RELEASE OF ATP IN RAT VAS-DEFERENS - ORIGIN AND ROLE OF CALCIUM

Release of endogenous ATP elicited by electrical (neural) stimulation and exogenous agonists was studied in the rat isolated vas deferens. The aims were to dissect neural and postjunctional contributions to the nerve activity-evoked overflow of ATP and to clarify the role of transmitter receptors and calcium in postjunctional ATP release. In tissues preincubated with [H-3]-noradrenaline, electrical stimulation (100 pulses/10 Hz) elicited contraction and an overflow of tritium and ATP. Contractions as well as ATP overflow were reduced by prazosin 0.3 mu M and even more so by prazosin 0.3 mu M combined with suramin 300 mu M. They were also reduced by nifedipine 10 mu M and even more so by nifedipine 10 mu M combined with ryanodine 20 mu M (the additional effect of ryanodine on ATP overflow was not significant). In tissues not pretreated with [H-3]-noradrenaline, exogenous noradrenaline 10 mu M and alpha,beta-methylene ATP 10 mu M elicited contraction and an overflow of ATP. Responses to noradrenaline were blocked by prazosin 0.3 mu M but not suramin 300 mu M and were greatly reduced by nifedipine 10 mu M and in Ca2+-free medium. Responses to alpha,beta-methylene ATP were blocked by suramin 300 mu M but not prazosin 0.3 mu M, were reduced by nifedipine 10 mu M (effect on ATP overflow not significant) and were reduced even more in Ca2+-free medium. Neuropeptide Y 0.3 mu M caused only very small contraction and ATP overflow. The electrically as well as the agonist-evoked ATP overflow correlated well with the contraction responses except in experiments with suramin which retarded the removal, by vas deferens tissue, of ATP from the medium. It is concluded that the overflow of ATP from rat vas deferens elicited by electrical (neural) stimulation is at least 90% postjunctional, presumably smooth muscle, in origin. ATP is released from postjunctional cells as a consequence of both alpha(1)-adrenoceptor and P-2-purinoceptor activation. Ca2+ is a second messenger in the postjunctional ATP release response; its major part enters through L-type channels. A ''purely neural'' overflow of ATP was not isolated under the conditions of the experiments.