Unexpected changes in the catecholamine content of platelets and plasma during exercise

Catecholamines are retained within platelets for several hours after plasma catecholamine concentrations have returned to baseline. To determine whether platelet catecholamine concentrations may provide an index of short-term elevations in plasma adrenaline (A) and noradrenaline (NA), the response of plasma and platelet catecholamines to an interval supramaximal, Max (107% VO(2) Max), and submaximal, Submax (37% VO(2) Max), cycling protocol was examined in seven healthy male volunteers, 22-34 years. Despite large rises in plasma NA and A in the Max study (12- and 8-fold increases above baseline, respectively) and smaller rises in the Submax study, the baseline platelet concentrations of A and NA fell significantly in the first 15 min of exercise in both groups. This fall was greater in the SubMax protocol. Catecholamine concentrations then increased slowly in the second half of exercise, but never returned to baseline. The circulating platelet count almost doubled during the exercise period, increasing from 308 to 569 X 10(3) platelets/ml plasma in both studies, returning close to baseline in recovery. These results indicate that at the beginning of exercise there is large rise in plasma catecholamines and the circulating platelet count, with a fall in the platelet catecholamine concentrations. This suggests that a sequestered platelet population, free of catecholamines, is released at the beginning of exercise. This release most probably occurs from the spleen. If this is the case, the reason for a propagation of platelets in the spleen, free of catecholamines, requires further investigation.