Attenuation of renovascular hypertension by cyclooxygenase-2 inhibitor partly through ANP release

Sex-dependent compensatory mechanisms preserve blood pressure homeostasis in prostacyclin receptor-deficient mice

Inhibitors of microsomal prostaglandin E synthase 1 (mPGES-1) are in the early phase of clinical development. Deletion of mPges-1 in mice confers analgesia, restrains atherogenesis, and fails to accelerate thrombogenesis, while suppressing prostaglandin E2 (PGE2), but increasing the biosynthesis of prostacyclin (PGI2). In low-density lipoprotein receptor-deficient (Ldlr-/-) mice, this last effect represents the dominant mechanism by which mPges-1 deletion restrains thrombogenesis, while suppression of PGE2 accounts for its antiatherogenic effect. However, the effect of mPges-1 depletion on blood pressure (BP) in this setting remains unknown. Here, we show that mPges-1 depletion significantly increased the BP response to salt loading in male Ldlr-/- mice, whereas, despite the direct vasodilator properties of PGI2, deletion of the I prostanoid receptor (Ipr) suppressed this response. Furthermore, combined deletion of the Ipr abrogated the exaggerated BP response in male mPges-1-/- mice. Interestingly, these unexpected BP phenotypes were not observed in female mice fed a high-salt diet (HSD). This is attributable to the protective effect of estrogen in Ldlr-/- mice and in Ipr-/- Ldlr-/- mice. Thus, estrogen compensates for a deficiency in PGI2 to maintain BP homeostasis in response to high salt in hyperlipidemic female mice. In male mice, by contrast, the augmented formation of atrial natriuretic peptide (ANP) plays a similar compensatory role, restraining hypertension and oxidant stress in the setting of Ipr depletion. Hence, men with hyperlipidemia on a HSD might be at risk of a hypertensive response to mPGES-1 inhibitors.

ANP-stimulated Na+ secretion in the collecting duct prevents Na+ retention in the renal adaptation to acid load

We have recently reported that type A intercalated cells of the collecting duct secrete Na⁺ by a mechanism coupling the basolateral type 1 Na⁺-K⁺-2Cl^- cotransporter with apical type 2 H⁺-K⁺-ATPase (HKA2) functioning under its Na⁺/K⁺ exchange mode. The first aim of the present study was to evaluate whether this secretory pathway is a target of atrial natriuretic peptide (ANP). Despite hyperaldosteronemia, metabolic acidosis is not associated with Na⁺ retention. The second aim of the present study was to evaluate whether ANP-induced stimulation of Na⁺ secretion by type A intercalated cells might account for mineralocorticoid escape during metabolic acidosis. In Xenopus oocytes expressing HKA2, cGMP, the second messenger of ANP, increased the membrane expression, activity, and Na⁺-transporting rate of HKA2. Feeding mice with a NH₄Cl-enriched diet increased urinary excretion of aldosterone and induced a transient Na⁺ retention that reversed within 3 days. At that time, expression of ANP mRNA in the collecting duct and urinary excretion of cGMP were increased. Reversion of Na⁺ retention was prevented by treatment with an inhibitor of ANP receptors and was absent in HKA2-null mice. In conclusion, paracrine stimulation of HKA2 by ANP is responsible for the escape of the Na⁺-retaining effect of aldosterone during metabolic acidosis.

Effect of asiatic acid on the Ang II-AT1R-NADPH oxidase-NF-κB pathway in renovascular hypertensive rats

Asiatic acid, a triterpenoid compound derived from Centella asiatica, has been demonstrated to have antioxidant and anti-inflammatory effects. The present study evaluated the effects of asiatic acid on hemodynamic alterations, renin-angiotensin system (RAS), oxidative stress, and inflammation in 2K-1C hypertensive rats. Renovascular hypertension was induced in male Sprague-Dawley rats and treated with vehicle, asiatic acid (30 mg/kg/day), or captopril (5 mg/kg/day) for 4 weeks. We observed that 2K-1C hypertensive rats exhibited hemodynamic alterations such as high blood pressure, heart rate, hindlimb vascular resistance, and low hindlimb blood flow. Signs of RAS activation, such as increased plasma angiotensin II and serum angiotensin-converting enzyme activity, enhanced AT₁R protein expression, and suppressed AT₂R expression was observed in 2K-1C hypertensive rats. Overproduction of vascular superoxide, high levels of plasma MDA, low levels of plasma nitric oxide metabolites (NOx), and upregulation of gp91^phox protein expression were observed in hypertensive rats. Furthermore, inflammation was observed in hypertensive rats, as evidenced by increased plasma TNF-α, NF-κB, and phospho-NF-κB protein expression. Asiatic acid or captopril alleviated hemodynamic alterations, RAS activation, oxidative stress, and inflammation in 2K-1C hypertensive rats. These findings indicate that asiatic acid is an antihypertensive agent that ameliorates hemodynamic alterations in 2K-1C hypertensive rats. This effect may involve one or both of the following mechanisms: the direct effect of asiatic acid on RAS activation, oxidative stress and inflammation, and/or asiatic acid acting as an ACE inhibitor agent to inhibit the Ang II-AT₁R-NADPH oxidase-NF-κB pathway.

The Role of Cyclooxygenase Enzymes in the Effects of Losartan and Lisinopril on the Contractions of Rat Thoracic Aorta

OBJECTIVE

It was suggested that prostaglandins which are synthesized by cyclooxygenase (COX) enzymes contribute to the actions of angiotensin-converting enzyme (ACE) inhibition and angiotensin AT1 receptor antagonism and there is an interaction between ACE signaling pathway and COX enzymes. We aim to investigate the role of COX enzymes in the effects of losartan, an angiotensin II (Ang II) receptor antagonist or lisinopril, an ACE inhibitor, on the contractions of rat thoracic aorta in isolated tissue bath.

MATERIALS AND METHODS

Responses of losartan (10^-6, 10^-5, 10^-4 M), lisinopril (10^-6, 10^-5, 10^-4 M), and non-selective COX inhibitor dipyrone (10^-4, 7 × 10^-4, 2 × 10^-3 M) alone to the contractions induced by phenylephrine (Phe) (10^-7 M), potassium chloride (KCl) (6 × 10^-2 M), Ang II (10^-8 M) and responses of losartan or lisinopril in combination with dipyrone to the contractions induced by Phe or KCl were recorded.

RESULTS

When used alone, dipyrone and losartan inhibited Phe, KCl, and Ang II-induced contractions, whereas lisinopril inhibited only Phe and Ang II-induced contractions. Inhibition of COX enzymes (COX-3, COX-3 + COX-1, COX-1+ COX-2 + COX-3 by dipyrone 10^-4, 7 × 10^-4, 2 × 10^-3 M, respectively) augmented the relaxant effects of losartan or lisinopril. Also, dipyrone potentiated the effect of lisinopril on KCl-induced contractions.

CONCLUSION

We suggest that dipyrone increases the smooth-muscle relaxing effects of losartan or lisinopril and that COX enzyme inhibition may have a role in the enhancement of this relaxation.