Mechanical responses to catecholamines in the longitudinal muscle of guinea‐pig gastric fundus

1 In the longitudinal muscle of guinea‐pig gastric fundus, adrenaline and phenylephrine (1–30 μ m ) both produced a slow contraction preceded by a relaxation. The slow contraction was strongly inhibited by prazosin (0.1 μ m ), but only weakly by yohimbine (1μ m ), suggesting main contribution of α 1 ‐adrenoceptors. 2 Most of the slow contraction was blocked by meclofenamate or indomethacin (0.1–0.3 μ m ). Both these drugs also inhibited spontaneously generated muscle tone. In some preparations, obtained from the apical fundus, a small contraction remained in the presence of meclofenamate. 3 During contraction induced by prostaglandin E 2 , adrenaline produced sustained relaxation and phenylephrine often transient relaxation, in the presence of meclofenamate. The transient relaxation, but not the sustained relaxation, was suppressed by prazosin. 4 In the presence of prostaglandin E 2 (5 n m ), after treating with phenoxybenzamine (30 μ m ) for 30 min, isoprenaline and adrenaline produced concentration‐dependent relaxation, with IC 50 s of 3.9 n m and 64 n m , respectively. Propranolol shifted these concentration‐response curves to the right, with apparent pA 2 s of 8.15 and 7.34, respectively. 5 It is suggested that in the fundic longitudinal muscle, adrenaline‐induced contraction is mediated mainly by an increase in endogenous prostaglandin production through activation of α 1 ‐adrenoceptors and that adrenaline produces transient relaxation through α 1 ‐adrenoceptors and sustained relaxation through β‐adrenoceptors. The β‐adrenoceptors in the longitudinal muscle are more sensitive to adrenaline and isoprenaline than those in the circular muscle.