| Popis: |
Although dopamine is known to be present in sympathetic ganglia, its role and mode of action as a peripheral neurotransmitter are still poorly understood. Dopaminergic agonists have been shown to inhibit adrenal catecholamine release and calcium uptake. However, the specific dopamine receptor subtype mediating these effects and the receptor transduction mechanism remain unknown. We now provide evidence demonstrating 1) that slowly inactivating, voltage-gated calcium channels serve as a target site for dopaminergic modulation of chromaffin cell function and 2) that it is the D2 receptor subtype which mediates dopaminergic inhibitory effects on catecholamine secretion, 45Ca uptake and voltage-gated calcium currents. Whole cell patch clamp electrophysiological techniques were used to monitor directly voltage-gated Ca++ channels. The D2 agonist apomorphine but not the D1 agonist SKF 38393 reduced reversibly a slowly inactivating, voltage-gated calcium current in cultured chromaffin cells and this effect was blocked by the D2 receptor antagonist haloperidol. The presence of D2 but not D1 dopamine receptors on chromaffin cell membranes was demonstrated by radioligand binding methods, using the specific D1 and D2 receptor radioligands, [3H]SCH23390 and [3H]N-methylspiperone, respectively. Nicotine- and KCl (60 mM)-evoked catecholamine secretion and 45Ca uptake were inhibited by the D2 agonist, apomorphine, but not by the D1 agonist, SKF 38393. These inhibitory effects were prevented by the D2 antagonist, sulpiride, but not by the D1 antagonist, SCH 23390. D2 dopamine receptors appear to function as inhibitory modulators of adrenal catecholamine secretion with a mode of action involving inhibition of calcium channel currents. |
