Prostaglandins, prostacyclin, and thromboxane in cardiovascular diseases

Aging-shifted prostaglandin profile in endothelium as a factor in cardiovascular disorders

Age-associated endothelium dysfunction is a major risk factor for the development of cardiovascular diseases. Endothelium-synthesized prostaglandins and thromboxane are local hormones, which mediate vasodilation and vasoconstriction and critically maintain vascular homeostasis. Accumulating evidence indicates that the age-related changes in endothelial eicosanoids contribute to decline in endothelium function and are associated with pathological dysfunction. In this review we summarize currently available information on aging-shifted prostaglandin profiles in endothelium and how these shifts are associated with cardiovascular disorders, providing one molecular mechanism of age-associated endothelium dysfunction and cardiovascular diseases.

Gender differences in cardiovascular disease: hormonal and biochemical influences

OBJECTIVE

Atherosclerosis is a complex process characterized by an increase in vascular wall thickness owing to the accumulation of cells and extracellular matrix between the endothelium and the smooth muscle cell wall. There is evidence that females are at lower risk of developing cardiovascular disease (CVD) as compared to males. This has led to an interest in examining the contribution of genetic background and sex hormones to the development of CVD. The objective of this review is to provide an overview of factors, including those related to gender, that influence CVD.

METHODS

Evidence analysis from PubMed and individual searches concerning biochemical and endocrine influences and gender differences, which affect the origin and development of CVD.

RESULTS

Although still controversial, evidence suggests that hormones including estradiol and androgens are responsible for subtle cardiovascular changes long before the development of overt atherosclerosis.

CONCLUSION

Exposure to sex hormones throughout an individual's lifespan modulates many endocrine factors involved in atherosclerosis.

Estrogen potentiates constrictor prostanoid function in female rat aorta by upregulation of cyclooxygenase-2 and thromboxane pathway expression

Estrogen potentiates vascular reactivity to vasopressin (VP) by enhancing constrictor prostanoid function. To determine the cellular and molecular mechanisms, the effects of estrogen on arachidonic acid metabolism and on the expression of constrictor prostanoid pathway enzymes and endoperoxide/thromboxane receptor (TP) were determined in the female rat aorta. The release of thromboxane A2 (TxA2) and prostacyclin (PGI2) was measured in male (M), intact-female (Int-F), ovariectomized-female (OvX-F), and OvX + 17beta-estradiol-replaced female (OvX + ER-F) rats. The expression of mRNA for cyclooxygenase (COX)-1, COX-2, thromboxane synthase (TxS), and TP by aortic endothelium (Endo) and vascular smooth muscle (VSM) of these four experimental groups was measured by RT-PCR. The expression of COX-1, COX-2, and TxS proteins by Endo and VSM was also estimated by immunohistochemistry (IHC). Basal release of TxA2 and PGI2 was similar in M (18.8 +/- 1.9 and 1,723 +/- 153 pg/mg ring wt/45 min, respectively) and Int-F (20.2 +/- 4.2 and 1,488 +/- 123 pg, respectively) rat aortas. VP stimulated the dose-dependent release of TxA2 and PGI2 from both male and female rat aorta. OvX markedly attenuated and ER therapy restored VP-stimulated release of TxA2 and PGI2 in female rats. No differences in COX-1 mRNA levels were detected in either Endo or VSM of the four experimental groups (P > 0.1). The expression of both COX-2 and TxS mRNA were significantly higher (P < 0.05) in both Endo and VSM of Int-F and OvX + ER-F, compared with M or OvX-F. Expression of TP mRNA was significantly higher in VSM of Int-F and OvX + ER-F compared with M or OvX-F. IHC revealed the uniform staining of COX-1 in VSM of the four experimental groups, whereas staining of COX-2 and TxS was greater in Endo and VSM of Int-F and OvX + ER-F than in OvX-F or M rats. These data reveal that estrogen enhances constrictor prostanoid function in female rat aorta by upregulating the expression of COX-2 and TxS in both Endo and VSM and by upregulating the expression of TP in VSM.

Sympathy for the devil: the role of thromboxane in the regulation of vascular tone and blood pressure

Historically, the vasodilatory prostanoids, especially prostacyclin and prostaglandin E(2), are believed to contribute significantly to the regulation of normal vascular tone and blood pressure (BP), primarily by counteracting the prevailing effects of the systemic vasoconstrictor systems, including angiotensin II, the catecholamines, and vasopressin. In contrast, the primary vasoconstrictor prostanoid thromboxane A(2) (TxA(2)) is produced in far smaller quantities in the normal state. While TxA(2) is believed to play a significant role in a variety of cardiovascular diseases, such as myocardial infarction, cerebral vasospasm, hypertension, preeclampsia, and various thrombotic disorders, its role in the regulation of vascular tone and BP in the normal physiological state is, at best, uncertain. Numerous studies have firmly established the dogma that TxA(2), while important in pathophysiological states in males, plays little or no role in the regulation of vascular tone or BP in females, except in the pulmonary vasculature. However, this concept is largely based on the predominant and preferential use of males in animal and human studies. Recent studies from our laboratory and others challenge this dogma and reveal that the TxA(2) pathway in the systemic vascular wall is an estrogen-dependent mechanism that appears to play an important role in the regulation of vascular tone and BP in females, in both normal and pathophysiological states. It is proposed that the potent vasoconstrictor action of TxA(2) is beneficial in the female in the normal state by acting as a local counterregulatory mechanism to increase vascular tone and BP and defend against hypotension that could result from the multiple estrogen-sensitive local vasodilator mechanisms present in the female vascular wall. Validation of this proposal must await further studies at the systemic, tissue, and molecular levels.

Sexual dimorphism in prostanoid-potentiated vascular contraction: roles of endothelium and ovarian steroids

The effects of constrictor prostanoid (CP) pathway inhibitors on vascular reactivity to vasopressin (VP) and phenylephrine (PE) were examined in thoracic aortas of male, female, and ovariectomized (OVX) female Sprague-Dawley rats. Maximal contractile response of control (Cont) aortas to VP was markedly higher in females (3,885 +/- 332 mg/mg ring wt) than in males (810 +/- 148 mg). Indomethacin (Indo; 10 microM) attenuated maximal response to VP in females (3,043 +/- 277 mg) but not in males. SQ-29,548 (SQ; 1 microM) attenuated maximal response to VP in females (3,042 +/- 290 mg) to a similar extent as Indo. Dazoxiben (Daz; 10 microM) alone had no effect, but Daz + SQ attenuated maximal contractile response to VP to a similar extent as SQ alone. Removal of the endothelium in female aortas attenuated contractile responses to VP in Cont aortas. OVX attenuated maximal contractile response to VP in Cont aortas (2,093 +/- 329 mg) and abolished the attenuating effects of Indo. Indo, SQ, and Daz exerted identical effects on contractile responses of male, female, and OVX female aortas to PE. These findings establish the following in the rat aorta: 1) CP, probably thromboxane and/or endoperoxide, is responsible for approximately 25-30% of contractile responses of females, but not males, to VP and PE; 2) CP production by the female aorta is primarily endothelial in origin; and 3) ovarian steroids modulate production and/or actions of CP in female aortas.

Beneficial action of beraprost sodium, a prostacyclin analog, in stroke-prone rats

Beraprost sodium is a stable analog of the vasodilator, platelet antiaggregatory eicosanoid, prostacyclin. Experiments were performed to determine whether long-term therapy with beraprost produces vascular protective effects in saline-drinking stroke-prone spontaneously hypertensive rats (SHRSPs). Oral beraprost at 30, 100, or 300 micrograms/kg/day starting at 8.4 weeks of age did not affect the progressive increase of systolic blood pressure (measured by tail-cuff plethysmography) in these rats. Additional experiments in SHRSPs, prepared for continuous monitoring of blood pressure by radiotelemetry, revealed that oral beraprost administration reduced mean arterial pressure but that these hypotensive responses were not sustained (< 4 h). In all SHRSPs receiving oral beraprost, proteinuria and cerebrovascular lesions developed. In contrast, continuous subcutaneous infusion of beraprost at 2.8 mg/kg/day from age 8.3-12.3 weeks reduced systolic blood pressure and markedly diminished the development of renal lesions and the occurrence of stroke in saline-drinking SHRSPs. Beraprost at 0.9 mg/kg/day reduced blood pressure less than did 2.8 mg/kg/day and provided partial protection against cerebral and renal lesions after a 4-week infusion period. These results indicate that long-term subcutaneous infusion of beraprost can protect saline-drinking SHRSPs against stroke and renal damage. This effect is not readily dissociated from the ability of beraprost to reduce blood pressure in SHRSPs.

Prostacyclin and thromboxane A2 synthesis are increased in acute lower limb ischaemia

Prostacyclin (PGI2) and thromboxane A2 (TXA2) play an important role in the pathophysiology of various cardiovascular diseases. The balance between PGI2 and TXA2 regulates the interaction between platelets and the vessel wall in vivo. In this study we measured PGI2 and TXA2 synthesis by analysing their urinary index metabolites 2,3-dinor-6-keto-PGF1 alpha and 11-dehydro-TXB2, respectively, in acute (10 patients) and chronic (10 patients) lower limb ischaemia. Both PGI2 and TXA2 synthesis were increased about two-fold in patients with acute lower limb ischaemia compared to chronic lower limb ischaemia. However, the PGI2/TXA2 ratio was more or less the same in acute and chronic lower limb ischaemia. In patients with acute lower limb ischaemia caused by thrombotic occlusion, PGI2 and TXA2 formation were about two times higher than in patients with acute lower limb ischaemia caused by embolic occlusion. Elevation of PGI2 and TXA2 synthesis in acute lower limb ischaemia may reflect increased platelet-vascular wall interactions without changing the PGI2/TXA2 ratio.

Neural regulation of coronary vascular resistance: role of nitric oxide in reflex cholinergic coronary vasodilation in normal and pathophysiologic states

A number of reflexes participate in the control of coronary vascular resistance through activation of the sympathetic or parasympathetic nervous system. Classically, activation of vagal efferent fibers to the heart results in vasodilation due to the release of acetylcholine and activation of muscarinic receptors. Recently, we have found that activation of a number of reflexes in conscious dogs, the Bezold-Jarisch reflex and the carotid chemoreflex in particular, results in cholinergic coronary vasodilation which is blocked by an inhibitor of nitric oxide synthesis, nitro-L-arginine. After the development of pacing-induced heart failure, the cholinergic dilation subsequent to activation of the Bezold-Jarisch or carotid chemoreflex is essentially abolished, since coronary blood vessels no longer produce nitric oxide. In contrast, after brief exercise training, there is a potentiation of Bezold-Jarisch reflex-induced coronary vasodilation since exercise upregulates nitric oxide production by coronary blood vessels. Since the Bezold-Jarisch reflex may be important as a compensatory mechanism during acute myocardial infarction, and the carotid chemoreflex is the acute mechanisms responsible for ameliorating systemic hypoxemia, the role of nitric oxide in reflex cholinergic coronary vasodilation may be essential in the compensatory vascular adjustments evoked by these and other reflexes.

The influence of prostacyclin (PGI2) on contractile properties of isolated right ventricle of rat heart

The mechanism of the positive inotropic effect of prostacyclin (PGI2) (2.6 x 10(-6) mol/l) on the isolated right ventricle of rat heart was studied. Our results show that the positive inotropic effect of prostacyclin is produced indirectly through beta adrenoceptors and slow Ca2+ channels, because blockade of slow Ca2+ channels with verapamil (10(-6) mol/l) and beta adrenoceptors with propranolol (10(-6) mol/l) abolishes this effect. Alpha adrenoceptors do not mediate the action of PGI2.

Effect of the ultrashort-acting beta-blocker Brevibloc on free-radical-mediated injuries during the early reperfusion state

The left descending coronary artery (LAD) was ligated for 45 min or 90 min followed by 1-h reperfusion. During experiments the animals in Group I (15 dogs) received saline infusion, in Group II (20 dogs) they received the ultrashort-acting beta-blocker Brevibloc (esmolol HCl). The marker of lipid peroxidation the malondialdehyde (MDH) as well as endogen scavengers, the glutathione (GSH), and superoxide dismutase (SOD) were measured in the heart tissue homogenates. In blood and heart tissue samples the 6-keto-prostaglandin F1 alpha (PGF1 alpha) and thromboxane B2 (TXB2) were determined. Biochemical measurements revealed that esmolol HCl has beneficial effect on the free-radical-meduated-damage reducing the MDA content in the ischemic area. In Group I the value of MDA after 90 min of LAD ligature was 138 +/- 5.6%, in Group II the elevation was only 107.4 +/- 3.2%. After treatment with Brevibloc the GSH content of ischemic-reperfused areas decreased slightly (81.75 +/- 3.5% of the normal value), moreover, in Group I the depletion of GSH was considerable (64.5 +/- 4.2%). Coronary reperfusion caused the release of eicosanoids in both groups, mainly in the first 10 min. The highest value of thromboxane in blood samples could be measured in Group I after 90 min of LAD ligature (24.8 +/- 3.6 pmol/ml; the normal value 8-12 pmol/ml). In Group II during the same period of experiments the TXB2 in the blood was 14 +/- 3.7 pmol/ml. In heart tissue samples the amount of endoperoxides increased in ischemic and non-ischemic areas of Group I and II. Nevertheless, the calculated ratio of PGF1 alpha and TXB2 was near to the normal after Brevibloc treatment (0.85-0.9; the normal values were 1-1.2). These results indicate that esmolol HCl can modulate both the free-radical-mediated reaction and arachidonic acid metabolism.

Evidence for metabolism of 3-methylindole by prostaglandin H synthase and mixed-function oxidases in goat lung and liver microsomes

3-Methylindole (3MI) is the causative agent of a naturally occurring lung disease in cattle. The effects of 3MI on the prostaglandin H synthase (PHS) and mixed-function oxidase (MFO) systems in lung and liver microsomes were investigated. Addition of 3MI in goat lung microsomes resulted in a pronounced increase in PHS activity as indicated by both the initial rate and total oxygen consumption. The effect of 3MI on PHS activity was dependent on arachidonic acid and inhibited by indomethacin. PHS was capable of activating [14C]3MI to a reactive intermediate as indicated by the covalent binding of 3MI to microsomal protein. [14C]3MI was converted to water-soluble 3MI metabolites in the PHS system. Biosynthesis of total prostaglandins was enhanced 69% with 0.5 mM 3MI. PHS-catalyzed co-oxidation of 3MI was shown to be independent of the MFO-catalyzed metabolism of 3MI. PHS and MFO enzyme activities were found to exist in both goat lung and liver microsomes. PHS activity was significantly higher in lung microsomes than in liver microsomes. Pulmonary MFO activity, unlike hepatic MFO activity, was enhanced with addition of 3MI. This indicates a tissue selectivity for the metabolism of 3MI. Lung microsomal PHS and MFO systems were capable of activating 3MI to a greater extent than liver microsomes. Thus, the combined effects of PHS and MFO systems in the activation of 3MI, the selectivity of pulmonary MFO to metabolize 3MI, and the capacity for altered prostaglandin biosynthesis in lung tissue may explain the tissue specificity of 3MI-induced toxicity.

Effect of varying concentrations of linoleic acid on alpha-adrenoceptor responses in spontaneously hypertensive rats

The effect of increased intake of linoleic acid on the alpha-adrenergic system was assessed by safflower oil supplementation to spontaneously hypertensive rats. Linoleic acid-enriched intake at 5%, 15% and 30% by weight of total food intake for 12 wk was associated with a reduction in resting arterial blood pressure, while heart rate and heart to body weight ratios were similar to control group values. A dose-response analysis to norepinephrine bitartrate administered intravenously indicated a significant reduction in the vascular reactivity to this alpha-adrenergic agonist in all groups given linoleic acid. Direct assessment of alpha-adrenoceptor number (Bmax) and affinity (KD) in cardiac sarcolemma with [3H]-prazosin indicated that receptor binding properties were not affected by linoleic acid intake. Our results suggest that short-term linoleic acid supplementation in the established hypertensive state may lower blood pressure through effects upon alpha-adrenergic reactivity in vascular tissue, without associated effects in cardiac tissue.