ENHANCEMENT OF SYMPATHETIC PURINERGIC NEUROTRANSMISSION IN THE GUINEA-PIG ISOLATED VAS-DEFERENS BY THE NOVEL ECTO-ATPASE INHIBITOR ARL-67156

1 Field stimulation of the sympathetic nerves of the guinea-pig isolat ed vas deferens with trains of pulses for 20 s at 1-8 Hz produced char acteristic biphasic contractions. The effect of the novel ecto-ATPase inhibitor, 6-N,N-diethyl-D-beta,gamma-dibromomethyleneATP (ARL 67156, formerly known as FPL 67156), on the magnitude of the initial, predomi nantly purinergic peak of this response was studied in order to determ ine the influence of enzymatic degradation of adenosine 5'-triphosphat e (ATP) on its action as a neurotransmitter. 2 The peak magnitude of t he response to nerve stimulation was significantly increased in a conc entration-dependent manner by ARL 67156 (5-100 mu M) and the size of t he neurogenic response at 4 Hz was approximately doubled in the presen ce of ARL 67156 (100 mu M). 3 ARL 67156 (100 mu M) has a rapid onset o f action. The enhancing effect on neurogenic contractions was maximal after 10 min, was well maintained for at least 30 min and was rapidly reversed, with responses returning to control levels 10 min after wash out. 4 The neurogenic contraction in the presence of prazosin (0.1 mu M) was purely purinergic, as it was abolished by the P-2-purinoceptor antagonist, PPADS (100 mu M). ARL 67156 (100 mu M) produced a similar degree of enhancement of neurogenic responses in the absence and prese nce of prazosin, supporting the view that the enhancing effects of ARL 67156 on neurogenic contractions result from potentiation of the acti on of ATP. 5 Exogenous ATP and alpha,beta-methyleneATP produced rapid transient contractions. Responses to ATP were increased in magnitude a nd duration in the presence of ARL 67156 (100 mu M), whereas those to the stable analogue, alpha,beta-methyleneATP were not significantly af fected. 6 Contractions to exogenous noradrenaline (10 mu M) and KCl (4 0 mM) were significantly enhanced by ARL 67156 (100 mu M), but this po tentiation was abolished by PPADS (100 mu M). Therefore, this effect o f the ecto-ATPase inhibitor may be due to a build up of endogenous ATP , increasing the sensitivity of the smooth muscle to other agonists. 7 It is concluded that ARL 67156 potentiates the action of ATP, and tha t when ATP acts as a neurotransmitter its postjunctional actions are g reatly attenuated by enzymatic degradation.