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

K⁺ released from exercisingmuscle via K⁺ channels needs to beremoved from the interstitium into the blood to maintain high musclecell membrane potential and allow normal muscle contractility. Uptakeby red blood cells has been discussed as one mechanism that would alsoserve to regulate red blood cell volume, which was found to be constantdespite increased plasma osmolality and K⁺ concentration([K⁺_pl]). We evaluatedexercise-related changes in[K⁺_pl], pH, osmolality, meancellular Hb concentration, cell water, and red blood cellK⁺ concentration during exhaustivehandgrip exercise. Unidirectional ⁸⁶Rb⁺(K⁺) uptake by red blood cellswas measured in media with elevated extracellularK⁺, osmolarity, andcatecholamines to simulate particularly those exercise-related changesin plasma composition that are known to stimulateK⁺ uptake. During exercise[K⁺_pl] increased from 4.4 ± 0.7 to 7.1 ± 0.5 mmol/l plasma water and red blood cell K⁺ concentration increased from137.2 ± 6.0 to 144.6 ± 4.6 mmol/l cell water(P  0.05), but the intracellularK⁺-to-mean cellularHb concentration ratio did not change.⁸⁶Rb⁺uptake by red blood cells was increased by ~20% on stimulation, caused by activation of theNa⁺-K⁺pump andNa⁺-K⁺-2Clcotransport. Results indicate theK⁺ content of red blood cells didnot change as cells passed the exhaustively exercising forearm muscledespite the elevated [K⁺_pl]. The tendency for an increase in intracellularK⁺ concentration was due to aslight, although statistically not significant, decrease in red bloodcell volume. K⁺ uptake, althoughelevated, was too small to move significant amounts ofK⁺ into red blood cells. Ourresults suggest that red blood cells do not contribute to the removalof K⁺ released from muscle and donot regulate their volume by K⁺uptake during exhaustive forearm exercise.