Changes in plasma and erythrocyte K+ during hypercapnia and different grades of exercise in trout

Trout were exposed to hypercapnia, two levels of aerobic exercise, or three successive periods of supramaximal exercise to evaluate the effects on erythrocyte and plasma K+. During aerobic exercise, plasma K+ increased slightly with the intensity of work, while no change was found in the erythrocyte K+ content. In contrast, both hypercapnia and supramaximal exercise induced a net erythrocyte K+ uptake. This uptake changed to a net loss of K+ as arterial pH and hemoglobin-bound oxygen saturation returned to control values during recovery. The maximal rates of net K+ uptake found during hypercapnia and supramaximal exercise corresponded to 195 and 350 mumol.kg fish-1.h-1, respectively, and the maximal rates of net K+ loss found during recovery corresponded in both cases to approximately 130 mumol.kg fish-1.h-1. Hypercapnia had only a minor effect on plasma K+, but return to normocapnic conditions induced a 0.8 mM rise in plasma K+. Of this increase, approximately 70% could be accounted for by the simultaneous net release of erythrocyte K+. Each period of supramaximal exercise induced an elevated plasma K+ level, resulting in accumulation of plasma K+ despite slight decreases in plasma K+ in between the exercise periods. At the same time the net erythrocyte K+ uptake caused an estimated reduction in plasma K+ of 1.5 mM. It is concluded that both hypercapnia and supramaximal exercise cause profound net changes in the erythrocyte K+ content with significant effects on plasma K+.