Interstitial anion distribution in striated muscle determined with [35S]sulfate and [3H]sucrose

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.