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Assessment of Na+/K+ ATPase Activity in Small Rodent and Human Skeletal Muscle Samples

JANNAS-VELA, SEBASTIAN1,2; BROWNELL, STUART1; PETRICK, HEATHER L.1; HEIGENHAUSER, GEORGE J. F.3; SPRIET, LAWRENCE L.1; HOLLOWAY, GRAHAM P.1

Medicine & Science in Sports & Exercise: November 2019 - Volume 51 - Issue 11 - p 2403–2409
doi: 10.1249/MSS.0000000000002063
SPECIAL COMMUNICATIONS: Methodological Advances
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Introduction In skeletal muscle, the Na+/K+ ATPase (NKA) plays essential roles in processes linked to muscle contraction, fatigue, and energy metabolism; however, very little information exists regarding the regulation of NKA activity. The scarcity of information regarding NKA function in skeletal muscle likely stems from methodological constraints, as NKA contributes minimally to total cellular ATP utilization, and therefore contamination from other ATPases prevents the assessment of NKA activity in muscle homogenates. Here we introduce a method that improves accuracy and feasibility for the determination of NKA activity in small rodent muscle samples (5–10 mg) and in human skeletal muscle.

Methods Skeletal muscle homogenates from mice (n = 6) and humans (n = 3) were used to measure NKA and sarcoplasmic reticulum Ca2+ ATPase (SERCA) activities with the addition of specific ATPase inhibitors to minimize “background noise.”

Results We observed that myosin ATPase activity was the major interfering factor for estimation of NKA activity in skeletal muscle homogenates, as the addition of 25 μM of blebbistatin, a specific myosin ATPase inhibitor, considerably minimized “background noise” (threefold) and enabled the determination of NKA maximal activity with values three times higher than previously reported. The specificity of the assay was demonstrated after the addition of 2 mM ouabain, which completely inhibited NKA. On the other hand, the addition of blebbistatin did not affect the ability to measure SERCA function. The coefficient of variation for NKA and SERCA assays were 6.2% and 4.4%, respectively.

Conclusion The present study has improved the methodology to determine NKA activity. We further show the feasibility of measuring NKA and SERCA activities from a common muscle homogenate. This methodology is expected to aid in our long-term understanding of how NKA affects skeletal muscle metabolic homeostasis and contractile function in diverse situations.

1Department of Human Health and Nutritional Sciences, University of Guelph, Guelph, Ontario, CANADA

2Exercise Science Laboratory, Faculty of Medicine, School of Kinesiology, Universidad Finis Terrae, Santiago, CHILE

3Department of Medicine, McMaster University, Hamilton, Ontario, CANADA

Address for correspondence: Sebastian Jannas-Vela, Ph.D., Human Health and Nutritional Sciences, University of Guelph, 50 Stone Rd. East, Guelph, Canada N1G 2W1; E-mail: sjannas@uoguelph.ca.

Submitted for publication October 2018.

Accepted for publication June 2019.

Online date: June 7, 2019

© 2019 American College of Sports Medicine