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 (4x4 min at 90 to 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 to 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/J) was unchanged. Muscle NKA content increased by 11% after IIT (p < 0.05). Single fiber measurements, increased in NKA alpha2 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 alpha2 abundance in type II fibers, coincident with increased muscle NKA content and enhanced exercise performance.

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