Positive inotropic effect of nicotine on electrically evoked contraction of isolated toad ventricle.

The mechanism of the responses of nicotine on electrically evoked contractions of the isolated spirally cut toad ventricle was investigated. Nicotine produced a concentration-dependent positive inotropic effect of the ventricle. Prior administration of propranolol, hexamethonium, hemicholinium-3 or guanethidine failed to antagonize the positive inotropic effect of nicotine, thus ruling out the possibility of the nicotine effect being mediated through activation of beta-adrenoceptors, ganglionic activation, presynaptic liberation of acetylcholine and liberation of norepinephrine from the sympathetic nerve endings respectively. The positive inotropic effect is probably mediated through mobilisation of calcium since prior incubation with verapamil--the calcium transport blocker, EDTA, which chelates the extracellular calcium and calcium-free Ringer prevented the positive inotropic effect of nicotine. These results suggest that the positive intropic effect of nicotine may presumably be due to the facilitating effects of nicotine on Ca++ exchange or mobilisation of membrane bound Ca++.