Stimulation of Ca2+ 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 microM 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 microM 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.