5'-(N-ETHYLCARBOXAMIDO) ADENOSINE INHIBITS CA2+ INFLUX AND ACTIVATES A PROTEIN PHOSPHATASE IN BOVINE ADRENAL CHROMAFFIN CELLS

We investigated the effect of the adenosine receptor agonist 5'-(N-eth ylcarboxamido) adenosine (NECA) in catecholamine secretion from adrena l chromaffin cells that exhibit only the A(2b) subtype adenosine recep tor. NECA reduced catecholamine release evoked by the nicotinic agonis t 1,1-dimethyl-4-phenylpiperazinium (DMPP) in a time-dependent manner. Inhibition reached 25% after 30-40-min exposure to NECA. This effect on DMPP-evoked catecholamine secretion was mirrored by a similar (27.7 +/- 3.3%), slowly developing inhibition of [Ca2+](i) transients induc ed by DMPP that peaked at 30-min preincubation with NECA. The capacity of the chromaffin cells to buffer Ca2+ load was not affected by the t reatment with NECA. Short-term treatment with NECA failed both to modi fy [Ca2+](i) levels and to increase endogenous diacylglycerol producti on, showing that NECA does not activate the intracellular Ca2+/protein kinase C signaling pathway. The inhibitory effects of NECA were accom panied by a 30% increase of protein phosphatase activity in chromaffin cell cytosol. We suggest that dephosphorylation of a protein involved in DMPP-evoked Ca2+ influx pathway (e.g., L-type Ca2+ channels) could be the mechanism of the inhibitory action of adenosine receptor stimu lation on catecholamine secretion from adrenal chromaffin cells.