INHIBITION BY ETHAVERINE OF CATECHOLAMINE SECRETION THROUGH BLOCKING L-TYPE CA2+ CHANNELS IN PC12 CELLS

Abstract

Ethaverine, a derivative of papaverine, is used as a vasodilator and antispasmodic drug. We have investigated the effects of ethaverine on the secretion of [H-3]norepinephrine from PC12 cells, of neuroendocrine origin. Treatment with ethaverine reduced catecholamine secretion in a concentration-dependent manner. The maximal inhibitory effect (90%) was achieved with 10 mu M ethaverine, and the IC50 for secretion was similar to 2 mu M. Ethaverine pretreatment for 1 min prior to stimulation by 70 mM K+ also decreased the level of intracellular Ca2+ in a concentration-dependent manner, as measured by fura-2 fluorescence. The IC50 for the inhibition of the increase in intracellular Ca2+ was similar to 2 mu M. Nifedipine, a dihydropyridine L-type Ca2+ channel blocker. did not enhance the inhibitory effect of ethaverine on the 70 mM K+-induced increase in [Ca2+](i) or catecholamine secretion. In contrast, the addition of the N-type voltage-sensitive Ca2+ channel antagonist omega-conotoxin with ethaverine resulted in further reductions in the increase in [Ca2+](i) and catecholamine release. Maximally effective concentrations of ethaverine and nifedipine showed the same inhibitory effect on the 70 mM K+-evoked responses. However, ethaverine pretreatment did not inhibit the bradykinin-induced secretion and [Ca2+](i) rise, which are known to he produced through the receptor-operated Ca2+ channels. We conclude that ethaverine reduces catecholamine secretion by blocking the L-type voltage-sensitive Ca2+ channel.