Relationship between intracellular pH and tension development in resting ventricular muscle and myocytes

The insulin receptor family in the heart: new light on old insights

Insulin was discovered over 100 years ago. Whilst the first half century defined many of the physiological effects of insulin, the second emphasised the mechanisms by which it elicits these effects, implicating a vast array of G proteins and their regulators, lipid and protein kinases and counteracting phosphatases, and more. Potential growth-promoting and protective effects of insulin on the heart emerged from studies of carbohydrate metabolism in the 1960s, but the insulin receptors (and the related receptor for insulin-like growth factors 1 and 2) were not defined until the 1980s. A related third receptor, the insulin receptor-related receptor remained an orphan receptor for many years until it was identified as an alkali-sensor. The mechanisms by which these receptors and the plethora of downstream signalling molecules confer cardioprotection remain elusive. Here, we review important aspects of the effects of the three insulin receptor family members in the heart. Metabolic studies are set in the context of what is now known of insulin receptor family signalling and the role of protein kinase B (PKB or Akt), and the relationship between this and cardiomyocyte survival versus death is discussed. PKB/Akt phosphorylates numerous substrates with potential for cardioprotection in the contractile cardiomyocytes and cardiac non-myocytes. Our overall conclusion is that the effects of insulin on glucose metabolism that were initially identified remain highly pertinent in managing cardiomyocyte energetics and preservation of function. This alone provides a high level of cardioprotection in the face of pathophysiological stressors such as ischaemia and myocardial infarction.

The insulin receptor family and protein kinase B (Akt) are activated in the heart by alkaline pH and α1-adrenergic receptors

Insulin and insulin-like growth factor stimulate protein synthesis and cardioprotection in the heart, acting through their receptors (INSRs, IGF1Rs) and signalling via protein kinase B (PKB, also known as Akt). Protein synthesis is increased in hearts perfused at alkaline pH_o to the same extent as with insulin. Moreover, α₁-adrenergic receptor (α₁-AR) agonists (e.g. phenylephrine) increase protein synthesis in cardiomyocytes, activating PKB/Akt. In both cases, the mechanisms are not understood. Our aim was to determine if insulin receptor-related receptors (INSRRs, activated in kidney by alkaline pH) may account for the effects of alkaline pH_o on cardiac protein synthesis, and establish if α₁-ARs signal through the insulin receptor family. Alkaline pH_o activated PKB/Akt signalling to the same degree as insulin in perfused adult male rat hearts. INSRRs were expressed in rat hearts and, by immunoblotting for phosphorylation (activation) of INSRRs/INSRs/IGF1Rs, we established that INSRRs, together with INSRs/IGF1Rs, are activated by alkaline pH_o. The INSRR/INSR/IGF1R kinase inhibitor, linsitinib, prevented PKB/Akt activation by alkaline pH_o, indicating that INSRRs/INSRs/IGF1Rs are required. Activation of PKB/Akt in cardiomyocytes by α₁-AR agonists was also inhibited by linsitinib. Furthermore, linsitinib inhibited cardiomyocyte hypertrophy induced by α₁-ARs in cultured cells, reduced the initial cardiac adaptation (24 h) to phenylephrine in vivo (assessed by echocardiography) and increased cardiac fibrosis over 4 days. We conclude that INSRRs are expressed in the heart and, together with INSRs/IGF1Rs, the insulin receptor family provide a potent system for promoting protein synthesis and cardioprotection. Moreover, this system is required for adaptive hypertrophy induced by α₁-ARs.

pH recovery from a proton load in rat cardiomyocytes: effects of chronic exercise

The ability of cardiomyocytes to recover from a proton load was examined in the hearts of exercise-trained and sedentary control rats in CO₂/[Formula: see text]-free media. Acidosis was created by the NH₄Cl prepulse technique, and intracellular pH (pH_i) was determined using fluorescence microscopy on carboxy-SNARF-1 AM-loaded isolated cardiomyocytes. CO₂-independent pH_i buffering capacity (β_i) was measured by incrementally reducing the extracellular NH₄Cl concentration in steps of 50% from 20 to 1.25 mM. β_i increased as pH_i decreased in both exercise-trained and sedentary control groups. However, the magnitude of increase in β_i as a function of pH_i was found to be significantly ( P < 0.001) greater in the exercise-trained group compared with the sedentary control group. The rate of pH_i recovery from an imposed proton load was found to not be different between the exercise-trained and control groups. The Na⁺/H⁺ exchanger-dependent H⁺ extrusion rate during the recovery from an imposed proton load, however, was found to be significantly greater in the exercise-trained group compared with the control group. By increasing β_i and subsequently the Na⁺/H⁺ exchanger-dependent H⁺ extrusion rate, exercise training may provide cardiomyocytes with the ability to better handle an intracellular excess of H⁺ generated during hypoxia/ischemic insults and may serve in a cardioprotective role. These data may be predictive of two positive outcomes: 1) increased exercise tolerance by the heart and 2) a protective mechanism that limits the degree of myocardial acidosis and subsequent damage that accompanies ischemia-reperfusion stress. NEW & NOTEWORTHY The enhanced ability to deal with acidosis conferred by exercise training is likely to improve exercise tolerance and outcomes in response to myocardial ischemia-reperfusion injury.

Sarcomere neutralization in inherited cardiomyopathy: small-molecule proof-of-concept to correct hyper-Ca2+-sensitive myofilaments

The sarcomere is the functional unit of the heart. Alterations in sarcomere activation lead to disease states such as hypertrophic and restrictive cardiomyopathy (HCM/RCM). Mutations in many of the sarcomeric genes are causal for HCM/RCM. In most cases, these mutations result in increased Ca(2+) sensitivity of the sarcomere, giving rise to altered systolic and diastolic function. There is emerging evidence that small-molecule sarcomere neutralization is a potential therapeutic strategy for HCM/RCM. To pursue proof-of-concept, W7 was used here because of its well-known Ca(2+) desensitizer biochemical effects at the level of cardiac troponin C. Acute treatment of adult cardiac myocytes with W7 caused a dose-dependent (1-10 μM) decrease in contractility in a Ca(2+)-independent manner. Alkalosis was used as an in vitro experimental model of acquired heightened Ca(2+) sensitivity, resulting in increased live cell contractility and decreased baseline sarcomere length, which were rapidly corrected with W7. As an inherited cardiomyopathy model, R193H cardiac troponin I (cTnI) transgenic myocytes showed significant decreased baseline sarcomere length and slowed relaxation that were rapidly and dose-dependently corrected by W7. Langendorff whole heart pacing stress showed that R193H cTnI transgenic hearts had elevated end-diastolic pressures at all pacing frequencies compared with hearts from nontransgenic mice. Acute treatment with W7 rapidly restored end-diastolic pressures to normal values in R193H cTnI hearts, supporting a sarcomere intrinsic mechanism of dysfunction. The known off-target effects of W7 notwithstanding, these results provide further proof-of-concept that small-molecule-based sarcomere neutralization is a potential approach to remediate hyper-Ca(2+)-sensitive sarcomere function.

Effect of nitric oxide donors S-nitroso-N-acetyl-DL-penicillamine, spermine NONOate and propylamine propylamine NONOate on intracellular pH in cardiomyocytes

1. Previous studies suggest that exogenous nitric oxide (NO) and NO-dependent signalling pathways modulate intracellular pH (pH(i)) in different cell types, but the role of NO in pH(i) regulation in the heart is poorly understood. Therefore, in the present study we investigated the effect of the NO donors S-nitroso-N-acetyl-DL-penicillamine, spermine NONOate and propylamine propylamine NONOate on pH(i) in rat isolated ventricular myocytes. 2. Cells were isolated from the hearts of adult Wistar rats and pH(i) was monitored using the pH-sensitive fluorescent indicator 5-(and-6)-carboxy seminaphtharhodafluor (SNARF)-1 (10 μmol/L) and a confocal microscope. To test the effect of NO donors on the Na⁺/H⁺ exchanger (NHE), basal pH(i) in Na⁺-free buffer and pH(i) recovery from intracellular acidosis after an ammonium chloride (10 mmol/L) prepulse were monitored. The role of carbonic anhydrase was tested using acetazolamide (50 μmol/L). 4,4-Diisothiocyanatostilbene-2,2'-disulphonic acid (0.5 mmol/L; DIDS) was used to inhibit the Cl⁻/OH⁻ and Cl⁻/HCO₃-exchangers. Acetazolamide and DIDS were applied via the superfusion system 1 and 5 min before the NO donors. 3. All three NO donors acutely decreased pH(i) and this effect persisted until the NO donor was removed. In Na⁺-free buffer, the decrease in basal pH(i) was increased, whereas inhibition of carbonic anhydrase and Cl⁻/OH⁻ and Cl⁻/HCO₃⁻ exchangers did not alter the effects of the NO donors on pH(i). After an ammonium preload, pH(i) recovery was accelerated in the presence of the NO donors. 4. In conclusion, exogenous NO decreases basal pH(i), leading to increased NHE activity. Carbonic anhydrase and chloride-dependent sarcolemmal HCO₃⁻ and OH⁻ transporters are not involved in the NO-induced decrease in pH(i) in rat isolated ventricular myocytes.

Influence of pH on Ca²⁺ current and its control of electrical and Ca²⁺ signaling in ventricular myocytes

Modulation of L-type Ca(2+) current (I(Ca,L)) by H(+) ions in cardiac myocytes is controversial, with widely discrepant responses reported. The pH sensitivity of I(Ca,L) was investigated (whole cell voltage clamp) while measuring intracellular Ca(2+) (Ca(2+)(i)) or pH(i) (epifluorescence microscopy) in rabbit and guinea pig ventricular myocytes. Selectively reducing extracellular or intracellular pH (pH(o) 6.5 and pH(i) 6.7) had opposite effects on I(Ca,L) gating, shifting the steady-state activation and inactivation curves to the right and left, respectively, along the voltage axis. At low pH(o), this decreased I(Ca,L), whereas at low pH(i), it increased I(Ca,L) at clamp potentials negative to 0 mV, although the current decreased at more positive potentials. When Ca(2+)(i) was buffered with BAPTA, the stimulatory effect of low pH(i) was even more marked, with essentially no inhibition. We conclude that extracellular H(+) ions inhibit whereas intracellular H(+) ions can stimulate I(Ca,L). Low pH(i) and pH(o) effects on I(Ca,L) were additive, tending to cancel when appropriately combined. They persisted after inhibition of calmodulin kinase II (with KN-93). Effects are consistent with H(+) ion screening of fixed negative charge at the sarcolemma, with additional channel block by H(+)(o) and Ca(2+)(i). Action potential duration (APD) was also strongly H(+) sensitive, being shortened by low pH(o), but lengthened by low pH(i), caused mainly by H(+)-induced changes in late Ca(2+) entry through the L-type Ca(2+) channel. Kinetic analyses of pH-sensitive channel gating, when combined with whole cell modeling, successfully predicted the APD changes, plus many of the accompanying changes in Ca(2+) signaling. We conclude that the pH(i)-versus-pH(o) control of I(Ca,L) will exert a major influence on electrical and Ca(2+)-dependent signaling during acid-base disturbances in the heart.

Platelet-activating factor stimulates sodium-hydrogen exchange in ventricular myocytes

Sodium-hydrogen exchanger (NHE), the principal sarcolemmal acid extruder in ventricular myocytes, is stimulated by a variety of autocrine/paracrine factors and contributes to myocardial injury and arrhythmias during ischemia-reperfusion. Platelet-activating factor (PAF; 1-o-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a potent proinflammatory phospholipid that is released in the heart in response to oxidative stress and promotes myocardial ischemia-reperfusion injury. PAF stimulates NHE in neutrophils and platelets, but its effect on cardiac NHE (NHE1) is unresolved. We utilized quiescent guinea pig ventricular myocytes bathed in bicarbonate-free solutions and epifluorescence to measure intracellular pH (pH(i)). Methylcarbamyl-PAF (C-PAF; 200 nM), a metabolically stable analog of PAF, significantly increased steady-state pH(i). The alkalosis was completely blocked by the NHE inhibitor, cariporide, and by sodium-free bathing solutions, indicating it was mediated by NHE activation. C-PAF also significantly increased the rate of acid extrusion induced by intracellular acidosis. The ability of C-PAF to increase steady-state pH(i) was completely blocked by the PAF receptor inhibitor WEB 2086 (10 μM), indicating the PAF receptor is required. A MEK inhibitor (PD98059; 25 μM) also completely blocked the rise in pH(i) induced by C-PAF, suggesting participation of the MAP kinase signaling cascade downstream of the PAF receptor. Inhibition of PKC with GF109203X (1 μM) and chelerythrine (2 μM) did not significantly affect the alkalosis induced by C-PAF. In summary, these results provide evidence that PAF stimulates cardiac NHE1, the effect occurs via the PAF receptor, and signal relay requires participation of the MAP kinase cascade.

Involvement of AE3 isoform of Na(+)-independent Cl(-)/HCO(3)(-) exchanger in myocardial pH(i) recovery from intracellular alkalization

Myocardial pH(i) recovery from intracellular alkalization results in part from the acid load (-J(H+)) carried by Cl(-)/HCO(3)(-) anion-exchangers (AE). Three AE isoforms, AE1, AE2 and AE3, have been identified in cardiac membranes, but the function of each isoform on pH(i) homeostasis is still under investigation. This work explored, by means of specific antibodies, the role of AE3 isoform in myocardial pH(i) regulation. We developed rabbit polyclonal antibodies against the extracellular "loops": one connecting the fifth to sixth and the other one the seventh to eighth transmembrane domains (loops 3 and 4, respectively) of AE3, and their effect on pH(i) regulation was studied in rat papillary muscles. The anti-AE3 loop 3 antibody decreased -J(H+) in response to myocardial alkalization (from a mean control value of 1.06+/-0.26 to 0.32+/-0.13 mmol/L/min, n=7, P<0.05) without affecting the baseline pH(i) (7.22+/-0.03 vs. 7.21+/-0.04). The anti-AE3 loop 4 antibody did not modify either pH(i) recovery or baseline pH(i). Under control conditions, endothelin-1 (ET-1) increased -J(H+) in response to myocardial alkalization from 1.30+/-0.18 to 2.01+/-0.33 mmol/L /min (n=5, P<0.05). This effect of ET-1 on -J(H+) was abolished by anti-AE3 loop 3 antibody. In addition, the MgATP-induced stimulation of AE activity was reduced by the anti-AE3 loop 3 antibody. These data support the key role of the AE3 isoform in myocardial pH(i) recovery from alkaline loads and also in the stimulatory effect of ET-1 on AE activity. To a lesser extent, it may also contribute to the effect of MgATP on pH(i).

Dendroaspis natriuretic peptide protects the post-ischemic myocardial injury

The present study used isolated rat hearts to investigate whether (1) Dendroaspis natriuretic peptide (DNP) is protective against post-ischemic myocardial dysfunction, and (2) whether the cardioprotective effects of DNP is related to alteration of Bcl-2 family protein levels. The excised hearts of Sprague-Dawley rats were perfused on a Langendorff apparatus with Krebs-Henseleit solution with a gas mixture of 95% O2 and 5% CO2. Left ventricular end-diastolic pressure (LVEDP, mmHg), left ventricular developed pressure (LVDP, mmHg) and coronary flow (CF, ml/min) were continuously monitored. In the presence of 50 nM DNP, all hearts were perfused for a total of 100 min consisting of a 20 min pre-ischemic period followed by a 30 min global ischemia and 50 min reperfusion. Lactate dehydrogenase (LDH) activity in the effluent was measured during reperfusion. Treatment with DNP alone improved the pre-ischemic LVEDP and post-ischemic LVEDP significantly comparing with the untreated control hearts during reperfusion. However, DNP did not affect the LVDP, heart rate (HR, beats/min), and CF. Bcl-2, an anti-apoptotic protein expressed in ischemic myocardium of DNP+ischemia/reperfusion (I/R) group, was higher than that in I/R alone group. Bax, a pro-apoptotic protein expressed in ischemic myocardium of DNP+I/R group, has no significant difference compared with I/R alone group. These results suggest that the protective effects of DNP against I/R injury would be mediated, at least in part, through the increased ratio of Bcl-2 to Bax protein after ischemia-reperfusion.

On the genesis of myocardial ischemia

About three quarters of myocardial ischemic events are triggered by the autonomic nervous system. The pathognomonic constellation is a combination of an almost complete withdrawal of tonic vagal activity with increased sympathetic activity. The reduction of tonic vagal activity, which is characteristic for ischemic heart disease, and the acute withdrawal of vagal drive preceding the onset of ischemia are not dependent on coronary artery disease. In this paper, the pathophysiological steps that lead from sympathetic-parasympathetic imbalance to myocardial ischemia shall be discussed. A considerable increase of aerobic glycolysis within the myocardium as a result of the autonomic imbalance is of special importance in this process.

Functional diversity of electrogenic Na+-HCO3- cotransport in ventricular myocytes from rat, rabbit and guinea pig

The Na(+)-HCO(3)(-) cotransporter (NBC) is an important sarcolemmal acid extruder in cardiac muscle. The characteristics of NBC expressed functionally in heart are controversial, with reports suggesting electroneutral (NBCn; 1HCO(3)(-) : 1Na(+); coupling coefficient N= 1) or electrogenic forms of the transporter (NBCe; equivalent to 2HCO(3)(-) : 1Na(+); N= 2). We have used voltage-clamp and epifluorescence techniques to compare NBC activity in isolated ventricular myocytes from rabbit, rat and guinea pig. Depolarization (by voltage clamp or hyperkalaemia) reversibly increased steady-state pH(i) while hyperpolarization decreased it, effects seen only in CO(2)/HCO(3)(-)-buffered solutions, and blocked by S0859 (cardiac NBC inhibitor). Species differences in amplitude of these pH(i) changes were rat > guinea pig approximately rabbit. Tonic depolarization (-140 mV to -0 mV) accelerated NBC-mediated pH(i) recovery from an intracellular acid load. At 0 mV, NBC-mediated outward current at resting pH(i) was +0.52 +/- 0.05 pA pF(-1) (rat, n= 5), +0.26 +/- 0.05 pA pF(-1) (guinea pig, n= 5) and +0.10 +/- 0.03 pA pF(-1) (rabbit, n= 9), with reversal potentials near -100 mV, consistent with N= 2. The above results indicate a functionally active voltage-sensitive NBCe in these species. Voltage-clamp hyperpolarization negative to the reversal potential for NBCe failed, however, to terminate or reverse NBC-mediated pH(i)-recovery from an acid load although it was slowed significantly, suggesting electroneutral NBC may also be operational. NBC-mediated pH(i) recovery was associated with a rise of [Na(+)](i) at a rate approximately 25% of that mediated via NHE, and consistent with an apparent NBC stoichiometry between N= 1 and N= 2. In conclusion, NBCe in the ventricular myocyte displays considerable functional variation among the three species tested (greatest in rat, least in rabbit) and may coexist with some NBCn activity.

Atrial natriuretic peptide protects against ischemia-reperfusion injury in the isolated rat heart

BACKGROUND

Atrial natriuretic peptide (ANP), a stimulator of particulate guanylate cyclase, has been found to protect against reoxygenation-induced hypercontracture in isolated cardiomyocytes by increasing cyclic guanosine monophosphate synthesis. The purpose of this study was to investigate the cardioprotective effects of ANP against ischemia-reperfusion injury in isolated rat hearts.

METHODS

Twenty-four hearts were perfused with ANP at 0.01, 0.1, and 1 micromol/L or without ANP (n = 6 each) in normoxic conditions. Because 0.1 micromol/L ANP induced a threefold increase in cyclic guanosine monophosphate release into the coronary effluent without any effect on cardiac function, we used the 0.1 micromol/L ANP dose for ischemia-reperfusion studies. Eighteen hearts were subjected to 15 minutes of normothermic global ischemia followed by 15 minutes of reperfusion. The hearts were divided into three groups (n = 6 each).

RESULTS

In group 1, ANP was added before ischemia. In group 2, ANP was added to the reperfusate. Hearts were untreated in the control group. In group 1, the postischemic recovery of cardiac output, coronary flow, and cyclic guanosine monophosphate release was similar to the control group. In group 2, the recovery of cardiac output was significantly better than the control group (82.1% +/- 9.8% vs 61.8% +/- 6.8%, respectively, p < 0.01) with a similar trend to recovery of coronary flow (90.7% +/- 8.5% vs 79.3% +/- 11.8%, respectively). The improved cardiac function was closely related to a significant increase in postischemic cyclic guanosine monophosphate release.

CONCLUSIONS

Administration of ANP at the time of reperfusion protects the myocardium from ischemia-reperfusion injury. The concentrations of administration must not only increase the release of cyclic guanosine monophosphate release, but also lack negative inotropic effects.

Proton permeation through the myocardial gap junction

Although protons can directly or indirectly gate solute permeability of the myocardial gap junction, there is little information regarding their own permeation, despite their importance in the regulation of myocardial contractility and rhythm. By pipette-loading of acid into guinea pig isolated, ventricular myocyte pairs while imaging pH(i) confocally using SNARF fluorescence, we have observed that protons permeate the junctional region. Permeation is inhibited by glycyrrhetinic acid, an agent that also increases intercellular electrical resistance, suggesting H+ permeation via gap junctions. The rate of spread of acid between cells appears to be limited by junctional permeation rather than by cytoplasmic diffusion. Mathematical analyses, combined with experiments using SNARF as a proton carrier, suggest that gap junctional H+ transmission may be accomplished physiologically by the permeation of intrinsic "proton-porter" molecules. We propose that proton flux through gap junctions will contribute to the dissipation of regional acid loads within the myocardium. This represents a mechanism for the local control of myocardial pH(i).

Improved myocardial function using a Na+/H+ exchanger inhibitor during cardioplegic arrest and cardiopulmonary bypass

INTRODUCTION

We have demonstrated that a component of post-cardiopulmonary bypass (CPB)/cardioplegic arrest (CPA) myocardial dysfunction is related to myocardial edema. Myocardial ischemia/reperfusion that occurs with CPB/CPA activates the Na(+)/H(+) exchanger to normalize intracellular pH, with intracellular Na(+) (and water) accumulation. We hypothesized that Na(+)/H(+) exchanger inhibition with a selective inhibitor (EMD 87580) would decrease myocardial edema and improve myocardial performance after CPB/CPA.

METHODS

Anesthetized dogs (n = 14) were instrumented with myocardial ultrasonic crystals, and left ventricular (LV) micromanometer, to study myocardial function. Myocardial tissue water (MWC) was determined using microgravimetry. Treated animals (n = 5) received EMD 87580 (5 mg/kg IV pretreatment and 10 mol/L cardioplegia); control animals (n = 9) received a saline vehicle. After baseline, hypothermic CPB/CPA was initiated for 2 h, followed by reperfusion/rewarming for 45 min and separation from CPB. Myocardial function parameters and MWC were measured at 30 min, 60 min, and 120 min after CPB.

RESULTS

Preload recruitable stroke work did not decrease from baseline in EMD 87580-treated animals, and was significantly greater in EMD 87580-treated animals than control animals at 120 min after CPB. At a similar LV end-diastolic volume, the maximal rate of rise of LV pressure (dp/dtMAX) was significantly decreased from baseline at all time points in control animals, and unchanged in EMD 87580-treated animals. MWC increased with CPB/CPA in both groups, with no difference between groups. There was no difference in - dp/dtMAX or slope of the end-diastolic pressure-volume relationship.

CONCLUSION

Na(+)/H(+) exchanger inhibition improves systolic but not diastolic function after CPB/CPA. This is not due to a reduction in MWC.

Intrinsic H(+) ion mobility in the rabbit ventricular myocyte

The intrinsic mobility of intracellular H(+) ions was investigated by confocally imaging the longitudinal movement of acid inside rabbit ventricular myocytes loaded with the acetoxymethyl ester (AM) form of carboxy-seminaphthorhodafluor-1 (carboxy-SNARF-1). Acid was diffused into one end of the cell through a patch pipette filled with an isotonic KCl solution of pH 3.0. Intracellular H(+) mobility was low, acid taking 20-30 s to move 40 microm down the cell. Inhibiting sarcolemmal Na(+)-H(+) exchange with 1 mM amiloride had no effect on this time delay. Net H(+)(i) movement was associated with a longitudinal intracellular pH (pH(i)) gradient of up to 0.4 pH units. H(+)(i) movement could be modelled using the equations for diffusion, assuming an apparent diffusion coefficient for H(+) ions (D(H)(app)) of 3.78 x 10(-7) cm(2) s(-1), a value more than 300-fold lower than the H(+) diffusion coefficient in a dilute, unbuffered solution. Measurement of the intracellular concentration of SNARF (approximately 400 microM) and its intracellular diffusion coefficient (0.9 x 10(-7) cm(2) s(-1)) indicated that the fluorophore itself exerted an insignificant effect (between 0.6 and 3.3 %) on the longitudinal movement of H(+) equivalents inside the cell. The longitudinal movement of intracellular H(+) is discussed in terms of a diffusive shuttling of H(+) equivalents on high capacity mobile buffers which comprise about half (approximately 11 mM) of the total intrinsic buffering capacity within the myocyte (the other half being fixed buffer sites on low mobility, intracellular proteins). Intrinsic H(+)(i) mobility is consistent with an average diffusion coefficient for the intracellular mobile buffers (D(mob)) of ~9 x 10(-7) cm(2) s(-1).

Na(+)/H(+)-exchange inhibition and aprotinin administration: promising tools for myocardial protection during minimally invasive CABG

OBJECTIVE

Minimally invasive coronary artery bypass grafting (CABG), carried out on the warm beating heart, does not allow conventional myocardial protection. The objective was to investigate the possibility of enhancing tolerance to ischemia during short episodes of coronary artery occlusion, based on a pharmacological approach using a selective Na(+)/H(+)-exchange inhibitor (cariporide) or a serine protease inhibitor (aprotinin).

METHODS

Four groups (n=6 in each group) of sheep were subjected to 20 min of normothermic regional ischemia (first lateral branch of the circumflex artery occlusion) followed by 1 h of reperfusion. Regional wall thickening was measured using sonomicrometry, and expressed as the percentage of thickening fraction compared with baseline. Group I was the control with no treatment, group II received cariporide (1 mg/kg administered over 2 min prior to ischemia), group III was treated with aprotinin (2.10(6) kallikrein inactivation units (KIU) load followed by 500.000 KIU/h). Group IV was treated with a combination of cariporide and aprotinin at the same concentrations as in groups II and III, respectively.

RESULTS

Wall thickening measurements showed that, compared with control, cariporide was largely able to suppress secondary loss of wall thickening after initial recovery during early reperfusion. Wall thickening in the ischemic/reperfused myocardial area improved from 10+/-31 to 51+/-17% at 1 h of reperfusion (P=0.002). Aprotinin improved wall thickening at the end of 1 h of reperfusion to 70+/-13% (P=0.0001). However, in this group, there was a transient loss of regional contractility similar in amplitude and time course to the one observed in the control group. A combination of cariporide and aprotinin suppressed transient contractile loss and resulted in improved wall thickening at the end of 1 h of reperfusion (65+/-22%, P=0.0002 vs. control). This value was not significantly different from the cariporide (P=0.263) or aprotinin (P=0.704) group.

CONCLUSION

These data indicate that both Na(+)/H(+)-exchange inhibition and aprotinin administration are promising tools for cardioprotection during minimally invasive CABG. A combination of both treatments is able to adequately suppress loss of contractility during early reperfusion as a consequence of reperfusion injury, and results in significantly improved wall thickening at the end of 1 h of reperfusion.

Generation of intracellular pH gradients in single cardiac myocytes with a microperfusion system

This study describes the use of a microperfusion system to create rapid, large regional changes in intracellular pH (pH(i)) within single ventricular myocytes. The spatial distribution of pH(i) in single myocytes was measured with seminaphthorhodafluor-1 fluorescence using confocal imaging. Changes in pH(i) were induced by local external application of NH(4)Cl, CO(2), or sodium propionate. Local application was achieved by simultaneously directing two parallel square microstreams, each 275 microm wide, over a single myocyte oriented perpendicular to the direction of flow. One stream contained the control solution, and the other contained a weak acid or base. End-to-end, stable pH(i) gradients as large as 1 pH unit were readily created with this technique. This result indicates that pH within a single cardiac cell may not always be spatially uniform, particularly when weak acid or base gradients are present, which can occur, for example, in regional myocardial ischemia. The microperfusion method should be useful for studying the effects of localized acidosis on myocyte function, estimating intracellular ion diffusion rates, and, possibly, inducing regional changes in other important intracellular ions.

Diacylglycerol delays pH(i) overshoot after reperfusion and attenuates contracture in isolated, paced myocytes

Although protein kinase C (PKC) plays a pivotal role in ischemic preconditioning, it is not clear what the end effector is that protects the myocardium. In isolated, paced (1.25 Hz, 36-37 degrees C) adult rat cardiomyocytes, the effects of PKC preactivation by diacylglycerol on cell motion, intracellular Ca(2+) concentration ([Ca(2+)](i); indo 1), and intracellular pH (pH(i); seminaphthorhodafluor-1) during simulated ischemia-reperfusion (I/R) were investigated. The degree of reperfusion-induced contracture was significantly attenuated in the myocytes pretreated with 10 microM 1, 2-dioctanoyl-sn-glycerol (DOG; n = 19) compared with the untreated myocytes (n = 23, P < 0.02). There were no differences in twitch amplitude, end-diastolic [Ca(2+)](i), or peak-systolic [Ca(2+)](i) during I/R between the DOG-pretreated and untreated myocytes. Although there were no differences in pH(i) during ischemia, the pH(i) overshoot during reperfusion was significantly delayed in the DOG-pretreated myocytes compared with the untreated myocytes (n = 17 for each, P < 0.01). Chelerythrine completely abolished the favorable effects of DOG on the reperfusion-induced contracture and the pH(i) overshoot. These data suggest that diacylglycerol attenuates I/R injury in isolated, paced cardiomyocytes, which may be related to the slower pH(i) overshoot during reperfusion.

Atrial natriuretic peptide has different effects on contractility and intracellular pH in normal and hypertrophied myocytes from pressure-overloaded hearts

BACKGROUND

Atrial natriuretic peptide (ANP) depresses contractility in left ventricular myocytes. Its expression is upregulated in pressure-overloaded hypertrophied hearts; however, the effects of ANP on contractility in hypertrophied myocytes are not known. Our aims were (1) to examine the cellular mechanisms of this depression in contractility in normal myocytes and (2) to test the hypothesis that the effects of ANP on contractility differ in hypertrophied myocytes from rats with ascending aortic stenosis.

METHODS AND RESULTS

We measured the myocyte shortening as an index of contractility, [Ca2+]i with fluo 3, and pHi with seminaphthorhodafluor-1 (SNARF-1). In normal control myocytes (n=26), ANP caused a concentration-dependent depression of contractility and reduction in pHi. In the presence of 10(-6) mol/L ANP, fractional cell shortening was 78+/-5% of baseline (P<0.05) and pHi was reduced by 0.16+/-0.04 U from baseline (P<0.01) without changes in [Ca2+]i. The magnitude of the depression of contraction caused by ANP was similar to that caused by intracellular acidification induced by an NH4Cl pulse. The effects of ANP on contractility and pHi were prevented in the presence of 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), which inhibits the Na+/H+ exchanger. In hypertrophied myocytes (n=23), ANP did not depress either myocyte contractility or pHi at concentrations of either 10(-8), 10(-7), or 10(-6) mol/L. ANP caused no change in pHi or the [Ca2+]i transient in hypertrophied myocytes. The cGMP level was increased and Na+/H+ exchanger mRNA levels were normal in left ventricles from aortic stenosis rats compared with controls.

CONCLUSIONS

ANP directly depresses contractility in normal myocytes via intracellular acidification, which decreases myofilament [Ca2+]i sensitivity. In contrast, ANP causes no effects on contractility and pHi in hypertrophied myocytes, suggesting a suppression in the coupling of the ANP-cGMP intracellular signaling pathway to the Na+/H+ exchanger.

pHi and pHo at different depths in perfused myocardium measured by confocal fluorescence microscopy

Confocal microscopy and the H+-sensitive fluorophore carboxyseminaphthorhodafluor-1 (SNARF-1) were used to measure either intracellular pH (pHi) or extracellular pH (pHo) in isolated, arterially perfused rabbit papillary muscles. Single-excitation, dual-emission fluorescent images of the endocardial surface and underlying myocardium to a depth of 300 micron were simultaneously recorded from perfused cylindrical muscles suspended in a controlled atmosphere oriented oblique to the focal plane. Contraction was inhibited by the addition of butanedione monoxime. In separate muscles, pHo was measured during continuous perfusion of SNARF-1 free acid. pHi measurements were made after the muscle was loaded with SNARF-1/AM and the extracellular space was cleared of residual fluorophore. Initial experiments demonstrated the uniformity of ratiometric measurements as a function of pH, image depth, and fluorophore concentration, thereby establishing the potential feasibility of this method for quantitative intramural pH measurements. In subsequent experiments, the method was validated in isolated, arterially perfused rabbit papillary muscle during normal arterial perfusion and as pHi and pHo were altered by applying CO2 externally, exchanging HEPES and bicarbonate buffers, and changing pHi with NH4Cl washout. We conclude that in situ confocal fluorescent microscopy can measure pHi and pHo changes at the endocardial surface and deeper endocardial layers in arterially perfused ventricular myocardium. This method has the potential to study pHi regulation in perfused myocardium at boundaries where diffusion of gases, metabolites, and peptides are expected to modify processes that regulate pHi.

Effect of ANG II on pHi, [Ca2+]i, and contraction in rabbit ventricular myocytes from infarcted hearts

In this study we examined Na+/H+ exchange activity, Ca2+ transients, and contractility in rabbit ventricular myocytes isolated from normal and chronically (8-12 wk) infarcted left ventricles. Myocytes from infarcted hearts (post-MI myocytes) were isolated from the peri-infarcted region of the left ventricle. Intracellular pH (pHi) and Ca2+ concentration ([Ca2+]i) were measured with the fluorescent pH indicators seminaphthorhodafluor 1 and fluo 3, respectively, and contractility was assessed from changes in cell shortening during field stimulation. Experiments were performed at extracellular pH 7. 4 in the presence and absence (HEPES buffer) of CO2 and HCO-3. Our findings demonstrate that 1) myocytes after myocardial infarction (post-MI) were significantly larger than normal, 2) post-MI hypertrophy was not accompanied by changes in non-CO2 intracellular buffering power, 3) post-MI hypertrophy did not significantly affect the ability of Na+/H+ exchange to mediate pHi recovery from intracellular acidosis, 4) the stimulatory effect of ANG II (100 nM) on Na+/H+ exchange was significantly reduced in post-MI myocytes, 5) in HCO-3-buffered solutions, ANG II did not significantly stimulate pHi recovery from acidosis in post-MI myocytes, 6) the angiotensin AT1 receptor mediates the stimulatory action of ANG II on Na+/H+ exchange in normal and post-MI myocytes, and 7) the stimulatory effect of ANG II on the Ca2+ transient and contraction was blunted in post-MI myocytes bathed in HEPES-buffered solution. A suppressed ventricular responsiveness to ANG II may be beneficial in the intact myocardium by attenuating ATP consumption and by reducing intracellular Na+ accumulation during ischemia-reperfusion.

Evidence for an electrogenic Na+-HCO3- symport in rat cardiac myocytes

1. The perforated whole-cell configuration of patch clamp and the pH fluorescent indicator SNARF were used to determine the electrogenicity of the Na+-HCO3- cotransport in isolated rat ventricular myocytes. 2. Switching from Hepes buffer to HCO3- buffer at constant extracellular pH (pHo) hyperpolarized the resting membrane potential (RMP) by 2.9 +/- 0.4 mV (n = 9, P < 0.05). In the presence of HCO3-, the anion blocker SITS depolarized RMP by 2.6 +/- 0.5 mV (n = 5, P < 0.05). No HCO3--induced hyperpolarization was observed in the absence of extracellular Na+. The duration of the action potential measured at 50 % of repolarization time (APD50) was 29.2 +/- 6.1 % shorter in the presence of HCO3- than in its absence (n = 6, P < 0.05). 3. Quasi-steady-state currents were evoked by voltage-clamped ramps ranging from -130 to +30 mV, during 8 s. The development of a novel component of Na+-dependent and Cl--independent steady-state outward current was observed in the presence of HCO3-. The reversal potential (Erev) of the Na+-HCO3- cotransport current (INa,Bic) was measured at four different levels of extracellular Na+. A HCO3-:Na+ ratio compatible with a stoichiometry of 2:1 was detected. INa,Bic was also studied in isolation in standard whole-cell experiments. Under these conditions, INa,Bic reversed at -96.4 +/- 1.9 mV (n = 5), being consistent with the influx of 2 HCO3- ions per Na+ ion through the Na+-HCO3- cotransporter. 4. In the presence of external HCO3-, after 10 min of depolarizing the membrane potential (Em) with 45 mM extracellular K+, a significant intracellular alkalinization was detected (0.09 +/- 0. 03 pH units; n = 5, P < 0.05). No changes in pHi were observed when the myocytes were pre-treated with the anion blocker DIDS (0.001 +/- 0.024 pH units; n = 5, n.s.), or when exposed to Na+-free solutions (0.003 +/- 0.037 pH units; n = 6, n.s.). 5. The above results allow us to conclude that the cardiac Na+-HCO3- cotransport is electrogenic and has an influence on RMP and APD of rat ventricular cells.

Mechanisms involved in the acidosis enhancement of the isoproterenol-induced phosphorylation of phospholamban in the intact heart

Previous experiments have shown that acidosis enhances isoproterenol-induced phospholamban (PHL) phosphorylation (Mundiña-Weilenmann, C., Vittone, L., Cingolani, H. E., Orchard, C. H. (1996) Am. J. Physiol. 270, C107-C114). In the present experiments, performed in isolated Langendorff perfused rat hearts, phosphorylation site-specific antibodies to PHL combined with the quantitative measurement of 32P incorporation into PHL were used as experimental tools to gain further insight into the mechanism involved in this effect. At all isoproterenol concentrations tested (3-300 nM), phosphorylation of Thr17 of PHL was significantly higher at pHo 6.80 than at pHo 7.40, without significant changes in Ser16 phosphorylation. This increase in Thr17 phosphorylation was associated with an enhancement of the isoproterenol-induced relaxant effect. In the absence of isoproterenol, the increase in [Ca]o at pHo 6.80 (but not at pHo 7.40) evoked an increase in PHL phosphorylation that was exclusively due to an increase in Thr17 phosphorylation and that was also associated with a significant relaxant effect. This effect and the phosphorylation of Thr17 evoked by acidosis were both offset by the Ca2+/calmodulin-dependent protein kinase II inhibitor KN-62. In the presence of isoproterenol, either the increase in [Ca]o or the addition of a 1 microM concentration of the phosphatase inhibitor okadaic acid was able to mimic the increase in isoproterenol-induced Thr17 phosphorylation produced by acidosis. In contrast, these two interventions have opposite effects on phosphorylation of Ser16. Whereas the increase in [Ca]o significantly decreased phosphorylation of Ser16, the addition of okadaic acid significantly increased the phosphorylation of this residue. The results are consistent with the hypothesis that the increase in phospholamban phosphorylation produced by acidosis in the presence of isoproterenol is the consequence of two different mechanisms triggered by acidosis: an increase in [Ca2+]i and an inhibition of phosphatases.

A conditionally immortalized cell line from murine proximal tubule

We have developed a conditionally immortalized murine cell line with proximal tubule characteristics (tsMPT) and a background suitable for genetic manipulations. tsMPT was derived from the F1 progeny of crosses between: [1] a transgenic mouse harboring a gamma-interferon (IFN-gamma)-inducible, temperature sensitive SV40 large T antigen gene (tsA58) and [2] mice of the 129/SvEv strain, the background from which most embryonic stem (ES) cells are derived. Under permissive conditions (33 degrees C and in the presence of IFN-gamma), tsMPT cells grow rapidly as monolayers with a doubling time of 23 hours; the large T antigen can be detected by immunocytochemistry and by Western blotting. When transferred to non-permissive conditions (39 degrees C, without IFN-gamma), the cells undergo differentiation coinciding with the disappearance of the large T antigen. By electron microscopy, tsMPT cells are polarized and show microvilli at their apical surface. tsMPT cells express brush border enzymes gamma-glutamyl transpeptidase and carbonic anhydrase IV. They possess Na(+)-dependent transport systems for Pi, D-glucose and L-proline as well as an amiloride-insensitive Na(+)-H+ exchanger. Intracellular cAMP generation is stimulated by parathyroid hormone but not by arginine vasopressin. Angiotensinogen mRNA and protein are present in tsMPT with markedly higher levels at non-permissive conditions. tsMPT cells should be a useful model for investigation of the functional features of the proximal tubule epithelium in relation to cellular differentiation.

Role of intracellular sodium overload in the genesis of cardiac arrhythmias

A number of clinical cardiac disorders may be associated with a rise of the intracellular Na concentration (Na(i)) in heart muscle. A clear example is digitalis toxicity, in which excessive inhibition of the Na/K pump causes the Na(i) concentration to become raised above the normal level. Especially in digitalis toxicity, but also in many other situations, the rise of Na(i) may be an important (or contributory) cause of increased cardiac arrhythmias. In this review, we consider the mechanisms by which a raised Na(i) may cause cardiac arrhythmias. First, we describe the factors that regulate Na(i), and we demonstrate that the equilibrium level of Na(i) is determined by a balance between Na entry into the cell, and Na extrusion from the cell. A number of mechanisms are responsible for Na entry into the cell, whereas the Na/K pump appears to be the main mechanism for Na extrusion. We then consider the processes by which an increased level of Nai might contribute to cardiac arrhythmias. A rise of Na(i) is well known to result in an increase of intracellular Ca, via the important and influential Na/Ca exchange mechanism in the cell membrane of cardiac muscle cells. A rise of intracellular Ca modulates the activity of a number of sarcolemmal ion channels and affects release of intracellular Ca from the sarcoplasmic reticulum, all of which might be involved in causing arrhythmia. It is possible that the increase in contractile force that results from the rise of intracellular Ca may initiate or exacerbate arrhythmia, since this will increase wall stress and energy demands in the ventricle, and an increase in wall stress may be arrhythmogenic. In addition, the rise of Na(i) is anticipated to modulate directly a number of ion channels and to affect the regulation of intracellular pH, which also may be involved in causing arrhythmia. We also present experiments in this review, carried out on the working rat heart preparation, which suggest that a rise of Na(i) causes an increase of wall stress-induced arrhythmia in this model. In addition, we have investigated the effect on wall stress-induced arrhythmia of maneuvers that might be anticipated to change intracellular Ca, and this has allowed identification of some of the factors involved in causing arrhythmia in the working rat heart.

Endothelin and angiotensin II stimulation of Na+-H+ exchange is impaired in cardiac hypertrophy

We compared the effects of endothelin-1 (ET-1) on intracellular pH, intracellular [Ca2+]i, and cell contraction in hypertrophied adult ventricular myocytes from ascending aortic banded rats and age-matched controls. Intracellular pH (pH(i)) was measured in individual myocytes with SNARF-1, and [Ca2+]i was measured with indo-1, simultaneous with cell motion. Experiments were performed at 36 degrees C in myocytes paced at 0.5 Hz in Hepes-buffered solution (pH(o) 7.40) containing 1.2 mM CaCl2. At baseline, calibrated pH(i), diastolic and systolic [Ca2+]i values, and the amplitude of cell contraction were similar in hypertrophied and control myocytes. Exposure of the control myocytes to 10 nM ET-1 caused an increase in the amplitude of cell contraction to 163+/-22% of baseline (P < 0.05), associated with intracellular alkalinization (pH(i) + 0.08+/-0.02 U, P < 0.05) and a slight increase in peak systolic [Ca2+]i (104+/-11% of baseline, P < 0.05). In contrast, in the hypertrophied myocytes, exposure to ET-1 did not increase the amplitude of cell contraction or cause intracellular alkalinization (-0.01+/-0.02 U, NS). Similar effects were observed in the hypertrophied and control myocytes in response to exposure to 10 nM angiotensin II. ET-1 also increased the rate of recovery from intracellular acidosis induced by the washout of NH4Cl in the control cells, but did not do so in the hypertrophied cells. In the presence of 10 microM 5-(N-ethyl-N-isopropyl)-amiloride, which inhibits Na+-H+ exchange, ET-1 did not cause a positive inotropic effect or intracellular alkalinization in control cells. The activation of protein kinase C by exposure to phorbol ester caused intracellular alkalinization and it increased the rate of recovery from intracellular acidification induced by an NH4Cl pulse in control cells but not in hypertrophied cells. ET-1, as well as angiotensin II, and phorbol ester, fail to stimulate forward Na+-H+ exchange in adult hypertrophied myocytes. These data suggest a defect in the coupling of protein kinase C signaling with Na+-H+ exchange in adult hypertrophied myocytes.

Role of an electrogenic Na(+)-HCO3- cotransport in determining myocardial pHi after an increase in heart rate

The contribution of electrogenic Na(+)-HCO3- cotransport to pHi regulation during changes in heart rate was explored in cat papillary muscles loaded with BCECF-AM in bicarbonate-free (HEPES) medium and in CO2/HCO3(-)-buffered medium. Stepwise increments in the frequency of contraction from 15 to 100 bpm induced a reversible increase in the pHi from 7.13 +/- 0.03 to 7.36 +/- 0.03 (P < .05, n = 5) in the presence of CO2/ HCO3- buffer. The same increase in the frequency of stimulation, however, decreased pHi from 7.10 +/- 0.02 to 6.91 +/- 0.06 (P < .05, n = 5), in the absence of bicarbonate. Moreover, in CO2/HCO3(-)-superfused muscles pretreated with SITS (0.1 mmol/L), this effect of increasing the contraction frequency was reversed, and a decrease of pHi from 7.03 +/- 0.04 to 6.88 +/- 0.06 (P < .05, n = 4) was observed when the pacing rate was increased stepwise from 15 to 100 bpm. High [K+]o-induced depolarization of cell membrane alkalinized myocardial cells in the presence of HCO3- ions, whereas acidification was observed as a consequence of hyperpolarization induced by low external [K+]o. Myocardial resting membrane potential became hyperpolarized upon exposure to HCO3(-)-buffered media. This HCO3(-)-induced hyperpolarization was not blocked by the inhibition of Na+,K(+)-ATPase activity by ouabain (0.5 mumol/L) but was prevented by SITS. The results suggested that membrane depolarization during cardiac action potential causes an increase in electrogenic Na(+)-HCO3- cotransport. Such depolarizations occurring as a consequence of increases in heart rate would thus, by means of elevated bicarbonate influxes, substantially increase the myocardial cell's ability to recover from an enhanced proton production.

Effects of angiotensin II on intracellular Ca2+ and pH in isolated beating rabbit hearts and myocytes loaded with the indicator indo-1

1. Angiotensin II increases myocardial contractility in several species, including the rabbit and man. However, it is controversial whether the predominant mechanism is an increase in free cytosolic [Ca2+]i or a change in myofilament Ca2+ sensitivity. To address this question, we infused angiotensin II in isolated perfused rabbit hearts loaded with the Ca2+ indicator indo-1 AM and measured changes in beat-to-beat surface transients of the Ca2+i-sensitive 400:500 nm ratio and left ventricular contractility. The effects of angiotensin II were compared with the response to a Ca(2+)-dependent increase in the inotropic state produced by a change in the perfusate [Ca2+] from 0.9 to 3.6 nM. 2. In the isolated beating heart, an increase in perfusate [Ca2+] caused an increase in left ventricular pressure +dP/dt in association with an increase in peak systolic [Ca2+]i. Angiotensin II perfusion caused a similar increase in left ventricular +dP/dt in the absence of any increase in peak systolic [Ca2+]i. 3. To exclude any contribution of non-myocyte sources of Ca(2+)-sensitive fluorescence which may be present in the intact heart, we also compared the effects of angiotensin II and a change in superfusate [Ca2+] in collagenase-dissociated paced adult rabbit ventricular myocytes loaded with indo-1 AM. In the isolated rabbit myocytes a change in perfusate [Ca2+] from 0.9 to 3.6 mM caused an increase in peak systolic cell shortening coincident with an increase in peak systolic [Ca2+]i. In contrast, angiotensin II caused a similar increase in peak systolic cell shortening whereas there was no increase in peak systolic [Ca2+]i. There was also no change in inward Ca2+ current (ICa) in response to angiotensin II. 4. To investigate further the mechanism of the positive inotropic action of angiotensin II, its effects on intracellular pH were studied in isolated rabbit myocytes loaded with the fluorescent H+ probe SNARF 1. These experiments demonstrated that angiotensin II induced a 0.2 pH unit increase coincident with the development of a positive inotropic effect in isolated rabbit myocytes. 5. In summary, angiotensin II has a direct positive inotropic effect in beating rabbit hearts and in isolated paced rabbit myocytes. These experiments provide support for the hypothesis that the predominant mechanism is not an increase in free cytosolic Ca2+ but is due in part to an increase in myofilament Ca2+ sensitivity due to intracellular alkalosis.

New Na(+)-H+ exchange inhibitor HOE 694 improves postischemic function and high-energy phosphate resynthesis and reduces Ca2+ overload in isolated perfused rabbit heart

BACKGROUND

Experiments were carried out using the new Na(+)-H+ exchange inhibitor (3-methylsulfonyl-4-piperidinobenzoyl)guanidine methanesulfonate (HOE 694) to assess the role of Na(+)-H+ exchange in myocardial ischemic and reperfusion injury.

METHODS AND RESULTS

Three groups of rabbit hearts (n = 5 in each) were perfused with blood and were subjected to 45 minutes of global normothermic (37 degrees C) ischemia, followed by 1 hour of reperfusion. Group 1 was the control group (vehicle only); in group 2, HOE 694 (1 mumol/L) was administered before ischemia (pretreatment group); and in group 3, HOE 694 was given only during reperfusion to separate actions exerted during ischemia from those specifically obtained during reperfusion. End-diastolic pressure rise at 1 hour of reperfusion was reduced by administration of HOE 694 starting before ischemia (from 52.2 +/- 8.5 mm Hg in group 1 to 17.6 +/- 4.5 mm Hg in group 2, P < .01) or starting on reperfusion (28.8 +/- 5.4 mm Hg in group 3, P < .05 versus group 1). Left ventricular developed pressure (LVDP) and its derivative (dP/dt) recovered better in HOE 694-pretreated hearts (LVDP, 79 +/- 9.9 mm Hg in group 2 versus 24.8 +/- 10 mm Hg in group 1; dP/dt, 1580 +/- 198 mm Hg/s versus 340 +/- 221 mm Hg/s, P < .01). In hearts treated only on reperfusion, some improvement was observed, which, however, did not reach statistical significance. Coronary flow on reperfusion was higher in groups 2 and 3 compared with controls, and no "no-reflow" was observed. Two additional groups of hearts were perfused with phosphate-free Krebs-Henseleit solution to enable studies with 31P nuclear magnetic resonance (NMR). ATP was better preserved in HOE 694-pretreated (62 +/- 4.9% of preischemic value) than in control hearts (44 +/- 3.3%) at the end of 30 minutes of reperfusion, and phosphocreatine resynthesis was higher (109 +/- 3.7% versus 86 +/- 5.4%). HOE 694 did not affect the time course of intracellular acidosis during ischemia but suppressed a small alkaline overshoot occurring early in reperfusion (pH 6.96 +/- 0.02 in HOE 694-pretreated hearts versus 7.14 +/- 0.05 in control hearts). Electron microscopy with Ca2+ staining of the blood-perfused hearts showed that clumping of Ca2+ aggregates in mitochondria was prevented by HOE 694.

CONCLUSIONS

Postischemic dysfunction was associated with a rise in end-diastolic pressure. This rise was effectively blocked by HOE 694. The drug was most effective when hearts were treated before ischemia, although partial protection was observed when administration was started on reperfusion. The action of HOE 694 strengthens the idea that Na(+)-H+ exchange during both ischemia and reperfusion contributes to contractile dysfunction.