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Saxitoxin Blocks L-Type ICa

Saxitoxin (STX) and tetrodotoxin (TTX) are frequently used to selectively block sodium channels. In this study, we provide evidence that commercial STX also inhibits L-type Ca2+ currents (ICa,L) in adult mouse ventricular myocytes (VMs) and tsA-201 cells that were transiently cotransfected with three calcium channel subunits. We measured inhibition of sodium currents (INa) in mouse VMs, of ICa,L in mouse VM and tsA-201 cells, and intracellular calcium concentration ([Ca2+]i) transients in single mouse VMs. STX or TTX was abruptly applied before the test voltage pulse using a rapid solution switcher device. STX (10 μM; Calbiochem) and TTX (60 μM; Sigma-Aldrich) completely blocked INa in mouse VMs. However, STX at 10 μM also reduced ICa,L in mouse VM by 39% (P < 0.0001; n = 14), whereas TTX at 60 μM had no effect on ICa,L. STX (10 μM; Calbiochem) reduced the amplitude of the [Ca2+]i transients in mouse VMs by 36% (P < 0.0001; n = 10). In contrast, TTX (60 μM; Sigma-Aldrich) only reduced the amplitude of the [Ca2+]i transients by 9% (P = 0.003; n = 5). STX (10 μM) obtained from Sigma-Aldrich showed a similar inhibitory effect on ICa,L (33%) (P < 0.0001; n = 5) in mouse VMs. STX (Calbiochem) inhibited the calcium currents of tsA-201 cells in a dose-dependent manner. This inhibition was voltage-independent. The current-voltage relationship of calcium currents in tsA-201 cells was not altered by STX. These results indicate that STX partially blocks L-type Ca2+ channels and thus provide further evidence that its effects are not specific for Na+ channels.