Distribution and functional role of renal ET receptor subtypes in normotensive and hypertensive rats

Experiments were designed to compare the distribution and physiological roles of endothelin (ET) receptor subtypes, ET(A) and ET(B), in the kidneys of normotensive Sprague-Dawley (SD) and spontaneously hypertensive (SH) rats. Using [I-125] ET-1 and subtype-selective ligands sarafotoxin 6c (S6c, ET(B)-selective agonist) and BQ123 (ET(A)-selective antagonist), the distribution of ET(A) and ET(B) receptors in SD rat kidney cortex, outer medulla and papilla was calculated to be 50:50, 30:70 and 10:90, respectively. The ET receptor subtypes in outer medulla and papilla of age-matched SH rats were similar to those of SD. However, in the cortex of SH rats, the ratio of ET(A) to ET(B) was 25:75 compared to 50:50 in SD rats. In addition, the affinity of the ET receptors was also higher in SH rats (117 pM vs. 235 pM). In the conscious SD rats, bolus i.v. injections of ET-1 and S6c elicited similar dose-dependent decrease in renal blood flow (RBF), which were unaffected by the infusion of the selective ET(A) receptor antagonist, BQ123. The SH rats were more sensitive to the renal vasoconstrictor effect of S6c and ET-1. Also, the dose-response curve to S6c was shifted to the left when compared to ET-1; however, BQ123 infusion abolished this difference. In renal clearance studies, BQ123 infusions decreased RBF and glomerular filtration rate (GFR) only in SH rats, and the fractional excretion of sodium only in SD rats. The combined data indicate that the distribution and functional roles of ET(A) and ET(B) receptor subtypes are altered in the kidneys of SH rats.