5′‐(N‐Ethylcarboxamido)adenosine Inhibits Ca2+ Influx and Activates a Protein Phosphatase in Bovine Adrenal Chromaffin Cells

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Abstract
We investigated the effect of the adenosine receptor agonist 5′‐(N‐ethylcarboxamido)adenosine (NECA) in catecholamine secretion from adrenal chromaffin cells that exhibit only the A2b subtype adenosine receptor. NECA reduced catecholamine release evoked by the nicotinic agonist 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 induced 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 treatment with NECA. Short‐term treatment with NECA failed both to modify [Ca2+]i levels and to increase endogenous diacylglycerol production, showing that NECA does not activate the intracellular Ca2+/protein kinase C signaling pathway. The inhibitory effects of NECA were accompanied 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 stimulation on catecholamine secretion from adrenal chromaffin cells.

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