ADP-induced platelet shape change: an investigation of the signalling pathways involved and their dependence on the method of platelet preparation.

Shape change is an important early event in platelet activation. In this study we show that the Ca2+ chelator BAPTA and the Rho-kinase inhibitor Y-27632 inhibit ADP-induced myosin light chain (MLC) phosphorylation and platelet shape change through distinct pathways and with distinct kinetics. Ca2+ is largely responsible for the initial onset of shape change, whilst Rho-kinase plays a major role in the maintenance of the response. The relative contribution of these two pathways to each stage of the response was dependent on the method of platelet preparation, but in all cases shape change was shown to be downstream of the P2Y1 receptor. Similar observations were made in murine platelets. The shape change response was modulated via changes in cAMP levels, possibly via the P2TAC receptor, but not by tyrosine phosphorylation. We conclude that ADP-induced shape change occurs via the P2Y1 receptor, which can be differentially coupled to Rho-kinase and Ca2+-linked pathways dependent on the method of platelet preparation.