Mechanisms underlying the differential sensitivity to alpha 1-adrenoceptor activation in the bisected rat vas deferens.

1. The factors underlying the different responsiveness of the prostatic and epididymal portions of rat vas deferens to alpha 1-adrenoceptor stimulation were investigated. 2. The alpha 1-adrenoceptors in membranes of both halves of rat vas deferens were labelled with 3H]-prazosin and the affinities of agonists and antagonists for these receptors were determined. In saturation studies, the Bmax and KD values for 3H]-prazosin in membranes of both portions were the same. 3. In competition studies, the inhibition curves for phentolamine were biphasic and consistent with the presence of both alpha 1a- and alpha 1b-adrenoceptor subtypes. The proportions of binding sites with high and low affinity for phentolamine in both halves of rat vas deferens were similar and in good agreement with the percentages of binding sites for WB-4101 and phentolamine previously reported in the whole rat vas deferens. 4. The phenylethylamines displaced 3H]-prazosin with a shallow inhibition curve. The data are compatible with the assumption of two affinity states for the binding sites. For the imidazoline compounds no such distinct affinity states could be demonstrated. 5. The affinity for, and the relative intrinsic efficacy on postsynaptic alpha 1-adrenoceptors of both portions of rat vas deferens were studied for noradrenaline, phenylephrine and methoxamine by irreversible inactivation of the alpha 1-adrenoceptors by phenoxybenzamine. The parameters for partial agonists were determined by comparing the responses to the partial agonist to those of a full agonist in the same tissue. Homogeneous estimates of the equilibrium dissociation constants (Ka) were obtained, indicating that these agonists bind to the receptors of both tissues in an identical manner.(ABSTRACT TRUNCATED AT 250 WORDS)