Cellular Cl content and concentration of amphibian skeletal and heart muscle

Toads (Bufo marinus) and frogs (Rana pipiens pipiens) were given intraperitoneal injections of Na36Cl and Na235SO4. After in vivo equilibration for 20--180 min, the animals were pithed, and their ventricular and semitendinosus muscles were excised. Measurements of total Cl (by titrimetry) and 36Cl (by radioassay) showed that specific radioactivities of plasma and mus les approached equality within 1 h after injection for toad skeletal and heart muscle and frog ventricles, indicating complete exchange of cellular Cl with 36Cl. From the simultaneously measured muscle water contents and 35SO4 spaces, intracellular Cl concentrations in vivo (in mumol/g cell water) for semitendinosus and ventricular muscles were calculated to be, respectively, 1.4 +/- 0.3 and 2.3 +/- 0.8 for Bufo and 1.7 +/- 0.7 and 4.8 +/- 2.4 for Rana. In view of these low values, active cellular Cl accumulation seems improbable, but cannot be rigorously excluded without simultaneous membrane potential and intracellular ion activity measurements. A high concentration of Cl in the sarcoplasmic reticulum of skeletal muscle is also inconsistent with these measurements.