Evidence for K+ and Cl− Binding Inside Muscle from Diffusion Studies

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Abstract
Intrafiber diffusion of 42K+, 36Cl−, and 14C-sorbitol was measured along the longitudinal axis of a single muscle fiber which had been placed inside the lumen of a glass capillary at least 24 h beforehand. The mean diffusion coefficients (× 10−5 cm2/s) in myoplasm at 10 °C and at pH 7.5 were 0.728 ± 0.008, 0.683 ± 0.006, and 0.216 ± 0.005 for K+, Cl−, and sorbitol, respectively. The K+ coefficient decreased, the Cl− coefficient increased, and the sorbitol coefficient remained unchanged as the pH of the muscle-capillary preparation was increased. By applying Wang's theory to explain diffusion in polyelectrolyte solutions (1954), we have estimated the diffusible volume (1 − ϕ) and the binding fractions (ƒK and ƒCl) of K+ and Cl− in myoplasm. From pH 5.2 to 10, ƒK varied from 0 to 0.13 and ƒCl varied from 0.13 to 0. Analysis of this K+ diffusion data along with the Na+ diffusion data from an earlier study (Can. J. Physiol. Pharmacol. 50, 228–237, 1972) leads to the prediction that myoplasm at physiological pH has a minimum binding capacity for Na+ and K+ of about 70 mmol/kg dry weight and a selectivity of 3.3 for Na+ over K+. Estimations of the diffusible volume ranged from 0.7 to 0.8, indicating that probably all the intrafiber water (74–78% by weight) is being utilized in the diffusion process.

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