CONTRIBUTION OF L-AND NON-L-TYPE CALCIUM CHANNELS TO VOLTAGE-GATED CALCIUM CURRENT AND GLUCOSE-DEPENDENT INSULIN-SECRETION IN HIT-T15 CELLS

The pharmacological properties of voltage-gated Ca current and glucose -dependent insulin secretion were determined using the HIT insulinoma line to understand the role of Ca channels in stimulus-secretion coupl ing. The L-type Ca channel antagonist nimodipine inhibited a maximum o f 50-55% of the peak Ca current, suggesting that L- and non-L-type cha nnels contribute to Ca current. The L-agonist BAY K 8644 increased Ca current by 155%, whereas the N-channel blocker omega-conotoxin MVIIA r eversibly blocked 35% of the peak Ca current. Total block with nimodip ine and MVIIA was additive. Conotoxin MVIIC did not affect HIT Ca curr ent. Prolonged depolarizations elicited rapidly and slowly inactivatin g Ca currents. Nimodipine partially inhibited transient current, but f ully inhibited slowly inactivating current, suggesting that the former is mediated by L- and N-channels, and the latter is mediated by L-cha nnels. Like slowly inactivating Ca current, glucose-dependent insulin secretion was fully inhibited by nimodipine and insensitive to MVIIA. BAY K potentiated secretion and antagonized nimodipine block. These re sults suggest that persistent Ca current is mediated by L-channels and is strongly coupled to insulin secretion, whereas transient Ca curren t is mediated by L- and N-channels and is weakly coupled. Sustained Ca influx may be preferentially coupled because glucose persistently dep olarizes HIT cells and inactivates more transient Ca channel pathways.