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(+) -- HCO(3)(-) -cotransport (NBC) transport rates. We fluorimetrically measured H(i)(+) 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 pH(i) 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 pH(i)'s 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 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 pH(i) or absence of additional Na(i)(+) loading during reperfusion, or both, limit ischemia-reperfusion injury.
van Borren
Marcel M G J
MM
Department of Physiology, Academic Medical Center, University of Amsterdam, Meibergdreef 15, 1105 AZ Amsterdam, The Netherlands. m.m.vanborren@amc.uva.nl
Baartscheer
Antonius
A
Wilders
Ronald
R
Ravesloot
Jan H
JH
D000818:Animals; D006352:Heart Ventricles / Q000166:cytology; D006859:Hydrogen / Q000378:metabolism; D066298:In Vitro Techniques; D015428:Myocardial Reperfusion Injury / Q000378:metabolism*; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D011817:Rabbits; D012964:Sodium / Q000378:metabolism; D012966:Sodium Cyanide / Q000494:pharmacology; D027982:Sodium-Bicarbonate Symporters / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*
D027982:Sodium-Bicarbonate Symporters; D017923:Sodium-Hydrogen Exchangers; C069139:growth factor-activatable Na-H exchanger NHE-1; D006859:Hydrogen; D012964:Sodium; D012966:Sodium Cyanide