Thioredoxin 1 (TRX1) Overexpression Cancels the Slow Force Response (SFR) Development.

The stretch of cardiac muscle increases developed force in two phases. The first phase occurs immediately after stretch and is the expression of the Frank-Starling mechanism, while the second one or slow force response (SFR) occurs gradually and is due to an increase in the calcium transient amplitude. An important step in the chain of events leading to the SFR generation is the increased production of reactive oxygen species (ROS) leading to redox sensitive ERK1/2, p90RSK, and NHE1 phosphorylation/activation. Conversely, suppression of ROS production blunts the SFR. The purpose of this study was to explore whether overexpression of the ubiquitously expressed antioxidant molecule thioredoxin-1 (TRX1) affects the SFR development and NHE1 phosphorylation. We did not detect any change in basal phopho-ERK1/2, phopho-p90RSK, and NHE1 expression in mice with TRX1 overexpression compared to wild type (WT). Isolated papillary muscles from WT or TRX1-overexpressing mice were stretched from 92 to 98% of its maximal length. A prominent SFR was observed in WT mice that was completely canceled in TRX1 animals. Interestingly, myocardial stretch induced a significant increase in NHE1 phosphorylation in WT mice that was not detected in TRX1-overexpressing mice. These novel results suggest that magnification of cardiac antioxidant defense power by overexpression of TRX1 precludes NHE1 phosphorylation/activation after stretch, consequently blunting the SFR development.

Silencing of epidermal growth factor receptor reduces Na+/H+ exchanger 1 activity and hypertensive cardiac hypertrophy.

Pathological cardiac hypertrophy (PCH) can be triggered by epidermal growth factor receptor (EGFR) transactivation. Progression of PCH can be prevented by inhibition of hyperactive Na+/H+ exchanger isoform 1 (NHE1). We first aimed, to limit PCH of spontaneously hypertensive rats (SHR) by specific and localized silencing of cardiac EGFR, and second to study the connection of its activation pathway with cardiac NHE1 activity. Short hairpin RNA (shRNA) against EGFR was delivered with a lentivirus (l-shEGFR) in the cardiac left ventricle (LV) wall. Protein expression was analyzed by immunoblots, and NHE1 activity was indirectly measured in isolated papillary muscles by rate of pHi recovery from transient acidification. EGFR protein expression in the LV was reduced compared to the group injected with l-shSCR (Scrambled sequence) without changes in ErbB2 or ErbB4. Hypertrophic parameters together with cardiomyocytes cross sectional area were reduced in animals injected with l-shEGFR. Echocardiographic analysis exhibited a reduced fractional shortening in the l-shSCR group 30 days following treatment that was not observed in l-shEGFR group. l-shEGFR treated rats presented a reduced basal production of reactive oxygen species and decreased lipid peroxidation. NHE1 activity was significantly diminished in hearts with a partial EGFR silencing, without modification of its protein expression. We conclude that specifically silencing cardiac EGFR expression prevents progression of PCH through a pathway that involves a decrease in the NHE1 activity. Lentiviral vectors prove to be a valuable tool for long term expression of shRNA, bringing the possibility to extend its use in clinical area.

p38-MAP Kinase Negatively Regulates the Slow Force Response to Stretch in Rat Myocardium through the Up-Regulation of Dual Specificity Phosphatase 6 (DUSP6).

BACKGROUND/AIMS: Myocardial stretch increases cardiac force in two consecutive phases: The first one due to Frank-Starling mechanism, followed by the gradually developed slow force response (SFR). The latter is the mechanical counterpart of an autocrine/paracrine mechanism involving the release of angiotensin II (Ang II) and endothelin (ET) leading to Na⁺/H⁺ exchanger 1 (NHE-1) phosphorylation and activation. Since previous evidence indicates that p38-MAP kinase (p38-MAPK) negatively regulates the Ang II-induced NHE1 activation in vascular smooth muscle and the positive inotropic effect of ET in the heart, we hypothesized that this kinase might modulate the magnitude of the SFR to stretch. METHODS: Experiments were performed in isolated rat papillary muscles subjected to sudden stretch from 92 to 98% of its maximal length, in the absence or presence of the p38-MAPK inhibitor SB202190, or its inactive analogous SB202474. Western blot technique was used to determine phosphorylation level of p38-MAPK, ERK1/2, p90RSK and NHE-1 (previously immunoprecipitated with NHE-1 polyclonal antibody). Dual specificity phosphatase 6 (DUSP6) expression was evaluated by RT-PCR and western blot. Additionally, the Na⁺-dependent intracellular pH recovery from an ammonium prepulse-induced acid load was used to asses NHE-1 activity. RESULTS: The SFR was larger under p38-MAPK inhibition (SB202190), effect that was not observed in the presence of an inactive analogous (SB202474). Myocardial stretch activated p38-MAPK, while pre-treatment with SB202190 precluded this effect. Inhibition of p38-MAPK increased stretched-induced NHE-1 phosphorylation and activity, key event in the SFR development. Consistently, p38-MAPK inhibition promoted a greater increase in ERK1/2-p90RSK phosphorylation/activation after myocardial stretch, effect that may certainly be responsible for the observed increase in NHE-1 phosphorylation under this condition. Myocardial stretch induced up-regulation of the DUSP6, which specifically dephosphorylates ERK1/2, effect that was blunted by SB202190. CONCLUSION: Taken together, our data support the notion that p38-MAPK activation after myocardial stretch restricts the SFR by limiting ERK1/2-p90RSK phosphorylation, and consequently NHE-1 phosphorylation/activity, through a mechanism that involves DUSP6 up-regulation.

Aldosterone stimulates the cardiac sodium/bicarbonate cotransporter via activation of the g protein-coupled receptor gpr30.

Some cardiac non-genomic effects of aldosterone (Ald) are reported to be mediated through activation of the classic mineralocorticoid receptor (MR). However, in the last years, it was proposed that activation of the novel G protein-coupled receptor GPR30 mediates certain non-genomic effects of Ald. The aim of this study was to elucidate if the sodium/bicarbonate cotransporter (NBC) is stimulated by Ald and if the activation of GPR30 mediates this effect. NBC activity was evaluated in rat cardiomyocytes perfused with HCO3(-)/CO2 solution in the continuous presence of HOE642 (sodium/hydrogen exchanger blocker) during recovery from acidosis using intracellular fluorescence measurements. Ald enhanced NBC activity (% of ΔJHCO3(-); control: 100±5.82%, n=7 vs Ald: 151.88±11.02%, n=5; P<0.05), which was prevented by G15 (GPR30 blocker, 90.53±7.81%, n=7). Further evidence for the involvement of GPR30 was provided by G1 (GPR30 agonist), which stimulated NBC (185.13±18.28%, n=6; P<0.05) and this effect was abrogated by G15 (124.19±10.96%, n=5). Ald- and G1-induced NBC stimulation was abolished by the reactive oxygen species (ROS) scavenger MPG and by the NADPH oxidase inhibitor apocynin. In addition, G15 prevented Ald- and G1-induced ROS production. Pre-incubation of myocytes with wortmannin (PI3K-AKT pathway blocker) prevented Ald- or G1-induced NBC stimulation. In summary, Ald stimulates NBC by GPR30 activation, ROS production and AKT stimulation.

Reactive oxygen species partially mediate high dose angiotensin II-induced positive inotropic effect in cat ventricular myocytes.

BACKGROUND: Reactive oxygen species, such as superoxide, are being increasingly recognized as key components of a vast array of signaling pathways. Angiotensin II is a well-recognized stimulus for superoxide production through NADPH oxidase activation and opening of the mitochondrial ATP-sensitive potassium channels (mKATP). A role for this mechanism has been proposed to explain several physiological effects of the peptide. The aim of this study was to evaluate the involvement of this mechanism in the inotropic response to 100nmol/L angiotensin II. METHODS: Sarcomere shortening and intracellular pH (BCECF-epifluorescence technique) were evaluated in isolated cat ventricular myocytes placed in a perfusion chamber on the stage of an inverted microscope. Myocardial superoxide production was evaluated by the lucigenin quimioluminiscence method. RESULTS: Angiotensin II (100nmol/L) increased~70% sarcomere shortening, effect that was only partially prevented by NADPH oxidase inhibition, mKATP channel blockade or inhibition of the cardiac Na(+)/H(+) exchanger (NHE-1). Moreover, angiotensin II stimulates NHE-1 activity by a NADPH oxidase-dependent mechanism. Myocardial superoxide production was also increased by angiotensin II, and this action was completely prevented either by NADPH oxidase inhibition or mKATP channel blockade. CONCLUSIONS: The positive inotropic response to 100nmol/L angiotensin II is due to both ROS/NHE-1 dependent and independent pathways, this being a point of divergence with the signaling previously described to be triggered by lower concentrations of angiotensin II (i.e.: 1nmol/L).

Physiological cardiac hypertrophy: critical role of AKT in the prevention of NHE-1 hyperactivity.

BACKGROUND: The involvement of NHE-1 hyperactivity, critical for pathological cardiac hypertrophy (CH), in physiological CH has not been elucidated yet. Stimulation of NHE-1 increases intracellular Na(+) and Ca(2+) favouring calcineurin activation. Since myocardial stretch, an activator of NHE-1, is common to both types of CH, we speculate that NHE-1 hyperactivity may also happen in physiological CH. However, calcineurin activation is characteristic only for pathological hypertrophy. We hypothesize that an inhibitory AKT-dependent mechanism prevents NHE-1 hyperactivity in the setup of physiological CH. METHODS: Physiological CH was induced in rats by swimming (90 min/day, 12 weeks) or in cultured isolated cardiomyocytes with IGF-1 (10 nmol/L). RESULTS: Training induced eccentric CH development (left ventricular weight/tibial length: 22.0±0.3 vs. 24.3±0.7 mg/mm; myocyte cross sectional area: 100±3.2 vs. 117±4.1 %; sedentary (Sed) and swim-trained (Swim) respectively; p<0.05] with decreased myocardial stiffness and collagen deposition [1.7±0.05 % (Sed) vs. 1.4±0.09 % (Swim); p<0.05]. Increased phosphorylation of AKT, ERK1/2, p90(RSK) and NHE-1 at the consensus site for ERK1/2-p90(RSK) were detected in the hypertrophied hearts (P-AKT: 134±10 vs. 100±5; P-ERK1/2: 164±17 vs. 100±18; P-p90(RSK): 160±18 vs. 100±9; P-NHE-1 134±10 vs. 100±10; % in Swim vs. Sed respectively; p<0.05). No significant changes were detected neither in calcineurin activation [calcineurin Aß 100±10 (Sed) vs. 96±12 (Swim)], nor NFAT nuclear translocation [100±3.11 (Sed) vs. 95±9.81 % (Swim)] nor NHE-1 expression [100±8.5 (Sed) vs. 95±6.7 % (Swim)]. Interestingly, the inhibitory phosphorylation of the NHE-1 consensus site for AKT was increased in the hypertrophied myocardium (151.6±19.4 (Swim) vs. 100±9.5 % (Sed); p<0.05). In isolated cardiomyocytes 24 hours IGF-1 increased cell area (114±1.3 %; p<0.05) and protein/DNA content (115±3.9 %, p<0.05), effects not abolished by NHE-1 inhibition with cariporide (114±3 and 117±4.4 %, respectively). IGF-1 significantly decreased NHE-1 activity during pHi recovery from sustained intracellular acidosis (JH+ at pHi 6.8: 4.08±0.74 and 9.09±1.21 mmol/L/min, IGF-1 vs. control; p<0.05), and abolished myocardial slow force response, the mechanical counterpart of stretch-induced NHE-1 activation. CONCLUSIONS: NHE-1 hyperactivity seems not to be involved in physiological CH development, contrary to what characterizes pathological CH. We propose that AKT, through an inhibitory phosphorylation of the NHE-1, prevents its stretch-induced activation. This posttranslational modification emerges as an adaptive mechanism that avoids NHE-1 hyperactivity preserving its housekeeping functioning.

Myocardial mineralocorticoid receptor activation by stretching and its functional consequences.

Myocardial stretch triggers an angiotensin II-dependent autocrine/paracrine loop of intracellular signals, leading to reactive oxygen species-mediated activation of redox-sensitive kinases. Based on pharmacological strategies, we previously proposed that mineralocorticoid receptor (MR) is necessary for this stretch-triggered mechanism. Now, we aimed to test the role of MR after stretch by using a molecular approach to avoid secondary effects of pharmacological MR blockers. Small hairpin interference RNA capable of specifically knocking down the MR was incorporated into a lentiviral vector (l-shMR) and injected into the left ventricular wall of Wistar rats. The same vector but expressing a nonsilencing sequence (scramble) was used as control. Lentivirus propagation through the left ventricle was evidenced by confocal microscopy. Myocardial MR expression, stretch-triggered activation of redox-sensitive kinases (ERK1/2-p90(RSK)), the consequent Na(+)/H(+) exchanger-mediated changes in pHi (HEPES-buffer), and its mechanical counterpart, the slow force response, were evaluated. Furthermore, reactive oxygen species production in response to a low concentration of angiotensin II (1.0 nmol/L) or an equipotent concentration of epidermal growth factor (0.1 µg/mL) was compared in myocardial tissue slices from both groups. Compared with scramble, animals transduced with l-shMR showed (1) reduced cardiac MR expression, (2) cancellation of angiotensin II-induced reactive oxygen species production but preservation of epidermal growth factor-induced reactive oxygen species production, (3) cancellation of stretch-triggered increase in ERK1/2-p90(RSK) phosphorylation, (4) lack of stretch-induced Na(+)/H(+) exchanger activation, and (5) abolishment of the slow force response. Our results provide strong evidence that MR activation occurs after myocardial stretch and is a key factor to promote redox-sensitive kinase activation and their downstream consequences.

Mitochondrial NHE1: a newly identified target to prevent heart disease.

Mitochondrial damage has been associated with early steps of cardiac dysfunction in heart subjected to ischemic stress, oxidative stress and hypertrophy. A common feature for the mitochondrial deterioration is the loss of the mitochondrial membrane potential (ΔΨ m) with the concomitant irreversible opening of the mitochondrial permeability transition pore (MPTP) which follows the mitochondrial Ca(2+) overload, and the subsequent mitochondrial swelling. We have recently characterized the expression of the Na(+)/H(+) exchanger 1 (mNHE1) in mitochondrial membranes. This surprising observation provided a unique target for the prevention of the Ca(2+)-induced MPTP opening, based on the inhibition of the NHE1 m. In this line, inhibition of NHE1 m activity and/or reduction of NHE1 m expression decreased the Ca(2+)-induced mitochondrial swelling and the release of reactive oxygen species (ROS) in isolated cardiac mitochondria and preserved the ΔΨ m in isolated cardiomyocytes. Mitochondrial NHE1 thus represents a novel target to prevent cardiac disease, opening new avenues for future research.

Aldosterone stimulates the cardiac Na(+)/H(+) exchanger via transactivation of the epidermal growth factor receptor.

The use of antagonists of the mineralocorticoid receptor in the treatment of myocardial hypertrophy and heart failure has gained increasing importance in the last years. The cardiac Na(+)/H(+) exchanger (NHE-1) upregulation induced by aldosterone could account for the genesis of these pathologies. We tested whether aldosterone-induced NHE-1 stimulation involves the transactivation of the epidermal growth factor receptor (EGFR). Rat ventricular myocytes were used to measure intracellular pH with epifluorescence. Aldosterone enhanced the NHE-1 activity. This effect was canceled by spironolactone or eplerenone (mineralocorticoid receptor antagonists), but not by mifepristone (glucocorticoid receptor antagonist) or cycloheximide (protein synthesis inhibitor), indicating that the mechanism is mediated by the mineralocorticoid receptor triggering nongenomic pathways. Aldosterone-induced NHE-1 stimulation was abolished by the EGFR kinase inhibitor AG1478, suggesting that is mediated by transactivation of EGFR. The increase in the phosphorylation level of the kinase p90(RSK) and NHE-1 serine703 induced by aldosterone was also blocked by AG1478. Exogenous epidermal growth factor mimicked the effects of aldosterone on NHE-1 activity. Epidermal growth factor was also able to increase reactive oxygen species production, and the epidermal growth factor-induced activation of the NHE-1 was abrogated by the reactive oxygen species scavenger N-2-mercaptopropionyl glycine, indicating that reactive oxygen species are participating as signaling molecules in this mechanism. Aldosterone enhances the NHE-1 activity via transactivation of the EGFR, formation of reactive oxygen species, and phosphorylation of the exchanger. These results call attention to the consideration of the EGFR as a new potential therapeutic target of the cardiovascular pathologies involving the participation of aldosterone.

Silencing of NHE-1 blunts the slow force response to myocardial stretch.

Myocardial stretch induces a biphasic force response: a first abrupt increase followed by a slow force response (SFR), believed to be the in vitro manifestation of the Anrep effect. The SFR is due to an increase in Ca²âº transient of unclear mechanism. We proposed that Na⁺/H⁺ exchanger (NHE-1) activation is a key factor in determining the contractile response, but recent reports challenged our findings. We aimed to specifically test the role of the NHE-1 in the SFR. To this purpose small hairpin interference RNA capable of mediating specific NHE-1 knockdown was incorporated into a lentiviral vector (l-shNHE1) and injected into the left ventricular wall of Wistar rats. Injection of a lentiviral vector expressing a nonsilencing sequence (scramble) served as control. Myocardial NHE-1 protein expression and function (the latter evaluated by the recovery of pH(i) after an acidic load and the SFR) were evaluated. Animals transduced with l-shNHE1 showed reduced NHE-1 expression (45 ± 8% of controls; P < 0.05), and the presence of the lentivirus in the left ventricular myocardium, far from the site of injection, was evidenced by confocal microscopy. These findings correlated with depressed basal pH(i) recovery after acidosis [(max)dpH(i)/dt 0.055 ± 0.008 (scramble) vs. 0.009 ± 0.004 (l-shNHE1) pH units/min, P < 0.05], leftward shift of the relationship between J(H⁺) (H⁺ efflux corrected by the intrinsic buffer capacity), and abolishment of SFR (124 ± 2 vs. 101 ± 2% of rapid phase; P < 0.05) despite preserved ERK1/2 phosphorylation [247 ± 12 (stretch) and 263 ± 23 (stretch l-shNHE1) % of control; P < 0.05 vs. nonstretched control], well-known NHE-1 activators. Our results provide strong evidence to propose NHE-1 activation as key factor in determining the SFR to stretch.

Antibodies against the cardiac sodium/bicarbonate co-transporter (NBCe1) as pharmacological tools.

BACKGROUND AND PURPOSE: Na(+) /HCO(3) (-) co-transport (NBC) regulates intracellular pH (pH(i) ) in the heart. We have studied the electrogenic NBC isoform NBCe1 by examining the effect of functional antibodies to this protein. EXPERIMENTAL APPROACH: We generated two antibodies against putative extracellular loop domains 3 (a-L3) and 4 (a-L4) of NBCe1 which recognized NBCe1 on immunoblots and immunostaining experiments. pH(i) was monitored using epi-fluorescence measurements in cat ventricular myocytes. Transport activity of total NBC and of NBCe1 in isolation were evaluated after an ammonium ion-induced acidosis (expressed as H(+) flux, J(H) , in mmol·L(-1) min(-1) at pH(i) 6.8) and during membrane depolarization with high extracellular potassium (potassium pulse, expressed as ΔpH(i) ) respectively. KEY RESULTS: The potassium pulse produced a pH(i) increase of 0.18 ± 0.006 (n= 5), which was reduced by the a-L3 antibody (0.016 ± 0.019). The a-L-3 also decreased J(H) by 50%. Surprisingly, during the potassium pulse, a-L4 induced a higher pH(i) increase than control,(0.25 ± 0.018) whereas the recovery of pH(i) from acidosis was faster (J(H) was almost double the control value). In perforated-patch experiments, a-L3 prolonged and a-L4 shortened action potential duration, consistent with blockade and stimulation of NBCe1-carried anionic current respectively. CONCLUSIONS AND IMPLICATIONS: Both antibodies recognized NBCe1, but they had opposing effects on the function of this transporter, as the a-L3 was inhibitory and the a-L4 was excitatory. These antibodies could be valuable in studies on the pathophysiology of NBCe1 in cardiac tissue, opening a path for their potential clinical use.

Silencing of cardiac mitochondrial NHE1 prevents mitochondrial permeability transition pore opening.

Inhibition of Na(+)/H(+) exchanger 1 (NHE1) reduces cardiac ischemia-reperfusion (I/R) injury and also cardiac hypertrophy and failure. Although the mechanisms underlying these NHE1-mediated effects suggest delay of mitochondrial permeability transition pore (MPTP) opening, and reduction of mitochondrial-derived superoxide production, the possibility of NHE1 blockade targeting mitochondria has been incompletely explored. A short-hairpin RNA sequence mediating specific knock down of NHE1 expression was incorporated into a lentiviral vector (shRNA-NHE1) and transduced in the rat myocardium. NHE1 expression of mitochondrial lysates revealed that shRNA-NHE1 transductions reduced mitochondrial NHE1 (mNHE1) by ∼60%, supporting the expression of NHE1 in mitochondria membranes. Electron microscopy studies corroborate the presence of NHE1 in heart mitochondria. Immunostaining of rat cardiomyocytes also suggests colocalization of NHE1 with the mitochondrial marker cytochrome c oxidase. To examine the functional role of mNHE1, mitochondrial suspensions were exposed to increasing concentrations of CaCl(2) to induce MPTP opening and consequently mitochondrial swelling. shRNA-NHE1 transduction reduced CaCl(2)-induced mitochondrial swelling by 64 ± 4%. Whereas the NHE1 inhibitor HOE-642 (10 µM) decreased mitochondrial Ca(2+)-induced swelling in rats transduced with nonsilencing RNAi (37 ± 6%), no additional HOE-642 effects were detected in mitochondria from rats transduced with shRNA-NHE1. We have characterized the expression and function of NHE1 in rat heart mitochondria. Because mitochondria from rats injected with shRNA-NHE1 present a high threshold for MPTP formation, the beneficial effects of NHE1 inhibition in I/R resulting from mitochondrial targeting should be considered.

The Anrep effect requires transactivation of the epidermal growth factor receptor.

Myocardial stretch elicits a biphasic contractile response: the Frank-Starling mechanism followed by the slow force response (SFR) or Anrep effect. In this study we hypothesized that the SFR depends on epidermal growth factor receptor (EGFR) transactivation after the myocardial stretch-induced angiotensin II (Ang II)/endothelin (ET) release. Experiments were performed in isolated cat papillary muscles stretched from 92 to 98% of the length at which maximal twitch force was developed (L(max)). The SFR was 123 +/- 1% of the immediate rapid phase (n = 6, P < 0.05) and was blunted by preventing EGFR transactivation with the Src-kinase inhibitor PP1 (99 +/- 2%, n = 4), matrix metalloproteinase inhibitor MMPI (108 +/- 4%, n = 11), the EGFR blocker AG1478 (98 +/- 2%, n = 6) or the mitochondrial transition pore blocker clyclosporine (99 +/- 3%, n = 6). Stretch increased ERK1/2 phosphorylation by 196 +/- 17% of control (n = 7, P < 0.05), an effect that was prevented by PP1 (124 +/- 22%, n = 7) and AG1478 (131 +/- 17%, n = 4). In myocardial slices, Ang II (which enhances ET mRNA) or endothelin-1 (ET-1)-induced increase in O(2)() production (146 +/- 14%, n = 9, and 191 +/- 17%, n = 13, of control, respectively, P < 0.05) was cancelled by AG1478 (94 +/- 5%, n = 12, and 98 +/- 15%, n = 8, respectively) or PP1 (100 +/- 4%, n = 6, and 99 +/- 8%, n = 3, respectively). EGF increased O(2)() production by 149 +/- 4% of control (n = 9, P < 0.05), an effect cancelled by inhibiting NADPH oxidase with apocynin (110 +/- 6% n = 7), mKATP channels with 5-hydroxydecanoic acid (5-HD; 105 +/- 5%, n = 8), the respiratory chain with rotenone (110 +/- 7%, n = 7) or the mitochondrial permeability transition pore with cyclosporine (111 +/- 10%, n = 6). EGF increased ERK1/2 phosphorylation (136 +/- 8% of control, n = 9, P < 0.05), which was blunted by 5-HD (97 +/- 5%, n = 4), suggesting that ERK1/2 activation is downstream of mitochondrial oxidative stress. Finally, stretch increased Ser703 Na(+)/H(+) exchanger-1 (NHE-1) phosphorylation by 172 +/- 24% of control (n = 4, P < 0.05), an effect that was cancelled by AG1478 (94 +/- 17%, n = 4). In conclusion, our data show for the first time that EGFR transactivation is crucial in the chain of events leading to the Anrep effect.

Early signals after stretch leading to cardiac hypertrophy. Key role of NHE-1.

The enhanced activity of the cardiac Na+/H+ exchanger (NHE-1) after myocardial stretch is considered a key step of the intracellular signaling pathway leading to the slow force response to stretch as well as an early signal for the development of cardiac hypertrophy. We propose that the chain of events triggered by stretch begins with the release of small amounts of Angiotensin II (Ang II)/endothelin (ET) and ends with the increase in intracellular Ca2+ concentration ([Ca2+]i) through the Na+/Ca2+ exchanger in reverse mode (NCX(rev)), which triggers cardiac hypertrophy by activation of widely recognized Ca2+-dependent intracellular signaling pathways.

The electrogenic Na+/HCO3- cotransport modulates resting membrane potential and action potential duration in cat ventricular myocytes.

Perforated whole-cell configuration of patch clamp was used to determine the contribution of the electrogenic Na+/HCO3- cotransport (NBC) on the shape of the action potential in cat ventricular myocytes. Switching from Hepes to HCO3- buffer at constant extracellular pH (pH(o)) hyperpolarized resting membrane potential (RMP) by 2.67 +/- 0.42 mV (n = 9, P < 0.05). The duration of action potential measured at 50% of repolarization time (APD50) was 35.8 +/- 6.8% shorter in the presence of HCO3- than in its absence (n = 9, P < 0.05). The anion blocker SITS prevented and reversed the HCO3- -induced hyperpolarization and shortening of APD. In addition, no HCO3- -induced hyperpolarization and APD shortening was observed in the absence of extracellular Na+. Quasi-steady-state currents were evoked by 8 s duration voltage-clamped ramps ranging from -130 to +30 mV. A novel component of SITS-sensitive current was observed in the presence of HCO3-. The HCO3- -sensitive current reversed at -87 +/- 5 mV (n = 7), a value close to the expected reversal potential of an electrogenic Na+/HCO3- cotransport with a HCO3-:Na+ stoichiometry ratio of 2: 1. The above results allow us to conclude that the cardiac electrogenic Na+/HCO3- cotransport has a relevant influence on RMP and APD of cat ventricular cells.

[Angiotensin II-induced endothelin-1 release in cardiac myocytes]. / Liberación de endotelina-1 por angiotensina II en miocitos cardíacos aislados.

Many of the effects thought to be due to angiotensin II (Ang II) are due to the release/formation of endothelin (ET). We tested whether Ang II elicits its positive inotropic effect (PIE) by the action of endogenous ET-1 and the role played by the reactive oxygen species (ROS) in this mechanism. Experiments were performed in cat isolated ventricular myocytes in which sarcomere shortening (SS) was measured to asses contractility after pharmacological interventions and the effect of Ang II on inotropism were analyzed. Ang II 1 nM increased SS by 31.8 +/- 3.8% (p < 0.05). This PIE was cancelled by AT1 receptor blockade, by ET-1 receptors blockade, by Na+/H+ exchanger (NHE) inhibition, by reverse mode Na+/Ca2+ exchanger (NCX) blockade or by ROS scavenging. Ang II 100 nM increases SS by 70.5 +/- 7.6% (p < 0.05). This PIE was completely abolished by AT1 receptors blockade and were partially bocked by ET-1 receptors blockade, by NHE inhibition, by reverse mode NCX blockade or by ROS scavenging. Ang II increased preproET-1 mRNA, effect that was blunted by AT1 receptors blockade. We conclude that Ang II induces (through its AT1 receptor) release/formation of ET-1, which acting in autocrine fashion on ET receptors of the isolated myocytes increases inotropism through NHE stimulation and NCX reverse mode activation. The participation of ROS is involved is this chain of events.

The positive inotropic effect of angiotensin II: role of endothelin-1 and reactive oxygen species.

Many effects believed to be because of angiotensin II (Ang II) are attributable to the action of endothelin (ET)-1, which is released/produced by Ang II. We investigated whether Ang II elicits its positive inotropic effect (PIE) by the action of endogenous ET-1, in addition to the role played by reactive oxygen species (ROS) in this mechanism. Cat cardiomyocytes were used for: (1) sarcomere shortening measurements; (2) ROS measurements by epifluorescence; (3) immunohistochemical staining for preproET-1, BigET-1, and ET-1; and (4) measurement of preproET-1 mRNA by RT-PCR. Cells were exposed to 1 nmol/L Ang II for 15 minutes. This low concentration of Ang II increases sarcomere shortening by 29.2+/-3.7% (P<0.05). This PIE was abrogated by Na+/H+ exchanger or Na+/Ca2+ exchanger reverse mode inhibition. The production of ROS increased in response to Ang II treatment (DeltaROS respect to control: 68+/-15 fluorescence units; P<0.05). The Ang II-induced PIE and ROS production were blocked by the Ang II type 1 receptor blocker losartan, the nonselective ET-1 receptor blocker TAK044, the selective ETA receptor blocker BQ-123, or the ROS scavenger N-(2-mercapto-propionyl)glycine. Exogenous ET-1 (0.4 nmol/L) induced a similar PIE and increase in ROS production to those caused by Ang II. Immunostaining for preproET-1, BigET-1, and ET-1 was positive in cardiomyocytes. The preproET-1 mRNA abundance increased from 100+/-4.6% in control to 241.9+/-39.9% in Ang II-treated cells (P<0.05). We conclude that the PIE after exposure to 1 nmol/L Ang II is due to endogenous ET-1 acting through the ETA receptor and triggering ROS production, Na+/H+ exchanger stimulation, and Na+/Ca2+ exchanger reverse mode activation.

Liberacion de endotelina-1 por angiotensina ii en miocitos cardiacos aislados / Angiotensin II-induced endothelin-1 release in cardiac myocytes

Muchos de los efectos de la angiotensina II (Ang II) son mediados en realidad por la acción de endotelina (ET) endógena liberada y/o producida en respuesta a la Ang II. En este trabajo evaluamos la interacción Ang II/ET-1, sus consecuencias en la contractilidad cardíaca y el papel de las especies reactivas del oxígeno (EROs). Se usaron cardiomiocitos aislados de gato. La Ang II, 1 nM, produjo un efecto inotrópico positivo (EIP) de 31.8±3.8% que fue cancelado por inhibición de los receptores AT1, de los receptores de ET, del intercambiador Na+/H+ (NHE), del modo inverso del intercambiador Na+/Ca2+ (NCX) o por el secuestro de EROs. La Ang II, 100 nM, produjo un EIP de 70.5±7.6% que fue cancelado por inhibición de los receptores AT1y bloqueado en parte por inhibición de los receptores de ET, del NHE, del modo inverso del NCX o por el secuestro de EROs. La Ang II, 1 nM, incrementó el ARNm de la preproET-1 lo cual fue anulado por el bloqueo de los receptores AT1. Los resultados permiten concluir que el EIP de la Ang II es debido a la acción de la ET-1 endógena liberada/formada por la Ang II. La ET-1 produce: estimulación del NHE, activación del modo inverso del NCX y un consecuente EIP. Dentro de esta cascada también participarían los EROs.

Many of the effects thought to be due to angiotensin II (Ang II) are due to the release/formation of endothelin (ET). We tested whether Ang II elicits its positive inotropic effect (PIE) by the action of endogenous ET-1 and the role played by the reactive oxygen species (ROS) in this mechanism. Experiments were performed in cat isolated ventricular myocytes in which sarcomere shortening (SS) was measured to asses contractility after pharmacological interventions and the effect of Ang II on inotropism were analyzed. Ang II 1 nM increased SS by 31.8±3.8% (p<0.05). This PIE was cancelled by AT1 receptor blockade, by ET-1 receptors blockade, by Na+/H+ exchanger (NHE) inhibition, by reverse mode Na+/Ca2+ exchanger (NCX) blockade or by ROS scavenging. Ang II 100 nM increases SS by 70.5±7.6% (p<0.05). This PIE was completely abolished by AT1 receptors blockade and were partially bocked by ET-1 receptors blockade, by NHE inhibition, by reverse mode NCX blockade or by ROS scavenging. Ang II increased preproET-1 mRNA, effect that was blunted by AT1 receptors blockade. We conclude that Ang II induces (through its AT1 receptor) release/formation of ET-1, which acting in autocrine fashion on ET receptors of the isolated myocytes increases inotropism through NHE stimulation and NCX reverse mode activation. The participation of ROS is involved is this chain of events.

Myocardial hypertrophy of normotensive Wistar-Kyoto rats.

In our studies with spontaneously hypertensive (SHR), Wistar-Kyoto (WKY), and Wistar rats, we observed normotensive WKY rats with cardiac hypertrophy determined by a greater left ventricular (LV) mass (LVM)-to-body weight (BW) ratio (LVM/BW) than that of normotensive Wistar rats. Thus we compared the following parameters in SHR, WKY, and Wistar rats: LVM/BW, cell capacitance as index of total surface area of the myocytes, length, width, and cross-sectional area of cardiac myocytes, LV collagen volume fraction, and myocardial stiffness. The LVM/BW of WKY (2.41 +/- 0.03 mg/g, n = 41) was intermediate between SHR (2.82 +/- 0.04 mg/g, n = 47) and Wistar rats (1.98 +/- 0.04 mg/g, n = 28). A positive correlation between blood pressure and LVM was found in SHR, whereas no such relationship was observed in WKY or Wistar rats. Cell capacitance and cross-sectional area were not significantly different in SHR and WKY rats; these values were significantly higher than those of Wistar rats. The cell length was smaller but the width was similar in WKY compared with SHR. Papillary muscles isolated from the LV of WKY and SHR were stiffer than those from Wistar rats. Consistently, a greater level of myocardial fibrosis was detected in WKY and SHR compared with Wistar rats. These findings demonstrate blood pressure-independent cardiac hypertrophy in normotensive WKY rats.