Protection of ischemic myocardium in diabetics by inhibition of electroneutral Na+-K+-2Cl- cotransporter.

Abstract

Diabetes increases both the incidence of cardiovascular disease and complications of myocardial infarction and heart failure. Studies using diabetic animals have shown that changes in myocardial sodium transporters result in alterations in intracellular sodium (Na(i)) homeostasis. Because the changes in sodium homeostasis can be due to increased entry of Na+ via the electroneutral Na+-K+-2Cl- cotransporter (NKCC), we conducted experiments in acute diabetic hearts to determine if 1) net inward cation flux via NKCC is increased, 2) this cotransporter contributes to a greater increase in Na(i) during ischemia, and 3) inhibition of NKCC limits injury and improves function after ischemia-reperfusion. These issues were investigated in perfused type I diabetic and nondiabetic rat hearts subjected to ischemia and 60 min of reperfusion. A group of diabetic and nondiabetic hearts was perfused with 5 microM of bumetanide, an inhibitor of NKCC. Flux via NKCC, Na(i), and ATP was measured in each group with the use of radiotracer 86Rb, 23Na, and 31P nuclear magnetic resonance spectroscopy, respectively, whereas ischemic injury was assessed by measuring creatine kinase release on reperfusion. Cation flux via NKCC, as measured by 86Rb uptake, was significantly increased in diabetic hearts. Inhibition of NKCC significantly reduced ischemic injury in diabetic hearts, improved functional recovery on reperfusion, attenuated the ischemic rise in Na(i), and conserved ATP during ischemia-reperfusion. Parallel studies in nondiabetic hearts showed that NKCC inhibition was not cardioprotective. These findings demonstrate that flux via NKCC is increased in type I diabetic hearts and that inhibition with bumetanide attenuates changes in Na(i) and ATP during ischemia and protects against ischemic injury. The data suggest a therapeutic role for pharmacological agents that inhibit flux via NKCC in diabetic patients with myocardial ischemia.