The ability of pimozide to prevent inhibition by dopamine analogs of cardioaccelerator nerves in cat hearts.

The dopamine receptor blocking agent, pimozide, antagonized inhibitory actions of apomorphine, N,N-dimethyldopamine and 5,6-dihydroxy-2-dimethyl-aminotetralin (M-7) on stimulation-induced positive chronotropic responses of cat hearts in both in vivo and in vitro experiments. Pimozide did not antagonize the inhibitory effect of clonidine in vitro experiments. Phentolamine, an alpha-adrenergic blocking agent, antagonized the inhibitory actions of dopamine analogs at 1.10 mug/ml concentration in in vitro experiments. Phentolamine, 0.3 mug/ml, only attenuated the response of N, N-dimethyldopamine but did not antagonize the effects of apomorphine and M-7. Lower concentration of phentolamine, 0.1 mug/ml, showed marked inhibition to the inhibitory effect of clonidine. Both phentolamine and pimozide potentiated stimulation-induced positive chronotropic responses of cat right atria. Possibility of the presence of dopaminergic inhibitory system near the alpha-adrenergic inhibitory system in cardioaccelerator nerve terminal is discussed.