Radioligand binding and functional assays demonstrate postsynaptic alpha 2-receptors on proximal tubules of rat and rabbit kidney.
We have explored the localization of renal alpha 2-adrenoceptors by radioligand binding techniques and studies of 22Na+ influx into isolated renal tubular cells. Initially, we validated the use of the alpha 2-adrenergic antagonist [3H]rauwolscine as a high-affinity (Kd = 2.5 +/- 0.65 nM) probe that binds to a single class (Hill slope = 0.94 +/- 0.06) of alpha 2-adrenoceptors in rat renal cortical membranes (Bmax = 340 +/- 50 fmol/mg). [3H]Rauwolscine and [3H]yohimbine identified an identical number of sites, but [3H]rauwolscine bound with a several-fold higher affinity. Treatment of rats with 6-hydroxydopamine depleted renal norepinephrine greater than 80% but failed to alter the number or affinity of renal cortical alpha 2-receptors, thus suggesting that these alpha 2-receptors are not located on presynaptic sites. Further evidence for the postsynaptic localization of these receptors was obtained by in vitro autoradiographic studies, which showed that [3H]rauwolscine sites were preferentially located on proximal tubular cells, and by functional assays with isolated rabbit proximal tubular cells. In the latter studies, Na+-H+ antiport activity was stimulated up to 2-fold by the selective alpha 2-adrenergic agonist guanabenz, and this stimulation was blocked by yohimbine but not by the alpha 1-blocker prazosin. Although amiloride was able to block this response to guanabenz, radioligand binding studies demonstrated that amiloride can compete for [3H]rauwolscine sites at the concentrations used to block Na+-H+ antiport activity. These data suggest that alpha 2-adrenoceptors are present on proximal tubular cells in rats and rabbits and that these receptors appear to stimulate Na+-H+ antiport activity.(ABSTRACT TRUNCATED AT 250 WORDS)
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D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001345:Autoradiography; D006143:Guanabenz / Q000494:pharmacology; D006892:Hydroxydopamines / Q000494:pharmacology; D066298:In Vitro Techniques; D007672:Kidney Cortex / Q000378:metabolism; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D016627:Oxidopamine; D011224:Prazosin / Q000494:pharmacology; D011817:Rabbits; D011869:Radioligand Assay; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000378:metabolism*; D015016:Yohimbine / Q000378:metabolism
D006892:Hydroxydopamines; D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D000584:Amiloride; D016627:Oxidopamine; D006143:Guanabenz; D011224:Prazosin