ALPHA(1A)-ADRENOCEPTOR MEDIATED CONTRACTION OF RAT PROSTATIC VAS-DEFERENS AND THE INVOLVEMENT OF RYANODINE STORES AND CA2+ INFLUX STIMULATED BY DIACYLGLYCEROL AND PKC

1 The present study has investigated the alpha(1)-adrenoceptor subtype mediating contraction of the rat isolated prostatic vas deferens and the possible effector mechanisms involved in this response by use of f unctional experiments. 2 Contractions to noradrenaline in the rat isol ated prostatic vas deferens were antagonized by prazosin (9.4, 1.04+/- 0.19, pA(2) and Schild plot slope), 5-methyl urapidil (8.9, 1.10+/-0.1 3), BMY 7378 (6.4, 1.53+/-0.07) and RS 17053 (8.3, 1.13+/-0.18). These affinities are consistent with the response being mediated by the alp ha(1A)-adrenoceptor subtype. 3 The contraction to noradrenaline at 37 degrees C consisted of an initial phasic response, composed of many rh ythmic contractile spikes and a more slowly developing tonic contracti on. When the temperature was lowered to 25 degrees C the phasic contra ction became a smooth single response which was increased in magnitude . 4 In Ca2+-free Krebs solution the tonic contraction to noradrenaline (10(-4) M) was abolished, suggesting that this response was dependent on influx of extracellular Ca2+. After 2 min in Ca2+-free Krebs solut ion at 37 degrees C and 25 degrees C the phasic response to noradrenal ine (10(-4) M) was 38+/-2% and 91+/-4%, respectively, compared with th e phasic contraction to noradrenaline (10(-4) M in normal Krebs soluti on) and after 30 min it was abolished at 37 degrees C and was 7+/-1% a t 25 degrees C. Ryanodine abolished the noradrenaline response in Ca2-free Krebs solution for 2 min at 25 degrees C, while cyclopiazonic ac id reduced it to 36+/-2%. 5 In normal Krebs solution at 25 degrees C t he protein kinase C inhibitor calphostin C reduced the tonic contracti on to noradrenaline (10(-5) M) from 36+/-8% to 14+/-3% compared with t he phasic contraction to noradrenaline (10(-4) M). The DAG kinase inhi bitor R 59022 increased the contraction following the initial phasic r esponse to a maximum of 107+/-17% after 35 s, before dropping down to a well maintained contraction which was still greater in magnitude com pared with the control. Nifedipine (3 x 10(-7) M) reduced the tonic co ntraction from 49+/-6% to 7+/-1% but did not reduce the phasic respons e. Ryanodine (10(-4) M) reduced the phasic contraction from 50+/-2% to 7+/-1% and the tonic response from 47+/-5% to 27+/-5%. 6 The phorbol ester phorbol-12,13-dibutyrate at 25 degrees C produced a transient co ntraction of the rat prostatic vas deferens, maximum response (10(-5) M) 48+/-4%, compared with the maximum tonic response to noradrenaline. The contraction to PDBu (10(-5) M) was reduced to 23+/-2% by calphost in C (10(-6) M) and to 15+/-1% by nifedipine (3 x 10(-7) M) and was ab olished after 2 min in Ca2+-free Krebs solution. 7 In conclusion, the alpha(1A)-adrenoceptor mediated contraction to noradrenaline of the ra t prostatic vas deferens appears to consist of an initial phasic compo nent due to the release of intracellular Ca2+ from ryanodine-sensitive stores. These stores are depleted in the absence of extracellular Ca2 + and this depletion is slower at 25 degrees C than at 37 degrees C. T he phasic contraction is followed by a tonic contraction involving act ivation of protein kinase C by diacylglycerol and influx of Ca2+ throu gh nifedipine-sensitive channels.