Fragmented sarcoplasmic reticulum (FSR) membranes isolated from rabbit skeletal muscle are impermeable to inulin-14C (mol wt 5,000), and dextran-14C (mol wt 15,000–90,000) at pH 7.0–9.0, yielding an excluded space of 4–5 µl/mg microsomal protein. In the same pH range urea and sucrose readily penetrate the FSR membrane. EDTA or EGTA (1 mM) increased the permeability of microsomes to inulin-14C or dextran-14C at pH 8–9, parallel with the lowering of the FSR-bound Ca++ content from initial levels of 20 nmoles/mg protein to 1–3 nmoles/mg protein. EGTA was as effective as EDTA, although causing little change in the Mg++ content of FSR. The permeability increase caused by chelating agents results from the combined effects of high pH and cation depletion. As inulin began to penetrate the membrane there was an abrupt fall in the rate of Ca++ uptake and a simultaneous rise in ATPase activity. At 40°C inulin penetration occurred at pH 7.0 with 1 mM EDTA and at pH 9.0 without EDTA, suggesting increased permeability of FSR membranes. This accords with the higher rate of Ca++ release from FSR at temperatures over 30°C. The penetration of microsomal membranes by anions is markedly influenced by charge effects. At low ionic strength and alkaline pH acetate and Cl are partially excluded from microsomes when applied in concentrations not exceeding 1 mM, presumably due to the Donnan effect. Penetration of microsomal water space by acetate and Cl occurs at ionic strengths sufficiently high to minimize charge repulsions.

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