Metabolic consequences of a species difference in Gibbs free energy of Na+/Ca2+ exchange: rat versus guinea pig.
The Gibbs free energy of the sarcolemmal Na+/Ca2+ exchanger (DeltaG(Na/Ca)) determines its net Ca2+ flux. We tested the hypothesis that a difference of diastolic DeltaG(Na/Ca) exists between rat and guinea pig myocardium. We measured the suprabasal rate of oxygen consumption (VO2) of arrested Langendorff-perfused hearts of both species, manipulating DeltaG(Na/Ca) by reduction of extracellular Na+ concentration, [Na+](o). Hill equations fitted to the resulting VO2-[Na+](o) relationships yielded Michaelis constant (K(m)) values of 67 and 25 mM for rat and guinea pig, respectively. We developed and tested a simple thermodynamic model that attributes this difference of K(m) values to a 7.84 kJ/mol difference of DeltaG(Na/Ca). The model predicts that reversal of Na+/Ca2+ exchange, leading to diastolic Ca2+ influx, should occur at a value of [Na+](o) about three times higher in rat myocardium. We verified this quantitative prediction using fura 2 fluorescence to index intracellular Ca2+ concentration in isolated ventricular trabeculae at 37 degrees C. The postulated difference in free energy of Na+/Ca2+ exchange explains a number of reported disparities of Ca2+ handling at rest between rat and guinea pig myocardia.
Department of Physiology, Faculty of Medicine and Health Science, University of Auckland, Private Bag 92019, Auckland, New Zealand.
P J
PJ
Cooper
M L
ML
Ward
P J
PJ
Hanley
G R
GR
Denyer
D S
DS
Loiselle
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002151:Calorimetry; D006168:Guinea Pigs; D006321:Heart / Q000502:physiology*; D006352:Heart Ventricles; D066298:In Vitro Techniques; D007700:Kinetics; D008954:Models, Biological; D009206:Myocardium / Q000378:metabolism*; D010101:Oxygen Consumption; D010477:Perfusion; D051381:Rats; D012508:Sarcolemma / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000737:chemistry / Q000378:metabolism*; D013045:Species Specificity; D013816:Thermodynamics
D019831:Sodium-Calcium Exchanger; D012964:Sodium; D002118:Calcium