Membrane ion conductances of frog sartorius fibers as a function of tonicity

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Abstract
Membrane resistances (Rm) of frog [Rana pipiens] sartorius fibers were measured in Cl--containing (40.1 mM) and Cl-free solutions with relative tonicities of 0.5 x (hypotonic), 1 x (isotonic), 2 x (hypertonic). Cation concentrations were identical in all solutions ([Na+]o was 52.2 mM), and sucrose was used to alter tonicity. Assuming Ri to vary with tonicity, the calculated Rm values were: 21.9 (0.5 x tonicity), 27.0 (2 x), and 7.5 kilohm-cm2 (2 x) for Cl--free solutions, and 2.1 (0.5 x), 5.0 (1 x), and 6.2 kilohm-cm2 (2 x) for Cl--containing solutions. Thus, hypertonicity caused a pronounced decrease of Rm in the absence of Cl-, but caused a small increase in its presence; hypotonicity caused a decrease in both the presence and absence of Cl. Partial conductance of chloride ions (gCl) increases in hypotonic and decreases in hypertonic solutions; in contrast, gK [partial conductance of potassium ions] increases markedly in hypertonic and only slightly in hypotonic solutions; the gCl/gK ratio is highest (9.3) in hypotonic and lowest (0.2) in hypertonic solutions. Electron microscopy showed that the transverse sarcotubules swell markedly in hypertonic and shrink slightly in hypotonic solutions. If the wall of the transverse tubules is not permeable to Cl-, then the Cl- permeability of a unit area of surface membrane must increase during fiber swelling in hypotonic solutions.