Alpha 1B-adrenergic receptors in rat renal microvessels

Although several alpha-adrenergic receptor genes are expressed in the rat kidney, their expression in the renal vasculature has not been studied. Since pharmacological studies have suggested that an alpha 1B-adrenergic receptor may mediate renal vasoconstriction, we studied the expression of alpha 1B-adrenergic receptors in renal microvessels, from 10- to 14-week-old male spontaneously hypertensive rats (SHR) and their normotensive control, the Wistar-Kyoto rat (WKY). In these microvessels, isolated by perfusion with iron, alpha 1B-adrenergic receptor mRNA levels (by ribonuclease protection assay) were similar in SHR and WKY rats. Photo-affinity labeling with [125I]-arylazidoprazosin demonstrated the presence of alpha 1B-adrenergic receptor protein. Maximum receptor density (determined by 3H-prazosin binding: Bmax 59.8 +/- 4.1 and 58.7 +/- 4.3; Kd 0.48 +/- 0.05 nM and 0.31 +/- 0.06 nM in SHR and WKY, respectively) and chloroethylclonidine (CEC)-sensitive binding sites (determined by [125I]-(2-beta(4-hydroxyphenyl)-ethylaminomethyl)-tetralone binding) (125I-HEAT) were similar in SHR and WKY rats. There are two novel findings in these studies: (1) the alpha 1B-adrenergic receptor gene is expressed in renal microvessels of WKY and SHR; (2) alpha 1B-adrenergic receptor gene expression in renal microvessels is not altered in adult SHR. The failure to down-regulate expression of the alpha 1B-adrenergic receptor at the mRNA and protein level in the SHR could result in persistence of alpha 1B-adrenergic receptor effects and contribute to the increased vascular resistance in hypertension.