A model of poorly controlled type 1 Diabetes Mellitus and its treatment with aerobic exercise training.

BACKGROUND: Modern exogenous insulin therapy can improve the quality of life of Type 1 Diabetic Mellitus (T1DM) patients, although maintenance of normal glycaemic levels is often a challenge given the variety of factors that alter it. A number of studies have examined the effect of exercise in T1DM; however, the majority of experimental studies have utilized diabetic rodents with severe hyperglycaemia. Given that T1DM patients are likely to refrain from hyperglycaemia, studies examining the effects of regular exercise in which blood glucose is poorly controlled would better represent the T1DM population. METHODS: The current study examined the ability of a ten-week aerobic exercise training program to modify markers of cardiovascular function and bone health in STZ-induced diabetic rodents maintained in the 9-15 mM glycaemic range through insulin therapy. RESULTS: Moderate hyperglycaemia, when prolonged, leads to significant changes in cardiac structure, bone health, and glucose handling capacity. Ten weeks of exercise was able to alleviate many of these deleterious events as no significant cardiovascular functional alterations were evident except a reduction in resting heart rate and an increase in stroke volume index. Further, despite changes in cardiac dimensions, exercise was able to elevate cardiac output index and increase the E/A ratio of exercising diabetic animals which would be indicative of improvements of cardiac function. CONCLUSIONS: Together, this study demonstrates that despite moderate hyperglycaemia, the combined role of a ten-week exercise training program coupled with insulin therapy is able to alleviate many of the well-known complications associated with diabetes progression.

Mechanistic basis of differences in Ca2+ -handling properties of sarcoplasmic reticulum in right and left ventricles of normal rat myocardium.

This study investigated Ca2+ -cycling properties of sarcoplasmic reticulum (SR) in right ventricle (RV) and left ventricle (LV) of normal rat myocardium. Intracellular Ca2+ transients and contractile function were monitored in freshly isolated myocytes from RV and LV. SR in RV displayed nearly fourfold lower rates of ATP-energized Ca2+ uptake in vitro than SR of LV. The Ca2+ concentration required for half-maximal activation of Ca2+ transport was nearly twofold higher in SR of RV. The lower Ca2+ -sequestering activity of SR in RV was accompanied by a matching decrement in Ca2+ -induced phosphoenzyme formation during the catalytic cycle of the Ca2+ -pumping ATPase (SERCA2). Western immunoblot analysis showed that protein levels of Ca2+ -ATPase and its inhibitor phospholamban (PLN) were only approximately 15% lower in SR of RV than in SR of LV. Coimmunoprecipitation experiments revealed that PLN-bound, functionally inert Ca2+ -ATPase molecules in SR of RV greatly exceed (> 50%) that in SR of LV. Endogenous Ca2+/calmodulin-dependent protein kinase-mediated phosphorylation of SR substrates did not abolish the huge disparity in SR Ca2+ pump function between RV and LV. Intracellular Ca2+ transients, evoked by electrical field stimulation, were significantly prolonged in RV myocytes compared with LV myocytes, mainly because of slow decay of intracellular Ca2+ concentration. The slow decay of intracellular Ca2+ concentration in RV and consequent decrease in the speed of RV relaxation may promote temporal synchrony of the end of diastole in RV and LV. The preponderance of functionally silent SR Ca2+ pumps in RV reflects a higher diastolic reserve required to protect and maintain RV function in the face of a sudden rise in afterload or resistance in the pulmonary circulation.

The Na(+)/H(+) exchanger: a target for cardiac therapeutic intervention.

The Na(+)/H(+) exchanger (NHE) is a ubiquitous protein present in mammalian cells. In higher eukaryotes this integral membrane protein removes one intracellular H(+) for one extracellular Na(+) protecting cells from intracellular acidification. NHE is of essential importance in the myocardium. It prevents intracellular acidosis that inhibits contractility. NHE also plays a key role in damage to the mammalian myocardium that occurs during ischemia and reperfusion and is involved in hypertrophy of the myocardium. NHE is composed of a membrane bound domain of approximately 500 amino acids plus a hydrophilic regulatory cytoplasmic domain of approximately 315 amino acids. The NHE1 isoform is the only significant plasma membrane isoform present in the myocardium. The activity of NHE1 is elevated in animal models of myocardial infarcts and in left ventricular hypertrophy. During ischemia and reperfusion of the myocardium, NHE activity catalyzes increased uptake of intracellular sodium. This in turn is exchanged for extracellular calcium by the Na(+)/Ca(2+) exchanger resulting in calcium overload and damage to the myocardium. Numerous inhibitors of NHE have been developed to attempt to break this cycle of calcium overload. In animal models excellent success has been obtained in this regard. However in humans, clinical trials have resulted in only modest success and recently, significant detrimental side effects were note of one NHE inhibitor. The mechanisms by which these inhibitors affect NHE activity are presently being investigated and regions of the protein important in NHE activity and inhibitor efficacy are related but not identical. Future studies may develop superior inhibitors that may circumvent recently reported side effects. Recently, NHE inhibition has been shown to be remarkably effective in preventing hypertrophy in some animal models. Whether this proves to be a practical treatment for hypertrophy in humans has yet to be determined.

Calpain, calpastatin activities and ratios during myocardial ischemia-reperfusion.

The purpose of this study was to test the hypothesis that myocardial ischemia-reperfusion (I/R) is accompanied by an early burst in calpain activity, resulting in decreased calpastatin activity and an increased calpain/calpastatin ratio, thereby promoting increased protein release. To determine the possibility of a 'calpain burst' impacting cardiac calpastatin inhibitory activity, rat hearts were subjected (Langendorff) to either 45 or 60 min of ischemia followed by 30 min of reperfusion with and without pre-administration (s.c.) of a cysteine protease inhibitor (E-64c). Myocardial function, calpain activities (casein release assay), calpastatin inhibitory activity and release of CK, LDH, cTnI and cTnT were determined (n = 8 for all groups). No detectable changes in calpain activities were observed following I/R with and without E-64c (p > 0.05). Both I/R conditions reduced calpastatin activity (p < 0.05) while E-64c pre-treatment was without effect, implicating a non-proteolytic event underlying the calpastatin changes. A similar result was noted for calpain-calpastatin ratios and the release of all marker proteins (p < 0.05). In regard to cardiac function, E-64c resulted in transient improvements (15 min) for left ventricular developed pressure (LVDP) and rate of pressure development (p < 0.05). E-64c had no effect on end diastolic pressure (LVEDP) or coronary pressure (CP) during I/R. These findings demonstrate that restricting the putative early burst in calpain activity, suggested for I/R, by pre-treatment of rats with E-64c does not prevent downregulation of calpastatin inhibitory activity and/or protein release despite a transient improvement in cardiac function. It is concluded that increases in calpain isoform activities are not a primary feature of l/R changes, although the role of calpastatin downregulation remains to be elucidated.

Propofol protection of sodium-hydrogen exchange activity sustains glutamate uptake during oxidative stress.

UNLABELLED: We investigated the role of intracellular pH in protection by propofol of glutamate uptake during oxidative stress. Exposure of primary astrocyte cultures to tert-butylhydroperoxide (t-BOOH, 300 microM) decreased the initial rate of Na-dependent glutamate uptake. Either propofol or alpha-tocopherol, administered 30 min after t-BOOH, attenuated this transport inhibition. These lipophilic antioxidants protected glutamate uptake whether the medium contained 25 mM bicarbonate or was nominally bicarbonate-free. t-BOOH also inhibited Na/H exchanger isoform 1 (NHE1) activation by intracellular protons and propofol prevented this inhibition. Blockade of NHE1 by the potent antagonist, 5-(N-ethyl-N-isopropyl) amiloride (1 microM), abolished the protective effects of small concentrations of propofol (1 microM) and alpha-tocopherol (40 microM) on glutamate uptake during oxidative stress in bicarbonate-free medium. 5-(N-ethyl-N-isopropyl) amiloride had no effect on antioxidant rescue of glutamate transport in medium containing 25 mM bicarbonate. These results indicate that regulation of intracellular pH may contribute to neuroprotection by propofol and other lipophilic antioxidants. Propofol concentrations that are associated with anesthesia and neuroprotection may prevent intracellular acidification during oxidative stress by preserving the NHE1 response to cytosolic protons. However, if intracellular acidification occurs nonetheless, then propofol protection of glutamate uptake activity becomes less effective and the extracellular glutamate concentration may increase to neurotoxic levels. IMPLICATIONS: Anesthetic concentrations of propofol maintain the capacity of brain cells to extrude protons during oxidative stress. However, if intracellular acidification occurs nonetheless, then propofol's protection of glutamate clearance mechanisms from oxidative damage becomes attenuated, and extracellular glutamate concentration may increase to neurotoxic levels.

Role of sodium-hydrogen exchange in cardiac hypertrophy and heart failure: a novel and promising therapeutic target.

The myocardial sodium-hydrogen exchanger (NHE), and more specifically the NHE-1 isoform is now well-recognized to be a major contributor to ischemic and reperfusion injury. Recent evidence suggests that NHE-1 is also potential candidate for targeted intervention in terms of attenuation of the remodelling and hypertrophic processes which contributes to heart failure. Experimental studies have shown that NHE-1 inhibitors attenuate cardiomyocyte hypertrophy induced by various factors and reduce heart failure in vivo, independently of infarct size reduction. Although the precise cellular mechanisms for NHE-1 involvement remain to be elucidated, current data suggest a potentially effective new therapeutic approach for the treatment of heart failure via NHE-1 inhibition.

Attenuation of compensatory right ventricular hypertrophy and heart failure following monocrotaline-induced pulmonary vascular injury by the Na+-H+ exchange inhibitor cariporide.

Pulmonary hypertension results in compensatory right ventricular (RV) hypertrophy. We studied the role of the Na+-H+ exchange (NHE) in the latter process by determining the effect of the NHE-1 inhibitor cariporide after monocrotaline-induced pulmonary artery injury. Sprague-Dawley rats received a control or cariporide diet for 7 days, at which time they were administered either monocrotaline (60 mg/kg) or its vehicle. Twenty-one days later, monocrotaline control, but not cariporide-fed animals, demonstrated increased RV weights and cell size of 65 and 52%, respectively. Monocrotaline alone significantly increased RV systolic pressure and end diastolic pressure by 70 and 94%, respectively, whereas corresponding values with cariporide were significantly reduced to 33 and 42%. Central venous pressure increased by 414% in control animals, which was significantly reduced by cariporide. Monocrotaline treatment produced a decrease in cardiac output of 28 and 8% in the absence or presence of cariporide (P < 0.05 between groups), respectively. Although body weights were significantly lower in both monocrotaline-treated groups compared with vehicle treatment, with cariporide the net gain in body weight was twice that seen in the monocrotaline-treated animals without cariporide. Monocrotaline also increased RV NHE-1 and atrial natriuretic peptide mRNA expression, which was abrogated by cariporide. Monocrotaline-induced myocardial necrosis, fibrosis, and mononuclear infiltration was completely prevented by cariporide. Cariporide had no effect on monocrotaline-induced pulmonary intimal wall thickening. Our results demonstrate that cariporide directly attenuates myocardial dysfunction after monocrotaline administration independent of pulmonary vascular effects. NHE-1 inhibition may represent an effective adjunctive therapy that selectively targets myocardial hypertrophic responses in pulmonary vascular injury.

Therapeutic potential of Na-H exchange inhibitors for the treatment of heart failure.

The Na-H exchanger (NHE) represents a family of transporters which regulate intracellular pH by removing protons in exchange for sodium influx in an electroneutral 1:1 stoichiometric relationship. Six isoforms have thus far been identified with the NHE-1 subtype representing the primary isoform in the cardiac cell. It is well-established that NHE-1 contributes to cardiac injury produced by ischaemia and reperfusion and inhibitors of the antiporter exert excellent cardioprotection. More recent evidence suggests that NHE-1 may also be important for cell growth and may contribute to the maladaptive remodelling which contributes to heart failure particularly the early hypertrophic responses. Evidence from in vitro studies suggest that NHE-1 inhibitors attenuate cardiomyocyte hypertrophy in response to various stimuli whereas in vivo studies report substantial attenuation of both hypertrophy and heart failure by these agents, especially after myocardial infarction. Accordingly, NHE-1 inhibitors could emerge as important therapeutic tools for the attenuation and treatment of heart failure.

The myocardial Na+/H+ exchanger: a potential therapeutic target for the prevention of myocardial ischaemic and reperfusion injury and attenuation of postinfarction heart failure.

The myocardial Na+/H+ exchange (NHE) represents a major mechanism for pH regulation during normal physiological processes but especially during ischaemia and early reperfusion. However, there is now very compelling evidence that its activation contributes to paradoxical induction of cell injury. The mechanism for this most probably reflects the fact that activation of the exchanger is closely coupled to Na+ influx and therefore to elevation in intracellular Ca2+ concentrations through the Na+/Ca2+ exchange. The NHE is exquisitely sensitive to intracellular acidosis; however, other factors can also exhibit stimulatory effects via phosphorylation-dependent processes. These generally represent various autocrine and paracrine as well as hormonal factors such as endothelin-1, angiotensin II and alpha1-adrenoceptor agonists, which probably act through receptor-signal transduction processes. Thus far, 6 NHE isoforms have been identified and designated as NHE1 through NHE6. All except NHE6, which is located intracellularly, are restricted to the sarcolemmal membrane. In the mammalian myocardium the NHE1 subtype is the predominant isoform, although NHE6 has also been identified in the heart. The predominance of NHE1 in the myocardium is of some importance since, as discussed in this review, pharmacological development of NHE inhibitors for cardiac therapeutics has concentrated specifically on those agents which are selective for NHE1. These agents, as well as the earlier nonspecific amiloride derivatives have now been extensively demonstrated to possess excellent cardioprotective properties, which appear to be superior to other strategies, including the extensively studied phenomenon of ischaemic preconditioning. Moreover, the salutary effects of NHE inhibitors have been demonstrated using a variety of experimental models as well as animal species suggesting that the role of the NHE in mediating injury is not species specific. The success of NHE inhibitors in experimental studies has led to clinical trials for the evaluation of these agents in high risk patients with coronary artery disease as well as in patients with acute myocardial infarction (MI). Recent evidence also suggests that NHE inhibition may be conducive to attenuating the remodelling process after MI, independently of infarct size reduction, and attenuation of subsequent postinfarction heart failure. As such, inhibitors of NHE offer substantial promise for clinical development for attenuation of both acute responses to myocardial as well as chronic postinfarction responses resulting in the evolution to heart failure.

Activation of Na+/H+ exchanger-directed protein kinases in the ischemic and ischemic-reperfused rat myocardium.

The activity of the Na(+)/H(+) exchanger has been implicated as an important contributing factor in damage to the myocardium that occurs during ischemia and reperfusion. We examined regulation of the protein in ischemic and reperfused isolated hearts and isolated ventricular myocytes. In isolated myocytes, extracellular signal-regulated kinases were important in regulating activity of the exchanger after recovery from ischemia. Ischemia followed by reperfusion caused a strong inhibitory effect on NHE1 activity that abated with continued reperfusion. Four major protein kinases of size 90, 55, 44, and 40 kDa phosphorylated the Na(+)/H(+) exchanger. The Na(+)/H(+) exchanger-directed kinases demonstrated dramatic increases in activity of 2-10-fold that was induced by 3 different models of ischemia and reperfusion in intact hearts and isolated myocytes. p90(rsk) was identified as the 90-kDa protein kinase activated by ischemia and reperfusion while ERK1/2 was identified as accounting for some of the 44-kDa protein kinase phosphorylating the Na(+)/H(+) exchanger. The results demonstrate that MAPK-dependent pathways including p90(rsk) and ERK1/2 and are important in regulating the Na(+)/H(+) exchanger and show their dramatic increase in activity toward the Na(+)/H(+) exchanger during ischemia and reperfusion of the myocardium. They also show that ischemia followed by reperfusion have important inhibitory effects on Na(+)/H(+) exchanger activity.

Na(+)/H(+) exchange inhibition reduces hypertrophy and heart failure after myocardial infarction in rats.

We investigated the effect of sodium/hydrogen exchange inhibition (NHE-1) on hypertrophy and heart failure after coronary artery ligation (CAL) in the rat. Animals were subjected to occlusion (or sham) of the left main coronary artery and immediately administered a control diet or one consisting of the NHE-1 inhibitor cariporide for 13-15 wk. Hearts were separated by small [30% of LV) infarcts. CAL depressed change in left ventricular increase in pressure over time (LV +dP/dt) in small and large infarct groups by 18.8% (P < 0.05) and 34% (P < 0.01), respectively, whereas comparative values for the cariporide groups were 8.7% (not significant) and 23.1% (P < 0.01), respectively. LV end-diastolic pressure was increased by 1,225% in the control large infarct group but was significantly reduced to 447% with cariporide. Cariporide also significantly reduced the degree of LV dilation in animals with large infarcts. Hypertrophy, defined by tissue weights and cell size, was reduced by cariporide, and shortening of surviving myocytes was preserved. Infarct sizes were unaffected by cariporide, and the drug had no influence on either blood pressure or the depressed inotropic response of infarcted hearts to dobutamine. These results suggest an important role for NHE-1 in the progression of heart failure after myocardial infarction.

Pharmacology and clinical assessment of cariporide for the treatment coronary artery diseases.

Myocardial protection through pharmacological approaches represents a large therapeutic challenge and is an important therapeutic strategy in patients with coronary artery disease, particularly after myocardial infarction. Extensive animal experiments have repeatedly demonstrated the efficacy of sodium-hydrogen exchange (NHE) inhibition as a potent cardioprotective approach. The heart possesses primarily the NHE1 isoform which has led to the development of NHE1 specific inhibitors for cardiovascular therapeutics. Cariporide (HOE 642) is the first of such agents to have been developed and subjected to clinical trial. Preclinical studies with cariporide revealed excellent protection against necrosis, apoptosis, arrhythmias and mechanical dysfunction in hearts subjected to ischaemia and reperfusion. Cariporide has recently been evaluated in a large dose-finding Phase II/Phase III clinical trial (GUARDIAN) to assess its efficacy in patients with acute coronary syndromes. Overall results failed to demonstrate protection but sub-group analysis revealed significant risk reductions with the highest cariporide dose (120 mg t.i.d.) especially in high risk patients undergoing coronary artery bypass surgery. This suggests that insufficient dosage may have accounted, at least in part, for the less than optimum results. Another NHE1 inhibitor, eniporide, is currently in Phase II clinical trial (ESCAMI) in patients with acute myocardial infarction (MI) who are given angioplasty or thrombolysis. Although the study has not been completed interim findings appear positive. Both drugs were well-tolerated and produced no excess side effects compared with placebo. Further studies are needed to confirm the efficacy of NHE1 inhibitors for the treatment of coronary heart disease, even so initial results are encouraging.

Diabetes-induced myocardial structural changes: role of endothelin-1 and its receptors.

Several metabolic abnormalities may be triggered secondary to hyperglycemia in diabetes. Some of these abnormalities may alter expression of vasoactive factors in the target organs of diabetic complications. We investigated alterations of endothelin-1 (ET-1) and its receptors, ET(A) and ET(B), and associated structural changes in the myocardium of streptozotocin-induced diabetic rats after 6 months of hyperglycemia. We further assessed the preventive effects of an ET-receptor antagonist bosentan on these changes. Compared to the non-diabetic, age- and sex-matched control animals, diabetic rats showed hyperglycemia, glucosuria, reduced body weight gain and elevated glycated Hb levels. Measurement of ET-1, ET(A) and ET(B) mRNAs by semiquantitative RT-PCR showed significantly increased mRNA levels in the hearts of diabetic rats. Treatment with bosentan failed to reduce ET-1 or ET(B) mRNA expression in diabetes, however ET(A) mRNA expression was reduced. Immunocytochemically, ET-1 was detected in the cardiomyocytes, endothelium and smooth muscle cells of the larger blood vessels and was increased in diabetes. Autoradiographic localization of ET-1 receptors, using (125)I-ET-1, showed increased binding in the endothelium and myocardium of diabetic animals. Histologically, focal fibrous scarring with apoptotic cardiomyocytes, consistent with changes secondary to microvascular occlusion, was only present in the diabetic rats. In keeping with focal fibrosis, myocardium from diabetic rats further showed significantly increased mRNA expression of two extracellular matrix protein transcripts, fibronectin and collagen alpha 1(IV) which were completely prevented by treatment with bosentan. These data suggest that hyperglycemia-induced upregulation of the ET-system in the heart may be important in the pathogenesis of cardiac involvement in diabetes.

Inhibition of alpha(1)-adrenergic-mediated responses in rat ventricular myocytes by adenosine A(1) receptor activation: role of the K(ATP) channel.

Both beta- and alpha(1)-adrenoceptors mediate the myocardial effects of catecholamines. It is well known that adenosine inhibits beta-dependent effects; however, whether alpha(1)-dependent responses can be similarly modulated is unclear. Accordingly, rat ventricular myocytes were exposed for 25 min to the alpha(1) agonist phenylephrine (2 microM, in the presence of 1 microM propranolol) in the absence or presence of adenosine (100 microM) or the A(1) receptor-selective agonist N(6)-cyclopentyladenosine (CPA, 1 microM). We also investigated the effects of K(ATP) blockade with glibenclamide (1 microM), the protein kinase C inhibitor bisindolylmaleimide (20 nM), and pertussis toxin (300 ng/ml), which uncouples G(i) protein/receptor interaction, and assessed whether effects of adenosine were mimicked by K(ATP) activation with either pinacidil or cromakalim (5 microM). Phenylephrine significantly increased cell shortening by 190% and the Ca(2+) transient by 24%, which was abolished by either adenosine or CPA, but not in the presence of the A(1) receptor-selective antagonist 8-cyclopentyl-1, 3-dipropylxanthine (1 microM), and was abolished by pertussis toxin. The effect of adenosine or CPA was reversed by glibenclamide and mimicked by either cromakalim or pinacidil. Bisindolylmaleimide was without effect. The A(2) or A(3) receptor agonists 2-(4-(2-carboxyethyl)phenylethylamino)-5'-N-ethylcarboxamidoade nos ine and N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (1 microM each), respectively, were without effect. Neither CPA nor adenosine modulated the effect of endothelin-1 (5 nM), which also acts via the phosphoinositide hydrolysis pathway. We conclude that adenosine selectively inhibits alpha(1)-adrenergic-mediated effects in rat ventricular myocytes through a G(i) protein-dependent mechanism involving A(1) receptor and K(ATP) activation. Our study further suggests that endogenous adenosine may modulate alpha(1)-mediated effects of catecholamines.

Na(+)/H(+) exchange inhibition attenuates hypertrophy and heart failure in 1-wk postinfarction rat myocardium.

Na(+)/H(+) exchange (NHE) represents a major mechanism for intracellular pH regulation, particularly in the ischemic myocardium. NHE has also been shown to be important in the regulation of cell proliferation and growth. We examined whether inhibition of NHE results in an attenuation of early postinfarction myocyte remodeling responses in the rat. Male Sprague-Dawley rats were randomized to receive either a control diet or an identical diet supplemented with the NHE inhibitor cariporide. After 1 wk, animals were anesthetized, subjected to ligation of the left main coronary artery, and maintained for an additional week, after which time they were anesthetized and intraventricular pressures were obtained. Hearts were removed, and myocytes were isolated to obtain cell dimensions and determine the response to isoproterenol. Body, heart, and lung weights were obtained. Coronary artery ligation in control animals resulted in a significant elevation in left ventricular end-diastolic pressure, as well as increased heart weight- and lung weight-to-body weight ratios, both of which were abrogated by cariporide. Cell length and area significantly increased by 14 and 19.2%, respectively, whereas cell width increased by 4.1% (P > 0.05). These cells exhibited a significant hyporesponsiveness to the positive inotropic responses to isoproterenol at the lower drug concentrations (3 and 10 nM). A <1% dimensional change occurred in myocytes from cariporide-fed animals, and the hyporesponse to isoproterenol was reversed. Cariporide had no effect on infarct size or blood pressure. These studies suggest that the early adaptive hypertrophic response of surviving myocytes is dependent on NHE activity. As such, it is attractive to suggest that NHE inhibition could be an effective therapeutic strategy for prevention of postinfarction remodeling, independent of infarct size or afterload reduction.

Cardioprotective effects of propofol and sevoflurane in ischemic and reperfused rat hearts: role of K(ATP) channels and interaction with the sodium-hydrogen exchange inhibitor HOE 642 (cariporide).

BACKGROUND: Sodium ion-hydrogen ion (Na(+)-H(+)) exchange inhibitors are effective cardioprotective agents. The N(+)-H(+) exchange inhibitor HOE 642 (cariporide) has undergone clinical trials in acute coronary syndromes, including bypass surgery. Propofol and sevoflurane are also cardioprotective via unknown mechanisms. The authors investigated the interaction between propofol and HOE 642 in the ischemic reperfused rat heart and studied the role of adenosine triphosphate-sensitive potassium (K(ATP)) channels in the myocardial protection associated with propofol and sevoflurane. METHODS: Isolated rat hearts were perfused by the Langendorff method at a constant flow rate, and left ventricular function and coronary pressures were assessed using standard methods. Energy metabolites were also determined. To assess the role of K(ATP) channels, hearts were pretreated with the K(ATP) blocker glyburide (10 microM). Hearts were then exposed to either control buffer or buffer containing HOE 642 (5 microM), propofol (35 microM), sevoflurane (2.15 vol%), the K(ATP) opener pinacidil (1 microM), or the combination of propofol and HOE 642. Each heart was then subjected to 1 h of global ischemia followed by 1 h of reperfusion. RESULTS: Hearts treated with propofol, sevoflurane, pinacidil, or HOE 642 showed significantly higher recovery of left ventricular developed pressure and reduced end-diastolic pressures compared with controls. The combination of propofol and HOE 642 provided superior protection toward the end of the reperfusion period. Propofol, sevoflurane, and HOE 642 also attenuated the onset and magnitude of ischemic contracture and preserved high-energy phosphates (HEPs) compared with controls. Glyburide attenuated the cardioprotective effects of sevoflurane and abolished the protection observed with pinacidil. In contrast, glyburide had no effect on the cardioprotection associated with propofol treatment. CONCLUSION: HOE 642, propofol, and sevoflurane provide cardioprotection via different mechanisms. These distinct mechanisms may allow for the additive and superior protection observed with the combination of these anesthetics and HOE 642.

The myocardial Na(+)-H(+) exchange: structure, regulation, and its role in heart disease.

The Na(+)-H(+) exchange (NHE) is a major mechanism by which the heart adapts to intracellular acidosis during ischemia and recovers from the acidosis after reperfusion. There are at least 6 NHE isoforms thus far identified with the NHE1 subtype representing the major one found in the mammalian myocardium. This 110-kDa glycoprotein extrudes protons concomitantly with Na(+) influx in a 1:1 stoichiometric relationship rendering the process electroneutral, and its activity is regulated by numerous factors, including phosphorylation-dependent processes. There is convincing evidence that NHE mediates tissue injury during ischemia and reperfusion, which probably reflects the fact that under conditions of tissue stress, including ischemia, Na(+)-K(+) ATPase is inhibited, thereby limiting Na(+) extrusion, resulting in an elevation in [Na(+)](i). The latter effect, in turn, will increase [Ca(2+)](i) via Na(+)-Ca(2+) exchange. In addition, NHE1 mRNA expression is elevated in response to injury, which may further contribute to the deleterious consequence of pathological insult. Extensive studies using NHE inhibitors have consistently shown protective effects against ischemic and reperfusion injury in a large variety of experimental models and has led to clinical evaluation of NHE inhibition in patients with coronary artery disease. Emerging evidence also implicates NHE1 in other cardiac disease states, and the exchanger may be particularly critical to postinfarction remodeling responses resulting in development of hypertrophy and heart failure.