Mechanisms of protection of the ischemic and reperfused myocardium by sodium-hydrogen exchange inhibition.

Sodium-hydrogen exchange (Na-H exchange) is a major regulator of intracellular pH and is one of the major mechanisms for restoring pH after ischemia-induced intracellular acidosis. However, activation of Na-H exchange during ischemia and reperfusion is also involved in paradoxical induction of cell injury. This likely reflects the fact that activation of the exchanger is closely coupled to sodium influx and, as a consequence, to elevation in intracellular calcium concentrations through sodium-calcium exchange. In addition to intracellular acidosis, other factors can also stimulate the exchanger, including various autocrine and paracrine factors, such as endothelin-1, angiotensin II, alpha(1)-adrenergic agonists, as well as toxic agents, such as hydrogen peroxide and lysophosphatidylcholine. Although at least six Na-H exchange isoforms have thus far been identified, it appears that the 1 subtype, termed NHE1, is the predominant isoform in the mammalian myocardium. Effective pharmacological inhibitors of Na-H exchange, including those that are NHE1 specific, have been extensively demonstrated to protect the ischemic and reperfused myocardium in terms of improved systolic and diastolic function, preservation of cellular ultrastructure, attenuation of the incidence of arrhythmias, and reduction of apoptosis. Moreover, the salutary effects of these agents have been demonstrated using a variety of experimental models as well as animal species, suggesting that the role of Na-H exchange in mediating injury is not species specific. Thus, Na-H exchange represents an important target for pharmacological intervention in attenuation of ischemia and reperfusion-induced cardiac injury. Coupled with the low potential for toxicity of the agents, Na-H exchange inhibition could emerge as an effective therapeutic strategy in cardiac disorders, particularly involving conditions associated with ischemia and reperfusion.