title: Tetrodotoxin blockade on canine cardiac L-type Ca²⁺ channels depends on pH and redox potential.
abstract: Tetrodotoxin (TTX) is believed to be one of the most selective inhibitors of voltage-gated fast Na⁺ channels in excitable tissues. Recently, however, TTX has been shown to block L-type Ca²⁺ current (I(Ca)) in canine cardiac cells. In the present study, the TTX-sensitivity of I(Ca) was studied in isolated canine ventricular myocytes as a function of (1) channel phosphorylation, (2) extracellular pH and (3) the redox potential of the bathing medium using the whole cell voltage clamp technique. Fifty-five micromoles of TTX (IC₅₀ value obtained under physiological conditions) caused 60% ± 2% inhibition of I(Ca) in acidic (pH = 6.4), while only a 26% ± 2% block in alkaline (pH = 8.4) milieu. Similarly, the same concentration of TTX induced 62% ± 6% suppression of ICa in a reductant milieu (containing glutathione + ascorbic acid + dithiothreitol, 1 mM each), in contrast to the 31% ± 3% blockade obtained in the presence of a strong oxidant (100 μM H₂O₂). Phosphorylation of the channel protein (induced by 3 μM forskolin) failed to modify the inhibiting potency of TTX; an IC₅₀ value of 50 ± 4 μM was found in forskolin. The results are in a good accordance with the predictions of our model, indicating that TTX binds, in fact, to the selectivity filter of cardiac L-type Ca channels.
authors.affiliation: Department of Physiology, University of Debrecen, P.O. Box 22, H-4012 Debrecen, Hungary.
authors.forename: Bence
authors.initials: B
authors.lastname: Hegyi
authors.forename: István
authors.initials: I
authors.lastname: Komáromi
authors.forename: Kornél
authors.initials: K
authors.lastname: Kistamás
authors.forename: Ferenc
authors.initials: F
authors.lastname: Ruzsnavszky
authors.forename: Krisztina
authors.initials: K
authors.lastname: Váczi
authors.forename: Balázs
authors.initials: B
authors.lastname: Horváth
authors.forename: János
authors.initials: J
authors.lastname: Magyar
authors.forename: Tamás
authors.initials: T
authors.lastname: Bányász
authors.forename: Péter P
authors.initials: PP
authors.lastname: Nánási
authors.forename: Norbert
authors.initials: N
authors.lastname: Szentandrássy
mesh_terms: D000818:Animals; D002121:Calcium Channel Blockers / Q000008:administration & dosage / Q000494:pharmacology*; D020746:Calcium Channels, L-Type / Q000187:drug effects* / Q000378:metabolism; D004285:Dogs; D006863:Hydrogen-Ion Concentration; D020128:Inhibitory Concentration 50; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000378:metabolism; D010084:Oxidation-Reduction; D018408:Patch-Clamp Techniques; D010766:Phosphorylation; D013779:Tetrodotoxin / Q000008:administration & dosage / Q000494:pharmacology*
chemical_list: D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D013779:Tetrodotoxin