Increase in Serum Ionized Calcium during Exercise

1. Normal subjects showed an average increase in serum ionized calcium (Ca 2+ ) concentration of 0·11 mmol/l in peripheral venous blood 10 min after onset of bicycle exercise at 70% of maximum aerobic capacity. The corresponding mean rise in serum total calcium concentration was 0·21 mmol/l. 2. The change in serum Ca 2+ as result of acidification was studied in 20 normal subjects by carbon dioxide equilibration in vitro followed by measurement of serum Ca 2+ . The log serum Ca 2+ was inversely proportional to serum pH. 3. The Δlog serum Ca 2+ /ΔpH in vitro was similar to the Δlog serum Ca a+ /ΔpH in vivo during exercise, this ratio, however, being somewhat greater during the first minute of exercise. 4. Serum Ca 2+ returned to normal values about 20 min after stopping exercise as the pH returned to normal, but the fall immediately after stopping exercise was more pronounced than that due to the change in pH, as predicted from the studies in vitro. 5. Blood lactate concentration rose from 0·86 to 8·41 mmol/l after 10 min exercise, but the rise in blood lactate during exercise was slower than the rise in serum Ca 2+ . Also the fall during the recovery period was delayed compared with the fall in serum Ca 2+ . 6. It is suggested that the rise in serum Ca 2+ during severe muscular exercise might be important for the physiological adaptations during work, and for bone metabolism.