The signaling pathway for aldosterone-induced mitochondrial production of superoxide anion in the myocardium.

Mineralocorticoid receptor (MR) antagonists decrease morbidity and mortality in heart failure patients for whom oxidative stress is usual; however, the underlying mechanism for this protection is unclear. Since aldosterone stimulates reactive oxygen species (ROS) production in several tissues, we explored its effect and the intracellular pathway involved in the rat myocardium. Aldosterone dose-dependently increased O2(-) production in myocardial slices. At 10 nmol/L, aldosterone increased O2(-) to 165 ± 8.8% of control, an effect prevented not only by the MR antagonists eplerenone and spironolactone (107 ± 7.8 and 103 ± 5.3%, respectively) but also by AG1478 (105 ± 8.0%), antagonist of the EGF receptor (EGFR). Similar results were obtained by silencing MR expression through the direct intramyocardial injection of a lentivirus coding for a siRNA against the MR. The aldosterone effect on O2(-) production was mimicked by the mKATP channel opener diazoxide and blocked by preventing its opening with 5-HD and glibenclamide, implicating the mitochondria as the source of O2(-). Inhibiting the respiratory chain with rotenone or mitochondrial permeability transition (MPT) with cyclosporine A or bongkrekic acid also canceled aldosterone-induced O2(-) production. In addition, aldosterone effect depended on NADPH oxidase and phosphoinositide 3-kinase activation, as apocynin and wortmannin, respectively, inhibited it. EGF (0.1 µg/mL) similarly increased O2(-), although in this case MR antagonists had no effect, suggesting that EGFR transactivation occurred downstream from MR activation. Inhibition of mKATP channels, the respiratory chain, or MPT did not prevent Akt phosphorylation, supporting that it happened upstream of the mitochondria. Importantly, cardiomyocytes were confirmed as a source of aldosterone induced mitochondrial ROS production in experiments performed in isolated cardiac myocytes. These results allow us to speculate that the beneficial effects of MR antagonists in heart failure may be related to a decrease in oxidative stress.

Mitochondrial reactive oxygen species (ROS) as signaling molecules of intracellular pathways triggered by the cardiac renin-angiotensin II-aldosterone system (RAAS).

Mitochondria represent major sources of basal reactive oxygen species (ROS) production of the cardiomyocyte. The role of ROS as signaling molecules that mediate different intracellular pathways has gained increasing interest among physiologists in the last years. In our lab, we have been studying the participation of mitochondrial ROS in the intracellular pathways triggered by the renin-angiotensin II-aldosterone system (RAAS) in the myocardium during the past few years. We have demonstrated that acute activation of cardiac RAAS induces mitochondrial ATP-dependent potassium channel (mitoKATP) opening with the consequent enhanced production of mitochondrial ROS. These oxidant molecules, in turn, activate membrane transporters, as sodium/hydrogen exchanger (NHE-1) and sodium/bicarbonate cotransporter (NBC) via the stimulation of the ROS-sensitive MAPK cascade. The stimulation of such effectors leads to an increase in cardiac contractility. In addition, it is feasible to suggest that a sustained enhanced production of mitochondrial ROS induced by chronic cardiac RAAS, and hence, chronic NHE-1 and NBC stimulation, would also result in the development of cardiac hypertrophy.

Insulin resistance in adipocytes from spontaneously hypertensive rats: effect of long-term treatment with enalapril and losartan.

Insulin responsiveness was studied in isolated adipocytes from the normotensive Wistar Kyoto (WKY) rat and the spontaneously hypertensive rat (SHR). The effect of insulin (0.1 to 5 nmol/L) on glucose uptake (glucose transport and lipogenesis) was measured, and the maximal effect of insulin (Emax) and the dose of insulin required to elicit 50% of the maximal response (EC50) were calculated. A diminished Emax on lipogenesis without changes in the EC50 was detected in SHRs. The Emax was 0.49 +/- 0.09 (SHR) and 1.16 +/- 0.14 (WKY) micromol/10(5) cells (P < .05), and the EC50 was 0.13 +/- 0.03 and 0.11 +/- 0.02 nmol/L for WKY and SHR, respectively. Similar results were obtained when measuring insulin-stimulated glucose transport. A 30-day long-term treatment with enalapril (20 mg/kg/d) normalized insulin responsiveness in adipocytes from SHRs. The effect of enalapril was suppressed when SHRs were pretreated with enalapril and 150 microg/kg/d of the bradykinin (BK) B2-receptor blocker Hoe 140. Pretreatment with losartan (40 mg/kg/d) did not improve insulin action in the SHR. Since these results were obtained with isolated cells in which glucose availability was not a function of blood flow, and the effect of insulin in the SHR was improved by pretreatment with an angiotensin-converting enzyme (ACE) inhibitor but not with the AT1-receptor blocker, it appears that the insulin resistance linked to the hypertension is not related to changes in blood flow.

Comparative effects of insulin and isoproterenol on lipoprotein lipase in rat adipose cells.

The effects of insulin and isoproterenol on lipoprotein lipase mass and enzyme activity were investigated in rat adipocytes. Cells were pulse labeled for 1 h with [35S]methionine to measure immunoprecipitable lipoprotein lipase. The results showed that 80% of the newly synthesized enzyme was membrane associated and 20% was secreted into the cell incubation medium. Enzyme activity was mainly associated with lipoprotein lipase secreted into the medium. A 10-min incubation with 10(-7) M insulin stimulated the secretion of lipoprotein lipase activity and the activity associated with adipocyte membranes. Conversely, 10(-6) M isoproterenol decreased the activity in all fractions. In addition, insulin increased lipoprotein lipase mass associated with cell membranes and decreased that in the incubation medium, whereas isoproterenol induced a decrease in both cell membranes and medium. Insulin and isoproterenol stimulated phosphorylation of lipoprotein lipase. These findings suggest that insulin stimulates the secretion of active lipoprotein lipase and a reuptake of inactive secreted enzyme, and isoproterenol decreases the activity by enzyme degradation. Moreover, because both agents stimulate phosphorylation of lipoprotein lipase, phosphorylation may play a role in the effect of insulin increasing enzyme activity, in secretion or reuptake, and in the effect of isoproterenol inducing degradation of lipoprotein lipase.