Frog toe muscles were bathed in isotonic, sodium-free Tris chloride, methanesulfonate, or sulfate solutions containing sucrose or mannitol and varying in ionic strength from 0.006 to 0.291. By decreasing the ionic strength the curve relating the peak tension of the K contractures to the log [K] was reversibly shifted to lower [K]. Increasing the [Ca] from 1 to 4 mM almost abolished this effect. The resting uptake of 45Ca was increased more than two times by decreasing the ionic strength from 0.125 to 0.039. It was not increased significantly by raising [Ca] from 1 to 4 mM at low or normal ionic strength. The additional uptake of 45Ca during contractures provoked by 120 mM K was not significantly different at the two levels of ionic strength. The rate of emergence of 45Ca from muscles loaded with 45Ca at reduced ionic strength, was decreased. The effects of low ionic strength are discussed in terms of changes in the potential difference across a membrane with fixed negative charges on the outer surface.

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