Relative abundance of two molecular forms of Na+,K+-ATPase in the ferret heart: developmental changes and associated alterations of digitalis sensitivity.

We have previously reported that adult ferret hearts have high affinity and low affinity ouabain-binding sites in approximately equal abundance corresponding, respectively, to alpha(+) and alpha subunits of Na+,K+-ATPase. In the present study, possible changes in relative abundance of the two molecular forms of Na+,K+-ATPase during postnatal development were examined. In 5-day-old or 7- to 10-day-old ferret heart, there was a predominance of the alpha form of Na+,K+-ATPase, the alpha(+)/alpha ratio being approximately 1:7 and 1:4 at 5 and 7-10 days, respectively, in contrast to a 1:1 ratio observed previously in adult heart. Corresponding to the reduced number of high affinity ouabain-binding sites associated with the alpha(+) form of Na+,K+-ATPase, ouabain sensitivity of the Na+,K+-ATPase isolated from newborn ferret heart was lower than that of adult ferret heart. Furthermore, the low dose positive inotropic effect, which was apparent in the adult ferret heart, could not be observed in the newborn ferret heart. The present finding may represent a plausible explanation for low digitalis sensitivity of neonatal heart observed in several mammalian species.