Effect of veratridine on membrane potential of sartorius muscle from Rana pipiens

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Abstract
The effect of veratridine on the membrane potential of sartorius muscles from Rana pipiens was studied. Membrane potential (Vm) was measured in Ringer solutions containing 2.5, 10, 30, 75, and 190 mM K+ in the absence and presence of veratridine. The product [K]o[Cl]o was kept at 300 mM2 to maintain Donnan equilibrium. External Na+ was lowered to 10 mM. Ouabain (100 microM) was present in all solutions. Vm vs. log [K]o curves were fit using the Goldman-Hodgkin-Katz equation with a single free parameter, alpha = PNa/PK (permeability ratio of Na to K). Veratridine (100 microM) causes alpha to increase 12.6 +/- 1.2-fold (n = 9), from 0.146 to 1.657. The effect of veratridine on Vm is dose dependent and reversible, with a time constant for washout of 40 min. The depolarization produced by veratridine is prevented by tetrodotoxin and by Mg, is sensitive to external Na concentration, and is insensitive to curare.