Modulation of leukotriene synthesis and actions by synthetic derivatives of arachidonic acid

Seven analogs of arachidonic acid were tested for their coronary vasoactivity and their ability to inhibit LTC4 and LTD4 synthesis by lung tissue and to antagonize LTD4 induced coronary constriction. None of the seven arachidonic acid analogs significantly altered peptide leukotriene production by minced cat lung. Two of the analogs (i.e., 7, 13-diethanoarachidonic acid and 7, 10, 13-triethanoarachidonic acid) exerted modest but significant coronary vasodilation in isolated cat coronary arteries, and significantly antagonized the coronary vasoconstrictor response to LTD4. These analogs may be of interest in modulating leukotriene actions.

Specificity of anti-leukotriene actions of nicardipine

The calcium channel blocker, nicardipine (100 ng/ml) markedly antagonized the coronary vasoconstrictor effect of the peptide leukotrienes LTC4 and LTD4 on the isolated perfused cat coronary artery. However, nicardipine even at 300 ng/ml failed to antagonize the leukotriene induced contraction of either tracheal or pulmonary parenchymal strips from guinea pigs. However, at higher concentrations (i.e., 10 micrograms/ml), nicardipine inhibited the production of peptide leukotrienes from minced cat lung incubated in the presence of A23187. Thus, nicardipine exerts some selectivity in its anti-leukotriene actions.

Leukotriene production in isolated tissues of diabetic rats

We examined the ability of heart, aorta and lung obtained from alloxan diabetic rats as well as control rats to produce peptide leukotrienes (LT). The isolated perfused heart preparation as well as incubated minced tissue preparations were studied. Upon infusion of the Ca++ ionophore A23187, hearts from diabetic rats produced significantly less peptide LT when compared to control hearts. Lung tissue from diabetic animals incubated with A23187 also produced less immunoreactive peptide leukotrienes (iLT) when compared to the control group. In both preparations, incubation with the lipoxygenase inhibitor propyl gallate significantly inhibited the production of iLT in both the diabetic and control group. The observed differences in production of leukotrienes may alter vascular reactivity and thus play a role in the cardiovascular complications observed in diabetes.

Coronary vascular responsiveness to non-eicosanoid vasoconstrictors in the perfused diabetic rat heart

We tested arginine vasopressin, and a dihydropyridine calcium agonist, BAY K-8644 in isolated perfused hearts from control and diabetic rats. Arginine vasopressin (1-100 ng/ml) and BAY K-8644 (100-500 ng/ml) significantly increased coronary perfusion pressure during constant flow perfusion indicative of coronary vasoconstriction. However, no significant potentiation was observed between diabetic rats and their weight matched controls for either vasoconstrictor.

Potentiation of leukotriene formation in pulmonary and vascular tissue

Leukotriene (LT) release from vascular and pulmonary tissue was assessed by a radioimmunoassay for peptide leukotrienes (i.e., LTC4, LTD4 and LTE4). The calcium ionophore A-23187 at 1-3 micrograms/ml and platelet activating factor (PAF) at 10 micrograms/ml produced marked formation of peptide leukotrienes in minced cat pulmonary tissue. This was also confirmed by bioassay of the incubates in isolated perfused cat coronary arteries. Rat pulmonary tissue was comparable to cat with regard to LT production, but guinea-pig lung produced about 30-50% less on a weight basis. In addition, aortic and coronary artery vessel walls produced significant amounts of LTs. The time course for maximal leukotriene production occurred at 45-60 min of incubation at 37 degrees C in both the radioimmunoassay and the bioassay. Cat coronary artery constricted markedly to LTC4 or LTD4 (30-40 mm Hg) and to the lung or blood vessel incubate. This constriction was virtually totally blocked by the leukotriene antagonist FPL-55712, but not by the thromboxane receptor antagonist, pinane thromboxane A2, the alpha-adrenergic receptor antagonist, phenoxybenzamine, or the angiotensin receptor antagonist, saralasin. Thus, pulmonary and vascular tissue produce leukotrienes that appear to exert coronary constrictor effects on specific leukotriene receptors. These results indicate that the ischemia of shock and anaphylaxis may be accentuated by the release of peptide leukotrienes.

Altered coronary vascular responsiveness to leukotrienes in alloxan-diabetic rats

Altered responsiveness to and metabolism of various eicosanoids in diabetic animals and patients has been reported by several investigators. The purpose of this investigation was to examine the coronary vascular responsiveness of alloxan-diabetic rats to the leukotrienes. Hearts from 12- to 16-week-old alloxan-diabetic rats and weight-matched controls were perfused at constant flow by the Langendorff method. Coronary vasoactivity to leukotrienes B4, C4, D4 and E4 was assessed by measuring the change in coronary perfusion pressure upon infusion of these eicosanoids. Hearts from diabetic rats showed increased responsiveness to leukotrienes C4 (4-40 nM) and D4 (10-100 nM). Both control and diabetic rat hearts were only slightly responsive to leukotriene E4, and no difference between the two groups existed in the reactivity to this leukotriene. Neither group was responsive to the chemotactic leukotriene, B4. Perfusion of the hearts with the cyclooxygenase inhibitor, ibuprofen, failed to alter the coronary vascular responses to the leukotrienes. The coronary constrictor effects of the leukotrienes are the primary effect of these agents on the rat heart, since heart rate does not change significantly, and changes in contractile force are secondary to the coronary vascular constriction. These alterations in responsiveness to leukotrienes may play a role in the cardiovascular complications associated with diabetes.

Effects of lipoxygenase inhibitors in arachidonate-induced sudden death

Both BW 755c, a cyclo-oxygenase and lipoxygenase inhibitor, and nordihydroguaiaretic acid (NDGA), a selective lipoxygenase inhibitor, were tested for their protection against arachidonate-induced sudden death in rabbits. 100% survival was seen with BW 755c (1 mg kg-1), while NDGA showed 0 and 17% survival (2 mg kg-1 and 4 mg kg-1). BW 755c prevented 12-fold increase in plasma thromboxane B2 concentrations and the formation of pulmonary artery thrombi normally seen with arachidonate-induced sudden death, while NDGA showed no such protective effect. Radioimmunoassay of rabbit plasma for leukotrienes (LTC4, LTD4 and LTE4) indicated that they do not accumulate in blood in the model and BW 755c had no effect, suggesting that the deleterious effects seen are caused by cyclo-oxygenase pathway metabolites such as thromboxane A2, but not by lipoxygenase pathway products such as leukotrienes.

Hyperreactivity of coronary vasculature in platelet-perfused hearts from diabetic rats

Coronary vascular responsiveness to platelet-produced eicosanoids was examined in isolated perfused hearts of alloxan-diabetic rats. Coronary perfusion pressure was increased in isolated hearts of control and diabetic rats on perfusion with platelets and arachidonic acid (AA). However, the increase in perfusion pressure was approximately twofold higher in hearts of diabetic rats when compared with those isolated from control rats. This was associated with increased thromboxane B2 (TxB2) and prostaglandin F2 alpha (PGF2 alpha) production that was comparable in platelet-perfused hearts of control and diabetic animals. Ibuprofen, a cyclooxygenase inhibitor, blocked the increase in perfusion pressure and TxB2 and PGF2 alpha production by greater than 90% in both control and diabetic hearts perfused with platelets and AA. Dazoxiben, a thromboxane synthetase inhibitor, blocked the increase in perfusion pressure by 50%, totally inhibited TxB2 production, but increased PGF2 alpha production by 60% in both groups of platelet-perfused hearts. Increased levels of PGF2 alpha and possibly other constrictor eicosanoids (e.g., leukotriene D4) may account for the partial constriction observed in platelet-perfused hearts with dazoxiben. Results of the present study suggest that vascular reactivity to vasoconstrictor eicosanoids is increased in hearts of diabetic animals.

Studies on the mechanism of leukotriene induced coronary artery constriction

Leukotriene (LT) C4, D4, and E4 at concentrations of 10 to 100 ng/ml were found to be potent coronary artery constrictors in the perfused cat coronary artery and perfused rat heart. In contrast, LTB4, was essentially inactive. The coronary constrictor effect of leukotrienes was not related to thromboxane release, but rather appeared to be due to a calcium mediated activation of specific leukotriene receptors.

Vascular responsiveness and eicosanoid production in diabetic rats

Vascular responsiveness to vasoactive eicosanoids as well as vascular prostacyclin and thromboxane production was investigated in 7-10 weeks alloxan-diabetic rats. Aortic rings from diabetic rats exhibited increased responsiveness to carbocyclic thromboxane A2, a thromboxane analogue, when compared to control rat aortae. Isolated perfused hearts of diabetic rats showed increased vascular responsiveness to 9,11-methanoepoxy PGH2 (U-46619), an endoperoxide analogue. Diabetes resulted in a reduction in prostacyclin generation by isolated incubated aortae which was overcome by the addition of arachidonic acid but not by homogenization of incubated aortic tissue. In contrast, prostacyclin, but not thromboxane, generation was elevated in isolated perfused hearts of diabetic animals in response to moderate doses of arachidonic acid, but at high doses of arachidonate, more thromboxane was formed by perfused hearts of diabetic rats. These results suggest that different vessels can either increase or decrease their prostaglandin production in response to diabetes. The alterations in prostanoid production may be due to differential changes in prostacyclin and thromboxane synthesis in vessels which, in turn, may be related to the changes in vascular responsiveness.

Antagonism of platelet aggregation by 13-azaprostanoic acid in acute myocardial ischemia and sudden death

The effects of 13-azaprostanoic acid (13-APA) were studied during acute myocardial ischemia in cats and in rabbit sudden death induced by sodium arachidonate (Na-Ar). To more clearly define the mechanism of action of 13-APA, we also examined its effects on isolated cat and rabbit coronary arteries, in vitro aggregation of cat and rabbit platelet-rich plasma (PRP) and circulating rabbit platelet count measured in vivo. 13-APA provided minimal protection during myocardial ischemia in cats, partially reversing ischemia-induced ST segment elevations by 3-5 hours after coronary artery occlusion. However, 13-APA was ineffective in inhibiting the rise in plasma creatine kinase (CK) activity or the loss of CK from ischemic myocardial tissue. 13-APA (1.0 - 100 microM) did not inhibit contraction of cat coronary arteries produced by a stable thromboxane A2 analog. However, 13-APA (100 microM) inhibited aggregation of cat PRP induced by AA (1.0 microM). 13-APA also provided significant protection against sudden death induced by Na-Ar in rabbits. While this agent was ineffective in reducing vasoconstriction of rabbit coronary arteries or inhibiting platelet aggregation in response to 500 microM AA, aggregation of rabbit PRP by 250 microM AA was completely inhibited. AA injection produced a significant decrease in circulating platelet count in vehicle-treated rabbits. However, 13-APA reduced the decrease in circulating platelet count in rabbits which survived AA injection during the 13-APA infusion. These results indicate that antagonism of thromboxane A2 receptors in platelets may be an important feature in protecting against sudden death. The difference in sensitivities of vascular and platelet thromboxane receptors as well as the accessability of 13-APA to these receptors may explain the lack of protection of 13-APA in myocardial ischemia.

Protective actions of ibuprofen in arachidonate-induced sudden death

Sodium arachidonate given intravenously at a dose of 2 mg/kg is uniformly lethal in rabbits. Rabbits die within 2-5 min following a dramatic decrease in mean arterial blood pressure (MABP), and in circulating platelets, and a large increase in plasma thromboxane B2 (TxB2) concentration. The nonsteroidal anti-inflammatory agent, ibuprofen, given 15 min prior to challenge with sodium arachidonate, significantly protects against the abrupt decrease in MABP and in circulating platelets and prevents the increase in circulating TxB2 concentrations. These rabbits all survive the lethal effects of arachidonic acid when given ibuprofen at doses of 0.75, 6.25 or 12.5 mg/kg intravenously 15 min prior to arachidonate challenge. At 0.375 mg/kg, only 20% of the animals survive. Concomitant with survival is a significant attenuation of the decrease in MABP (84 +/- 8 mm Hg without ibuprofen vs. 1 +/- 1 to 33 +/- 12 mm Hg with 12.5-0.75 mg/kg ibuprofen). Similarly, these rabbits show a greatly reduced loss of circulating platelets, and virtually no increases in the formation of TxB2. Ibuprofen protects against arachidonate-induced sudden death in a dose-related manner. The mechanism of the protection appears to involve prevention of adherence or aggregation of platelets and the subsequent formation of thromboxane A2. The net result is prevention of pulmonary thrombosis, the major pathological event in triggering sudden death.

Anti-thromboxane A2 actions of pinane thromboxane A2 derivatives

Six pinane-thromboxane A2 analogs have been synthesized and tested for their ability to antagonize carbocyclic thromboxane A2 (CTA2) induced coronary vasoconstriction and prostaglandin endoperoxide analog induced platelet aggregation. Two of the derivatives, 5C-15S BPTA2 and 5T-15S BPTA2 and 5T-15S BPTA2 (1 microM) showed 76 +/- 3 and 72 +/- 9 percent inhibition of CTA2 (15 nM) induced vasoconstriction of cat coronary arteries respectively, while the other compounds showed between 15 and 50 percent inhibition at 1.0 and 2.0 microM. 5C-15S BPTA2 also antagonized prostaglandin-endoperoxide analog induced human platelet aggregation, although the other compounds showed little or no antagonism of aggregation in this system.