Exercise‐Induced Changes in Plasma Composition Increase Erythrocyte Na + ,K + ‐ATPase, but not Na + –K + –2Cl − Cotransporter, Activity to Stimulate net and Unidirectional K + Transport in Humans

We tested the hypothesis that exercise‐induced changes in plasma composition result in peak stimulation of erythrocyte unidirectional K + ( J K,in ) and net K + ( J K,net ) transport within the first 120 s. In experimental series 1 (7 men; 2 women), plasma [K + ] was continuously measured in vitro (37 °C) after the addition of red blood cells (RBCs) obtained from rested subjects (resting RBCs) into an exercise‐simulated plasma (ESP; increased plasma osmolality, [K + ], [H + ], [lactate] and [adrenaline] (epinephrine)), and J K,net calculated. In experimental series 2 (7 men; 4 women), resting RBCs were incubated in true exercise plasma (TEP) obtained after two 30 s bouts of high intensity leg cycling exercise to determine J K,net and J K,in (via RBC 86 Rb accumulation). J K,net of resting RBCs increased from 0.9 ± 28.7 in resting plasma to 285 ± 164 mmol (l RBCs) −1 h −1 in ESP and to 178 ± 60 mmol (l RBCs) −1 h −1 after 10 s in TEP. Both J K,net and J K,in peaked within 10 s of incubation and decreased rapidly during the initial 120 s. The use of inhibitors for the Na + ,K + ‐ATPase (ouabain) and the Na + ‐K + ‐2Cl − cotransporter (NKCC; bumetanide) indicated that rapid increases in J K,in and J K,net upon incubation of resting RBCs in TEP were due primarily to increased Na + ,K + ‐ATPase activity; the NKCC appeared to be involved only when the Na + ,K + ‐ATPase was blocked. It is concluded that RBCs rapidly increase J K,in and J K,net in response to exercise‐induced changes in plasma composition.