Inhibition by alpha 2-adrenoceptor agonists of the secretion of catecholamines from isolated adrenal medullary cells.

In adrenal medullary cells, carbachol evokes the secretion of catecholamines with simultaneous uptake of 45Ca. Highly selective agonists for alpha 2-adrenoceptors, clonidine, naphazoline, guanfacine, tramazoline and oxymetazoline inhibited carbachol evoked secretion of catecholamines in a dose-dependent manner. These alpha 2-agonists also inhibited the uptake of 45Ca evoked by carbachol with similar dose-response curve to inhibition of catecholamine secretion. On the contrary, highly selective agonists for alpha 1-adrenoceptors, phenylephrine and norfenefrine did not inhibit the secretion of catecholamines and cellular uptake of 45Ca. The inhibition by alpha 2-agonists was not restored either by the increase in carbachol or Ca concentrations, suggesting that the mode of inhibition was distinct from competition at cholinergic receptors or Ca-channels. It is likely that alpha 2-agonists inhibited the secretion of catecholamines via the inhibition of Ca uptake which was probably caused through the activation of alpha 2-adrenoceptors.