ω-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. ω-Conotoxin GVIA (ω-CgTx, 100 nM) inhibited action-potential-mediated Ca²⁺ influx by 79%, while nitrendipine (1 μM) had little effect. ω-Grammotoxin SIA (ω-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 ω-CgTx, suggesting that ω-GsTx blocks N-type Ca²⁺ channels. In contrast to block by ω-CgTx, the block produced by ω-GsTx reversed upon washout of the peptide. ω-GsTx (270 nM) blocked 80%, and ω-CgTx (1 μM) blocked 64%, of whole-cell Ca²⁺ current (I_Ca) elicited by step depolarization to 0 mV from a holding potential of -80 mV. ω-GsTx completely occluded inhibition of I_Ca by ω-CgTx. However, when applied after ω-CgTx, ω-GsTx produced an additional inhibition of 27%, indicating that ω-GsTx also blocked a non-N-type Ca²⁺ channel. BayK8644 (1 μM) elicited an increase in I_Ca in the presence of maximally effective concentrations of ω-GsTx, suggesting that ω-GsTx does not block L-type channels. Thus, ω-GsTx displays a selectivity for Ca²⁺ channel subtypes which should prove useful for studying Ca²⁺ channels and Ca²⁺-channel-mediated processes.