Intense interval training in healthy older adults increases skeletal muscle [3H]ouabain-binding site content and elevates Na+,K+-ATPase α2 isoform abundance in Type II fibers.

Victoria L. Wyckelsma,Michael J. McKenna,Mitchell J. Anderson,Aaron C. Petersen,Ben D. Perry,Robyn M. Murphy,Christopher P. Hedges,Itamar Levinger

doi:10.14814/phy2.13219

Victoria L. Wyckelsma, Michael J. McKenna + Show 6 more

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https://doi.org/10.14814/phy2.13219

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Abstract

Young adults typically adapt to intense exercise training with an increased skeletal muscle Na+,K+‐ATPase (NKA) content, concomitant with reduced extracellular potassium concentration [K+] during exercise and enhanced exercise performance. Whether these changes with longitudinal training occur in older adults is unknown and was investigated here. Fifteen older adults (69.4 ± 3.5 years, mean ± SD) were randomized to either 12 weeks of intense interval training (4 × 4 min at 90–95% peak heart rate), 3 days/week (IIT, n = 8); or no exercise controls (n = 7). Before and after training, participants completed an incremental cycle ergometer exercise test until a rating of perceived exertion of 17 (very hard) on a 20‐point scale was attained, with measures of antecubital venous [K+]v. Participants underwent a resting muscle biopsy prior to and at 48–72 h following the final training session. After IIT, the peak exercise work rate (25%), oxygen uptake (16%) and heart rate (6%) were increased (P < 0.05). After IIT, the peak exercise plasma [K+]v tended to rise (P = 0.07), while the rise in plasma [K+]v relative to work performed (nmol.L−1.J−1) was unchanged. Muscle NKA content increased by 11% after IIT (P < 0.05). Single fiber measurements, increased in NKA α2 isoform in Type II fibers after IIT (30%, P < 0.05), with no changes to the other isoforms in single fibers or homogenate. Thus, intense exercise training in older adults induced an upregulation of muscle NKA, with a fiber‐specific increase in NKA α2 abundance in Type II fibers, coincident with increased muscle NKA content and enhanced exercise performance.

Highlights

Sustained skeletal muscle contractions require repeated action potential (AP) propagation along sarcolemmal and t-tubular membranes
We report that 12 weeks of intense interval training (IIT) increased muscle NKA content by 11% in older adults and that fiber-type specific upregulation occurred for the NKA a2 isoform, being elevated by 30% in Type II fibers after IIT
While we found that IIT increased VO2peak by 16% and peak workrate by 11%, there was no significant reduction in the rise in venous [K+] relative to work performed during incremental exercise, suggesting that at least in antecubital venous blood, IIT did not improve circulating K+ homeostasis

Summary

Introduction

Sustained skeletal muscle contractions require repeated action potential (AP) propagation along sarcolemmal and t-tubular membranes. The Na+,K+-ATPase (NKA) protein is critical in regulating these Na+/K+ fluxes in skeletal muscle and acts to preserve muscle membrane excitability and contractile function (McKenna et al 2008; Clausen 2013). Three NKA isoforms are expressed for each of the catalytic a (a1-3) and the regulatory b (b1-3) subunits (Murphy et al 2004), with a c subunit known as FXYD1 is an important regulator of NKA activity (Bibert et al 2008). The b1 isoform is critical in NKA integration into the cell membrane (Geering 2001) and plays a key role in regulating NKA enzymatic activity (Lavoie et al 1997; Blanco and Mercer 1998)

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