Discrete events measure single quanta of adenosine 5'-triphosphate secreted from sympathetic nerves of guinea-pig and mouse vas deferens.

Intracellularly recorded excitatory junction potentials in smooth muscle cells, and the first time differentials of their rising phases ("discrete events") were used to analyse transmitter secretion from the postganglionic sympathetic nerves of guinea-pig and mouse vas deferens. The aim was to determine whether the transmitter causing these responses is noradrenaline or adenosine 5'-triphosphate (ATP). Depletion of the noradrenaline stores following treatment with reserpine reduced the frequency of occurrence, but not the amplitude, of spontaneous junction potentials and discrete events. Nerve stimulation could still evoke "fast" junction potentials and discrete events, normal in appearance, but "slow" junction potentials were reduced in amplitude and had shorter times to peak. In contrast, desensitization of ATP receptors by alpha, beta-methylene ATP abolished spontaneous and stimulus-induced "fast" (but not "slow") junction potentials and discrete events, reversibly. It is concluded that it is not noradrenaline, but ATP or some related compound which causes spontaneous and "fast" stimulus-induced junction potentials, and discrete events. The present and earlier data show that discrete events reflect the secretion of individual quanta of ATP (or quanta of "ATP + noradrenaline", if both are secreted in parallel from the same vesicle) from postganglionic sympathetic nerve terminals in guinea-pig and mouse vas deferens.