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.

The distributions of a charged and an uncharged extracellular tracer in the interstitial spaces of skeletal and heart muscles were examined in vivo by a double-labeling technique. 35SO4(2-) and [3H]sucrose were simultaneously injected intraperitoneally into rats and toads, and extracellular volume was determined in the rat gastrocnemius and left ventricle and in the toad semitendinosus. In nephrectomized rats and in toads with intact kidneys, sucrose and SO4(2-) spaces were constant for several hours. Sucrose and SO4(2-) spaces did not significantly differ in rat ventricle (P greater than 0.80); in rat gastrocnemius the sucrose space was much larger than SO4(2-) space (2P less than 0.0005), while in toad semitendinosus sucrose space was somewhat smaller than SO4(2-) space (2P less than 0.005). These observations suggest that fixed charges in the interstitial compartment can lead to extracellular anion exclusion in some tissues and perhaps to accumulation in others. The magnitude and direction of these effects differ for different striated muscles.

Studies were done to investigate the transepithelial current-voltage (IT-VT) relationships of urinary bladder and colon of the toad Bufo marinus. Like several other Na transporting epithelia, the IT-VT plots characteristically showed a break at voltage E1, averaging near 124 mV for urinary bladder and 110 mV for colon. With bladders treated with antidiuretic hormone, estimates of ENa and shunt resistance, Rs, were obtained according to a method outlined by Yonath and Civan, 1971 (J Membr. Biol. 5:336-385). Our results not only confirmed their observations, but were consistent with the notion that the values of E1 (IT-VT plots) were the same as those of ENa. In addition, the values of Rs were found to be the same as those estimated from the quotient E1/I1 obtained from the voltage and current coordinates at the break of the IT-VT plot of bladders studied in both stretched and unstretched states. Amiloride at concentrations up to 10(-5) M caused a small decrease of both E1 and E1/I1 of urinary bladder. Similarly, amiloride caused small but significant changes of ENa and RNa of the colon. For both epithelia, the values of E1 and E1/I1 of the IT-VT plots were the same as those of ENa and Rs estimated by an independent method. In general, these findings are similar to those of several other epithelia where the ENa and Rs can be estimated directly from their IT-VT relationships.

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