SENSITIVITY TO DIHYDROPYRIDINES, OMEGA-CONOTOXIN AND NORADRENALINE REVEALS MULTIPLE HIGH-VOLTAGE-ACTIVATED CA2+ CHANNELS IN RAT INSULINOMA AND HUMAN PANCREATIC BETA-CELLS

High-voltage-activated (HVA) Ba2+ currents of rat insulinoma (RINm5F) and human pancreatic beta-cells were tested for their sensitivity to dihydropyridines (DHPs), omega-conotoxin (omega-CgTx) and noradrenaline. In RINm5F cells, block of HVA currents by nimodipine, nitrendipine and nifedipine was voltage- and dose-dependent (apparent K(D) < 37 nM) and largely incomplete even at saturating doses of DHPs (mean 53%, at 10 muM and 0 mV). Analysis of slow tail currents in Bay K 8644-treated cells indicated the existence of Bay K 8644-insensitive channels that turned on at slightly more positive voltages and deactivated more quickly than Bay K 8644-modified channels. DBP Ca2+ agonists and antagonists in human beta-cells had similar features to RINm5F cells except that DHP block was more pronounced (76%, at 10 muM and 0 mV) and Bay K 8644 action was more effective, suggesting a higher density of L-type Ca2+ channels in these cells. In RINm5F cells, but not in human beta-cells, DHP-resistant currents were sensitive to omega-CgTx. The toxin depressed 10-20% of the DHP-resistant currents sparing a ''residual'' current (25-35%) with similar voltage-dependent characteristics and Ca2+/Ba2+ permeability. Noradrenaline (10 muM) exhibited different actions on the various HVA current components: (1) it prolonged the activation kinetics of omega-CgTx-sensitive currents, (2) it depressed by about 20% the size of DHP-sensitive currents, and (3) it had little or no effects on the residual DHP- and omega-CgTx-resistant current although intracellularly applied guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) prolonged its activation time course. The first action was clearly voltage-dependent and most evident in RINm5F cells that displayed neuronal-like processes. The second was observed more frequently, was voltage-independent and fully blocked by saturating doses of nifedipine (10 muM). Both actions were prevented by intracellular perfusion with guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S). Our data suggest that beside a majority of L-type channels, RINm5F and human pancreatic beta-cells may express a variable fraction of DHP-insensitive channels that may be involved in the control of insulin secretion during beta-cell activity.