title: Saxitoxin blocks L-type ICa.
abstract: 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 (I(Ca,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 I(Ca,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 microM; Calbiochem) and TTX (60 microM; Sigma-Aldrich) completely blocked INa in mouse VMs. However, STX at 10 microM also reduced I(Ca,L) in mouse VM by 39% (P < 0.0001; n = 14), whereas TTX at 60 microM had no effect on I(Ca,L). STX (10 microM; Calbiochem) reduced the amplitude of the [Ca2+]i transients in mouse VMs by 36% (P < 0.0001; n = 10). In contrast, TTX (60 microM; Sigma-Aldrich) only reduced the amplitude of the [Ca2+]i transients by 9% (P = 0.003; n = 5). STX (10 microM) obtained from Sigma-Aldrich showed a similar inhibitory effect on I(Ca,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.
authors.affiliation: Cardiology Division, University of Utah Health Sciences Center, Salt Lake City, Utah 84132, USA.
authors.forename: Zhi
authors.initials: Z
authors.lastname: Su
authors.forename: Michael
authors.initials: M
authors.lastname: Sheets
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Ishida
authors.forename: Fenghua
authors.initials: F
authors.lastname: Li
authors.forename: William H
authors.initials: WH
authors.lastname: Barry
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D020746:Calcium Channels, L-Type / Q000187:drug effects / Q000378:metabolism*; D006352:Heart Ventricles / Q000166:cytology; D066298:In Vitro Techniques; D008670:Metals / Q000032:analysis; D051379:Mice; D009206:Myocardium / Q000378:metabolism; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000378:metabolism; D012530:Saxitoxin / Q000494:pharmacology*; D013779:Tetrodotoxin / Q000494:pharmacology
chemical_list: D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D008670:Metals; D012530:Saxitoxin; D013779:Tetrodotoxin; D002118:Calcium