There has recently been rapid progress in understanding receptors that generate intracellular signals from inositol lipids. One of these lipids, phosphatidylinositol 4,5-bisphosphate, is hydrolysed to diacylglycerol and inositol trisphosphate as part of a signal transduction mechanism for controlling a variety of cellular processes including secretion, metabolism, phototransduction and cell proliferation. Diacylglycerol operates within the plane of the membrane to activate protein kinase C, whereas inositol trisphosphate is released into the cytoplasm to function as a second messenger for mobilizing intracellular calcium.

Inositol trisphosphate, a novel second messenger in cellular signal transduction.

The biology of platelet-derived growth factor

The intracellular role of ATP has been recognized for many years; it is an energy source for many important reactions. Cellshavecomplexmechanismsfor synthesizing ATP and maintaining or rapidly restoring the intracellular level, and one might therefore expect that release ofATP to the extracellular milieu would rarely occur; teleological reasoning suggests that cells should guard their intracellular ATP at all costs, and the established view is that ATP does not cross the plasma membrane of viable cells a suggestion that the selective release ofcytoplasmic ATP might be part of a physiological regulatory mechanism would, in the opinion of many, verge on the heretical. Nonetheless, extracellular ATP (and/or ADP) at low micromolar concentrations can influence many biological processes including platelet aggregation, vascular tone, neurotransmission (peripheral and central), cardiac function and muscle contraction. Nucleotides can be released from the cytoplasm of several cell types (and by exocytosis from platelets and neurones); they interact with specific receptors on the surface of many different cells and they aremetabolized locallybyectonucleotidases. The main purpose of this Review is to summarize current knowledge of these topics and to indulge in some modest speculation about the role ofextracellular ATP in biology and pathology.

Extracellular ATP: effects, sources and fate.

There is abundant evidence that changes in diet and various types of vessel wall injury can independently induce the growth of arterial lesions in experimental animals. These lesions closely resemble those found in humans with atherosclerosis. Whether endothelial injury or accumulation of lipoprotein in the arterial intima is the initial event, the progression of the disease is characterized by changes in the neointima that favor the deposition of lipid. The metabolism of proteoglycans may be especially important in this process; this is relevant to diabetes because changes in proteoglycan metabolism are associated with this disease. Insulin and growth hormone may favor the proliferation of smooth muscle cells in the arteries of diabetic patients. Many agents, which are potentially injurious to the endothelium, accentuate the response of the vessel wall to injury. Modifications of the thrombotic process, such as increased production of thromboxane by platelets, decreased production of prostacyclin by the endothelium, and increased production of von Willebrand factor further enhance the thrombotic process and may be important in the initiation and subsequent progression of atherosclerosis in diabetics. Alterations in lipoprotein metabolism may also facilitate the development of endothelial injury.

Pathogenesis of atherosclerosis.

Extracellular ATP, at micromolar concentrations, induces significant functional changes in a wide variety of cells and tissues. ATP can be released from the cytosol of damaged cells or from exocytotic vesicles and/or granules contained in many types of secretory cells. There are also efficient extracellular mechanisms for the rapid metabolism of released nucleotides by ecto-ATPases and 5'-nucleotidases. The diverse biological responses to ATP are mediated by a variety of cell surface receptors that are activated when ATP or other nucleotides are bound. The functionally identified nucleotide or P2-purinergic receptors include 1) ATP receptors that stimulate G protein-coupled effector enzymes and signaling cascades, including inositol phospholipid hydrolysis and the mobilization of intracellular Ca2+ stores; 2) ATP receptors that directly activate ligand-gated cation channels in the plasma membranes of many excitable cell types; 3) ATP receptors that, via the rapid induction of surface membrane channels and/or pores permeable to ions and endogenous metabolites, produce cytotoxic or activation responses in macrophages and other immune effector cells; and 4) ADP receptors that trigger rapid ion fluxes and aggregation responses in platelets. Current research in this area is directed toward the identification and structural characterization of these receptors by biochemical and molecular biological approaches.

https://www.physiology.org/doi/pdf/10.1152/ajpcell.1993.265.3.C577

Signal transduction via P2-purinergic receptors for extracellular ATP and other nucleotides

Extensive aggregation of human platelets can be induced by ADP without secondaryaggregation or release of granule contents. This occurs with washed platelets in Tyrode solution containing 0.35% alb...

Factors responsible for ADP-induced release reaction of human platelets

Fibrinogen and ADP-induced platelet aggregation

Mechanisms of platelet shape change, aggregation, and release induced by collagen, thrombin, or A23,187.

Platelet interaction with damaged rabbit aorta.

Publisher Summary This chapter describes the process of isolating human platelets from plasma by centrifugation and washing. Through the centrifugation and washing methods, large volumes of blood can be handled readily and the platelet count in the final suspension can be adjusted to any desired number. By using these methods, platelets respond to aggregating and release-inducing agents in a manner similar to platelets in native plasma, or plasma anticoagulated with hirudin, and maintain this responsiveness for hours. The constituents of a medium can be varied or additions can be made, as required, for experimental purposes. Thrombin can be used as an agonist without the problem of the formation of large amounts of fibrin that occurs when thrombin is added to platelet-rich plasma. The morphological appearance of platelets by electron microscopy is similar to that of platelets in plasma. Because platelet suspensions are kept at 37°, experiments can readily be done at this temperature so that conditions more closely resemble an in vivo situation.

Raelene L. Kinlough-Rathbone

Papers

Factors responsible for ADP-induced release reaction of human platelets

Fibrinogen and ADP-induced platelet aggregation

Mechanisms of platelet shape change, aggregation, and release induced by collagen, thrombin, or A23,187.

Platelet interaction with damaged rabbit aorta.

Isolation of human platelets from plasma by centrifugation and washing.