Stimulation of Ca efflux from fura-2-loaded platelets activated by thrombin or phorbol myristate acetate

We investigated the restoration of Ca²⁺]_i in fura-2-loaded human platelets following discharge of internal Ca²⁺ stores in the absence of external Ca²⁺. After stimulation by thrombin Ca²⁺]_i returned from a peak level of 0.6 μM to resting levels within 4 min. When ionomycin discharged the internal stores the recovery was slower with Ca²⁺]_i still elevated at around 0.5 μM after 5 min. Thrombin added shortly after ionomycin could accelerate the recovery of Ca²⁺]_i and restore resting levels within 5 min, an effect that was mimicked by phorbol-12-myristate-13-acetate (PMA). Since the continued presence of ionomycin precluded reuptake into the internal stores we conclude that thrombin and PMA stimulate Ca²⁺ efflux, perhaps via protein kinase C actions on a plasma membrane Ca²⁺ pump.