Species variation in serum levels of prostaglandins and their precursor acids

Basic aggregation properties of washed rat platelets: correlation between aggregation, phospholipid degradation, malondialdehyde, and thromboxane formation

The basic characteristics of washed rat platelets in aggregation and biochemical responses to thrombin and collagen were investigated. Maximum aggregation response to thrombin and collagen was observed in the presence of 1-3 mM extracellular CA2+. Breakdown of endogenous phospholipids was followed by HPTLC. Phosphatidylinositol degraded most rapidly and greatly, followed by phosphatidylethanolamine, and phosphatidylcholine. Sphingomyelin did not change on stimulation. Formation of malondialdehyde was closely associated with thrombin- and collagen-induced aggregation within the range of the concentration employed. In addition, it was correlated with TXB2 formation and phosphatidylinositol degradation. These results indicated that, despite known species variation, thrombin-induced aggregation is mediated by TXA2 and malondialdehyde formation reflects both cyclo-oxygenase and thromboxane synthase activities, and phospholipid degradation in rat platelets as well as in human platelets.

Characteristics of the formation of the platelet lipoxygenase product from endogenous arachidonic acid

The concentration of 12-hydroxyeicosatetraenoic acid (12-HETE) formed in rat platelets aggregated by collagen suspension increased continuously during a 115-min incubation period, whereas the concentration of TXB2 or PGF2 alpha reached the maximum within 3 min and stayed at the plateau for the remaining incubation period. These data indicate that platelet lipoxygenase is not completely inactivated as is cyclooxygenase by the oxidizing agent. Platelets of essential fatty acid deficient (EFAD) rats resuspended in plasma of control rats produced more 12-HETE than platelet-rich plasma (PRP) of EFAD rats, whereas platelets of control rats resuspended in plasma of EFAD rats formed less 12-HETE than PRP of control rats. However, the concentration of TXB2 or PGF2 alpha produced was not changed in both cases implying that platelet cyclooxygenase preferentially utilizes arachidonic acid (AA) derived from platelet lipids. Radioactivity of phosphatidylcholine (2-arachidonyl-1-14C) suspended in the plasma of PRP was incorporated into 12-HETE but not to TXB2, indicating again that only lipoxygenase can utilize AA derived from plasma phospholipids. The significance of this observation is that the effects of platelet lipoxygenase products, although their physiological roles are not known, would be much more persistent than cyclooxygenase products after platelets are stimulated or aggregated in vivo.

In vitro and in vivo effects of ethanol on the formation of endoperoxide metabolites in rat platelets

Preincubation of rat platelet-rich plasma (PRP) with ethanol resulted in dose-dependent inhibition of the formation of endoperoxide metabolites (EPM) when the PRP was aggregated by collagen suspension. The inhibition was manifested at concentrations normally attainable in blood of rats or humans by tolerable amounts of ethanol ingestion. Paradoxically, chronic ingestion of ethanol caused enhanced synthesis of EPM in platelets, indicating that the inhibitory effect of ethanol would be temporary, and that it can be reversed as soon as ethanol is eliminated. The level of arachidonic acid in platelet phospholipids of rats fed the ethanol diet was not different from that of the control, indicating that availability of immediate precursor acid would not be a factor for the enhanced synthesis of EPM in the ethanol group. This result suggested that platelets from rats subjected to chronic ethanol ingestion become hyperactive in synthesizing EPM through an unknown mechanism. When citrated whole blood was incubated in the presence of collagen suspension, amounts of EPM synthesized in the ethanol group were not different from those of the control group, but this was due to significant reduction of platelet counts in the ethanol group. Whether the effect of ethanol on other tissues would be similar to that on platelets is unknown. It is tempting to speculate that some of the pathological changes resulting from alcoholism might be mediated through the effect of ethanol on EPM formation.