Species and ionic influences on the accumulation of digitalis glycosides by isolated perfused hearts.

1. The ability of isolated perfused guinea-pig (digitalis-sensitive species) and rat (digitalis-resistant species) hearts to accumulate radio-labelled digitalis glycosides was studied in relation to the ionic composition of the perfusion medium.2. It was observed that in both species much less digoxin was accumulated than was digitoxin or proscillaridin.3. The accumulation of digoxin was markedly inhibited in a low sodium or in high potassium medium. These effects were similar, but relatively less marked, with digitoxin and proscillaridin. Calcium and magnesium removal had relatively smaller effects on the accumulation of both polar and non-polar glycosides.4. The low accumulation of all digitaloids by the rat heart in comparison to the guinea-pig heart may be due to the formation of unstable complexes between the cellular membranes in the rat heart and the various digitaloids used in this study. Although digitaloids have a reduced affinity for rat hearts and rat heart membranes in comparison to guinea-pigs, the order of the accumulation of different glycosides in both species is the same, i.e. much less with polar glycosides than with non-polar glycosides.5. It was concluded that non-polar glycosides such as digitoxin and proscillaridin demonstrate the same ion-dependent accumulation mechanism as do the more polar glycosides such as digoxin and ouabain. In addition, the non-polar glycosides possess high capacity for ion-independent binding presumably due to lipophilic interactions with membranes.