Platelet Aggregation Mechanisms and Their Implications in Haemostasis and Inflammatory Disease

A new family of thromboxane receptor antagonists with secondary thromboxane synthase inhibition

We report herein a novel class of thromboxane receptor (TP receptor) antagonists modeled on unstable natural lipids that we identified several years ago, the hepoxilins. These antagonists have been rendered chemically and biologically more stable than the natural compounds through structural modification by chemical synthesis. We demonstrate that the analogs inhibit the aggregation of human platelets in vitro evoked by the thromboxane receptor agonists, I-BOP ([1S-[1alpha,2alpha(Z),3beta(1E,3S*),4alpha]]-7-[3-[3-hydroxy-4-(4-iodophenoxy)-1-butenyl]-7-oxabi-cyclo[2.2.1]hept-2-yl]5-heptenoic acid) and U46619 (9,11-dideoxy-9alpha,11alpha-methanoepoxy-prosta-5Z,13E-dien-1-oic acid). The most potent of the analogs described, PBT-3 [10(S)-hydroxy-11,12-cyclopropyl-eicosa-5Z,8Z,14Z-trienoic acid methyl ester], has an IC(50) versus aggregation by I-BOP = 0.6 x 10(-7) M and versus U46619 = 7 x 10(-7) M, representing one of the most potent anti-aggregating substances so far described. PBT-3 also inhibits thromboxane formation and aggregation evoked by collagen with an IC(50) = 8 x 10(-7) M. Other PBT (hepoxilin cyclopropane) analogs so far tested were 5- to 10-fold less active, and the native hepoxilins were about 500-fold less active. Neither PBT-3 nor the other analogs inhibited 12-lipoxygenase, phospholipase A(2), or cyclooxygenase 1 or 2, and weakly stimulated adenyl cyclase (threshold stimulation at 10(-7) M and little selectivity for each of the PBT compounds). TP antagonism by PBT-3 was further demonstrated in receptor binding studies through use of (125)I-BOP, where the IC(50) for PBT-3 was 8 x 10(-9) M, approximately 16-fold less than for I-BOP itself. These findings identify a new mode of action of PBT-3 and other related analogs as primarily TP antagonists. These studies identify a new family of compounds useful in further development as novel therapeutics for thromboxane-mediated diseases.

Hepoxilins: a review on their cellular actions

This review is intended to summarize the biological actions of the hepoxilins reported to date. These actions appear to have, as their basis, changes in intracellular concentrations of ions including calcium and potassium ions as well as changes in second messenger systems. Recent evidence suggests that the biological actions of the hepoxilins may be receptor-mediated as indicated from data showing the existence of hepoxilin-specific binding proteins in the human neutrophil. Such evidence also implicates the association of G-proteins both in hepoxilin-binding as well as in hepoxilin action. The potential use of stable analogs of the hepoxilins is discussed as well as the directions in which this area is heading.

A topographical study of increased vascular permeability in passive cutaneous anaphylaxis in the chicken

Passive cutaneous anaphylaxis was produced in chickens with bovine serum albumin (BSA) and anti-BSA chicken serum. Colloidal carbon was then given intravenously to identify the leaky vessels. Microscopic examination of cleared unstained skin revealed the topography of labelled vessels. The carbon labelling was confined to venules and small veins only. Examination of the time-course of permeability response revealed a biphasic pattern.

Anti-inflammatory effects of blood platelets in the rat

A selective accumulation of blood platelets occurs at the sites of intradermal injection of zymosan and carrageenan in the rat. The inflammatory response to zymosan, assessed by the local accumulation of systemic radioactive human serum albumin, was significantly enhanced in platelet-depleted rats. Similar enhancements of the paw oedemas, caused by the subplantar injection of either carrageenan or zymosan, were also observed in thrombocytopenic animals. It is concluded that blood platelets may exert protective, i.e., anti-inflammatory, effects in some models of acute inflammation in the rat.