Dependence of membrane potential on extracellular ionic concentrations in myotonic goats and rats

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Abstract
Intracellular K concentration ([K]i) and the resting Na to K permeability ratio (.alpha. = PNa/PK) were determined by recording resting potentials at 25.degree. C in 6 solutions with variable K+ concentrations, constant [K+] + [Na+] and constant [K+] .times. [Cl-] product and fitting the results by nonlinear least squares to the Goldman-Hodgkin-Katz equation. Fits to models including Cl- permeability terms suggest that Cl- is in Donnan equilibrium in the muscles studied. External intercostal muscle was biopsied from anesthetized normal goats and goats with hereditary myotonia. In normal goat muscle, .alpha. was 0.012 .+-. 0.001 (mean .+-. SE, n = 11) and [K]i was 133 .+-. 6 mM (n = 9); myotonic goat muscle did not differ from this. Extensor digitorum longus (EDL) muscle was removed from anesthetized male Wistar rats and from like rats pretreated with a single large dose of 20,25-diazacholesterol (DAC). In normal rat EDL, .alpha. was 0.019 .+-. 0.002 (n = 12), and [K]i was 142 .+-. 4 mM (n = 12). In the 16 day DAC-treated rats, .alpha. was significantly reduced (P < 0.002) to 0.010 .+-. 0.002 (n = 8), and [K]i was slightly reduced by DAC. DAC apparently reduced PNa in rat fibers. Tetrodotoxin at 5 .mu.M did not affect either parameter in the rats.