Identification of alpha 1-adrenoceptor subtypes in the rat vas deferens: binding and functional studies

1. The alpha 1-adrenoceptor subtypes of the prostatic and epididymal portion of rat vas deferens were characterized in binding and functional experiments. 2. In saturation experiments, [3H]-prazosin bound to two distinct affinity sites in the epididymal portion of rat vas deferens (pKD = 10.1 +/- 0.13 and 9.01 +/- 0.15, Bmax = 507 and 1231 fmol mg-1 protein, respectively). In the prostatic portion [3H]-prazosin bound to a single affinity site (pKD = 9.82 +/- 0.04, Bmax = 924 fmol mg-1 protein). 3. In the displacement experiments, unlabelled prazosin displaced biphasically the binding of 200 pM [3H]-prazosin to the epididymal portion; the resulting two pKI values were consistent with the affinity constants obtained in the saturation experiments. WB4101 (2-(2,6-dimethoxy-phenoxyethyl)-amino-methyl-1,4-benzodioxane) and benoxathian also discriminated the two affinity sites in the epididymal portion and the population of low affinity sites for the three antagonists was approximately 40%. On the other hand, the prostatic portion predominantly showed a single affinity site for prazosin, WB4101 and benoxathian, although the presence of a small proportion (less than 10%) of the low affinity site could be detected. HV723 (alpha-ethyl-3,4,5-trimethoxy-alpha-(3-((2-(2-methoxyphenoxy)ethyl)-a min o)- propyl) benzeneacetonitrile fumarate) displaced the [3H]-prazosin binding monophasically with a low affinity in both halves. 4. Pretreatment with chlorethylclonidine (CEC) at concentrations higher than 1 microM inhibited 700 pM [3H]-prazosin binding to the prostatic portion by approximately 50%. However, the inhibition in the epididymal portion was much less (approximately 21% at 50 microM CEC).5. In the functional study, the contractile response to noradrenaline was competitively inhibited by prazosin, WB4101, benoxathian and HV723 with similar and low affinities (pKB value ranging from 8.0to 9.0) in the epididymal portion of rat vas deferens. In the prostatic portion of rat vas deferens,noradrenaline also produced a contraction, but the maximal amplitude of contraction developed was approximately one-fourth of that in the epididymal portion. Prazosin and WB4101 also inhibited the contractile response of the prostatic portion with the pKB values similar to those obtained in the epididymal portion. The contractions to noradrenaline in both portions were potently attenuated by 1 LM nifedipine but were not affected by pretreatment with 1O LM CEC.6. Under conditions where P2