NHE-1 and NBC during pseudo-ischemia/reperfusion in rabbit ventricular myocytes

Despite many studies into the pathophysiology of cardiac ischemia-reperfusion injury, a number of key details are as yet undisclosed. These include the timing and magnitude of the changes in both Na+/H+ exchange (NHE-1) and Na+ - HCO3--cotran sport (NBC) transport rates. We fluorimetrically measured H+ fluxes (J(NHE-1) and J(NBc)) and Na-i(+) fluxes in single contracting rabbit ventricular myocytes subjected to metabolic inhibition, pseudo-ischemia (i.e. metabolic inhibition and extracellular acidosis of 6.4), and pseudo-reperfusion. Metabolic inhibition and pseudo-ischemia inhibited NHE-1 by 43 +/- 3.1% and 91 +/- 3.6%, and NBC by 66 +/- 5.4% and 100%, respectively. Inhibition was due to both an acidic shift of the pH(i) at which NHE-1 and NBC become quiescent (set-point pH(i)) and a reduction of the steepness of the pH(i) - H-i(+) flux profiles. NHE-1 and NBC did not contribute to Na-i(+) loading during metabolic inhibition (Na-i(+) 18 +/- 1.7 mM) or pseudo-ischemia (Na-i(+) 21 +/- 1.7 mM), because pHi acidified less than set-point pH(i)'s. Upon pseudo-reperfusion NBC recovered to 54 +/- 7.3% but NHE-1 to 193 +/- 11% of aerobic control flux, and set-point pHi's returned to near neutral values. Metabolic inhibition and reperfusion caused an acid load of 18 +/- 3.2 mM H-i(+) 94% of which were extruded by the hyperactive NHE- 1. At pseudo-reperfusion Na-i(+) rose sharply to 31 +/- 5.8 mM within 1.5 min and that coincided with hypercontracture. Cariporide not only prevented the Na-i(+) transient, but also inhibited pH(i) recovery and prevented hypercontracture. Our results are consistent with the view that NHE-1 is active during metabolic inhibition if, like in whole hearts, pH(i) is driven more acidic than NHE-1 set-point pH(i). Furthermore, either an acidic pHi or absence of additional Na-i(+) loading during reperfusion, or both, limit ischernia-reperfusion injury. (C) 2004 Elsevier Ltd. All rights reserved