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

Renin-angiotensin system in vertebrates: phylogenetic view of structure and function

Renin substrate, biological renin activity, and/or renin-secreting cells in kidneys evolved at an early stage of vertebrate phylogeny. Angiotensin (Ang) I and II molecules have been identified biochemically in representative species of all vertebrate classes, although variation occurs in amino acids at positions 1, 5, and 9 of Ang I. Variations have also evolved in amino acid positions 3 and 4 in some cartilaginous fish. Angiotensin receptors, AT₁ and AT₂ homologues, have been identified molecularly or characterized pharmacologically in nonmammalian vertebrates. Also, various forms of angiotensins that bypass the traditional renin-angiotensin system (RAS) cascades or those from large peptide substrates, particularly in tissues, are present. Nonetheless, the phylogenetically important functions of RAS are to maintain blood pressure/blood volume homeostasis and ion-fluid balance via the kidney and central mechanisms. Stimulation of cell growth and vascularization, possibly via paracrine action of angiotensins, and the molecular biology of RAS and its receptors have been intensive research foci. This review provides an overview of: (1) the phylogenetic appearance, structure, and biochemistry of the RAS cascade; (2) the properties of angiotensin receptors from comparative viewpoints; and (3) the functions and regulation of the RAS in nonmammalian vertebrates. Discussions focus on the most fundamental functions of the RAS that have been conserved throughout phylogenetic advancement, as well as on their physiological implications and significance. Examining the biological history of RAS will help us analyze the complex RAS systems of mammals. Furthermore, suitable models for answering specific questions are often found in more primitive animals.

Cardiovascular and thermoregulatory responses to vasotocin and angiotensin II in the pigeon

The cardiovascular and thermoregulatory effects of intrahypothalamically (preoptic/anterior hypothalamus) and intravenously injected arginine vasotocin (AVT) and [Val5]angiotensin II (ANG II) were measured at 2 degrees C in the pigeon (Columba livia). In addition, the effects of intrahypothalamic and intravenous injections of AVT on respiratory rates were measured at 10-15 degrees C. Intrahypothalamic and intravenous AVT (500 ng and 20 micrograms/kg, respectively) reduced shivering and body temperature but had no effects on blood pressure, heart rate or respiratory rate. Intrahypothalamic (500 ng and 1 microgram) and intravenous (3 micrograms/kg) ANG II elevated blood pressure. If the blood pressure increased slowly, the shivering and body temperature also increased, whereas a rapid rise in blood pressure inhibited shivering and lowered body temperature. Intravenous ANG II produced tachycardia but intrahypothalamic ANG II did not affect the heart rate.

Angiotensin, thirst, and sodium appetite

Angiotensin (ANG) II is a powerful and phylogenetically widespread stimulus to thirst and sodium appetite. When it is injected directly into sensitive areas of the brain, it causes an immediate increase in water intake followed by a slower increase in NaCl intake. Drinking is vigorous, highly motivated, and rapidly completed. The amounts of water taken within 15 min or so of injection can exceed what the animal would spontaneously drink in the course of its normal activities over 24 h. The increase in NaCl intake is slower in onset, more persistent, and affected by experience. Increases in circulating ANG II have similar effects on drinking, although these may be partly obscured by accompanying rises in blood pressure. The circumventricular organs, median preoptic nucleus, and tissue surrounding the anteroventral third ventricle in the lamina terminalis (AV3V region) provide the neuroanatomic focus for thirst, sodium appetite, and cardiovascular control, making extensive connections with the hypothalamus, limbic system, and brain stem. The AV3V region is well provided with angiotensinergic nerve endings and angiotensin AT1 receptors, the receptor type responsible for acute responses to ANG II, and it responds vigorously to the dipsogenic action of ANG II. The nucleus tractus solitarius and other structures in the brain stem form part of a negative-feedback system for blood volume control, responding to baroreceptor and volume receptor information from the circulation and sending ascending noradrenergic and other projections to the AV3V region. The subfornical organ, organum vasculosum of the lamina terminalis and area postrema contain ANG II-sensitive receptors that allow circulating ANG II to interact with central nervous structures involved in hypovolemic thirst and sodium appetite and blood pressure control. Angiotensin peptides generated inside the blood-brain barrier may act as conventional neurotransmitters or, in view of the many instances of anatomic separation between sites of production and receptors, they may act as paracrine agents at a distance from their point of release. An attractive speculation is that some are responsible for long-term changes in neuronal organization, especially of sodium appetite. Anatomic mismatches between sites of production and receptors are less evident in limbic and brain stem structures responsible for body fluid homeostasis and blood pressure control. Limbic structures are rich in other neuroactive peptides, some of which have powerful effects on drinking, and they and many of the classical nonpeptide neurotransmitters may interact with ANG II to augment or inhibit drinking behavior. Because ANG II immunoreactivity and binding are so widely distributed in the central nervous system, brain ANG II is unlikely to have a role as circumscribed as that of circulating ANG II. Angiotensin peptides generated from brain precursors may also be involved in functions that have little immediate effect on body fluid homeostasis and blood pressure control, such as cell differentiation, regeneration and remodeling, or learning and memory. Analysis of the mechanisms of increased drinking caused by drugs and experimental procedures that activate the renal renin-angiotensin system, and clinical conditions in which renal renin secretion is increased, have provided evidence that endogenously released renal renin can generate enough circulating ANG II to stimulate drinking. But it is also certain that other mechanisms of thirst and sodium appetite still operate when the effects of circulating ANG II are blocked or absent, although it is not known whether this is also true for angiotensin peptides formed in the brain. Whether ANG II should be regarded primarily as a hormone released in hypovolemia helping to defend the blood volume, a neurotransmitter or paracrine agent with a privileged role in the neural pathways for thirst and sodium appetite of all kinds, a neural organizer especially in sodium appetit

Effects of angiotensin II and its blockers Sar1-Ile8-angiotensin II and DuP 753 on drinking in ducks in relation to properties of subfornical organ neurons

Properties of systemically applied angiotensin II in stimulating water intake of normally hydrated ducks were studied and the results compared with properties of angiotensin II-responsive neurons of the subfornical organ which are considered as targets for circulating angiotensin II acting as a dipsogen. Following intravenous infusion of hypertonic saline (2000 mosmol.kg-1 at 0.3 ml.min-1 for 1 h), intravenous infusion of 0.3 ml.min-1 isotonic saline with angiotensin II (200 ng.min-1), starting 1 h later, stimulated drinking in each case at an angiotensin II plasma level of about 1400 pg.ml-1. Without hypertonic priming, the same angiotensin II infusion did not stimulate drinking in each experiment; however, if effective, repeated infusions of ANGII induced stable dipsogenic responses. Angiotensin II infusions did not alter plasma levels of antidiuretic hormone. Sar1-Ile8-angiotensin II, a non-selective angiotensin II antagonist, acted weakly as a partial agonist when infused at a dose 200-fold higher than angiotensin II and effectively blocked the dipsogenic action of angiotensin II; this corresponds to the inhibition of angiotensin II-induced excitation by Sar1-Ile8-angiotensin II observed in duck subfornical organ neurons. DuP 753 (losartan), an angiotensin II antagonist specifically blocking AT1 receptors in mammals, had equivocal effects on angiotensin II-induced drinking in ducks at rates 50- and 200-fold higher than angiotensin II, which corresponds to the weak inhibitory action of this compound on angiotensin II-induced neuronal excitation in the duck SFO. Blood pressure was only marginally elevated by the applied angiotensin II dose and Sar1-Ile8-angiotensin II had no effect.

Differential responses to angiotensin-(1-7) in the feline mesenteric and hindquarters vascular beds

Regional vascular responses to angiotensin (Ang)-(1-7), a heptapeptide derivative of Ang II were investigated in the feline hindquarters and mesenteric vascular beds under conditions of controlled flow. In the mesenteric vascular bed, injections of Ang-(1-7) in doses of 1, 3 and 10 micrograms produced dose-dependent decreases in mesenteric perfusion pressure whereas at doses of 30 and 100 micrograms, increases were observed. In contrast, in the hindquarters circulation, low doses produced increases while high doses produced decreases in perfusion pressure. In both vascular beds the degree of vasoconstriction was weak, being less than 1% of that elicited by Ang II. The vasoconstrictor effect of Ang-(1-7) in both the mesenteric and hindquarters vascular bed was blocked by DuP 753 (1 mg/kg i.v.), an Ang receptor subtype 1 (AT1) antagonist. The vasodilator responses in both vascular beds were partially blocked by the nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (100 mg/kg i.v.) but were unaffected by the cyclooxygenase inhibitor, meclofenamate (2.5 mg/kg i.v.). The present results show that in the peripheral vascular bed of the cat, Ang-(1-7) causes vasodilation or modest vasoconstriction, depending on the dose and the regional vascular bed studied. The present data also suggest that the vasodilator effect of the peptide may be mediated in part by the release of endothelium-derived relaxing factor and the vasoconstrictor effect by activation of the AT1 receptor subtype.

Response to angiotensin II after selective lesioning of brain regions believed to be involved in water intake regulation

The subfornical organ (SFO) and organum vasculosum lamina terminalis (OVLT) are regions of the brain that border the third ventricle (outside the blood-brain barrier) and have been implicated in the control of water intake elicited by angiotensin II (ANGII). Studies were conducted in which the response to injected ANGII following lesions of the SFO (LSFO) or OVLT (LOVLT) in broiler chicks was observed. Three groups of birds were used for each trial: lesioned and ANGII-injected (LI); not lesioned and ANGII-injected (NLI); and not lesioned and saline-injected (NLC). In Experiment 1, water intake of LISFO was decreased through 3 h postinjection (P less than .01). Intakes of LISFO and NLCSFO were not significantly different through 1 h postinjection. The OVLT did not have an effect on cumulative water intake in response to intramuscular ANGII. Water intakes of LIOVLT and NLIOVLT did not differ from each other, but were significantly higher than NLCOVLT (P less than .05) at .5 and 1 h postinjection. Feed intake was unaltered by SFO or OVLT lesions. Feed intake was suppressed and water intake increased by ANGII injection. The present study indicates that the SFO, but not the OVLT, plays a role in ANGII-induced water intake in broiler chicks.

Renin and angiotensin converting enzyme in elasmobranchs

Renin-like activity (RLA) and angiotensin I converting enzyme-like activity (ACELA), the two key enzymes of the renin-angiotensin system (RAS), were sought in the elasmobranch Scyliorhinus canicula. Renal extracts were desalted in a G-25 and eluted in a G-100 Sephadex column (calibration 15,000-70,000). The fractions were concentrated in a vacuum device. A 48,000-MW fraction incubated with synthetic and porcine angiotensiongen generated angiotensin I estimated by RIA. This same fraction was vasopressor in rats and dogfish. ACELA was sought in gill, heart, liver, spleen, pancreas, intestine, kidney, gonads, brain, skin, and muscle of dogfish using a spectrophotometric assay. The highest level of ACELA was found in the gills followed by spleen, kidney, and brain (33.79 +/- 2.3, 29.56 +/- 1.0, 14.62 +/- 1.0, and 13.80 +/- 2.3 nmol hippurate/min/mg protein, respectively). Intestine, gonads, skin and muscle contained no measurable amounts of ACELA. Captopril inhibited enzymatic activity from all ACELA containing tissues.

The effect of analogues of angiotensin II on drinking and cardiovascular responses to central angiotensin II in the rat

1. Intracerebroventricular (I.C.V.) infusion (60 ng h-1) of Isoleu5-angiotensin II (Isoleu5--AngII) and des-amine-angiotensin II (des-amine-AngII) in rats caused increased drinking behaviour and an increase in arterial blood pressure. 2. Des-amine-AngII caused similar increases in heart rate and arterial blood pressure as AngII. 3. Previous I.C.V. injection of the antagonists [Leu8]-AngII, des-amine-[Leu8]-AngII and octanoyl-[Leu8]-AngII prevented the increases in heart rate and blood pressure produced by I.C.V. infusion of AngII and caused partial reduction of the dipsogenic response. 4. The three antagonists had no effect on the increase in arterial blood pressure and heart rate caused by des-amine-AngII. The drinking response was reduced by previous injection of [Leu8]-AngII and des-amine-[Leu8]-AngII but not by octanoyl-[Leu8]-AngII. 5. In conclusion, Isoleu5-AngII and des-amine-AngII increase drinking behaviour, arterial blood pressure and heart rate when infused into the cerebral ventricle of rats. The study with the antagonists showed that des-amine-AngII probably binds more strongly to AngII-receptors.

Increased blood pressure induced by central application of aminopeptidase inhibitors is angiotensinergic-dependent in normotensive and hypertensive rat strains

Two aminopeptidase inhibitors, amastatin (AM) and bestatin (BE), were employed in 3 strains of rats, spontaneously hypertensive (SHR), Wistar-Kyoto (WKY), and Sprague-Dawley (SD), to investigate the central angiotensinergic system. The results indicate that intracerebroventricular (i.c.v.) injections of AM and BE induced pressor elevations in all 3 strains of rats. In order to test for the possibility of spillage into peripheral vasculature, members from all 3 strains were peripherally infused with AM, BE, or 0.15 NaCl via jugular vein catheters. The SHRs were significantly more responsive to the aminopeptidases than the normotensive strains, however their overall pressor responses were only 33% of those to i.c.v. infusion. Next, in order to test the notion that these aminopeptidase inhibitors are having their effect via the central angiotensinergic system, and not some other peptidergic system, the specific angiotensin receptor antagonist, Sar1, Thr8-AII (sarthran) was employed. Intracerebroventricular pretreatment with sarthran prevented subsequent pressor responses to i.c.v. AM and BE in members of all 3 strains, thereby suggesting that these aminopeptidase inhibitors are having their effect via the central angiotensinergic system.

Central and systemic antidiuretic hormone and angiotensin II in salt and fluid balance of birds as compared to mammals

1. Tonicity dominates the release of ADH with similar sensitivities (0.2-1 pg/ml per mOsm/kg) for both birds and mammals. 2. There is an inverse relationship between the volume of the extracellular fluid compartments and the plasma level of ADH. 3. Angiotensin II formation is governed by volume factors. 4. In birds the factors reducing the delivery of Na+ to the nephron distal tubules stimulate ANGII formation. 5. Mammals have a high vascular constrictor sensitivity to ADH and ANGII; there is little or no vascular sensitivity to these in birds. 6. In birds and mammals the subfornical organ and other circumventricular organs have receptors that specifically bind ANGII. 7. Dog and duck CSF levels of ADH and AII indicate their function as specific mediators of intrinsic neuronal systems controlling salt and fluid balance.

Food and water intake response of turkeys to intracerebroventricular injections of angiotensin II

The effects of intracerebroventricular (ICV) injections of angiotensin II on food and water intake were studied in adult Medium White turkey hens. Food intake was not significantly affected by ICV injections of angiotensin II. The ICV injection of 50, 100, and 200 ng of angiotensin II significantly increased water intake in a dose-dependent manner. The dipsogenic effect of angiotensin II was significantly attenuated by Saralasin, an angiotensin blocker. These results suggest that angiotensin II functions in neural pathways within the central nervous system to control water intake, but not food intake, in turkeys.

Central angiotensin-induced water intake and salt appetite in the pig

Single intracranial injections of the peptide analogues of angiotensin and the enzyme renin induced drinking of water and 1.8% NaCl solution in prepubertal female pigs maintained ad libitum on a sodium-free diet with unrestricted access to both fluids. Over the dose range 10(-12)-10(-9) mol angiotensin (AI), angiotensin II (AII), angiotensin III (AIII), and renin substrate (RS), the volumes of water and salt solution ingested were dose-dependent. As in other vertebrates, AII was the most potent and rapidly acting dipsogen. However, unlike in the rat, dog and pigeon, AIII was highly effective in stimulating both water intake and salt appetite, whereas AI and RS were relatively weak. Pretreatment of intracranial sites with 10(-9) mol of the AII competitive antagonist, Sar1-Ala8-angiotensin, had no effect on the volume of water or 1.8% NaCl ingested after 10(-10) mol AIII, suggesting that in the pig AIII can exert its dipsogenic effects without acting on central AII receptors. Single microinjections of 1 and 10 mU of renin also elicited dose-dependent drinking of water and 1.8% NaCl, but compared with the peptide analogues of angiotensin the responses had a longer latency and duration and were more variable between individual animals.

Central angiotensin III-induced dipsogenicity in rats and gerbils

Previous findings from our laboratory demonstrated [125I]angiotensin II (AII) binding to plasma membranes from rat but not gerbil circumventricular organs (CVOs), the presumed location of brain receptors for angiotensin-induced dipsogenicity. Since members of both species drink to intracranially applied AII, a degradation product of AII was suspected to be the active ligand in gerbils. High specific [125I]angiotensin III (AIII) binding capacity was presently determined in CVOs taken from both rats and gerbils. Nearly identical dose-response curves were obtained for members of each species following the intracerebroventricular injection of AIII; however, rats drank more water than gerbils following the administration of AII. These results were interpreted to suggest that the dipsogenically active ligand in gerbils is AIII or derived from AIII, and that this analogue also contributes to angiotensin-induced drinking in rats. Since the distribution of specific angiotensin binding capacity represented by gerbil closely approximates that seen in non-human primate brain, these findings are of particular relevance and encourage future efforts directed toward understanding the role of AII metabolites in the central control of dipsogenicity.

Central effects of angiotensins on drinking and blood pressure: structure-activity relationships

The dipsogenic and the pressor effect following intracerebroventricular injection of angiotensins and several C-terminal fragments were studied. Angiotensin I (ANG I), ANG II, ANG III and C-terminal hexa-, penta, tetra- and tripeptide stimulated water intake in water-replete rats and induced significant pressor responses. In both paradigms the most active peptides (in the pmol range) were ANG II, ANG I and ANG III, in that order. Shorter C-terminal peptides appeared to be active, but had to be injected in the nmol range. Latencies to the onset of drinking were less than 45 s for all peptides tested. The C-terminal dipeptide and other dipeptide fragments did not possess detectable dipsogenic activity. The dipsogenic effect of ANG I was inhibited by pretreatment of animals with the converting enzyme inhibitor SQ 14,225. Drinking induced by both ANG I and ANG (4-8) was antagonized by the ANG II-receptor blocking agent Sar1-Ala8-ANG II. It is concluded that conversion of ANG I into ANG II is a prerequisite for the expression of the observed biological activity in the brain. Short C-terminal fragments are capable of stimulating the ANG II receptors, but a peptide chain of 7 amino acids appears necessary for maximal agonistic activity.

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.