Mohan Viswanathan

Changes in expression of angiotensin receptor subtypes in the rat aorta during development

Quantitative autoradiography was used to characterize angiotensin AT1 and AT2 receptors, in the rat aorta at three developmental ages; embryonic day 18 (E18), and postnatal weeks 2 and 8. The expression of angiotensin receptors was higher in the aorta of E18 and 2-week-old rat. A major proportion of the angiotensin receptors expressed in the aorta at these two ages was AT2 (84 and 81% respectively). Conversely, in the aorta of 8-week-old rats, AT1 was the predominant angiotensin receptor subtype (71%). In 8-week-old rats, the AT2 subtype was also present (28%). In pre- and postnatal rats, [125I]Sar1-angiotensin II binding to AT1 receptors was sensitive to GTP gamma S whereas binding to AT2 receptors was not. AT2 receptors may serve an important role during stages of rapid growth of the aorta, and also have a significant function in the adult vasculature.

Expression of angiotensin II AT2 receptors in the rat skin during experimental wound healing.

We localized and characterized angiotensin II AT1 and AT2 receptors in the skin of 2-week-old rats during experimental wound healing. Both AT1 and AT2 were present in the skin. Three days after wounding, the expression of angiotensin II receptors was significantly enhanced in the dermis as well as in a localized band within the superficial dermis of the skin surrounding the wound. The major proportion of this increase was due to angiotensin II AT2 receptors. Our results suggest a physiological role for AT2 receptors in the process of tissue repair.

Balloon angioplasty enhances the expression of angiotensin II AT1 receptors in neointima of rat aorta.

Angiotensin II is a vasoactive peptide and may act as a growth factor in vascular smooth muscle cells. Experimental injury of the rat aorta causes rapid migration of medial smooth muscle cells and their proliferation resulting in the formation of neointima. We have examined, using quantitative autoradiography, the expression of angiotensin II receptor subtypes AT1 and AT2, and angiotensin-converting enzyme, in the neointima formed in the rat thoracic aorta 15 d after balloon-catheter injury. In contrast to the normal aortic wall, which contained both AT1 and AT2 receptors (80% and 20%, respectively), neointimal cells expressed almost exclusively angiotensin II AT1 receptors. The apparent number of these receptors was fourfold higher in the neointima compared to that in the normal aortic wall. The affinities of the neointimal receptors to angiotensin II or to the AT1 receptor antagonist, losartan, were not different from those in the normal aortic wall. Angiotensin-converting enzyme binding in the neointima was not different from that in the media of the uninjured aorta. Our data suggest that angiotensin II AT1 receptors may have a significant role in injury-induced vascular smooth muscle proliferation and migration.

TYPE-1 AND TYPE-2 ANGIOTENSIN II RECEPTORS IN FETAL RAT BRAIN

Brain angiotensin II receptors were located in 18-day-old rat embryos by quantitative autoradiography. In the nucleus of the solitary tract and choroid plexus, binding was displaced by the type-1 antagonist DuP 753. In the inferior olive, paratrigeminal and hypoglossal nuclei, binding was displaced by the type-2 antagonist CGP 42112 A. Meninges and cephalic soft tissues contained predominantly type-2 angiotensin II receptors. Our results indicate a role for type-1 and type-2 angiotensin II receptors during brain development.

Vascular Melatonin Receptors

High-affinity melatonin receptors are expressed in the tail artery of the rat and arteries forming the circle of Willis of the rat and certain primates. The characteristics of the vascular melatonin receptors seem to be similar to the ones described in the central nervous system. The expression of vascular melatonin receptors in the rat is differentially regulated by factors such as strain and age, and in the female by reproductive hormones. Functional studies using the caudal artery of the rat suggest that melatonin regulates vascular tone. The highly restricted distribution of melatonin receptors in arteries involved in regulating blood flow to areas involved in heat dissipation suggests that vascular melatonin receptors may have a specific function in thermoregulatory homeostasis.

Renal and vascular injury induced by exogenous angiotensin II is AT1 receptor-dependent.

Angiotensin II (Ang II) infusion in rats augments vascular injury in balloon-injured carotid arteries and induces marked vascular and tubulointerstitial injury in kidneys. We examined how the AT1 receptor is modulated and whether blockade of the receptor with losartan could prevent the phenotypic and cellular changes. We also examined the role of the local renin-angiotensin system (RAS) by examining the expression of angiotensin-converting enzyme (ACE) and the effect of treatment with the ACE inhibitor, ramipril. Ang II infusion resulted in systemic hypertension and accelerated intimal and medial thickening in balloon-injured carotid arteries. Renal injury was manifested by proteinuria, glomerular phenotypic changes (mesangial expression of α-actin and podocyte expression of desmin), and tubulointerstitial injury with the tubular upregulation of the macrophage-adhesive protein, osteopontin, the interstitial accumulation of macrophages and myofibroblasts, and the deposition of collagen types III and IV. Ang II infusion decreased AT1 receptor number in the renal interstitium but not in glomeruli. Losartan completely blocked the Ang II-mediated hypertension, proteinuria, and injury to both carotid and kidney. Ang II infusion was also associated with an increase in ACE protein in both the proximal tubular brush border as well as at interstitial sites of injury, but despite evidence for activation of the local RAS, treatment with ramipril was without effect. These studies demonstrate that the renal and vascular injury induced by Ang II infusion is mediated by the AT1 receptor despite downregulation of the receptor in the interstitium. In addition, although there is evidence for local RAS activation, the injury appears to be mediated solely by the exogenous Ang II.

Differential expression of melatonin receptors in spontaneously hypertensive rats.

Quantitative autoradiography was used to compare melatonin receptors in brain areas and arteries of young (4 weeks old) and adult (14 weeks old) spontaneously hypertensive rats (SHR) to those in age-matched normotensive controls, Wistar-Kyoto (WKY) rats. Age and strain influenced the number of melatonin receptors in an anatomically selective manner, and the most striking changes occurred in arterial receptors. Melatonin receptors were not detectable in the anterior cerebral arteries of adult SHR. In the caudal artery, melatonin receptors decreased with age in both strains, but the decrease was more pronounced in SHR. When compared to age-matched WKY rats, the number of caudal artery receptors was higher in young and lower in adult SHR. The number of melatonin receptors was higher in the area postrema of adult SHR when compared to adult WKY rats, but in the suprachiasmatic nucleus, no such differences between the two strains were present. Alterations in receptor density were not accompanied by changes in binding affinity. Our results indicate that in the rat melatonin receptors show different developmental patterns according to location and that the receptors may be expressed differentially in genetic hypertension.

Kidney angiotensin II receptors and converting enzyme in neonatal and adult Wistar-Kyoto and spontaneously hypertensive rats

The aim of the present study was to correlate the development of the renin angiotensin system (RAS) in the kidney of the rat with the development of genetic hypertension. Immature (1-week-old) and adult (12-week-old) normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive kidney rats (SHR) were used for quantification of angiotensin II (ANG II) receptors and angiotensin converting enzyme (ACE) binding sites using quantitative autoradiography. In both neonatal and adult animals of either strain, ANG II receptors were of AT1 subtype. In all kidney areas of 1-week-old rats. ANG II receptor density was higher in SHR than WKY. Binding density increased with age in WKY rats; thus, in the glomeruli and the outer stripe of the outer medulla of 12-week-old WKY, binding was significantly higher than that present in age-matched SHR. [125I]351A binding to ACE was highest in the outer medulla and not detectable in glomeruli. In 1-week-old rats, binding to ACE was higher in WKY than in SHR strain. No differences in ACE binding were found between adult SHR and WKY rats, with the exception of the inner stripe of the outer medulla, where no binding was detected in SHR. Our results support the hypothesis that the RAS in kidney is developmentally regulated and is involved in the development and maintenance of genetic hypertension in SHR.

Angiotensin II AT1 receptors in rat superior cervical ganglia : characterization and stimulation of phosphoinositide hydrolysis

Angiotensin II receptor number was higher in superior cervical ganglia of 2-week-old when compared to 8-week-old rats. In both young and adult rats, specific binding of [125I][Sar1]angiotensin II was displaced competitively by the AT1-receptor antagonist DuP 753 but not by the AT2-receptor competitor PD 123177. In ganglia from adult rats, DuP 753 competed with an IC50 of 113 nM. The stable guanine nucleotide GTP gamma S inhibited binding of [125I][Sar1]angiotensin II in young and adult rats by approximately 50% with IC50 values of 105 and 120 nM, respectively, suggesting that the angiotensin receptor is G-protein linked. Angiotensin II at a dose of 1 microM stimulated inositol phosphate formation 58% over control values in superior cervical ganglia from 8-week-old rats. This effect was totally blocked by 10 microM DuP 753 but not by 10 microM PD 123177. Our findings demonstrate that rat superior cervical ganglia contain AT1-type angiotensin receptors that are probably G-protein linked, and their stimulation results in increased inositol phospholipid metabolism.

Melatonin receptors mediate contraction of a rat cerebral artery

MELATONIN receptors are expressed in the cerebral arteries of the rat which form the circle of Willis. We report here that melatonin induces contraction of in vitro preparations of pressurized rat posterior communicating artery, in a concentration-dependent manner. This action of melatonin is inhibited by S-20928, a specific melatonin receptor antagonist. Our results demonstrate that the contractile action of melatonin is mediated by its receptors in the cerebral artery, and suggest a role for melatonin in the regulation of cerebral circulation.