EFFECT OF EXERCISE INTENSITY ON POTASSIUM-BALANCE IN MUSCLE AND BLOODOF MAN

1. The rise of plasma [K+] during high intensity exercise is due to an intially rapid loss of K+ from the exercising muscle to the circulati on. The K+ loss is primarily governed by the balance between K+ efflux rate from the muscle cells and the reuptake rate. It has been assumed that the reuptake rate is proportional to [K+] in the femoral vein ([ K+]fv) during short-lasting uphill running, but this may not hold true for other types and durations of exercise. 2. In four subjects, initi al rates of increase and decay of [K+]fv at start and end of bicycle e xercise were quantified by means of K+-sensitive electrodes inserted i nto the femoral vein. Responses to exercise intensities between 90 and 440 W were examined. Both the initial rate of rise and the rate of de cay of [K+]fv were linearly related to power. 3. In six subjects, exer cising at 60, 85 and 110 % of maximal oxygen uptake, blood was obtaine d from the femoral artery and vein. The veno-arterial concentration di fference for K+ across the exercising leg decayed with half-times of a bout 3 min at all exercise levels and became not significantly differe nt from zero at low powers. This fits with a good match between K+ eff lux and reuptake rates at the cellular level. 4. Arterial plasma [K+] ([K+]a) rose faster with increasing exercise intensity, reaching peak values of 5.7 +/- 0.1, 6.0 +/- 0.2 and 8.0 +/- 0.2 mmol 1-1. [K+]a fel l again over the subsequent 5 min at the lowest intensity in spite of significant loss of muscle K+. Hence, released K+ was redistributed to other compartments outside the vascular bed. 5. While K+ loss increas ed linearly with increasing power, [K+]. showed a curvilinear relation ship. Thus redistribution of K+ is less efficient at high intensities. [K+]a correlated better with relative work load than with absolute wo rk load. 6. Reuptake of K+ after the end of the high intensity bout of exercise caused [K+]a to fall with a half-time of 31 s. The rate of K + reuptake in the exercising muscle was not proportional to [K+]a or [ K+]fv. However, at the level of the muscle cell, the rate of K+ reupta ke was probably inversely related to intracellular [K+].