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

• Published in: American Journal of Physiology: Cell Physiology 2007-09, Vol.293 (3), p.C967-C977
• Main Authors: Dutka, T. L, Lamb, G. D
• Format: Article
• Language: English
• Subjects: Adenosine triphosphatase; Adenosine Triphosphate - metabolism; Animals; Chlorides - pharmacology; Energy Metabolism - drug effects; Energy Metabolism - physiology; Glyburide - pharmacology; Glycolysis - drug effects; Glycolysis - physiology; Hypoglycemic Agents - pharmacology; Kinases; Male; Muscle Contraction - drug effects; Muscle Contraction - physiology; Muscle Fatigue - physiology; Muscle Fibers, Skeletal - metabolism; Muscle Fibers, Skeletal - ultrastructure; Muscle, Skeletal - cytology; Muscle, Skeletal - metabolism; Musculoskeletal system; Phosphoenolpyruvate - pharmacology; Potassium; Pyruvate Kinase - pharmacology; Rats; Rats, Long-Evans; Rodents; Sodium; Sodium-Potassium-Exchanging ATPase - metabolism
• ISSN: 0363-6143; 1522-1563
• DOI: 10.1152/ajpcell.00132.2007

Department of Zoology, La Trobe University, Melbourne, Victoria, Australia Submitted 1 April 2007 ; accepted in final form 31 May 2007 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 Ca 2+ 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 Address for reprint requests and other correspondence: T. L. Dutka, Dept. of Zoology, La Trobe Univ., Melbourne 3086, Victoria, Australia (e-mail: t.dutka{at}latrobe.edu.au )