α1-Adrenoceptor subtypes on rat afferent arterioles assessed by radioligand binding and RT-PCR

We utilized [H-3]prazosin saturation and competition radioligand binding studies to characterize the expression of alpha (1)-adrenoceptors in preglomerular vessels. mRNA for adrenoceptor subtypes was assayed using RT-PCR. The vessels were isolated using an iron oxide-sieving method. [H-3]prazosin bound to a single class of binding sites (K-d 0.087 +/- 0.012 nM, B-max 326 +/- 56 fmol/mg protein). Phentolamine displaced [H-3]prazosin (0.2 nM) with a pK(i) of 8.37 +/- 0.09. Competition with the selective alpha (1A)- adrenoceptor antagonist 5-methylurapidil fit a two-site model (pK(i) 9.38 +/- 0.21 and 7.94 +/- 0.15); 59 +/- 3% of the sites were high-affinity, and 41 +/- 3% were low-affinity binding sites. Competition with the alpha (1D)-adrenoceptor antagonist 8-(2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-8-azaspiro[4.5] decane-7,9-dione dihydrochloride (BMY-7378) fit a one-site model with low affinity (pK(i) 6.83 +/- 0.03). The relative contents of alpha (1A)- alpha (1B)-, and alpha (1D)-adrenoceptor mRNAs were 64 +/- 5, 25 +/- 5, and 11 +/- 1%, respectively. Thus there was a very good correlation between mRNA and receptor binding for the subtypes. These data indicate a predominance of the alpha (1A)-adrenoceptor subtype in rat renal resistance vessels, with smaller densities of alpha (1B)- and alpha (1D)-adrenoceptors.