[The studies on the mechanism of platelet activation induced by exogenous arachidonic acid with special reference to intraplatelet Ca2+ mobilization].
It has been reported that the dose-response curves for exogenous arachidonic acid (AA)-induced human platelet aggregation and secretion are biphasic. Low concentration (0.5-10 microM) and high concentration (30 microM-50 microM) of AA increase both platelet aggregation and secretion. In contrast, middle concentration (10 microM-20 microM) of AA reduces the both phenomena. This mechanism, however, has not been elucidated in detail. Since calcium ion has been reported to act as an important second messenger for platelet activation, intracellular Ca2+ concentration ([Ca2+]cyt) using fura-2 and aequorin, the plasma membrane-bound Ca2+ using chlortetracycline(CTC), and Ca2+ flux across the plasma membrane using 45Ca2+, in human platelet suspensions were measured in the presence and/or in the absence of external Ca2+. Cyclic AMP, inhibitory substance of platelet activation, was also measured. The increase of [Ca2+]cyt induced by low concentration of AA was mainly derived from intracellular Ca2+ storage site via thromboxane A2-phospholipase C and the increase of [Ca2+]cyt induced by high concentration of AA was mainly derived from both membrane bound Ca2+ and influx of extracellular Ca2+. The membrane bound Ca2+ release was induced by the direct effect to the plasma membrane of AA in a dose dependent manner, and it was suggested that CTC molecules were not bound to the cytoplasmic organella membrane but only to the plasma membrane. [Ca2+]cyt measured using aequorin method was thought to reflect the change in the concentration of Ca2+ area directory under the plasma membrane. The time course of intracellular 45Ca2+ flux suggested that the plasma membrane Ca2+ pump might operate in AA-stimulated platelets. Inhibition of increase of [Ca2+]cyt, and the decrease of platelet aggregation observed in 10-20 microM AA stimulation were related to the increase of cyclic AMP level. These results support the clinical usefulness of drugs that increase the cyclic AMP level in platelet.
Koyama
M
M
Third Department of Internal Medicine, Hokkaido University School of Medicine, Sapporo, Japan.
D016718:Arachidonic Acid / Q000494:pharmacology*; D002118:Calcium / Q000097:blood* / Q000502:physiology; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000097:blood / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D006801:Humans; D015539:Platelet Activation / Q000187:drug effects*; D010974:Platelet Aggregation / Q000187:drug effects
D016718:Arachidonic Acid; D000242:Cyclic AMP; D002118:Calcium