Neomycin inhibits inositol phosphate formation in human platelets stimulated by thrombin but not other agonists

Abstract

Neomycin (0.1-l mM) added to human platelet-rich plasma or washed platelets prelabeled with [3H]inositol inhibits aggregation, ATP secretion (ID,, 0.2 mM) and formation of [3H]inositol mono-, bis-and trisphosphate (ID,, 060.8 mM) in response to thrombin (0.25 U/ml). The production of inositol phosphates in response to other platelet agonists (vasopressin, platelet activating factor, prostaglandin endoperoxide analogs and collagen) is not inhibited by neomycin, even at a concentration of 2 mM. At this concentration neomycin reduces the secretion of ATP stimulated by these agents (by up to 50%). The results indicate that neomycin has multiple effects on platelets that are unrelated to a specific inhibition of inositol phospholipid degradation by phospholipase C. Low concentrations (0. l-l mM) of neomycin might selectively inhibit the interaction of thrombin with the platelet surface, and high concentrations (> 2 mM) might unspecifically reduce platelet secretion in response to various platelet agonists. Neomycin Platelet aggregation Platelet release reaction

G Proteins and Phospholipase C Mediate Thrombin-lnduced Generation of Plasminogen Activator Inhibitor-1 from Vascular Smooth Muscle Cells

“…Receptor cleavage by thrombin is tightly coupled with PLC and that is associated with the generation of inositol 1,4,5-trisphosphate, diacylglycerol and PKC activation in endothelial cells [36], Neomycin inhibits thrombin-induced polyphosphoinositide metabo lism in platelets [37]. Neomycin is a potent PLC inhibitor but it also affects the activity of PLD [38].…”

Epinephrine and the Ca2+ ionophore A23187 synergistically induce platelet aggregation without protein kinase C activation

“…Synergism has been demonstrated for each pair of these pathways: (a) Ca 2+ mobilization and protein kinase C activation [2,3], (b) epinephrine and protein kinase C activation [5], and (c) epinephrine and Ca 2+ mobilization (this study). Interestingly, each pathway can synergize with the other independently of the third pathway.…”

“…Interestingly, each pathway can synergize with the other independently of the third pathway. The synergism of the Ca 2+ pathway and the a¢2-adrenergic receptor/Gi-protein pathway concerns platelet aggregation but not secretion, and it is, in that regard, comparable to the synergism between the protein kinase C and Gi-protein pathways [5]. In contrast, the Ca 2+ and protein kinase C pathways synergize in inducing both platelet aggregation and secretion [2,3].…”

“…The synergism of epinephrine and protein kinase C activators in inducing platelet aggregation involved a novel mechanism [5]. The present study was undertaken to determine whether and how te2-adrenoceptor activation by epinephrine would synergize with the Ca 2÷ pathway stimulated by Ca 2+ ionophores in human platelets.…”

“…It does not induce phospholipase C activation in human platelets, if the formation or action of positive feedback stimulators such as ADP and endoperoxides/thromboxane A2 is inhibited [4]. Also, epinephrine does not -in aspirinized platelets -elevate significantly cytosolic Ca 2+ and activate Ca2+-dependent kinases and protein kinase C [5]. Epinephrine binds to cr2-adrenoceptors on human platelets [6,7].…”