Effects of St−587 on the α-adrenoceptors in the Bisected Rat Vas Deferens

Abstract

Abstract The effects of the α-adrenoceptor agonist St−587 have been studied on the twitch responses induced by field stimulation in the prostatic portion of rat vas deferens. Moreover the drug’s influence on the unstimulated prostatic and epididymal halves of rat vas deferens has also been determined. Alone and after addition of yohimbine (0·3 μM) it enhanced in a concentration-dependent manner the twitch responses in the prostatic half. Prazosin competitively antagonized (pA2 = 8·41 · 0·03) this effect. The enhancing effect of St−587 was not reduced in reserpinized animals. These results suggest that post-synaptic α1-adrenoceptors are involved in the potentiation of twitch responses induced by St−587. When α1-adrenoceptors were blocked by prazosin (0·1 μM), St−587 partially inhibited the twitch responses of the prostatic portion of rat vas deferens (Emax=49·5 ± 3·5%). Yohimbine completely reversed the inhibitory effects of both St-587 and Clonidine. Furthermore St−587 antagonized the inhibitory effects of Clonidine on twitch responses. Thus it appears that St−587 also behaves as a partial agonist of presynaptic α2-adrenoceptors in this portion of rat vas deferens, but it did not induce contractions in the unstimulated prostatic half of the vas deferens. However, it competitively antagonized the α1-adrenoceptor agonist phenylephrine by acting as an antagonist of prostatic postsynaptic α1 adrenoceptors. These α1-adrenoceptors are probably different from those that mediate the twitch enhancing response to St−587 in that portion. On the other hand, St−587 was a partial agonist of α1-adrenoceptors in the epididymal half. These results suggest that a different efficiency of stimulus-response mechanism is operating after postsynaptic α1-adrenoceptor activation in each one of the unstimulated portions of rat vas deferens.