A TOXIN FRACTION (FTX) FROM THE FUNNEL-WEB SPIDER POISON INHIBITS DIHYDROPYRIDINE-INSENSITIVE CA-2+ CHANNELS COUPLED TO CATECHOLAMINE RELEASE IN BOVINE ADRENAL CHROMAFFIN CELLS

In adrenal chromaffin cells, depolarization-evoked Ca2+ influx and catecholamine release are partially blocked by blockers of L-type voltage-sensitive Ca2+ channels. We have now evaluated the sensitivity of the dihydropyridine-resistant components of Ca2+ influx and catecholamine release to a toxin fraction (FTX) from the funnel-web spider poison, which is known to block P-type channels in mammalian neurons. FTX (1:4,000 dilution, with respect to the original fraction) inhibited K+-depolarization-induced Ca2+ influx by 50%, as monitored with fura-2, whereas nitrendipine (0.1-1 muM) and FTX (3:3), a synthetic FTX analogue (1 mM), blocked the [Ca2+]i transients by 35 and 30%, respectively. When tested together, FTX and nitrendipine reduced the [Ca2+]i transients by 70%. FTX or nitrendipine reduced adrenaline and noradrenaline release by approximately 80 and 70%, respectively, but both substances together abolished the K+-evoked catecholamine release, as measured by HPLC. The omega-conotoxin GVIA (0.5 muM) was without effect on K+-stimulated Ca-45(2+) uptake, Our results indicate that FTX blocks dihydropyridine- and omega-conotoxin-insensitive Ca2+ channels that, together with L-type voltage-sensitive Ca2+ channels, are coupled to catecholamine release.