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Contracting muscle mass and inactive muscle effects on K+ dynamics during exercise

M.J. McKenna1, S. Sostaric1, C.A. Goodman1, X. Gong1, J. Aw2, J. Leppik1, C.H. Steward1, S.F. Fraser3, H. Krum2, R.J. Snow4, M.J. Brown5, 1Human Movement Recreation And Performance, Victoria University of Technology, Melbourne, Australia, 2Epidemiology and Preventive Medicine, Monash University, Alfred Hospital, Melbourne, VIC, Australia, 3Medical Sciences, RMIT University, Melbourne, Australia, 4Exercise Science and Nutrition, Deakin University, Melbourne, VIC, Australia, 5Dept. Anaesthesia, Austin and Repatriation Medical Centre, Melbourne, VIC, Australia

Little is known about contracting muscle mass and inactive muscle effects on K+ dynamics during exercise. We investigated differences in arterial plasma [K+] and in the arterio-venous [K+] difference across the forearm, during small muscle mass exercise with the forearm finger flexors and large muscle mass exercise during two-legged cycling.

Eight healthy males gave written informed consent. Concentric, dynamic forearm finger flexor contractions were conducted on a custom-built ergometer and comprised three 1-min bouts, then a final bout to fatigue, at their peak incremental finger flexion workrate. After 2 h, subjects underwent two-legged cycling exercise comprising 10 min at each of 33% and 67% VO2peak, then to fatigue at 90% VO2peak. Radial arterial (a) and deep antecubital venous (v) blood was sampled simultaneously at rest, before and during each exercise bout and in recovery, for both exercise tests. Plasma [K+] was analysed using a K+-selective electrode.

During finger flexion exercise [K+]a rose only 0.29 ± 0.03 mM to peak at 4.19 ± 0.07 mM at fatigue. The wide negative [K+]a-v difference during exercise bouts reverted to resting levels post-exercise, and to a positive [K+]a-v in recovery (see Figure). During leg exercise the rise in [K+]a at fatigue was ∼9-fold greater (2.69 ± 0.28 mM, P<0.001), but the peak of 6.66 ± 0.26 mM was only 59% greater than during finger flexion (P<0.001). A positive [K+]a-v difference across the resting forearm occurred during leg exercise, being 0.39 ± 0.04, 0.70 ± 0.08 and 1.50 ± 0.15 mM, at 33%, 67% and 90% VO2peak, respectively.


Thus despite a large net K+ release from contracting muscle, [K+]a barely rose above rest during small muscle mass exercise. During large muscle mass exercise, [K+]a increased markedly, constrained by K+ uptake into inactive muscle.

Funded by NH&MRC