Lithium ion as a probe of Na+ channel activity in isolated rat hearts: a multinuclear NMR study

The aim of this study was to analyze Na+ fluxes in whole perfused hearts using Li+ as a Na+ congener and 7Li-nuclear magnetic resonance as a detection method. Hearts were equilibrated for 32 min with 15 mM LiCl added to P1-free Krebs-Henseleit buffer (intracellular space (ICS) [Li+] = 21.5 +/- 3.4 mM). Li+ efflux was monitored using a Li(+)-free perfusate. The effects of drugs on Li+ efflux were studied by adding the compounds 4 min prior to initiating Li+ washout. 7Li-NMR spectra were collected every 2 min at 139.95 MHz. Li+ efflux was biphasic with rate constants (k +/- SD, min-1) of 0.5 +/- 0.1 (extracellular) and 0.09 +/- 0.01 (ICS). Li+ efflux from ICS was dependent on heart rate (HR): cardiac arrest produced by 1 mM lidocaine or 20 mM KCl reduced k to 1/3 of its control value (Lidocaine, 0.030 +/- 0.004; KCl, 0.035 +/- 0.003). Increasing concentrations of carbachol (0.2-3.0 microM) caused a gradual decrease in HR and revealed a linear relationship between k and HR. In KCl-arrested hearts the Na+ channel opener veratridine increased k by 60% (10 microM, 0.057 +/- 0.006). Dimethylamiloride did not affect k (10 microM, 0.024 +/- 0.006) in Lidocaine-arrested hearts. Bumetanide (30 microM, 0.094 +/- 0.013), nifedepine (0.33 microM, 0.088 +/- 0.009), Bay K8644 (0.1 microM, 0.080 +/- 0.002), 4-aminopyridine (1.5 mM, 0.076 +/- 0.006) and cromakalim (10 microM, 0.088 +/- 0.006) did not significantly affect either k or HR. Li+ efflux from myocytes in perfused rat heart is mediated mainly by voltage-dependent Na+ channels.