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

During the investigation of fibrin deposition onto hydrophobic polymers in contact with human blood, a model was developed in which fibrinogen was denatured and irreversibly coated onto a polyethylene surface by heating to 70°C for 10 min. The denatured fibrinogen-polyethylene surface is resistant to fluid wall shear rates of up to 550 s-' and the fibrinogen does not desorb in the presence of plasma proteins. Compared to uncoated polyethylene, little albumin or fibrinogen adsorbs to heat-denatured fibrinogen. Thrombin binds to the denatured fibrinogen-coated polyethylene with low affinity and also acts on the surface-bound denatured fibrinogen and cleaves fibrinopeptide A (FPA) quantitatively. Washed, "Cr-labeled platelets do not adhere to the thermally denatured fibrinogen at either low or high shear rates compared to surfaces coated with undenatured fibrinogen (p < 0.01). These observations support the role of the D domain of fibrinogen in platelet adhesion because this is the region that is denatured by heating. In contrast, the E domain of fibrinogen is not altered by heating to 70°C and hence remains susceptible to thrombin a nd/or plasmin cleavage. The characteristics of this surface are such that it can be used to develop fibrin-coated surfaces for use in studies of thrombus formation on artificial surfaces. 6 1992 John Wiley & Sons, Inc.

Interactions of thermally denatured fibrinogen on polyethylene with plasma proteins and platelets.

Most Apyrase Preparations Are Impure and Contain Inhibitors of Cathepsin G: Suggestions for Use of Apyrase in Preparation and Stabilization of Platelet Suspensions -

Most apyrase preparations are impure and contain inhibitors of cathepsin G: suggestions for use of apyrase in preparation and stabilization of platelet suspensions.

Responses to aggregating agents after cleavage of GPIb of human platelets by the O-sialoglycoprotein endoprotease from Pasteurella haemolytica- potential surrogates for Bernard-Soulier platelets?

Raelene L. Kinlough-Rathbone

Papers

Interactions of thermally denatured fibrinogen on polyethylene with plasma proteins and platelets.

Most apyrase preparations are impure and contain inhibitors of cathepsin G: suggestions for use of apyrase in preparation and stabilization of platelet suspensions.

Responses to aggregating agents after cleavage of GPIb of human platelets by the O-sialoglycoprotein endoprotease from Pasteurella haemolytica- potential surrogates for Bernard-Soulier platelets?

The effect of prostaglandins E1, I2 and F2α on the shape and phosphatidylinositol-4,5-bisphosphate metabolism of washed rabbit platelets

Effect of calcium concentration and inhibitors on the responses of platelets stimulated with collagen : contrast between human and rabbit platelets