OMEGA-GRAMMOTOXIN BLOCKS ACTION-POTENTIAL-INDUCED CA2+ INFLUX AND WHOLE-CELL CA2+ CURRENT IN RAT DORSAL-ROOT GANGLION NEURONS

Field-potential stimulation of rat dorsal-root ganglion (DRG) neurons evoked action-potential-mediated transient increases in intracellular free calcium concentration ([Ca2+](i)) as measured by indo-1-based microfluorimetry. Field-potential-evoked [Ca2+](i) transients were abolished by tetrodotoxin, and their dependence on stimulus intensity exhibited an abrupt threshold. omega-Conotoxin GVIA (omega-CgTx, 100 nM) inhibited action-potential -mediated Ca2+ influx by 79 %, while nitrendipine (1 mu M) had little effect, omega-Grammotoxin SIA (omega-GsTx, 267 nM), a peptide toxin purified from the venom of the tarantula spider, Grammostola spatulata, blocked action-potential-mediated Ca2+ influx as effectively as did omega-CgTx, suggesting that omega-GsTx blocks N-type Ca2+ channels. In contrast to block by omega-CgTx, the block produced by omega-GsTx reversed upon washout of the peptide. omega-GsTx (270 nM) blocked 80%, and omega-CgTx (1 mu M) blocked 64%, of whole-cell Ca2+ current (I-ca) elicited by step depolarization to 0 mV from a holding potential of -80 mV. omega-GsTx completely occluded inhibition of I-ca by omega-CgTx. However, when applied after omega-CgTx, omega-GsTx produced an additional inhibition of 27%, indicating that omega-GsTx also blocked a non-N-type Ca2+ channel. BayK8634 (1 mu M) elicited an increase in I-ca in the presence of maximally effective concentrations of omega-GsTx, suggesting that omega-GsTx does not block L-type channels. Thus, omega-GsTx displays a selectivity for Ca2+ channel subtypes which should prove useful for studying: Ca2+ channels and Ca2+-channel-mediated processes.