Dissociation of vasoconstrictor and platelet aggregatory activities of thromboxane by carbocyclic thromboxane A2, a stable analog of thromboxane A2

Carbocyclic thromboxane A2 [2 beta (Z),3 alpha- (1E,3R*)-3-(3-hydroxy(1-octenyl)-bicyclo[3.1.1]hept-2-yl-5-heptenoic acid], a stable analog of thromboxane A2, has been tested for its physiologic properties. Carbocyclic thromboxane A2 is a potent coronary vasoconstrictor, stimulating cornonary vascular smooth muscle at concentrations as low as 29 pM. At 1-5 micro M it is also an inhibitor of arachidonic-acid- and endoperoxide-induced aggregation of platelets. At 200 nM it stimulated the release of lysosomal hydrolases from large granule fractions of liver homogenate. It inhibited thromboxane synthesis in platelets, although it did not inhibit synthesis of prostacyclin in ram seminal vesicles. Thus, carbocyclic thromboxane A2, a molecule closely related to thromboxane A2, separates coronary vasoconstrictor from platelet-aggregating activity. The constrictor activity predominates in vivo; carbocyclic thromboxane A2 induces coronary vasoconstriction leading to myocardial ischemia and sudden death in rabbits in the absence of pulmonary or coronary thrombosis.

Influence of thromboxane inhibition on the severity of myocardial ischemia in cats

The effects of thromboxane (Tx) inhibition or arachidonic acid (AA) infusion were studied in anesthetized cats during acute myocardial ischemia (MI). AA (7.2 mg kg-1 h-1) or imidazole (25 mg kg-1 h-1) infusions were initiated 30 min after occlusion of the left anterior descending coronary artery. Assessment of the degree of protection of the ischemic myocardium was made by measurement of S-T segment elevation, plasma and myocardial creatine phosphokinase (CPK) activities, and myocardial amino-nitrogen content. Assessment of Tx inhibition was performed by radioimmunoassay. Administration of imidazole inhibited the sevenfold increase in plasma thromboxane B2 (TxB2) levels occurring in MI (p less than 0.001 at 2-5 h), markedly decreased S-T segment elevations at 2-k h (p less than 0.025), significantly prevented the elevation in plasma CPK (p less than 0.05, at 4 and 5 h), the increase in TxB2 post-MI, significantly decreased (p less than 0.025) S-T segment evaluations at 2-5 h, caused a decrease in plasmaCPK levels (p less than 0.05 at 5 h), but did not prevent loss of myocardial CPK or amino-nitrogen. In summary, the administration of imidazole resulted in significant protection of the myocardium in all indices of ischemic damage measured, while AA infusion resulted in only a partial protection. The mechanism of the imidazole protection of ischemic myocardial tissue appears to be via inhibition of Tx synthesis althoug we cannot exclude a hemodynamic or cytoprotective mechanism. These results suggest that specific inhibition of Tx formation is beneficial during acute MI.

Salutary effects of prostacyclin in endotoxic shock

Endotoxin shock was induced in anesthetized cats with E. coli endotoxin (5 mg/kg, i.v.) This produced a severe decline in mean arterial blood pressure and a marked decrease in superior mesenteric artery flow (SMAF) within 1 h. The plasma activity of cathepsin D, a lysosomal protease, increased 6-fold by 2 h. At 5 h, myocardial depressant factor (MDF), a toxic of 0.75 nmol.kg-1.min-1 dilated the splanchnic circulation and significantly increased SMAF. In addition, PGI2 almost completely prevented the accumulation of cathepsin D and MDF in the circulating blood of cats given endotoxin. These findings suggest that PGI2 exerts a variety of beneficial actions in endotoxin shock including vasodilation and stabilization of lysosomal membranes. In addition, PGI2 is known to prevent platelet aggregation and suppress thromboxane formation, two additional effects that may be of positive survival value in endotoxin shock.

Preservation of ischemic myocardium by pinane thromboxane A2

Pinane thromboxane A2 (PTA2), a thromboxane A2 analog has been shown to antagonize the vasoconstriction and platelet aggregation induced by thromboxane A2, in addition to specifically inhibiting thromboxane synthetase. Because thromboxane A2 generation would be detrimental in acute myocardial ischemia (MI) by both decreasing coronary blood flow and increasing platelet aggregation, inhibition of thromboxane production and action may be beneficial in myocardial ischemia. In pentobarbital-anesthetized cats, the left anterior descending coronary artery was ligated, and PTA2 (0.5 mumol . kg-1 . h-1) or a Na2CO3 vehicle was infused 30 min post-MI for 270 min. Compared to vehicle-treated MI cats, PTA2 prevented the increase in plasma thromboxane levels seen at 2 through 5 h (P less than 0.005 at 2 through 5 h) and prevented the large increase in plasma CK activities at 4 and 5 h (P less than 0.025). In addition, PTA2 treatment abolished the differences in myocardial CK activities between ischemic and nonischemic regions and prevented the decrease in percent-bound cathepsin D in the ischemic region. Moreover, ECG analysis revealed a decreased incidence of premature beats in PTA2-treated MI cats as compared to MI-vehicle cats. In summary, these data indicate that PTA2 protects the ischemic myocardium and provide further evidence that inhibition of thromboxane formation, in addition to antagonism of its activity, is beneficial during the early stages of acute myocardial ischemia.

Biosynthetic studies on the collagenous components of basement membranes

The biosynthesis of the collagenous components of the rat lens capsule and glomerular basement membrane (GBM) has been investigated. Intact lenses, isolated lens capsules and isolated renal glomeruli incorporate [3H]proline into 4-hydroxy[3H]proline containing polypeptides of approximate molecular weight 180,000 (analysed under reducing conditions). These polypeptides are deposited in the deoxycholate-insoluble basement membrane matrix and correspond to subunits of the intact, non-radioactive basement membranes. With time a proportion of the newly synthesised collagenous polypeptides become assembled into high molecular weight aggregates which are stabilized by lysine- and/or hydroxylysine-derived cross-links. Despite the similar patterns of collagen biosynthesis in the rat lens capsule and GMB, the latter has a more complex polypeptide composition the origin of which remains uncertain.

Synthesis and biological properties of pinane-thromboxane A2, a selective inhibitor of coronary artery constriction, platelet aggregation, and thromboxane formation

Pinane-thromboxane A2 (PTA2, [1alpha,2 beta(Z),-3 alpha (1E,3R*),5 alpha]-7-(3-(3-hydroxy-1-octenyl)-6,6-dimethylbicyclo[3.1.1]hept-2-yl)-5-heptenoic acid) has been synthesized and tested for biological activity in systems responsive to thromboxane A2, stable prostaglandin endoperoxide (PGH2) analogs, and prostatacyclin (PGI2). At low concentrations, PTA2 inhibited cat coronary artery constriction induced by stable prostaglandin endoperoxide analogs, and it stabilized liver lysosomes. At slightly higher concentrations, it inhibited platelet aggregation. At still higher concentrations, PTA2 inhibited thromboxane synthetase, but it had no effect on prostacyclin synthetase. The analog also had no effect on the inhibition of platelet aggregation by PGI2 or prostaglandin D2. It is suggested that PTA2 has a suitable biochemical profile for use as an antithrombotic agent.