Red blood cells do not contribute to removal of K1 released from exhaustively working forearm muscle

Maassen, N., M. Foerster, and H. Mairbäurl. Red blood cells do not contribute to removal of K1 released from exhaustively working forearm muscle. J. Appl. Physiol. 85(1): 326–332, 1998.—K1 released from exercising muscle via K1 channels needs to be removed from the interstitium into the blood to maintain high muscle cell membrane potential and allow normal muscle contractility. Uptake by red blood cells has been discussed as one mechanism that would also serve to regulate red blood cell volume, which was found to be constant despite increased plasma osmolality and K1 concentration ([Kpl ]). We evaluated exercise-related changes in [Kpl ], pH, osmolality, mean cellular Hb concentration, cell water, and red blood cell K1 concentration during exhaustive handgrip exercise. Unidirectional 86Rb1 (K1) uptake by red blood cells was measured in media with elevated extracellular K1, osmolarity, and catecholamines to simulate particularly those exercise-related changes in plasma composition that are known to stimulate K1 uptake. During exercise [Kpl ] increased from 4.4 6 0.7 to 7.1 6 0.5 mmol/l plasma water and red blood cell K1 concentration increased from 137.2 6 6.0 to 144.6 6 4.6 mmol/l cell water (P # 0.05), but the intracellular K1-to-mean cellular Hb concentration ratio did not change. 86Rb1 uptake by red blood cells was increased by ,20% on stimulation, caused by activation of the Na1-K1 pump and Na1-K1-2Cl2 cotransport. Results indicate the K1 content of red blood cells did not change as cells passed the exhaustively exercising forearm muscle despite the elevated [Kpl ]. The tendency for an increase in intracellular K1 concentration was due to a slight, although statistically not significant, decrease in red blood cell volume. K1 uptake, although elevated, was too small to move significant amounts of K1 into red blood cells. Our results suggest that red blood cells do not contribute to the removal of K1 released from muscle and do not regulate their volume by K1 uptake during exhaustive forearm exercise.