Modulation by endogenously released ATP and opioids of chromaffin cell calcium channels in mouse adrenal slices

Abstract

Modulation of high-threshold voltage-dependent calcium channels by neurotransmitters has been the subject of numerous studies in cultures of neurons and chromaffin cells. However, no studies on such modulation exist in chromaffin cells in their natural environment, the intact adrenal medullary tissue. Here we performed such a study in voltage-clamped chromaffin cells of freshly prepared mouse adrenal slices under the whole cell configuration of the patch-clamp technique. The subcomponents of the whole cell inward Ca2+ current ( ICa) accounted for 49% for L-, 28% for N-, and 36% for P/Q-type channels. T-type Ca2+ channels or residual R-type Ca2+ currents were not seen. However, under the perforated-patch configuration, 20% of ICa accounted for a toxin-resistant R-type Ca2+ current. Exogenously applied ATP and methionine-enkephalin (Met-enk) inhibited ICa by 33%. Stop-flow and Ca2+ replacement by Ba2+, which favored the release of endogenous ATP and opioids, also inhibited ICa, with no changes in activation or inactivation kinetics. This inhibition was partially voltage independent and insensitive to prepulse facilitation. Furthermore, in about half of the cells, suramin and naloxone augmented ICa in the absence of exogenous application of ATP/Met-enk. No additional modulation of ICa was obtained after bath application of exogenous ATP and opioids to these already inhibited cells. Augmentation of ICa was also seen upon intracellular dialysis of guanosine 5′-[β-thio]diphosphate (GDPβS), indicating the existence in the intact slice of a tonic inhibition of ICa in resting conditions. These results suggest that in the intact adrenal tissue a tonic inhibition of ICa exists, mediated by purinergic and opiate receptors.