Role of autocrine stimulation on the effects of cyclic AMP on protein and lipid phosphorylation in collagen‐activated and thrombin‐activated platelets

We compared several responses in thrombin‐stimulated and collagen (type I)‐stimulated platelets with and without forskolin and inhibitors of autocrine stimulation (IAS: an ADP‐removing system of creatine phosphate/creatine phosphokinase, Arg‐Gly‐Asp‐Ser peptide to prevent fibrinogen/fibronectin binding to GPIIb/IIIa, SQ 29.548 as a thromboxane A 2 receptor antagonist, cyproheptadine as a serotonin receptor antagonist, BN 52021 as a platelet‐activating factor receptor antagonist). The pattern of tyrosine‐phosphorylated proteins, the phosphorylation of lipids in the polyphosphoinositide cycle and phosphorylation of pleckstrin (P47) were studied as markers for signal‐transducing responses, exposure of CD62 (P‐selectin) and CD63 (Glycoprotein 53), as well as secretion of ADP + ATP and β‐ N ‐acetyl‐glycosaminidase were studied as final activation responses. Clear differences between thrombin‐stimulated and collagen‐stimulated platelets were observed. First, practically all protein‐tyrosine phosphorylation induced by thrombin was inhibited by IAS, while a partial inhibition was observed for collagen; the phosphorylation due to collagen alone was apparently stimulated by elevation of cAMP. Secondly, the other responses to thrombin were inhibited by increased levels of cAMP, independent of autocrine stimulation. In contrast, only the autocrine part of the collagen‐induced responses was inhibited by elevation of cAMP. Thus, the inhibition by elevated cAMP seen in collagen‐stimulated platelets seems to be due to removal of the G‐protein‐mediated activation from secreted autocrine stimulators either by IAS or forskolin. The remaining activity is a pure collagen effect which is not affected by elevated levels of cAMP.