The uptake of cardiac glycosides by intestinal smooth muscle of the guinea‐pig in relation to digitalis receptors

Summary 1 . The accumulation and release of 3H‐digitoxin, 3H‐digoxin and 3H‐ouabain by isolated guinea‐pig intestinal smooth muscle has been studied and compared with a pharmacological action due to inhibition of the sodium pump. 2 . The uptake of labelled cardiac glycosides can be described by means of an exponential function. The t of uptake was similar for the three compounds and did not depend on the concentration. 3 . Analysis of the curve relating the uptake of cardiac glycosides at equilibrium to the bath concentration enabled a non‐saturable and a saturable binding site to be distinguished. 4 . In contrast to the uptake observations, the onset of the pharmacological effect was dependent on the concentration, and furthermore the t½; for this effect was shorter. 5 . The release of cardiac glycosides proceeded more slowly than the uptake. 6 . The uptake of a labelled glycoside was reduced in the presence of another glycoside. The amount of displaceable glycoside was nearly equivalent to the capacity of the saturable binding site. 7 . The significance of these results is discussed.