Actions of polypeptides at the neuromuscular junction

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Abstract
The effects of several polypeptides, e.g. angiotensin II, substance P, oxytocin and vasopressin, on the isolated frog gastrocnemius, chick biventer cervicis and rat hemodiaphragm preparations were studied using electrophysiological and neurochemical techniques. The effects of angiotensin II, substance P, oxytocin and vasopressin on neuromuscular transmission and muscle contraction were investigated by studying the following parameters: the directly and indirectly-elicited twitch and tetanic contractions, nerve compound action potential, uptake of3H-methylcholine into nerve-muscle preparations, the contractures produced by depolarizing drugs, e.g. ACh or TEA. The results showed that angiotensin II (10−10–10−6 M) and substance P (10−7–10−6 M) enhanced neuromuscular transmission and muscle contraction by increasing the amplitudes of the indirectly-elicited twitch and tetanic contractions. Oxytocin and vasopressin (1–100 mU/ml−1) both depressed neuromuscular transmission by reducing the contractile and electrical response in the frog, chick and rat skeletal muscle. It was concluded that, like their effects on ganglionic transmission, the peptides can modify neuromuscular transmission. The mechanism by which these peptides produce their effects may be dependent on external calcium concentration. These peptides may affect both pre- and postjunctional mechanisms; prejunctionally by increasing/decreasing the release of ACh, and postjunctionally by affecting the sensitivity of the postjunctional membrane to depolarizing drugs and/or producing a contracture in the skeletal muscle.