The reversal by metoclopramide of apomorphine-induced inhibition of responses to stimulation of the cardioaccelerator nerves in the cat.

The effect of metoclopramide on presynaptic dopamine receptors was investigated in the cat cardioaccelerator nerve preparation. Metoclopramide, a substituted benzamide derivative, antagonized inhibitory action of apomorphine on positive chronotropic responses induced by sympathetic nerve stimulation in cat hearts, in vivo. Neither phentolamine, an alpha-adrenergic blocking agent, nor indomethacin a prostaglandin synthesis inhibitor, antagonized the effect of apomorphine. Apomorphine did not alter the positive chronotropic effects of intravenously administered noradrenaline. Metoclopramide potentiated stimulation-induced positive chronotropic responses. These results suggest that metoclopramide blocks the presynaptic dopamine receptors at the cat heart.