Effects of alpha 2-adrenergic agonists on carbachol-stimulated catecholamine synthesis in cultured bovine adrenal medullary cells.

We examined the effects of alpha 2- and alpha 1-adrenergic agonists on synthesis of catecholamines in cultured bovine adrenal medullary cells. Clonidine, an alpha 2-adrenergic agonist, inhibited carbachol-stimulated synthesis of [14C]catecholamines from [14C]tyrosine in a concentration-dependent manner. Clonidine also inhibited carbachol-induced uptake of 45Ca2+ into cells at concentrations similar to those that inhibited the synthesis of [14C]catecholamines. Other alpha 2-adrenergic agonists, oxymetazoline and guanfacine, also strongly inhibited carbachol-stimulated synthesis of [14C]catecholamines. alpha 1-Adrenergic agonists, phenylephrine and norfenefrine, did not affect the synthesis. Tyrosine hydroxylase (EC 1.14.16.2) activity in a soluble fraction of cultured bovine adrenal medullary cells was assayed after gel filtration on a Sephadex G-25 column. Stimulation of the cells with carbachol increased the activity of tyrosine hydroxylase. Clonidine, oxymetazoline, and guanfacine all suppressed the carbachol-induced increase in activity of tyrosine hydroxylase in the cells. These results suggest that alpha 2-adrenergic agonists inhibit carbachol-stimulated synthesis of catecholamines by suppression of tyrosine hydroxylase activity, probably through the inhibition of Ca2+ uptake. However, the involvement of alpha 2-adrenoceptors in the inhibitory effects of alpha 2-agonists on catecholamine synthesis is still unsettled, since yohimbine failed to antagonize the inhibitory effect of clonidine on the synthesis in cultured bovine adrenal medullary cells.