 mg/kg i.v.) stimulated drinking without increasing plasma osmolality or sodium concentration, hematocrit, blood hemoglobin, or plasma protein concentration and without decreasing arterial pressure. Similarly, a dipsogenic dose of S. minnesota endotoxin (0.25 mg/kg i.v.) did not reduce arterial or venous pressures or change heart rate. Blocking the renin-angiotensin system with captopril or blocking histamine receptors with pyrilamine and cimetidine did not reduce drinking or urinary fluid retention caused by E. coli endotoxin. Injections of 10 or 450 ng E. coli endotoxin into a lateral cerebral ventricle increased body temperature but not water intake. In contrast to its stimulatory effect in water-replete rats, E. coli endotoxin (0.45 mg/kg i.v.) inhibited drinking in 24-h water-deprived rats. Thus we find no evidence to support the hypothesis that endotoxin causes thirst by changing known physiological signals of cellular or extracellular dehydration. The mechanism remains unknown.

Animals↗

PubMed1992

Investigating the role of angiotensin II in thirst: interactions between arterial pressure and the control of drinking.

Several lines of evidence suggest that angiotensin II plays a physiological role in the control of thirst. Establishing that, however, has been surprisingly difficult, given our current knowledge about the renin-angiotensin systems in the circulation and the brain and the variety of techniques available to measure and manipulate them. A major problem is that stimulating or blocking the renin-angiotensin system affects several physiological variables simultaneously. Since several of these variables also influence the controls of water intake directly or indirectly, the interpretation of the effect on drinking becomes more difficult. To illustrate the problem and recent developments, this paper describes some of the interactions between the effects of angiotensin II on arterial pressure and thirst, and it shows how they have contributed to the controversy over the physiological role of the peptide.

Angiotensin II↗

PubMed1990

Relationship between thirst and diazoxide-induced hypotension in rats.

Diazoxide, a potent vasodilator and antidiuretic, was used to examine the relationship between hypotension and thirst in conscious rats with indwelling arterial and venous catheters. Bolus iv. injections (5-50 mg/kg) caused prompt, long-lasting, and dose-dependent reductions in mean arterial pressure (MAP) and stimulated drinking. Water intake and degree of hypotension were closely correlated when MAP was 10-65 mmHg below normal. At the time of drinking there were no significant changes in central venous pressure, plasma osmolality, or Na+ or K+ concentration. Plasma glucose increased approximately 35%, and blood volume increased approximately 10% (based on hematocrit changes and dilution of Evans blue). Captopril (100 mg/kg sc to block the renin-angiotensin system) enhanced the depressor response to diazoxide but abolished the dipsogenic response over the same range of arterial pressures tested in controls. Angiotensin II iv infusion restored drinking in captopril-treated animals. The combination of captopril and diazoxide did not block drinking to iv infusions of hypertonic saline or water deprivation. These results confirm that hypotension potently stimulates thirst and support the hypothesis that angiotensin II mediates the dipsogenic response in rats.

Angiotensin II↗

PubMed1988

Effect of arterial pressure on drinking and urinary responses to angiotensin II.

To investigate the relationship between angiotensin II (ANG II) and mean arterial pressure (MAP) in the control of drinking in rats, we infused ANG II intravenously at constant rates (either 50 or 100 ng.kg-1.min-1 for 90 min) and varied MAP by intravenous injections of diazoxide (5-20 mg/kg). Rats were pretreated with captopril to block the endogenous synthesis of ANG II. When given alone, low and high doses of ANG II increased MAP approximately 30 and 50 mmHg, respectively. The low but not the high dose significantly increased water intake above control levels. Both doses caused such a large diuresis and natriuresis that the net effect was fluid loss. Reducing MAP toward normal greatly increased the drinking response to the high but not the low dose of ANG II and reduced the urinary solute and water loss to both doses. These results support the hypothesis that water intake and net fluid gain are inhibited when MAP is above normal. When MAP was reduced below normal in rats given constant infusions of ANG II the amount of water drunk and net fluid gain was proportional to the dose of ANG II but not the dose of diazoxide, the degree of hypotension, or urinary losses. This is consistent with previous reports that ANG II is essential for the drinking response to hypotension. Furthermore, it demonstrates that ANG II is not merely permissive but probably the signal controlling water intake when arterial pressure is reduced below normal.

Angiotensin II↗

PubMed1987

Pressor action of intravenous angiotensin II reduces drinking response in rats.

We investigated whether the pressor response to intravenous angiotensin II (ANG II) suppresses drinking. All experiments were done on conscious water-replete rats (200-400 g) with chronic vascular cannulas. Two rates of ANG II infusion (16.7 and 100 ng/min for 90 min) were tested; captopril (0.33 mg/min) was infused simultaneously to prevent endogenous production of ANG II. Both doses of ANG II increased mean arterial pressure (MAP) by 40-50 mmHg for the duration of the infusions, but water intakes were small. The drinking response was increased as much as fivefold, however, when the pressor response was reduced by injecting either isoproterenol (0.01 or 0.1 mg/kg, sc), diazoxide (20, 30, or 75 mg/kg, sc), or minoxidil (10 mg/kg, ip) 15 min after starting the ANG II infusion. The closer MAP was returned to normal, the greater was the drinking response. Since lowering MAP also reduced urinary water losses, net fluid intake increased even more dramatically. It is unlikely that the vasodilators directly stimulated thirst in the experiments because the dose of captopril used completely blocked drinking to these agents given alone. A situation of high circulating levels of ANG II but with MAP near or below normal more closely resembles physiological conditions of dehydration. Our results demonstrate that intravenous ANG II is a very potent dipsogen under these conditions.

Angiotensin II↗

PubMed1985

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.

Angiotensin II↗

PubMed1984

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)

Angiotensin-Converting Enzyme Inhibitors↗

PubMed1983

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

We have investigated the effects of separate and combined s.c. and intracerebroventricular (i.c.v.) injections of captopril, an inhibitor of angiotensin I-converting enzyme, on isoproterenol-induced thirst. Whereas s.c. injections of captopril (0.5 mg/kg) increased drinking, combined s.c. and i.c.v. (20 micrograms) injections of captopril nearly abolished drinking to isoproterenol (0.1 mg/kg s.c.). This inhibition was not caused by general debility of the rats since the same treatment did not reduce drinking to 12h water deprivation. Intracerebroventricular injection of 20 micrograms captopril alone also greatly reduced isoproterenol-induced drinking, perhaps because it leaked into the circulation; captopril i.c.v. also reduced the pressor response to i.v. injection of hog renin (0.1 Goldblatt Unit) by about 65%. These results support the hypotheses that the renin-angiotensin system participates in the stimulation of drinking by isoproterenol and that the enhancement of drinking caused by inhibition of CE only in the circulation is the result of increased synthesis of angiotensin II in the brain.

Angiotensin II↗

PubMed1983

Neuropeptides and thirst.

A number of neuropeptides have been found to affect fluid intake when injected directly into the brain of various vertebrate species. These include: angiotensin II and its peptide precursors; the tachykinins Substance P, eledoisin and physalaemin; the opioid peptides met- and leu-enkephalin and beta-endorphin; bombesin; neurotensin; and vasopressin. Some of these stimulate drinking, some inhibit water intake, and the tachykinins have opposite effects on thirst depending on the species tested. Very little is known about the site or mechamism of action of most of these peptides or if their effects on thirst are physiological. The exception is angiotensin II, a peptide hormone that is synthesized in the blood in response to hypovalaemia or hypotension and is involved in many aspects of the regulation of blood volume and pressure. Angiotensin II injected intravenously or intracranially stimulates drinking in all reptiles, birds and mammals tested. In addition to its role as a hormone, angiotensin II may also function as a neurotransmitter or neuromodulator, since all of the enzymes and precursors necessary for its synthesis have been found in the central nervous system.

Angiotensin II↗

PubMed1983

Effects of captopril on salt appetite in sodium-replete rats and rats treated with desoxycorticosterone acetate (DOCA).

Captopril (30 mg/kg daily by gavage), an orally active inhibitor of angiotensin-converting enzyme, reduced the intake of 0.15 M NaCl in rats treated with desoxycorticosterone acetate (2.5 mg/day S.C.). However, this is probably not a specific effect on salt appetite as captopril (30 or 60 mg/kg daily) did not reduce the intake of less palatable 0.5 M NaCl in desoxycorticosterone acetate-treated rats (1 or 2.5 mg daily). In contrast, captopril given alone (30 or 60 mg/kg daily) consistently caused a 3- to 5-fold increase in intake of 0.5 M NaCl that usually began on the 1st day and persisted until treatment stopped (1-3 weeks). Sodium intake and sodium excretion increased concomitantly, but the stimulation of salt appetite occurred even in the absence of sodium depletion. Also, the effect on salt appetite was specific; captopril did not increase the intake of 0.5 M KCl or 0.03 M sucrose. This potent and specific stimulation of sodium intake by oral treatment with an inhibitor of the renin-angiotensin system may be caused by a paradoxical increase in the synthesis of angiotensin II in the brain.

Animals↗

PubMed1981

Drinking and changes in blood pressure in response to angiotensin II in the pigeon Columba livia.

1. Angiotensin II is as potent a stimulus to drink in pigeons as it is in mammals. There are striking similarities in the action of this peptide in pigeons and mammals. 2. Angiotensin II injected intracranially, I.V. or I.P. consistently caused short-latency and vigorous drinking in pigeons but no other behaviour. Drinking was completed rapidly and intakes were very large, sometimes in excess of 10% of the bird's body weight. 3. The latency to drink and the amount drunk were dose dependent for all routes of injection. Angiotensin II was most effective when injected directly into the brain. As little as 10(-4) mol angiotensin II injected into the cerebral ventricles caused birds to drink. 4. The rapid cessation of drinking after intracranial injection of angiotensin II was not caused by rapid loss of activity of the peptide in the brain but by the actual ingestion of the water. 5. The brain sites most sensitive to the dipsogenic action of angiotensin II in the pigeon were the dorsal and ventral third ventricle, the tissue adjacent and anterior to these sites, and the lateral ventricles. The lateral hypothalamic area was only slightly less sensitive. Negative sites for drinking were found in the lateral forebrain and the hind brain. These findings are similar to those in mammals. 6. Pigeons drank during I.V. infusion of as little as 16 X 10(-12) mol angiotensin II kg-1 min-1. This was near the threshold for increasing arterial pressure in pigeons and is near the threshold for drinking in rats and dogs. 7. The Asn1, Asp1, Val5 and Ile5 analogues of angiotensin II were equipotent as stimuli to drink but a wide range of other peptides and drugs injected into the brain failed to increase water intake. An exception was eledoisin which was, comparing molecule with molecule, only 10-100 times less potent than angiotensin II in the pigeon. 8. Injections of angiotensin II into brain sites which caused drinking failed to alter heart rate or arterial pressure in pigeons. 9. This and other recent studies demonstrate the wide phylogenetic distribution of the dipsogenic action of angiotensin II and support the idea that the control of water intake is an important physiological function of the renin-angiotensin system in vertebrates.

Angiotensin II↗

PubMed1981

Drinking and changes in blood pressure in response to precursors, fragments and analogues of angiotensin II in the pigeon Columba livia.

1. The pigeon drank as vigorously in response to intracranial injection of synthetic renin substrate and angiotensin I as to angiotensin II. 2. Mammalian renin injected into the brain caused the water-replete pigeon to drink but it was a less effective dipsogen than in the mammal. As in the mammal, renin-induced drinking was slower in onset and continued for longer than angiotensin-induced drinking. 3. The converting enzyme inhibitor SQ 20881 attenuated drinking in response to intracranial renin, synthetic renin substrate and angiotensin I but enhanced intracranial angiotensin II-induced drinking. Therefore drinking induced by the intracranial injection of precursors of angiotensin II is mediated through local generation of angiotensin II. 4. I.V. injection of angiotensin I was as effective as angiotensin II in causing the pigeon to drink, but synthetic renin substrate was less effective. I.V. doses of angiotensin I and II had to be about 100 times greater than the intracranial doses in order to produce similar intakes. 5. Angiotensin I and II were equally effective pressor agents by I.V. injection in the pigeon but synthetic renin substrate was much less effective. I.V. SQ 20881 inhibited the pressor response to I.V. synthetic renin substrate or angiotensin I but enhanced the angiotensin II-induced response. 6. Aliphatic position 8-substituted analogues of angiotensin II which are competitive antagonists of angiotensin II-induced drinking and pressor responses in the mammal in antagonist:agonist mole ratios as low as 10:1, failed to reduce drinking in response to intracranial synthetic renin substrate or angiotensin II, although not themselves agonists, nor did they prevent the pressor to infusion of angiotensin II even with antagonist:agonist mole ratios as high as 10,000:1. 7. Shortening the angiotensin octapeptide from the N-terminus caused a progressive reduction in intracranial dipsogenic activity. Activity was completely abolished by removing the C-terminal phenylalanine. 8. These results demonstrate that in pigeons, as in mammals, it is angiotensin II which is the biologically active peptide in the control of drinking behaviour and blood pressure by the renin-angiotensin system. Precursors of angiotensin II can be converted to the octapeptide in the avian brain as well as in the circulation. The angiotensin receptors for drinking and blood pressure responses are similar to each other in the pigeon and they are very similar but not identical with the angiotensin receptors for the dipsogenic, pressor and myotropic actions of angiotensin II in mammals.

Angiotensin II↗