Interaction between Na+ and H+ ions on Na-H exchange in sheep cardiac Purkinje fibers.
The interaction between Na+ and H+ ions upon Na-H exchange (NHE) was examined in sheep cardiac Purkinje fibers. Acid equivalent fluxes through NHE were examined using recordings of intracellular pH and Na+ in isolated preparations measured with ion selective microelectrodes. The extent of acid-extrusion by NHE was estimated from pH(i) recovery-rate, multiplied by beta(i) (intracellular buffering power) in response to an internal acid load induced by 20 mm NH4Cl removal (nominally HCO3- free media). A mixed inhibitory effect was found of extracellular H+ on external Na+-activation of NHE (i.e. an increase, at low pH(o), in the apparent Michaelis constant for external Na+ ions [K(Nao)(0.5)] and a decrease in the maximum transport rate [V(Nao)(max)]). In addition, we confirmed that the stoichiometry of Na(o) binding is unaffected by the pH(o) (between 7.5 and 6.5), showing a Hill coefficient close to one. The interaction between Na+ and H+ ions at the internal face of the cardiac NHE was also studied. Our evidence suggests that an increase in the intracellular Na+ ion concentration ([Na+]i) inhibits acid efflux and that this inhibition can be approximated by the decrease in thermodynamic driving force caused by reducing the transmembrane Na+ gradient. It appears, however, that small variations in [Na+]i from the normal resting level (intracellular sodium activity, a(i)Na = 7 to 13 mm) have little or no effect on acid efflux, suggesting that variation of a(i)Na is not a physiologically important controller of NHE activity in heart.
Wu
M L
ML
Institute of Physiology, College of Medicine, National Taiwan University, Taipei, Republic of China.
Vaughan-Jones
R D
RD
D000818:Animals; D002021:Buffers; D004594:Electrophysiology / Q000295:instrumentation / Q000379:methods; D006531:HEPES; D006863:Hydrogen-Ion Concentration; D008839:Microelectrodes; D011522:Protons; D011690:Purkinje Fibers / Q000187:drug effects / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D012756:Sheep; D012964:Sodium / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*
D002021:Buffers; D011522:Protons; D000644:Quaternary Ammonium Compounds; D017923:Sodium-Hydrogen Exchangers; D012964:Sodium; D006531:HEPES