Studies on the kinetics of (3H)-ouabain uptake and exchange in the isolated papillary muscle of the guinea-pig.

1 The uptake, wash-out and exchange of [3H]-ouabain was studied in isolated, resting, and electrically stimulated papillary muscles of the guinea-pig. 2 At the equilibrium level of uptake, a different tissue/medium ratio was obtained for each of the concentrations used, i.e. 3.4, 1.8 and 0.82 for 1 times 10-minus 7M, 7.7 times 10-minus 7M, and 5 times 10-minus 6M ouabain, respectively. Equilibrium was reached more rapidly at high concentrations of ouabain. 3 The maximum number of binding sites for ouabain was estimated to be 1 times 10-15 binding sites/g wet weight. 4 No difference in [3H]-ouabain uptake could be detected between resting and electrically stimulated papillary muscles. 5 The kinetics of the ouabain uptake, wash-out and exchange are discussed. The results suggest that there is a saturable compartment in papillary muscle which can best be demonstrated if low concentrations of ouabain are used. Because of its small size, the saturable compartment submerges in the process of the filling of the extracellular space at high concentrations (5 times 10-minus 6M).