Purinergic activation of rat skeletal muscle membranes increases Vmax and Na+ affinity of the Na,K-ATPase and phosphorylates phospholemman and α1 subunits.

Muscle activity is associated with an increase in extracellular purines (ATP, ADP), which are involved in signalling mechanisms. The present study investigates the effect of purines on the function of Na,K-ATPase (Na,K-pump) in rat skeletal muscle. Na,K-ATPase activity was quantified by measuring the release of inorganic phosphate in the presence of ATP and variable Na(+) concentrations. In membranes purified from glycolytic muscle fibres, purinergic stimulation increases V (max) and decreases the K (m) (higher Na(+) affinity) of the Na,K-ATPase. Stimulatory effects were obtained using ATP, ADP, 2-methylthio-ADP and UPT, but not UDP and adenosine. The effect of ADP on V (max) can be inhibited by the non-specific P2Y receptor antagonists, suramin and PPADS. Moreover, the P2Y(13) receptor antagonist MRS 2211 strongly inhibited the response to ADP, whereas the specific P2Y(1) receptor antagonist MRS 2500 had less effect. Based on results from these agonists and antagonists, we conclude that P2Y(13) receptors mediate the main effects observed, that P2Y1 receptors are also involved and that some P2Y(2)/P2Y(4) receptors also appear to be involved. Receptor antagonists had no effect on ADP-induced subunit (phospholemman and α1) phosphorylation and changes in K (m) (Na(+) affinity). Thus, the stimulatory effects of purines are mediated by two independent mechanisms: P2Y receptor-mediated increase in Na,K-ATPase capacity (increased V (max)) and P2Y receptor-independent phosphorylation of Na,K-ATPase phospholemman and α1 subunits, which induce changes in ion affinity. These mechanisms may contribute to up-regulation of Na,K-ATPase during muscle activity.