Norepinephrine stimulates renal tubular sodium reabsorption, probably through an alpha1-adrenoceptor-mediated mechanism. Although the distribution of alpha1-adrenoceptors in the kidney has been studied with autoradiography, the precise location of these receptors in isolated nephron segments is unclear. Using a microassay we determined the specific binding of [I-125]iodoarylazidoprazosin ([I-125]prazosin), a high specific radioactivity analog of the selective alpha1-antagonist prazosin, to microdissected glomeruli and tubule segments. Specific binding of [I-125]prazosin (3 nM) in the proximal convoluted tubule was time- and concentration-dependent, saturable, and reversible. In this segment the apparent K(D) by association and dissociation rate constants of [I-125]prazosin binding was 0.47 nM, and the maximum receptor density was approximately 0.19 fmol/mm, or 720 fmol/mg protein. Binding specificity was verified in competition studies with excess (3 muM) unlabeled prazosin and probes for alpha2-(yohimbine), beta-(propranolol), dopamine1- (SCH23390), and dopamine2- (S-sulpiride) receptors. [I-125]Prazosin binding was inhibited significantly only by unlabeled prazosin. Mapping of prazosin binding along the nephron revealed that the highest density was in the proximal convoluted tubule, followed by the proximal straight tubule. Lesser binding was found in the thick ascending limb and in the distal convoluted tubule, whereas in the cortical and outer medullary collecting duct and in glomeruli, binding was not significantly different from zero. These results demonstrate specific prazosin binding sites in the proximal and early distal nephron where direct innervation by monoaminergic nerves is most abundant, and suggest that portions of the nephron beyond the proximal tubule, specifically the diluting segment, may also be under alpha1-agonist influence.
