Inhibition of voltage-dependent Ca²⁺ channels via α₂-adrenergic and opioid receptors in cultured bovine adrenal chromaffin cells

Adrenal chromaffin cells secrete catecholamindes and opioids. The effects of these agents on whole-cell Ca²⁺ channel currents were studied, using bovine adrenal chromaffin cells kept in short term culture. Ca²⁺ channel currents recorded during voltageclamp pulses from a holding potential of -80 mV to 0 mV were reversibly reduced by 10 μM epinephrine (in the presence of 1 μM propranolol) or 5 μM of the synthetic opioid, d-Ala²-d-Leu⁵-enkephalin (DADLE) by approximately 35% and 25%, respectively. The inhibitory action of epinephrine was mimicked by clonidine, reduced by yohimbine but not affected by prazosin. The DADLE-induced reduction of the Ca²⁺ channel current was antagonized by naloxone. The dihydropyridine (+)PN 200-110 (5 μM) reduced the Ca²⁺ channel current by approximately 40%; the Ca²⁺ channel current inhibited by (+)PN 200-110 was not further reduced by epinephrine. Intracellular infusion of guanosine-5′-O-(2-thiodiphosphate) and pretreatment of cells with pertussis toxin abolished the inhibitory effect of both epinephrine and DADLE. In membranes of adrenal chromaffin cells, four pertussis-toxin-sensitive G-proteins were identified, including G_i1, G_i2, G_o1 and another G_o subtype, possibly G_o2. The data show that activation of α₂-adrenergic and opioid receptors causes an inhibition of dihydropyridine-sensitive Ca²⁺ channels in adrenal chromaffm cells. These inhibitory modulations are mediated by pertussis-toxin-sensitive G-proteins and may represent a mechanism for a negative feedback signal by agents released from the adrenal medulla.