Intracellular sodium homeostasis in relation to the effective free-energy change of ATP hydrolysis: studies of crayfish muscle fibers

Abstract

We investigated the impact of reductions in the effective free-energy change of ATP hydrolysis (dG/dATP) on intracellular sodium homeostasis in bundles of fibers from the abdominal extensor muscle of the crayfish Procambarus clarkii. 31P nuclear magnetic resonance (NMR) spectroscopy was used to monitor high-energy phosphate levels and intracellular pH while interleaved 23Na-NMR spectra were acquired to monitor changes in sodium. Previous work has shown that the bulk of intracellular Na+ is NMR visible (see Ivanics et al. 1994). The 23Na-NMR spectra were diffusion-weighted which effectively filtered out signal contributions from extracellular sodium. The efficacy of this procedure was validated using a relatively non-toxic chemical shift reagent which allowed resolution of extracellular and intracellular Na+ signals. Metabolic inhibition (cyanide/iodoacetate) produced pronounced reductions in dG/dATP coincident with dramatic increases in intracellular Na+ levels ([Na+]i). However, the increases in [Na+]i occurred at dG/dATP values well above the threshold value of −46 kJ · mol−1 required by the existing Na+ gradient and the membrane potential. These results suggest that the global dG/dATP value may not reflect the dG/dATP value in the vicinity of the pump. Alternatively, other factors, including low molecular modulators of Na+, K+-ATPase activity, may be important in this context.