Contractile effects of substance P and neurokinin A on the rat stomach in vivo and in vitro

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Abstract
1 Substance P and neurokinin A (substance K) were infused into the coeliac artery of anaesthetized rats at doses of 0.06–20 nmol min−. Both tachykinins caused contractions of the stomach, the threshold dose of neurokinin A being 10 times lower than of substance P. The dose-response curve for substance P was flatter than that for neurokinin A. 2 On circular muscle strips from the rat gastric corpus in vitro, the dose-response curves for both tachykinins were parallel, neurokinin A being 10 times more potent than substance P. The contractions in response to 10 μm neurokinin A and to 30 μm substance P were 58 and 54%, respectively, of the maximal contraction to bethanechol (1 mm). 3 The effect of substance P was reduced by atropine both in vivo and in vitro. In vitro, the contractions to substance P were also reduced by tetrodotoxin but left unaffected by methysergide. The action of neurokinin A was not affected by these drugs. 4 It is concluded that neurokinin A contracts rat stomach by a direct action on the circular smooth muscle, whereas the action of substance P is mediated, at least in part, by cholinergic interneurones.

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