Na+-K+ pumps in the transverse tubular system of skeletal muscle fibers preferentially use ATP from glycolysis

The Na⁺-K⁺ pumps in the transverse tubular (T) system of a muscle fiber play a vital role keeping K⁺ concentration in the T-system sufficiently low during activity to prevent chronic depolarization and consequent loss of excitability. These Na⁺-K⁺ pumps are located in the triad junction, the key transduction zone controlling excitation-contraction (EC) coupling, a region rich in glycolytic enzymes and likely having high localized ATP usage and limited substrate diffusion. This study examined whether Na⁺-K⁺ pump function is dependent on ATP derived via the glycolytic pathway locally within the triad region. Single fibers from rat fast-twitch muscle were mechanically skinned, sealing off the T-system but retaining normal EC coupling. Intracellular composition was set by the bathing solution and action potentials (APs) triggered in the T-system, eliciting intracellular Ca²⁺ release and twitch and tetanic force responses. Conditions were selected such that increased Na⁺-K⁺ pump function could be detected from the consequent increase in T-system polarization and resultant faster rate of AP repriming. Na⁺-K⁺ pump function was not adequately supported by maintaining cytoplasmic ATP concentration at its normal resting level (8 mM), even with 10 or 40 mM creatine phosphate present. Addition of as little as 1 mM phospho(enol)pyruvate resulted in a marked increase in Na⁺-K⁺ pump function, supported by endogenous pyruvate kinase bound within the triad. These results demonstrate that the triad junction is a highly restricted microenvironment, where glycolytic resynthesis of ATP is critical to meet the high demand of the Na⁺-K⁺ pump and maintain muscle excitability. muscle fatigue; sodium-potassium-adenosinetriphosphatase; excitation-contraction coupling; T-system; excitability