DOPAMINE CONTRACTS THE RAT ISOLATED SEMINAL-VESICLE BY ACTIVATION OF POSTJUNCTIONAL ALPHA-1-ADRENOCEPTORS

The contractile effect of dopamine in the rat isolated seminal vesicle was investigated to determine the nature of the receptors involved. N oradrenaline (NA; 10(-8) to 10(-4) mol/l), phenylephrine (PE; 10(-7) t o 10(-4) mol/l) and dopamine (DA; 10(-5) to 10(-2) mol/l) produced con centration-dependent contractions with no sign of tachyphylaxis. The r elative potencies of NA:PE:DA derived from their EC(50) values were 1: 0.23:0.01. However, while NA and PE produced similar maximal contracti ons, the E(max), obtained with DA was significantly less (intrinsic ac tivity = 0.066). Depletion of tissue catecholamine stores by pretreati ng rats with reserpine did not significantly influence the responses o f the seminal vesicle to DA or PE. Moreover, cocaine (10(-5) mol/l) di d not significantly affect the course of the concentration-response cu rve to DA. Prazosin (10(-9) to 10(-8) mol/l), phentolamine (10(-7) to 10(-6) mol/l), haloperidol (10(-7) to 10(-6) mol/l) and yohimbine (10( -6) to 10(-5) mol/l) all produced a rightward displacement of the conc entration-response curves for the agonists. The Arunlakshana and Schil d plots of the data were linear and had slopes not significantly diffe rent from unity. The potency estimates obtained for each antagonist we re always similar irrespective of the agonist used. The order of poten cy of the antagonists was: prazosin > phentolamine > haloperidol > yoh imbine. In addition, phenoxybenzamine (10(-8) mol/l) produced a rightw ard shift of the concentration-response curves for both NA and DA. The displacement was greater in the case of DA, of which the maximum resp onse was also reduced by 45%. Sulpiride (10(-6) to 10(-5) mol/l) had n o effect on the concentration-response curves of either PE or DA. Simi larly, apomorphine (10(-6) mol/l) neither produced contraction on its own nor influenced those of PE or DA. These results taken together ind icate that the contractile effect of DA in the rat isolated seminal ve sicle is mediated through activation of postjunctional alpha(1)-adreno ceptors without involvement of specific DA receptors or release of end ogenous NA.