Release of sphingosine-1-phosphate from human platelets is dependent on thromboxane formation.

BACKGROUND: Platelets release the immune-modulating lipid sphingosine-1-phosphate (S1P). However, the mechanisms of platelet S1P secretion are not fully understood. OBJECTIVES: The present study investigates the function of thromboxane (TX) for platelet S1P secretion during platelet activation and the consequences for monocyte chemotaxis. METHODS: S1P was detected using thin-layer chromatography in [(3)H]sphingosine-labeled platelets and by mass spectrometry. Monocyte migration was measured in modified Boyden chamber chemotaxis assays. RESULTS: Release of S1P from platelets was stimulated with protease-activated receptor-1-activating peptide (PAR-1-AP, 100 µM). Acetylsalicylic acid (ASA) and two structurally unrelated reversible cyclooxygenase inhibitors diclofenac and ibuprofen suppressed S1P release. Oral ASA (500-mg single dose or 100 mg over 3 days) attenuated S1P release from platelets in healthy human volunteers ex vivo. This was paralleled by inhibition of TX formation. S1P release was increased by the TX receptor (TP) agonist U-46619, and inhibited by the TP antagonist ramatroban and by inhibitors of ABC-transport. Furthermore, thrombin-induced release of S1P was attenuated in platelets from TP-deficient mice. Supernatants from PAR-1-AP-stimulated human platelets increased the chemotactic capacity of human peripheral monocytes in a S1P-dependent manner via S1P receptors-1 and -3. These effects were inhibited by ASA-pretreatment of platelets. CONCLUSIONS: TX synthesis and TP activation mediate S1P release after thrombin receptor activation. Inhibition of this pathway may contribute to the anti-inflammatory actions of ASA, for example by affecting activity of monocytes at sites of vascular injury.

Oceanic basalt leads: a new interpretation and an independent age for the Earth.

A modified form of a model proposed by Gerling and Shukolyukow is used to study the development of lead isotopic ratios in oceanic basalts. This modified model yields an age for the earth which is calculated to be 4530 +/- 40 million years. The significance of this calculation is that it is independent of the knowledge of the ages of the samples studied. It is also found that although the source of the basalts is not generally homogeneous, the differentiation of this source from a closed system has occurred fairly recently geologically. The oldest time for the differentiation, 1230 +/- 350 million years ago, has been found for Mid-Atlantic tholeiites.