Myosin light-chain kinase inhibitor, 1-(5-chlornaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-9), inhibits catecholamine secretion from adrenal chromaffin cells by inhibiting Ca2+ uptake into the cells

For determination of whether myosin light-chain kinase (MLCK) is involved in the secretory mechanism of adrenal chromaffin cells, the effect of a preferential inhibitor of the enzyme, 1-(5-chlornaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-9), on catecholamine secretion from cultured bovine adrenal chromaffin cells was studied. ML-9 did not affect basal catecholamine secretion, but inhibited catecholamine secretion stimulated by acetylcholine, high K+, veratridine or palytoxin. At similar concentrations to those inhibiting the secretion of catecholamine, ML-9 also inhibited increased [45Ca]2+ uptake by the cells induced by these stimulants. However, it did not inhibit catecholamine secretion induced by the Ca2+ ionophore A23187. Moreover, it did not affect catecholamine secretion from digitonin-permeabilized cells induced by a micromolar Ca2+ concentration in the presence of Mg ATP. These results indicate that ML-9 inhibits catecholamine secretion from adrenal chromaffin cells by inhibiting the transmembrane Ca2+ uptake mechanism, but not by inhibiting the intracellular Ca2+-dependent mechanism. The possible role of MLCK in stimulus-secretion coupling in adrenal chromaffin cells is discussed.