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