Significance of skeletal muscle digitalis receptors for [³H]ouabain distribution in the guinea pig

The importance of specific digitalis glycoside binding sites in skeletal muscle for the digitalis glycoside distribution in the guinea pig was evaluated using [³H]ouabain and [³H]digoxin binding assays. Measurements of [³H]ouabain binding capacity (EO(max)) in gastrocnemius and heart muscles in vitro gave values of 474 ± 15 and 1,092 ± 39 pmol/g wet wt., respectively, in 4-week-old guinea pigs. Hence the total amount of [³H]ouabain binding sites in skeletal muscle and the heart was around 42,700 and 1,200 pmol, respectively. The apparent dissociation constants (K(d)) for ouabain receptor interaction was 0.7 x 10^-7 and 1.5 x 10^-7 M for skeletal muscle and heart, respectively. Comparison of [³H]ouabain and [³H]digoxin binding revealed that these drugs are competitive. From birth to maturity the concentration of [³H]ouabain binding sites in guinea pigs decreased from 803 ± 58 to 304 ± 28 pmol/g wet wt. in gastrocenmius muscle and from 1,458 ± 31 to 1,079 ± 19 pmol/g wet wt. in the heart. After i.p. injection, measurements of the distribution of [³H]ouabain in plasma, skeletal muscle and the heart showed an almost equal relative specific occupancy of digitalis glycoside receptors in skeletal muscle and the heart: When 10% of the digitalis receptors in the heart were occupied by [³H]ouabain, 13% of those in the skeletal muscles were occupied. It was calculated that 1 hr after the i.p. administration of [³H]ouabain the amount of [³H]ouabain specifically bound to the skeletal muscles and the heart corresponded to 5 times and 1/10 the amount available in the extracellular pool, respectively. Taken together the observations emphasize the importance of the skeletal muscle digitalis glycoside receptors as a major pool for the distribution of cardiac glycosides.