Gonadotropin response to naloxone in the mare: effect of time of year and reproductive status

Effects of a novel dihydropyridine type of antihypertensive drug, cilnidipine, on the regulation of the catecholamine secretion closely linked to the intracellular Ca2+ were examined using nerve growth factor (NGF)-differentiated rat pheochromocytoma PC12 cells. By measuring catecholamine secretion with high-performance liquid chromatography coupled with an electrochemical detector, we showed that high K+ stimulation evoked dopamine release from PC12 cells both before and after NGF treatments. Cilnidipine depressed dopamine release both from NGF-treated and untreated PC12 cells in a concentration-dependent manner. In contrast, inhibition by nifedipine was markedly decreased in the differentiated PC12 cells. With intracellular Ca2+ concentration (Ca2+]i) measurements using fura 2, the elevation of high K+-evoked Ca2+]i was separated into nifedipine-sensitive and -resistant components. The nifedipine-resistant Ca2+]i increase was also blocked by cilnidipine, as well as omega-conotoxin-GVIA. By the use of the conventional whole-cell patch-clamp technique, the compositions of the high-voltage-activated Ca2+ channel currents in the NGF-treated PC12 cells were divided into types: L-type, N-type, and residual current components. It was also estimated that cilnidipine at 1 and 3 micromol/L strongly blocked the N-type current without affecting the residual current. These results suggest that cilnidipine inhibits catecholamine secretion from differentiated PC12 cells by blocking Ca2+ influx through the N-type Ca2+ channel, in addition to its well-known action on the L-type Ca2+ channel.