Insulin- and Glucose-Induced Phosphorylation of the Na+,K+-Adenosine Triphosphatase α-Subunits in Rat Skeletal Muscle

Phosphorylation of the alpha-subunits of Na(+),K(+)-adenosine triphosphatase in response to insulin, high extracellular glucose concentration, and phorbol 12-myristate 13-acetate was investigated in isolated rat soleus muscle. All three stimuli increased alpha-subunit phosphorylation approximately 3-fold. Phorbol 12-myristate 13-acetate- and high glucose-induced phosphorylation of the alpha-subunit was completely abolished by the PKC inhibitor GF109203X, whereas insulin-stimulated phosphorylation was only partially reduced. Notably, insulin stimulation resulted in phosphorylation of the alpha-subunit on serine, threonine, and tyrosine residues, whereas high extracellular glucose or phorbol 12-myristate 13-acetate stimulation mediated phosphorylation only on serine and threonine residues. Insulin stimulation resulted in translocation of Na(+),K(+)-adenosine triphosphatase alpha(2)-subunit to the plasma membrane and increased Na(+),K(+)-adenosine triphosphatase activity in the same membrane fraction. High glucose had no effect on alpha-subunits distribution. Immunoprecipitation with antiphosphotyrosine antibody and subsequent Western blot analysis with anti-alpha(1)- and -alpha(2)-subunit antibodies revealed that both alpha(1)- and alpha(2)-subunit isoforms underwent phosphorylation on tyrosine residues in response to insulin, although with different time course and magnitude. Thus, we show that insulin-stimulated phosphorylation of Na(+),K(+)-adenosine triphosphatase alpha-subunit occurs via a PKC- and tyrosine kinase-dependent mechanism, whereas high glucose-induced phosphorylation is only PKC-dependent. Phosphorylation of Na(+),K(+)-adenosine triphosphatase alpha-subunits may be involved in regulation of Na(+),K(+)-adenosine triphosphatase activity by insulin or high extracellular glucose in skeletal muscle.