RNA from heart of young and old rats leads to the expression of protein(s) in Xenopus oocytes that alter the transport activity of rat Na+,K+-ATPases differently

To address the question of whether the function of Na+,K+-ATPases differs in the heart of young and old rats, enzymes formed from the alpha1 or alpha2 isoform with the beta1 subunit of rat were expressed in Xenopus oocytes. In addition to injections of the cRNA coding for the respective subunits, oocytes were co-injected with total RNA from the left ventricle of young or old rats. To assess alterations in transport activity due to the co-injections, ouabain-sensitive 86Rb+ uptake was measured. Co-injection of the RNA from young rats led to 31% inhibition of 86Rb+ uptake into oocytes with the alpha1/beta1 pumps while uptake into oocytes with the alpha2/beta1 pumps was hardly affected. Co-injection of the RNA from old rats, on the other hand, reduced 86Rb+ uptake only in cells with the alpha2/beta1 isoform (to 85%). The steady-state current generated in the absence of external Na+ by the alpha1/beta1 ATPase was significantly reduced by co-injection of RNA only from young rats to 70%, and this inhibition was hardly affected by membrane potential. For the alpha2/beta1 ATPase co-injection of RNA only from old rats also led to a significant reduction of pump-mediated current at potentials more negative than -70 mV to 70-80%. In the presence of Na+, inhibition of the alpha1 isoform by co-injection of RNA from young rats is voltage-dependent, increasing with more negative potentials. For the alpha2/beta1 pump, co-injection of RNA from old rats was no longer effective, but voltage-dependent inhibition by co-injection of RNA from young rats became apparent. The data indicate that changes in protein expression occurring in young and old rat hearts may modulate transport activity of the Na+,K+-ATPase and this modulation depends on membrane potential and the presence of external Na+. We propose that the described mechanisms may play a functional role in working myocardium, and may form a basis for processes involved in heart aging.