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--cotransport (NBC) transport rates. We fluorimetrically measured Hi+ fluxes (JNHE-1 and JNBC) and Nai+ 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 pHi at which NHE-1 and NBC become quiescent (set-point pHi) and a reduction of the steepness of the pHi-Hi+ flux profiles. NHE-1 and NBC did not contribute to Nai+ loading during metabolic inhibition (Nai+ 18 ± 1.7 mM) or pseudo-ischemia (Nai+ 21 ± 1.7 mM), because pHi acidified less than set-point pHiʹs. Upon pseudo-reperfusion NBC recovered to 54 ± 7.3% but NHE-1 to 193 ± 11% of aerobic control flux, and set-point pHiʹs returned to near neutral values. Metabolic inhibition and reperfusion caused an acid load of 18 ± 3.2 mM H+ 94% of which were extruded by the hyperactive NHE-1. At pseudo-reperfusion Nai+ rose sharply to 31 ± 5.8 mM within 1.5 min and that coincided with hypercontracture. Cariporide not only prevented the Nai+ transient, but also inhibited pHi recovery and prevented hypercontracture. Our results are consistent with the view that NHE-1 is active during metabolic inhibition if, like in whole hearts, pHi is driven more acidic than NHE-1 set-point pHi. Furthermore, either an acidic pHi or absence of additional Nai+ loading during reperfusion, or both, limit ischemia–reperfusion injury.