Effects of NHE-1 inhibition on cardioprotection and impact on protection by K/Mg cardioplegia

A halocin acting on Na+/H+ exchanger of haloarchaea as a new type of inhibitor in NHE of mammals

The capability of halocin H6 (a bacteriocin-like protein produced by haloarchaea Haloferax gibbonsii) to inhibit Na+/H+ exchanger (NHE) in mammalian cells and its cardio-protective efficacy on the ischemic and reperfused myocardium were evaluated in the present study. H6 inhibits NHE activity (measured by a flow cytometry method) in a dose-dependent form of cell lines of mammalian origin (HEK293, NIH3T3, Jurkat and HL-1) as well as in primary cell culture from human skeletal muscle (myocytes and fibroblasts). In vivo, an ischemia-reperfusion model in dogs by coronary arterial occlusion was used (two hours of regional ischemia and three hours of reperfusion). In animals treated with halocin H6 there was a significant reduction of premature ventricular ectopic beats and infarct size, whereas blood pressure and heart rate remained unchanged. Up to date, halocin H6 is the only described biological molecule that exerts a specific inhibitory activity in NHE of eukaryotic cells.

Mitochondrial ATP-sensitive potassium channels in surgical cardioprotection

ATP-sensitive potassium channels allow for the coupling of membrane potential to cellular metabolic status. Two K(ATP) channel subtypes coexist in the myocardium with one subtype located in the sarcolemma membrane and the other in the inner membrane of the mitochondria. The ATP-sensitive potassium channels can be pharmacologically modulated by a family of structurally diverse agents of varied potency and selectivity, collectively known as potassium channel openers and blockers. Sufficient evidence exists to indicate that the ATP-sensitive potassium channels and in particular the mitochondrial ATP-sensitive potassium channels play an important role both as a trigger and an effector in surgical cardioprotection. In this review, the biochemistry and specificity of the ATP-sensitive potassium channels is examined in relation to surgical cardioprotection.

Is timing everything? Therapeutic potential of modulators of cardiac Na(+) transporters

Sodium ion (Na(+)) transporters have roles in the modulation of cardiomyocyte pH and Na(+) and Ca(2+) handling. Activation of the cardiac Na(+)-H(+) exchanger 1 (NHE1) during ischaemia induces arrhythmias, myocardial stunning and irreversible cell injury. As the benefits of NHE1 inhibitors (e.g., amiloride, cariporide) in models of myocardial infarction are usually much greater when used as pretreatment, rather than during or after ischaemia, it is probably not surprising that clinical trials with cariporide in ischaemia have shown little shortterm benefit. NHE1 inhibitors have been shown to be beneficial in animal models of ventricular fibrillation and resuscitation, cardioplegia, hypertrophy and heart failure, and their therapeutic potential in these conditions should be further developed. The Na(+)-HCO(3)(-) cotransporter (NBC) is also stimulated by intracellular acidification, and part of the benefit of angiotensin-converting enzyme inhibitors after myocardial infarction may be due to inhibition of the NBC. Selective inhibitors of the NBC are required to determine the therapeutic potential of this mechanism. The Na(+)-Ca(2+) exchanger (NCX) has a major role in cardiac Na(+) and Ca(2+) homeostasis and influences cardiac electrical activity. The NCX also has a role in ischaemia/infarction, arrhythmias, hypertrophy and heart failure. NCX inhibitors may have beneficial effects in animal models of ischaemia and reperfusion injury and the therapeutic benefit of these should be further studied in animal models.

Cardioprotective effects and the mechanisms of terminal warm blood cardioplegia in pediatric cardiac surgery

OBJECTIVES

Terminal warm blood cardioplegia has been shown to enhance myocardial protection in adult patients. However, the cardioprotective effects and the mechanisms of terminal warm blood cardioplegia in pediatric heart surgery were still unknown.

METHODS

One hundred three consecutive patients were prospectively randomized to one of two groups. In the control group (n = 52), myocardial protection was achieved with intermittent hyperkalemic cold blood cardioplegia and topical cardiac cooling. In the terminal warm blood cardioplegia group (n = 51), this was supplemented with terminal warm blood cardioplegia before the aorta was declamped. Arterial and coronary sinus blood samples were analyzed to determine myocardial energy metabolism and tissue injury.

RESULTS

There were no significant differences between the two groups in age (5.5 +/- 0.6 years in the control group vs 5.6 +/- 0.5 years in the terminal warm blood cardioplegia group), body weight (17.2 +/- 1.4 kg in the control group vs 19.8 +/- 1.7 kg in the terminal warm blood cardioplegia group), percentage of cyanotic heart diseases (50% in the control group vs 51% in the terminal warm blood cardioplegia group), number of patients who required right ventriculotomy (33% in the control group vs 39% in the terminal warm blood cardioplegia group), cardiopulmonary bypass time (194 +/- 12.1 minutes in the control group vs 177 +/- 8.6 minutes in the terminal warm blood cardioplegia group), aortic crossclamp time (83.3 +/- 5.9 minutes in the control group vs 82.3 +/- 5 minutes in the terminal warm blood cardioplegia group), lowest rectal temperature (27.4 +/- 0.3 degrees C in the control group vs 28.1 +/- 0.3 degrees C in the terminal warm blood cardioplegia group), and myocardial temperature (9.6 +/- 0.6 degrees C in the control group vs 9.6 +/- 0.7 degrees C in the terminal warm blood cardioplegia group). Spontaneous defibrillation occurred after reperfusion in 80% in the terminal warm blood cardioplegia group, which was significantly (P <.05) higher than the control group (62%). The lactate extraction rate at 60 minutes of reperfusion was significantly (P <.05) higher in the terminal warm blood cardioplegia group (9.0 +/- 2.8%) than the control group (-3.3 +/- 2.4%). The postreperfusion values of cardiac troponin T (7.4 +/- 0.6 ng/mL vs 11.2 +/- 1.0 ng/mL at 6 hours; 4.6 +/- 0.6 ng/mL vs 9.3 +/- 1.6 ng/mL at 18 hours) and heart-type fatty acid binding protein (137 +/- 28 ng/mL vs 240 +/- 30 ng/mL at 2 hours; 88 +/- 19 ng/mL vs 162 +/- 26 ng/mL at 3 hours) were significantly (P <.05 vs the control group) lower in the terminal warm blood cardioplegia group.

CONCLUSION

Terminal warm blood cardioplegia enhances myocardial protection in pediatric cardiac surgery by an improvement in aerobic energy metabolism and a reduction of myocardial injury or necrosis.

The mitochondrial K(ATP) channel and cardioprotection

Adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels allow coupling of membrane potential to cellular metabolic status. Two K(ATP) channel subtypes coexist in the myocardium, with one subtype located in the sarcolemma (sarcK(ATP)) membrane and the other in the inner membrane of the mitochondria (mitoK(ATP)). The K(ATP) channels can be pharmacologically modulated by a family of structurally diverse agents of varied potency and selectivity, collectively known as potassium channel openers and blockers. Sufficient evidence exists to indicate that the K(ATP) channels and, in particular, the mitoK(ATP) channels play an important role both as a trigger and an effector in surgical cardioprotection. In this review, the biochemistry and surgical specificity of the K(ATP) channels are examined.