Effect of prolonged physical exercise on intra-erythrocyte and plasma potassium

Summary The intracellular concentrations of sodium [Na⁺] and potassium [K⁺] and the water content in human erythrocytes were investigated in 21 male runners before and after a marathon. From 2 to 5 min after the race, the intra-erythrocyte [K⁺] was significantly decreased (p<0.001) by 7% whereas the plasma [K⁺], intra-erythrocyte [Na⁺] and the erythrocyte water content were unchanged. The change in the intra-erythrocyte [K⁺] observed immediately after the marathon, was negatively correlated with the race time (r=−0.44;p<0.05). Furthermore, the change in the plasma [K⁺] (r=−0.64;p<0.001) and the amount of K⁺ excreted in the urine during the race (r=0.54;p<0.05) were also, respectively, negatively and positively correlated with the race time. It is concluded that during prolonged physical exercise the erythrocytes could serve as a kind of K⁺ reservoir that is drained with increasing magnitude of body K⁺ loss. This might explain why in the faster marathon runners, in whom the urinary K⁺ loss is smaller and the K⁺ intake is greater than in the slower runners during race, the intra-erythrocyte [K⁺] is unchanged after a marathon whereas in the slower runners it is decreased.