1 We have compared the receptors mediating the contractions of rings of rat thoracic aorta or rabbit pulmonary artery and rat stomach strips in response to the endothelin/sarafotoxin (ET/SX) family of peptides and to those mediating endothelium-dependent vasodilatations within the isolated perfused mesentery of the rat. To discriminate ETA receptors from ETB receptors we have used the criteria that ET-1 is more active than SX6c on ETA receptors, and that the ET/SX peptides are equiactive on ETB receptors. We have also assessed the effects of the ETA receptor-selective antagonist BQ-123, and the non-selective ET receptor antagonist PD 142893 on the responses of each preparation to the ET/SX peptides.2 ET-1-induced constrictions of the rat thoracic aorta (ECm 3 x 10-10 M), a prototypic ETA receptormediated response, or isolated perfused mesentery of the rat were antagonized by BQ-123 (10-5 M) or PD 142893 (10-5 M). SX6c did not constrict either the rat isolated perfused mesentery or the rat thoracic aorta. Thus, ETA receptors mediate these constrictions. 5 In the rat isolated perfused mesentery ET-1 or SX6c (0.3-300pmol) were equipotent in producing dose-related vasodilatations that were unaffected by BQ-123 (10-6 M), indicative of an ETB receptormediated response. In contrast to the other ETB-mediated responses, PD 142893 (10-6 M) strongly antagonized these vasodilatations. 6 Thus, ETA receptors mediate constrictions of the rat thoracic aorta and rat isolated perfused mesentery whereas ETB receptors mediate constrictions of the rabbit pulmonary artery and rat stomach strip and endothelium-dependent dilatations within the mesentery. However, within the group of ETB receptor-mediated responses, endothelium-dependent vasodilatations are sensitive to PD 142893, whereas contractions of the isolated smooth muscle preparations are not. Thus, the receptor present on the endothelium responsible for the release of nitric oxide in response to the ET/SX peptides is most probably different from that present on smooth muscle that mediates BQ-123-insensitive contractions.

Experiments were designed to elucidate the role of endothelin B receptors (ET(B)) on arterial pressure and renal function in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Male Sprague-Dawley rats underwent uninephrectomy and were treated with either DOCA and salt (0.9% NaCl to drink) or placebo. DOCA-salt rats given the ET(B)-selective antagonist, A-192621, for 1 wk (10 mg. kg(-1). day(-1) in the food) had significantly greater systolic arterial pressure compared with untreated DOCA-salt rats (208 +/- 7 vs. 182 +/- 4 mmHg) whereas pressure in placebo rats was unchanged. In DOCA-salt, but not placebo rats, A-192621 significantly decreased sodium and water excretion along with parallel decreases in food and water intake. To determine whether the response in DOCA-salt rats was due to increased expression of ET(B) receptors, endothelin receptor binding was performed by using membranes from renal medulla. Maximum binding (B(max)) of [(125)I]ET-1, [(125)I]ET-3, and [(125)I]IRL-1620 increased from 227 +/- 42, 146 +/- 28, and 21 +/- 1 fmol/mg protein, respectively, in placebo rats to 335 +/- 27, 300 +/- 38, and 61 +/- 6 fmol/mg protein, respectively, in DOCA-salt hypertensive rats. The fraction of receptors that are the ET(B) subtype was significantly increased in DOCA-salt (0.88 +/- 0.07) compared with placebo (0.64 +/- 0.01). The difference between [(125)I]ET-3 and [(125)I]IRL-1620 binding is consistent with possible ET(B) receptor subtypes in the kidney. These results indicate that ET(B) receptors in the renal medulla are up-regulated in the DOCA-salt hypertensive rat and may serve to maintain a lower arterial pressure by promoting salt and water excretion.

Endothelin (ET)-1 has potent renal and systemic vasoconstrictor properties, and thus we investigated whether ET-1 plays a role in increasing blood pressure and decreasing renal function in DOCA-salt hypertension. After a right nephrectomy, rats had DOCA or placebo pellets implanted subcutaneously and were given saline or tap water to drink, respectively. Additional groups of rats were given the ETA receptor antagonist A-127722 in their water. Rats were maintained in metabolic cages for monitoring excretory function and food and water intake. Three weeks after surgery, mean arterial pressure (MAP) was recorded in the conscious rats via a carotid artery catheter. As expected, DOCA-salt rats had significantly higher MAP compared with uninephrectomized controls (197 ± 6 vs. 133 ± 3 mmHg). Creatinine clearance, used as an estimate of glomerular filtration rate, was significantly reduced in DOCA-salt rats (2.9 ± 0.4 vs. 6.8 ± 0.3 dl ⋅ day−1 ⋅ 100 g−1 body wt in controls). ETA receptor blockade with A-127722 significantly reduced MAP (156 ± 8 mmHg) but had no effect on creatinine clearance of DOCA-salt-treated rats (2.8 ± 0.3 dl ⋅ day−1 ⋅ 100 g−1 body wt). Plasma ET-1 levels were significantly raised after DOCA-salt treatment (1.4 ± 0.5 pg/ml vs. 0.4 ± 0.1 pg/ml in controls). A-127722 treatment increased circulating ET-1 levels in both placebo (2.3 ± 0.5 pg/ml) and DOCA-salt (5.6 ± 0.7 pg/ml) rats. However, ET-1 mRNA expression in renal cortical and medullary tissue was not affected by either A-127722 or DOCA-salt treatments. Thus ETA receptors appear to play a role in the maintenance and development of DOCA-salt hypertension but not in the accompanying reduction of renal function.

1 Vascular responses to acetylcholine and sodium nitroprusside in vivo and in vitro, in the isolated perfused kidney and in rings of rat thoracic aorta, were measured in rats treated chronically with N0-nitro-L-arginine methyl ester (L-NAME; approx, 70 mg l-') and compared to responses in agematched control animals, and age-matched animals after the acute administration of L-NAME (3-100 gtmol kg-1). Parallel experiments examined alterations in responsiveness in rings of trachea and anococcygeus muscles taken from the same animals. 2 Chronic oral administration of L-NAME elevated the blood pressure in anaesthetized animals from 114 ± 5 mmHg to 153 ± 11 mmHg (n = 5). The hypotensive responses to both acetylcholine (1 nmol kg-') and sodium nitroprusside (10 nmol kg-') were enhanced by chronic L-NAME treatment (n = 5-7) whereas acute L-NAME administration enhanced only the response to sodium nitroprusside (n = 5).3 After chronic treatment with L-NAME, the basal perfusion pressure in the isolated perfused kidney was elevated. However, vasodilator responses to either acetylcholine (1 nmol) or sodium nitroprusside (3 nmol) were unaltered (n = 5-7). The vasodilatation induced by acetylcholine was inhibited in a concentration-dependent manner by the administration of acute L-NAME (0.1-100 pM; n = 5), such that significant inhibition was seen at 10 JIM L-NAME. The response to sodium nitroprusside was unaffected by L-NAME. 4 The relaxations of isolated rings of rat thoracic aorta induced by acetylcholine were inhibited in tissues prepared from rats treated chronically with L-NAME (n = 5-7). Acute administration of L-NAME (0.1-100 pM) concentration-dependently inhibited the relaxations induced by acetylcholine in this preparation, with significant inhibition occurring at 1 f1M L-NAME (n = 5). Responses to sodium nitroprusside were unaffected by either chronic or acute exposure to L-NAME (n = 5-7). 5 Relaxations of precontracted anococcygeus muscles induced by electrical field stimulation, or contractions of rings of trachea induced by carbachol or endothelin-1, were unaffected by chronic oral administration of L-NAME (n = 4-6). Acute addition of L-NAME (0.1-100 gM) to the organ baths inhibited in a concentration-dependent manner the relaxations of anococcygeus muscles taken from control animals, with a significant effect being seen at a concentration of 10 f.M (n = 4-6).6 Our cardiovascular data are consistent with chronic oral administration of L-NAME inhibiting the production of nitric oxide (NO) within the vasculature, although the pattern of inhibition is not uniform between different tissues. Despite the inhibition of endothelial NO production, chronic L-NAME does not alter the vasodepressor activity of acetylcholine in vivo or in the isolated perfused kidney. This may be explained by an enhanced responsiveness of guanylyl cyclase pathways, the increased release of vasodilators other than nitric oxide or a decreased importance of nitric oxide in resistance vessels compared with conductance vessels. The resistance of pe...