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

Abstract

The effects of field stimulation and ionophoretic application of ATP and noradrenaline [norepinephrine] were studied in the rat anococcygeus by means of an intracellular microelectrode. Field stimulation at room temperature produced 3 types of electrical membrane response: a fast excitatory junction potential (e.j.p.) which had a latency of < 100 ms and a time to peak of 300 ms; a slow e.j.p. which had a latency of several hundred ms and a time to peak of 1-2 s, and an inhibitory junction potential (i.j.p.) which had a time to peak of .apprx. 1.5 s. All 3 responses were blocked by tetrodotoxin. The ionophoretic application of ATP produced both monophasic and biphasic depolarizations; these responses had a latency of < 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.