Effects of thromboxane synthase inhibition on vascular responsiveness in the in vivo rat mesentery

Cytosolic Phospholipase A2α Is Essential for Renal Dysfunction and End-Organ Damage Associated With Angiotensin II-Induced Hypertension

BACKGROUND

The kidney plays an important role in regulating blood pressure (BP). cPLA2α in the kidney is activated by various agents including angiotensin II (Ang II) and selectively releases arachidonic acid (AA) from tissue lipids, generating pro- and antihypertensive eicosanoids. Since activation of cPLA2α is the rate-limiting step in AA release, this study was conducted to determine its contribution to renal dysfunction and end-organ damage associated with Ang II-induced hypertension.

METHODS

cPLA2α(+/+) and cPLA2α(-/-) mice were infused with Ang II (700 ng/ kg/min) or its vehicle for 13 days. Mice were placed in metabolic cages to monitor their food and water intake, and urine was collected and its volume was measured. Doppler imaging was performed to assess renal hemodynamics. On the 13th day of Ang II infusion, mice were sacrificed and their tissues and blood collected for further analysis.

RESULTS

Ang II increased renal vascular resistance, water intake, and urine output and Na(+) excretion, decreased urine osmolality, and produced proteinuria in cPLA2α(+/+) mice. Ang II also caused accumulation of F4/80(+) macrophages and CD3(+) T cells and renal fibrosis, and increased oxidative stress in the kidneys of cPLA2α(+/+) mice. All these effects of Ang II were minimized in cPLA2α(-/-) mice.

CONCLUSION

cPLA2α contributes to renal dysfunction, inflammation, and end-organ damage, most likely via the action of pro-hypertensive eicosanoids and increased oxidative stress associated with Ang II-induced hypertension. Thus, cPLA2α could serve as a potential therapeutic target for treating renal dysfunction and end-organ damage in hypertension.

Arthur C. Corcoran Memorial Lecture. The role of eicosanoids in angiotensin-dependent hypertension

Many eicosanoids produced in vascular and renal structures are endowed with the ability to influence vascular and renal mechanisms of blood pressure regulation. Eicosanoids subserve both prohypertensive and antihypertensive mechanisms. The development of angiotensin-dependent hypertension in rats is accompanied by increased vascular production of thromboxane A2 (TXA2) and of lipoxygenase-derived products with the ability to inhibit prostacyclin synthase. As a result of these abnormalities, the activity of pressor mechanisms mediated by TXA2 and/or prostaglandin (PG) H2 is increased. The cancellation of TXA2- and/or of PGH2-mediated pressor mechanisms, after treatment with thromboxane synthase inhibitors or TXA2/PGH2 receptor blockers, lowers blood pressure in rats with angiotensin-dependent hypertension. Inhibitors of lipoxygenase also lower blood pressure in such animals, in part by decreasing the synthesis of lipoxygenase-derived inhibitors of prostacyclin synthase. Thus, the vasodepressor effect of these agents is accompanied by increased vascular formation of PGI2 and can be prevented by cyclooxygenase inhibitors. Cyclooxygenase-derived eicosanoids, PGE2 and PGI2, also subserve antihypertensive mechanisms in angiotensin-dependent models of hypertension. The level of blood pressure in such models of hypertension reflects, in part, the interplay among prohypertensive and antihypertensive functions subserved by cyclooxygenase- and lipoxygenase-derived eicosanoids.

Effects of thromboxane agonists on cardiac adrenergic neurotransmission

The effects of thromboxane B2 (TxB2) and of two thromboxane mimetics, dl-(9,11), (11,12)-dimethano-TxA2 (ONO 11006) and 9,11-dideoxy-11,9-epoxymethano prostaglandin F2 alpha (U46619) on the cardiac response to adrenergic nerve stimulation in isolated guinea-pig atria were evaluated. All the agonists dose dependently reduced the positive inotropic effect induced by field stimulation, U46619 being the most active. The inhibitory effect of U46619 was reduced by the thromboxane receptor antagonists, sulotroban and AH 23848B. U46619 did not significantly reduce the positive inotropic effect induced by exogenous noradrenaline. However U46619 was unable to modify the tritium overflow induced by field stimulation in preparations preloaded with [3H]noradrenaline. In addition to this influence on adrenergic neurotransmission, U46619 also had a direct positive inotropic effect on cardiac contractility, which was antagonized by AH 23848B. These results indicate that U46619 reduces the cardiac response to sympathetic nerve stimulation and that is also has a direct stimulatory effect on cardiac muscle.

Thromboxane mediates renal hemodynamic response to infused angiotensin II

Since we had found that angiotensin II (Ang II), but not phenylephrine (PE), increased the excretion of thromboxane (Tx) and raised mean arterial pressure (MAP) by a Tx-dependent mechanism, we tested the role of TxA2 in mediating Ang II-induced changes in renal hemodynamics. For series 1, groups of anesthetized rats received an i.v. infusion of Ang II (50 ng.kg-1.min-1). When infused with a vehicle, Ang II increased MAP, renal vascular resistance (RVR) and the excretion of TxB2 factored by GFR. A PGH2-TxA2 receptor antagonist, SQ-29,548, or three days of pretreatment with a TxA2 synthase inhibitor UK-38,485, which reduced excretion of TxB2 by 80%, blunted the rise in MAP and RVR induced by Ang II. In contrast, three days of pretreatment with indomethacin did not alter the renal vascular response to Ang II. For series 2, groups of rats received Ang II at a higher rate (500 ng.kg-1.min-1) while the RPP was stabilized at +11 to +15 mm Hg with a suprarenal aortic clamp. SQ-29,548 and UK-38,485 both prevented Ang II-induced reductions in GFR and blocked 80% of the increase in RVR. For series 3, infusions of phenylephrine at an equipressor dose to series 2 of 30 micrograms.kg-1.min-1 with control of RPP at +14 mm Hg also increased RVR but this was not blunted by SQ-29,548.

IN CONCLUSION

1.) infusion of Ang II increases excretion of filtered TxB2, causes dose-dependent increases in RVR and, at high doses, reduces GFR.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of prostaglandin I2 and a selective thromboxane synthetase inhibitor on the arteriolar constriction caused by norepinephrine

Effects of prostaglandin (PG) I2 and thromboxane (Tx) A2 on the systemic blood pressure, the internal diameter of arterioles with a diameter of 20 to 30 microns, and the change caused by the topical administration of norepinephrine (NE) were studied in the rat mesentery. The change in diameter was measured by an image-shearing eyepiece and television microscopy. The rats were divided into three groups. Group I (9 rats) served as the non-treated, control group. In group II (6 rats), PGI2 was given at the rate of 0.6 micrograms/kg/min for ten minutes. In group III (6 rats), (E)-3-[4-(1-imidazolylmethyl)phenyl]-2-propenoic acid hydrochloride monohydrate (OKY 046), which is a highly selective TxA2 synthetase inhibitor, was administered at the rate of 0.5 mg/kg/min for ten minutes. The hypotension caused by PGI2 administration was accompanied by arteriolar constriction, although it disappeared with the increase in the systemic blood pressure five minutes after administration of PGI2 was stopped. The systemic blood pressure and the internal diameter of arterioles showed no change in the OKY 046 and in the control group. The vasoconstrictive response to NE was inhibited by PGI2 and OKY 046 for thirty to forty-five minutes and the inhibitory effects were similar in both cases. These results show that PGI2 and a selective Tx synthetase inhibitor, OKY 046, attenuate the vascular response to NE.

Increased bioavailability of enzymes of eicosanoid synthesis in hepatic and extrahepatic tissues after portacaval shunting

Metabolites of arachidonic acid have been attributed to severe circulatory, metabolic and hormonal alterations in patients with chronic liver disease. In order to study changes of the tissue-specific availability of enzymes of eicosanoid synthesis, we used portacaval-shunted rats, as this model exhibits many clinical and biochemical similarities to patients suffering from cirrhosis of the liver. Microsomal mass and maximal velocity of prostaglandin H synthase, the initial enzyme of prostaglandin synthesis, were markedly and permanently increased after shunting in both hepatic and extrahepatic tissues as compared to those of sham-operated rats. Maximal velocity of thromboxane synthase and prostacyclin synthase, two more peripheral enzymes of the arachidonic acid cascade, were tissue-specifically enhanced, whereas the apparent affinities (Km) remained unchanged. Determination of 5-lipoxygenase activity in tissue preparations disclosed a preferential increase in the liver, lung and renal cortex after portacaval shunting. Furthermore, exposure to endotoxin closely mimicked the shunting-induced changes. These results suggest that after portacaval shunting and possibly in patients with advanced liver disease, profound abnormalities at the level of local enzyme expression might play a pathophysiologically important role in the control of eicosanoid synthesis.

Effects of thromboxane synthase inhibitors on renal function

In general the effects of thromboxane A2(TXA2) on renal function are opposite those produced by other prostanoids. TXA2 synthase inhibitors decrease the biosynthesis of TXA2 and may increase the production of other prostanoids by causing endoperoxide shunting. Therefore, in situations of increased kidney arachidonate mobilization, inhibition of renal TXA2 synthase might alter renal function by reducing TXA2 production and/or increasing prostaglandin (PG) biosynthesis. This hypothesis was tested by comparing the changes in renal function induced by suprarenal aortic constriction in anesthetized dogs pretreated with either a TXA2 synthase inhibitor (UK38,485; n = 7 or OKY1581; n = 7) or vehicle (0.1 M Na2CO3; n = 9). Several renal function parameters were compared in control versus treated animals by analysis of variance. Neither UK38,485 (1 mg/kg, i.v.) nor OKY1581 (10 mg/kg, i.v.) significantly altered renal artery hypotension-induced changes in mean arterial blood pressure, heart rate, renal blood flow, renal vascular resistance, glomerular filtration, filtration fraction, urine flow rate, sodium excretion rate, fractional sodium excretion, potassium excretion, or fractional potassium excretion. However, both UK38,485 and OKY1581 seemed to attenuate the increase in renal renin secretion rate induced by suprarenal aortic constriction. We conclude that acute administration of TXA2 synthase inhibitors does not modify acute renal artery hypotension-induced changes in either electrolyte excretion or renal hemodynamics. However, acute administration of TXA2 inhibitors attenuates suprarenal aortic constriction-induced increases in renin release in anesthetized dogs by unknown mechanisms.

Caffeine enhances the slow-pressor response to angiotensin II in rats. Evidence for a caffeine-angiotensin II interaction with the sympathetic nervous system

The purpose of this study was to determine if caffeine augments the slow-pressor response to chronic low-dose infusions of angiotensin II (AII) or the rapid-pressor response to acute infusions of AII. AII was infused (125 ng/min i.p.) for 12 d via mini-osmotic pumps in four groups of rats: group I, intact rats not treated with caffeine (n = 9); group II, intact rats treated with caffeine (0.1% in drinking water, n = 9); group III, rats previously sympathectomized with 6-hydroxydopamine, but not treated with caffeine (n = 10); and group IV, rats previously sympathectomized with 6-hydroxydopamine and treated with caffeine (n = 10). Chronic low-dose AII infusions slowly elevated systolic blood pressure in all groups. Caffeine greatly augmented this slow-pressor response to AII in intact animals; however, caffeine failed to enhance AII-induced hypertension in sympathectomized rats. Caffeine pretreatment did not enhance the rapid-pressor response to acute intravenous infusions of AII. We conclude that caffeine augmented the slow-pressor effect of chronic low-dose infusions of AII via a mechanism that involved the sympathetic nervous system.

Pharmacotherapy of circulatory shock

The rubric "shock" encompasses a wide spectrum of critical events, which if untreated, result in morbidity and mortality. Understanding of the various forms of shock has evolved rapidly in the past 20 years as new laboratory and clinical observations have been published. In this article, the authors discuss the physiology of the shock state, review the circumstances in which shock becomes likely, and review the etiologies and diagnostic characteristics of distributive (septic, spinal, anaphylactoid/anaphylactic), cardiogenic, hypovolemic, and obstructive shock. The rationale and applications of conventional and controversial therapies are discussed. The therapeutic potentials of current lines of shock research are also discussed.