Pertussis toxin attenuates clonidine inhibition of catecholamine release in adrenal chromaffin cells

Characteristics of the inhibitory action of clonidine on catecholamine release in bovine adrenal chromaffin cells were investigated. Clonidine at 3 x 10(-5) M inhibited acetylcholine (ACh)-evoked release by about 50%, but not catecholamine release evoked by high K+. Another alpha 2-agonist alpha-methyladrenaline was ineffective at inhibiting ACh-evoked release. The inhibition by clonidine of ACh-evoked release was not reversed by alpha 2-antagonists. Treatment of these cells with pertussis toxin reversed the inhibitory effect of clonidine, while it did not affect the inhibitory action of hexamethonium and of nifedipine. Therefore, clonidine inhibition of catecholamine release in these cells seems not to be mediated by the alpha 2-adrenoceptor, but might be mediated by a specific receptor for clonidine.