Taming platelets with cyclic nucleotides

Cardiovascular diseases are often accompanied and aggravated by pathologic platelet activation. Tight regulation of platelet function is an essential prerequisite for intact vessel physiology or effective cardiovascular thera py. Physiological platelet antagonists as well as various pharmacological v asodilators inhibit platelet function by activating adenylyl and guanylyl c yclases and increasing intracellular cyclic AMP (cAMP) and cyclic GMP (cGMP ) levels, respectively. Elevation of platelet cyclic nucleotides interferes with basically all known platelet activatory signaling pathways, and effec tively blocks complex intracellular signaling networks, cytoskeletal rearra ngements, fibrinogen receptor activation, degranulation, and expression of pro-inflammatory signaling molecules. The major target molecules of cyclic nucleotides in platelets are cyclic nucleotide-dependent protein kinases th at mediate their effects through phosphorylation of specific substrates. Th ey directly affect receptor/G-protein activation and interfere with a varie ty of signal transduction pathways, including the phospholipase C, protein kinase C, and mitogen-activated protein kinase pathways. Regulation of thes e pathways blocks several steps of cytosolic Ca2+ elevation and controls a multitude of cytoskeleton-associated proteins that are directly involved in organization of the platelet cytoskeleton. Due to their multiple sites of action and strong inhibitory potencies, cyclic nucleotides and their regula tory pathways are of particular interest for developing new approaches for the treatment of thrombotic and cardiovascular disorders. (C) 2001 Elsevier Science Inc. All rights reserved.