The adrenal gland contains resident macrophages, some of which lie adjacent to the catecholamine producing chromaffin cells. Because macrophages release a variety of secretory products, it is possible that paracrine signaling between these two cell types exists. Of particular interest is the potential paracrine modulation of voltage-gated calcium channels (I(Ca)), which are the main calcium influx pathway triggering catecholamine release from chromaffin cells. We report that prostaglandin E(2) (PGE(2)), one of the main signals produced by macrophages, inhibited I(Ca) in cultured bovine adrenal chromaffin cells. The inhibition is rapid, robust, and voltage dependent; the activation kinetics are slowed and inhibition is largely reversed by a large depolarizing prepulse, suggesting that the inhibition is mediated by a direct G-protein betagamma subunit interaction with the calcium channels. About half of the response to PGE(2) was sensitive to pertussis toxin (PTX) incubation, suggesting both PTX-sensitive and -insensitive G proteins were involved. We show that activation of macrophages by endotoxin rapidly (within minutes) releases a signal that inhibits I(Ca) in chromaffin cells. The inhibition is voltage dependent and partially PTX sensitive. PGE(2) is not responsible for this inhibition as blocking cyclooxygenase with ibuprofen did not prevent the production of the inhibitory signal by the macrophages. Nor did blocking the lipoxygenase pathway with nordihydroguaiaretic acid alter production of the inhibitory signal. Our results suggest that macrophages may modulate I(Ca) and catecholamine secretion by releasing PGE(2) and other chemical signal(s).