INFLUENCE OF CARDIAC PACING ON INTRACELLULAR SODIUM IN THE ISOLATED PERFUSED RAT-HEART

Increased myocardial contractile tension has been described as being associated with increased heart rate. This phenomenon is believed due to greater sodium influx than efflux, resulting in accumulation of intracellular sodium. Sodium-23 nuclear magnetic resonance spectroscopy in combination with extracellular shift reagents offers near-continuous measurements of intracellular sodium that may be correlated with mechanical performance. In this study, the influence of cardiac pacing on intracellular sodium (Na+i) was examined in the isolated perfused rat heart using the paramagnetic shift reagent, Tm(DOTP)5-. The effect of changing heart rate on mechanical performance was measured using a pressure transducer-tipped catheter or a fluid-filled catheter during spectroscopic observation. There was no significant change in Na+i with increasing heart rate over a wide range of heart rates, and a fall in developed pressure with increasing heart rate (negative force-frequency relationship) was observed. It is concluded that the concentration of intracellular sodium monitored by this method is not sensitive to changes in heart rate.