Comparison of the protective effects of ischemic preconditioning and the Na+ /H+ exchanger blockade

The protective effects of ischemic preconditioning (IP) and Na+/H+ exchanger blockade (NHEb) by two blockers [ethylisopropylamiloride (EIPA) and HOE 642] were compared in the isovolumic perfused rat heart. The impairment in systolic and diastolic function detected in control ischemic hearts (C) exposed to 20 min of ischemia and 30 min of reperfusion was diminished in similar extent by IP and by NHEb with EIPA and HOE 642. At the end of the reperfusion period +dP/dtmax values were 57±9% in C hearts and 94±6%, 82±6% and 104±6% after IP and NHEb with EIPA and HOE 642, respectively. A depletion of ATP levels detected in C hearts after reperfusion (from 20.2±0.8 µmol/g dry weight before ischemia to 6.9±0.7 µmol/g dry weight) was partially prevented by both IP and NHEb with EIPA (9.2±0.7 µmol/g dry weight and 11.1±0.5 µmol/g dry weight, respectively). The ischemic contracture (IC), assessed by the left ventricular end diastolic pressure (LVEDP), observed in C hearts (35±4 mmHg) was not decreased by IP (40±4 mmHg) but it was prevented by NHEb (18±4 mmHg and 10±3 mmHg with EIPA and HOE 642, respectively). The ATP levels at the end of the ischemic period were similar in C and IP hearts (4.1±0.2 µmol/g dry wt vs. 3.3±0.4 µmol/g dry wt) but they were significantly higher after NHEb with HOE 642 (7.0±1.0 µmol/g dry wt). PKC inhibition by chelerythrine abolished the protection induced by IP after reperfusion although not the improvement induced by NHEb with EIPA.Facultad de Ciencias Exacta

Sodium-hydrogen exchanger, cardiac overload, and myocardial hypertrophy

Overload of neonatal and adult cardiomyocytes and multicellular myocardial preparations, which include whole hearts, are accompanied by an enhanced activity of the Na+/H+ exchanger 1 (NHE-1). Exogenous administration of prohypertrophic agents such as angiotensin II (Ang II), endothelin-1 (ET-1), and α1-adrenergic agonists also stimulates NHE-1 activity, which leads to an increased concentration of intracellular Na+ ([Na+]i). Moreover, inhibition of NHE-1 activity prevents the increase in [Na+]i, induces the regression of cardiac hypertrophy, and exerts beneficial effects in experimental heart failure. The present review summarizes the current knowledge of the causative factors and pathophysiological correlation of cardiac overload and NHE-1 activity.Centro de Investigaciones Cardiovasculare

Sodium-hydrogen exchanger, cardiac overload, and myocardial hypertrophy

Overload of neonatal and adult cardiomyocytes and multicellular myocardial preparations, which include whole hearts, are accompanied by an enhanced activity of the Na+/H+ exchanger 1 (NHE-1). Exogenous administration of prohypertrophic agents such as angiotensin II (Ang II), endothelin-1 (ET-1), and α1-adrenergic agonists also stimulates NHE-1 activity, which leads to an increased concentration of intracellular Na+ ([Na+]i). Moreover, inhibition of NHE-1 activity prevents the increase in [Na+]i, induces the regression of cardiac hypertrophy, and exerts beneficial effects in experimental heart failure. The present review summarizes the current knowledge of the causative factors and pathophysiological correlation of cardiac overload and NHE-1 activity.Centro de Investigaciones Cardiovasculare

Interaction between calcium and slow channel blocking drugs on atrial rate

The relationship between extracellular calcium concentration and the chronotropic effect of prenylamine, verapamil and nifedipine was studied in isolated spontaneously beating rat atria. The three slow channel blocking drugs produced a concentration-dependent decrease in atrial rate, though with different relative potencies. The order of potency for decreasing atrial rate, independently of the calcium level (1.0, 3.0, 6.0 or 9.0 mmol/l) was: verapamil > nifedipine > prenylamine. Increasing calcium from 1.0 to 6.0 and 9.0 mmol/l increased atrial rate from 251±beats·min−1 to 265±6 beats·min−1 and 285±9 beats·min−1 (mean±1 standard error) respectively (P<0.05). Despite their positive chronotropic effect high calcium levels failed to reverse the negative chronotropic effect of the slow channel blockers. Furthermore, the negative chronotropic effect of both verapamil and nifedipine was enhanced at high calcium levles. Raising calcium from 1.0 to 6.0 mmol/l in the presence of verapamil (1×10−7 mol/l) or nifedipine (3×10−7 mol/l) increased 2-fold the negative chronotropic effect of the calcium channel blockers. In addition, the concentration-effect curves for verapamil and nifedipine shifted to the left by 0.50±0.14 and 0.50±0.16 log units, respectively, when calcium increased from 1.0 to 6.0 mmol/l.Centro de Investigaciones Cardiovasculare

NHE-1 and NHE-6 Activities : Ischemic and Reperfusion Injury

The study published in this issue of Circulation Research showing that a null mutation of NHE-1 improves the tolerance of the heart to ischemia and reperfusion (I/R) is an important contribution for the following reasons: (1) In the animals with null mutation, contracture during the ischemic period was less and ATP levels were preserved compared with wild-type animals. This observation, on the one hand, provides evidence that protection by downregulation of NHE-1 during the ischemic period itself is indeed possible and, on the other hand, it argues against the suggestion that the exchanger is inactive during this same period. (2) In contrast with chronic blockade of the NHE-1 by pharmacological interventions, the long-term absence of the exchanger does not elicit major compensatory changes that, in turn, might negate the cardioprotective effect of blocking its activity for a relative short term. This point is related to a recent publication showing that long-term treatment with the NHE-1 blocker cariporide is followed by an upregulation of the functional units of the exchanger in a similar way to the well-known tolerance phenomenon following β-adrenergic receptor blockade. The absence of such upregulation negates possible hypersensitivity to ischemia upon withdrawal of the medication. The risk is evident in hearts with upregulation of NHE-1, which gain Na+i more rapidly during ischemia, and show impaired recovery after reperfusion. (3) No additional protection was obtained by adding the NHE-1 blocker eniporide to the NHE-1 null mice, suggesting that there is not another NHE isoform that can be blocked with this compound to add additional protection; the findings additionally hint that the attenuation of the injury obtained by the absence of the sarcolemmal NHE-1 is maximal and, therefore, no further beneficial effect will be detected by blocking the mitochondrial NHE (MNHE).Facultad de Ciencias MédicasCentro de Investigaciones Cardiovasculare

Positive chronotropic effect of Bay K 8644: participation of endogenous norepinephrine

The chronotropic effect of Bay K 8644, a dihydropyridine known to increase the slow inward current, was studied in spontaneously beating rat atria. Increases in atrial rate were concentration-dependent and the maximal increase (106 ± 10 beats/min) was obtained at 3 × 10⁻⁶ mol/l. Reserpine pretreatment, or propranolol 3 × 10⁻⁷ mol/l, or propranolol plus prazosin 10⁻⁶ mol/l decreased the maximum chronotropic effect of Bay K 8644 by about 60%. Blockade of the removal mechanisms of catecholamines (hydrocortisone 3 × 10⁻⁵ mol/l plus cocaine 10⁻⁵ mol/1) did not prevent the chronotropic effect of the compound. Exposure to Bay K 8644 increased the spontaneous outflow of tritium from atria preloaded with [3H]-norepinephrine by 30%. The results indicate that Bay K 8644 produces positive chronotropic effects through two mechanisms: a direct one and an indirect mechanism that involves the participation of norepinephrine released from sympathetic nerve endings.Centro de Investigaciones Cardiovasculare

Dissociation between myocardial relaxation and diastolic stiffness in the stunned heart: its prevention by ischemic preconditioning

The effects of myocardial stunning and ischemic preconditioning on left-ventricular developed pressure and end-diastolic pressure (diastolic stiffness) as well as on coronary-perfusion pressure were examined in isolated isovolumic rabbit hearts. The isovolumic relaxation was evaluated, and the time constant of pressure decay during the isovolumic period was calculated. Our experimental protocol comprised: 1) myocardial stunning-global ischemia (15 min) followed by reperfusion (30 min); 2) myocardial stunning-global ischemia (20 min) followed by reperfusion (30 min); and 3) ischemic preconditioning — a single cycle of brief global ischemia and reperfusion (5 min each), before a second ischemic period, of 20-min duration. There was no effect upon systolic and diastolic parameters when 15 and 20 minutes of ischemia were evaluated. In both stunned groups the left ventricular developed pressure first recovered to near control values, but then stabilized at only 60% of the control values. Whereas the isovolumic relaxation time constant was increased after 5 min of reperfusion, and return to control values at late reperfusion, the end diastolic pressure remained elevated during the entire period. Values of dP/dV calculated at common pressure levels, were used as a second index of diastolic stiffness. They were increased after stunning, as also was the coronary perfusion pressure. When the heart was preconditioned with a single episode of ischemia, the systolic and diastolic alterations were completely abolished. We thus concluded that diastolic abnormalities incurred by myocardial stunning consist in both an increase in diastolic stiffness and an early impairment of isovolumic relaxation. The increase in stiffness cannot result from incomplete relaxation since these two parameters become temporally dissociated during the reperfusion period.Centro de Investigaciones Cardiovasculare

Paradoxical effect of hypercapnia on toad heart muscle

Experiments were performed on strips dissected from toad ventricles driven at a constant frequency of 12 beats/min. After equilibration, the Pco2 of the medium was altered from 25 to 95 mm Hg or from 95 to 25 mm Hg. Developed tension (DT) and maximal rate of tension development (dT/dtmax) were recorded during a 30-minute period after the change in Pco2. In the first experimental series at 30°C, increasing Pco2, resulted in a decrease in DT and dT/dtmax, followed by a recovery of contractility that reached levels higher than controls. In the second and third series, performed after the addition to the bath of practolol (1 x 10-6M) or after reserpinization, high Pco2, depressed contractility but the recovery did not surpass control values after 30 minutes of hypercapnia. In this series, when high PcO2 was replaced by low Pco2 there was an increase in DT and dT/dtmax followed by a decrease that reached control levels within a 30-minute period. In the fourth series, at 22°C the significant decreases in DT and dT/dtmax observed after increasing Pco2 were followed by a recovery that surpassed control values. These results define the existence in toad cardiac muscle of a mechanism that tends to return contractility to control levels after a change in Pco2. Within 30 minutes this mechanism, present even after inhibition of catecholamine action, completely counteracts the primary negative inotropic effect of high Pco2 and the positive inotropic action of hypocapnia.Facultad de Ciencias Médica

Comparison of the protective effects of ischemic preconditioning and the Na+ /H+ exchanger blockade

The protective effects of ischemic preconditioning (IP) and Na+/H+ exchanger blockade (NHEb) by two blockers [ethylisopropylamiloride (EIPA) and HOE 642] were compared in the isovolumic perfused rat heart. The impairment in systolic and diastolic function detected in control ischemic hearts (C) exposed to 20 min of ischemia and 30 min of reperfusion was diminished in similar extent by IP and by NHEb with EIPA and HOE 642. At the end of the reperfusion period +dP/dtmax values were 57±9% in C hearts and 94±6%, 82±6% and 104±6% after IP and NHEb with EIPA and HOE 642, respectively. A depletion of ATP levels detected in C hearts after reperfusion (from 20.2±0.8 µmol/g dry weight before ischemia to 6.9±0.7 µmol/g dry weight) was partially prevented by both IP and NHEb with EIPA (9.2±0.7 µmol/g dry weight and 11.1±0.5 µmol/g dry weight, respectively). The ischemic contracture (IC), assessed by the left ventricular end diastolic pressure (LVEDP), observed in C hearts (35±4 mmHg) was not decreased by IP (40±4 mmHg) but it was prevented by NHEb (18±4 mmHg and 10±3 mmHg with EIPA and HOE 642, respectively). The ATP levels at the end of the ischemic period were similar in C and IP hearts (4.1±0.2 µmol/g dry wt vs. 3.3±0.4 µmol/g dry wt) but they were significantly higher after NHEb with HOE 642 (7.0±1.0 µmol/g dry wt). PKC inhibition by chelerythrine abolished the protection induced by IP after reperfusion although not the improvement induced by NHEb with EIPA.Facultad de Ciencias Exacta

Letter by Cingolani et al. regarding article, "Ventricular phosphodiesterase-5 expression is increased in patients with advanced heart failure and contributes to adverse ventricular remodeling after myocardial infarction in mice"

Pokreisz et al. recently published an interesting article in which they show increased cardiac left ventricular phosphodiesterase-5A (PDE5A) expression in patients with heart failure. They also generated a PDE5A transgenic mouse in which overexpression of this cGMP-selective phosphodiesterase worsened ventricular remodeling and function after myocardial infarction. Therefore, the obvious conclusion seems to be that inhibition of PDE5A might protect against postmyocardial infarction remodeling. Although the authors show cGMP to be related to these cardiac architectural changes, they raise new questions about the possible cGMP downstream signaling mechanisms involved.Facultad de Ciencias Médica