15-hydroxy-11-alpha-9-alpha-(epoxymethano)prosta-5-13-dienoic-acid and thrombin-receptor-peptide-(42-55)

The regulatory mechanism of thrombin receptor responsiveness in in situ endothelial cells was investigated by evaluating elevations of cytosolic Ca(2+) concentration ([Ca(2+)](i)) in fura-2-loaded porcine aortic valvular strips. Once stimulated with thrombin, endothelial cells did not respond to the second thrombin stimulation within 90 min. However, applying thrombin receptor activating peptide (TRAP7) at 15 min after the thrombin stimulation caused [Ca(2+)](i) elevation, which was smaller than that seen without preceding stimulation. After 90 min, response to TRAP7 recovered to the control level. When stimulated with TRAP7, the subsequent responses to thrombin and TRAP7 were attenuated at 15 min, and fully recovered after 90 min. Staurosporine partially prevented the TRAP7-induced desensitization. The recovery of responsiveness was inhibited completely by calyculin-A and partially by okadaic acid. Proteolysis and phosphorylation thus play an important role in thrombin receptor desensitization in in situ endothelial cells. Both cleaved and uncleaved receptors were desensitized through phosphorylation in part by staurosporine-sensitive kinase, and restored the responsiveness through dephosphorylation by type 1 phosphatase. The mechanism of regulation of thrombin receptor activity in in situ endothelial cells differed from those reported in cultured endothelial cells. We suggest that the cell-specific regulatory mechanism may be altered by culture conditions.

Topics: 15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; Animals; Calcium; Cattle; Coronary Vessels; Endothelium, Vascular; Enzyme Inhibitors; In Vitro Techniques; Muscle Relaxation; Muscle, Smooth, Vascular; Peptide Fragments; Phosphoprotein Phosphatases; Phosphorylation; Protein Kinase Inhibitors; Receptors, Thrombin; Swine; Thrombin; Vasoconstrictor Agents

Inhibition of aggregation by Ro 44-9883, a potent and selective non-peptide GPIIb/IIIa antagonist, resulted in inhibition of serotonin secretion induced by weak agonists such as ADP or low doses of either thrombin receptor agonist peptide (TRAP) or collagen. In contrast, alpha granule secretion was inhibited to different extents dependent on donor, averaging 60% inhibition. Inhibition of serotonin secretion correlated with an inhibition of thromboxane A2 (TxA2) formation, both of which were overcome by higher doses of TRAP or collagen. Ro 44-9883 had no effect on the already reduced serotonin secretion and TxA2 formation in Glanzmann's thrombasthenic platelets. Restoration of serotonin secretion in the absence of aggregation requires both TxA2 and lysophosphatidic acid. In addition, Ro 44-9883 inhibition of TxA2 formation was not due to a lack of phospholipase A2 (PLA2) phosphorylation and activation as assayed in vitro. These results suggest that aggregation is required for weak or low dose agonist induced in vivo activity of PLA2, possibly by either regulating phospholipid substrate availability or interaction of PLA2 with platelet membranes.

Topics: 15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; Acetates; Adenosine Diphosphate; Blood Platelets; Cytoplasmic Granules; Cytosol; Dose-Response Relationship, Drug; Enzyme Activation; Humans; Lysophospholipids; Peptide Fragments; Phosphorylase a; Phosphorylation; Plasma; Platelet Aggregation Inhibitors; Platelet Glycoprotein GPIIb-IIIa Complex; Serotonin; Thrombasthenia; Thromboxane A2; Tyrosine; Vasoconstrictor Agents