Prostanoids and aldosterone-induced mild experimental hypertension in rats

Blood pressure has only minor influence on aldosterone-induced oxidative stress and DNA damage in vivo

Epidemiological studies found an increased kidney cancer risk in hypertensive patients. These patients frequently present an increase in the mineralocorticoid aldosterone (Ald) due to a stimulated renin angiotensin aldosterone system (RAAS). Recently, we showed pro-oxidative and genotoxic effects of Ald in vitro. Here, we investigated the influence of blood pressure on aldosterone-induced oxidative damage. To distinguish whether effects in Sprague-Dawley rats treated with Ald were caused by Ald or by increased blood pressure, the mineralocorticoid receptor (MR) antagonist spironolactone was administered in a subtherapeutical dose, not lowering the blood pressure, and hydralazine, a RAAS-independent vasodilator, was given to normalize the pressure. With the antioxidant tempol, oxidative stress-dependent effects were demonstrated. Ald treatment caused kidney damage and oxidative and nitrative stress. Structural DNA damage and the mutagenic oxidative base modification 7,8-dihydro-8-oxoguanine were increased, as well as DNA repair activity and nuclear NF-κB translocation. Spironolactone and tempol decreased all markers significantly, whereas hydralazine had just slight effects. These data comprise the first report of essentially blood pressure-independent tissue- and DNA-damaging effects of Ald. A fully activated MR and the production of reactive oxygen and nitrogen species were crucial for these effects.

Chronic administration of aldosterone depresses baroreceptor reflex function in the dog

In a previous study it was shown that acute perfusion of aldosterone into the isolated carotid sinus decreased baroreceptor activity. The aim of the present study was to determine whether chronic, systemic administration of aldosterone also depresses baroreflex function. In six conscious dogs, the baroreflex was determined before and 10 days after an osmotic minipump containing aldosterone (100 micrograms/kg in 2 mL) was implanted. The slope of the relation between systolic arterial pressure and heart rate was significantly blunted after aldosterone administration (9.1 +/- 0.7 versus 13.3 +/- 1.2 for nitroglycerin, P < .01; 23.4 +/- 5.0 versus 40.1 +/- 5.0 for phenylephrine, P < .01). Baroreflex slopes did not change in a sham group (minipump with saline) and an aldosterone plus spironolactone (600 mg/d) group. Plasma aldosterone levels were significantly elevated after the aldosterone minipump was implanted (443 +/- 72 versus 37 +/- 11 pg/mL, P < .001). Mean arterial pressure was not significantly increased after aldosterone (106.5 +/- 3.8 versus 100.4 +/- 2.6 mm Hg, P = .2). On the 10th day after aldosterone or saline infusion, an acute experiment was carried out. Single baroreceptor fibers were recorded from the carotid sinus nerve. Compared with the sham group, the threshold was significantly elevated in the aldosterone group (111.3 +/- 2.1 versus 85.8 +/- 2.8 mm Hg), and the peak discharge rate was markedly decreased (32.5 +/- 1.5 versus 54.7 +/- 2.5 spikes per second, P < .01). The depressed baroreceptor function could be partially restored after a bolus injection of the Na+,K(+)-ATPase inhibitor ouabain (5 micrograms/kg i.v.).(ABSTRACT TRUNCATED AT 250 WORDS)

Aldosterone reduces baroreceptor discharge in the dog

We have previously demonstrated that baroreceptor discharge sensitivity is depressed in dogs with experimental heart failure and that this depressed sensitivity can be reversed by the Na+,K(+)-ATPase inhibitor ouabain. This suggests that enhanced Na+,K(+)-ATPase activity in baroreceptors is responsible for the blunted baroreceptor discharge sensitivity seen in heart failure state. Because aldosterone, a known stimulator of Na+,K(+)-ATPase, is elevated in heart failure the present study was undertaken to determine the effects on baroreceptor discharge of perfusion of the carotid sinus with aldosterone in normotensive dogs. Single unit baroreceptor activity was recorded as well as carotid sinus pressure and the diameter of the carotid sinus. Perfusion of the carotid sinus with aldosterone (in Krebs-Henseleit solution) significantly elevated threshold pressure (108.5 +/- 3.1 mm Hg versus 92.7 +/- 4.6 mm Hg, p less than 0.05) and reduced peak discharge rate (40.3 +/- 3.9 spikes/sec, p less than 0.05). These effects appeared 15 minutes after aldosterone perfusion and remained constant for the next 60 minutes. There was no change in the carotid sinus pressure-diameter curve during perfusion with aldosterone. Perfusion of the carotid sinus with ouabain (0.1 microgram/ml) during aldosterone perfusion did not reverse the blunted baroreceptor discharge. The blunted baroreceptor activity induced by perfusion of the carotid sinus with aldosterone was prevented by removal of the endothelial cells in the carotid sinus area with a balloon-tipped catheter or by perfusion with saponin. Finally, perfusion of the carotid sinus with spironolactone (10 ng/ml), a mineralocorticoid receptor antagonist, prevented the inhibitory effect of aldosterone. These data suggest that aldosterone reduces maximum baroreceptor discharge.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of prostanoids in renin-dependent and renin-independent hypertension

We investigated the role of prostanoid-mediated pressor mechanisms in setting the level of blood pressure in renin-dependent and renin-independent models of hypertension in unanesthetized rats. Intravenous administration of a blocker of thromboxane A2/prostaglandin endoperoxide receptors, SQ29548 (2 mg/kg bolus injection plus 2 mg/kg/hr for 3 hours), reduced from 162 +/- 4 to 144 +/- 5 mm Hg (p less than 0.05) the blood pressure of rats with aortic coarctation-induced hypertension at 7-14 days after coarctation when plasma renin activity is greatly increased. In contrast, treatment with SQ29548 was without effect on the blood pressure of either normotensive or hypertensive rats (i.e., aortic coarctation-induced hypertension at 90-113 days after coarctation, deoxycorticosterone-salt-induced hypertension) having normal or depressed values of plasma renin activity. The blood pressure-lowering effect of SQ29548 in the early phase of aortic coarctation-induced hypertension was positively correlated with the prevailing plasma renin activity and could not be demonstrated in hypertensive rats pretreated with indomethacin. We attribute the hypotensive effect of SQ29548 to interference with pressor mechanisms that depend on activation of thromboxane A2/prostaglandin endoperoxide receptors and suggest that such prostanoid-mediated mechanisms are operational and contribute to an increase in blood pressure in angiotensin-dependent forms of hypertension. Also prostanoid-mediated vasodepressor mechanisms are operational in the early phase of aortic coarctation-induced hypertension since the blood pressure of rats pretreated with SQ29548 was increased by the subsequent administration of indomethacin. Accordingly, the blood pressure of rats with aortic coarctation-induced hypertension is influenced by the interplay of prostanoid-mediated pressor and vasodepressor mechanisms.