Sphingolipid Metabolism during Human Platelet Activation

The effect of calcium on human platelet polypeptides was investigated. When lysed platelets were incubated with mM Ca++, two major intracellular polypeptides (Mr = 255,000 and 230,000) were found to rapidly disappear. A similar phenomenon was also observed when intact platelets were treated with the calcium ionophore A-23187 in the presence of mM Ca++. Determinations of lactic dehydrogenase activity in supernatant fractions demonstrated that these losses occurred before platelet lysis. Investigations into the identity of the high molecular weight polypeptides revealed that one (Mr = 255,000) had similar properties to actin binding protein. The loss of the high molecular weight polypeptides was accompanied by formation of lower molecular weight polypeptides (Mr = 135,000, 93,000 and 48,000), indicating that Ca++ activates a polypeptide cleavage mechanism. The Ca++-activated polypeptide cleavages were rapid, with significant changes being observed within the first 0.5 min of incubation. An obvious explanation for these effects is. that there is Ca++-activated proteolytic activity within platelets. The Ca++-activated proteolytic activity was determined by the hydrolysis of the artificial substrate azocasein. We found that more than 90% of the proteolytic activity in lysed platelets was due to Ca++-activated proteases. These studies show that Ca++-activated proteases may play an important role in platelet activation.

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Human platelet activation in the absence of aggregation: a calcium- dependent phenomenon independent of thromboxane formation

Blood ◽

10.1182/blood.v59.5.1078.1078 ◽

1982 ◽

Vol 59 (5) ◽

pp. 1078-1085 ◽

Cited By ~ 33

Author(s):

S Levy-Toledano ◽

J Maclouf ◽

P Bryon ◽

E Savariau ◽

RM Hardisty ◽

...

Keyword(s):

Platelet Activation ◽

Ultrastructural Change ◽

Creatine Phosphokinase ◽

Light Transmission ◽

Human Platelet ◽

Shape Change ◽

Creatine Phosphate ◽

Intracellular Membrane ◽

Calcium Dependent ◽

Intracellular Membrane Transport

Abstract In response to ionophore A 23187, thrombasthenic and EDTA-treated control platelet-rich plasmas (PRP) undergo a change in light transmission (LT) accompanied by a normal 14C-serotonin (5HT) release and thromboxane (TX) synthesis in the absence of aggregation. Ultrastructural qualitative electron microscopy revealed central apposition of organelles and loosely packed platelets in both models, while a central gel mass appeared only in thrombasthenic patients. Quantitative analysis of this ultrastructural change showed an increase in the elongation and a decrease in the circularity coefficients of thrombasthenic platelets, indicating a shape change with pseudopod formation, while EDTA-treated platelets underwent a shape change in the absence of pseudopod formation. Morphometric analysis showed that the ionophore caused extensive degranulation in both types of platelets (decrease of the granule volume), which occurred in the presence of contraction of thrombasthenic PRP (decrease of the SCS plus granule volume) but in its absence in EDTA-treated platelets. The change in LT was not inhibited by aspirin, suggesting a dissociation between release of 14C-5HT and TX formation. Moreover, it was not inhibited by creatine phosphate plus creatine phosphokinase, prostaglandin E1, or cytochalasin and/or colchicine. It was not dependent on ADP, cAMP, or the integrity of microfilaments and microtubules. However, chlorpromazine, TMB 8, and dibucaine, which interfere with intracellular membrane transport of Ca2+, inhibited this platelet activation (change in LT, 14C-5HT release and TX synthesis.

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