actions of quinine on the rat isolated rectum the actions of quinine on the rat isolated rectum was studied quinine 25 x 105 25 x 104 m initiated rhythmic phasic contractions rpc which were prevented by verapamil lanthanum indomethacin or ca2withdrawal but were unaffected by tetrodotoxin lpropranolol phentolamine or atropine higher quinine concentrations 25 x 104 103 m relaxed the rat rectum and inhibited the rpc induced by lower concentrations exposure to high ca2tyrode solution 6 ca2tyrode increased the baseline tone of the muscle under this condition quinine 25 x 104 103 m but not lower concentrations relaxed the muscle quinine 25 x 105 103 m concentrationdependently inhibited kcl and acetylcholineinduced contractions but more readily blocked kcl than acetylcholineinduced responses the inhibitory effect of quinine against kcl but not acetylcholine was largely reversed by increasing the ca2 concentration of the tyrode solution it is concluded that quinineinduced rpc occurred independently of adrenergic or cholinergic mechanisms and are likely to be due to enhanced ca2 influx into the rectum while the relaxation encountered with higher concentrations may be due to inhibition by quinine of transmembranal ca2 influx evidence was obtained that quinine may preferentially inhibit ca2 influx through a pathway linked to high k depolarisation