Comparison of the biphasic excitatory junction potential with membrane responses to adenosine triphosphate and noradrenaline in the rat anococcygeus muscle.

The effects of field stimulation and ionophoretic application of adenosine triphosphate (ATP) and noradrenaline were studied in the rat anococcygeus by means of an intracellular microelectrode. Field stimulation at room temperature produced three types of electrical membrane response: (a) a 'fast' excitatory junction potential (e.j.p.) which had a latency of less than 100 ms and a time to peak of 300 ms; (b) a 'slow' e.j.p. which had a latency of several hundred ms and a time to peak of 1-2 s, and (c) an inhibitory junction potential (i.j.p.) which had a time to peak of about 1.5 s. All three responses were blocked by tetrodotoxin. The ionophoretic application of ATP produced both monophasic and biphasic depolarizations; these responses had a latency of less than 30 ms and a time to peak of 150-300 ms. In contrast, ionophoretically-applied noradrenaline produced a depolarization which had a mean latency of 471 ms and a time to peak of 861 ms. The 'slow' e.j.p. and the noradrenaline-induced depolarization were blocked by prazosin whereas the 'fast' e.j.p. and the ATP responses were resistant to this antagonist and also to atropine. These results are further evidence that the 'fast' e.j.p. in some smooth muscle tissues is mediated by ATP.