The specific activity of platelet monoamine oxidase varies with platelet count during severe exercise and noradrenaline infusion.

During severe exercise or noradrenaline infusion in healthy male volunteers, the platelet count first rose and then fell. Platelet monoamine oxidase activity per unit protein rose and fell in parallel and the correlation with the platelet count was highly significant (P less than 0.001). The increase in observed specific monoamine oxidase activity might be due to the release of a population of platelets with higher specific activity, and the decrease to their selective removal.