Effect of calcium concentration and inhibitors on the responses of platelets stimulated with collagen: Contrast between human and rabbit platelets

1. Variations in the concentration of Ca2+ [Ca2+] in the suspending medium have different effects on the responses of human and rabbit platelets to collagen. 2. When rabbit platelets are stimulated with a low concentration of collagen (0.5 micrograms/ml), aggregation, release of granule contents, and formation of thromboxane are maximal when the suspending medium contains [Ca2+] in the physiological range (0.5-2.0 mM), and very slight in a medium with no added Ca2+. 3. In contrast, human platelets respond most strongly when the suspending medium contains no added Ca2+ [( Ca2+] approx. 20 microM); this is attributable to the enhanced formation of thromboxane A2 (TXA2) upon close platelet-to-platelet contact in this medium. 4. When TXA2 formation is blocked by inhibition of cyclo-oxygenase with aspirin or indomethacin, rabbit platelet aggregation and release in response to 1.25-10 micrograms/ml collagen is also maximal at [Ca2+] of 0.5-2.0 mM and least at 20 microM; human platelets do not aggregate and the extent of release is relatively independent of [Ca2+]. 5. In 1 mM [Ca2+], use of apyrase and/or ketanserin with rabbit platelets in which TXA2 formation is blocked shows that released ADP and serotonin make large contributions to aggregation and release in response to high concentrations of collagen; human platelet aggregation is largely dependent on TXA2. 6. Use of fura-2-loaded platelets shows that the collagen-induced rise in cytosolic [Ca2+] is only slightly inhibited by aspirin or indomethacin in rabbit platelets, but almost completely inhibited in human platelets. 7. Responses of rabbit platelets to collagen are less dependent on TXA2 than those of human platelets. Released ADP and serotonin make major contributions to the responses of rabbit platelets to collagen.