Effect of tumour-promoting phorbol ester on calcium homeostasis in human platelets.

The more interesting features of the effects or PMA on Ca2+]i and ATP release were the following: 1. preincubation with PMA inhibited thrombin-evoked calcium transients; 2. PMA stimulated slightly the release of calcium and ATP whereas inhibited calcium and ATP pools sensitive to thrombin; 3. A23187 reversed the inhibitory effect of PMA; 4. subsaturating thrombin concentrations gave results similar to PMA on thrombin-induced calcium and ATP release but not on Ca2+]i.