Brain extracellular γ-hydroxybutyrate concentrations are decreased by L-lactate in rats: role in the treatment of overdoses

Abstract

Purpose: L-lactate represents a potential treatment for GHB overdose by inhibiting GHB renal reabsorption mediated by monocarboxylate transporters. Our objective was to assess the dose-dependence of L-lactate treatment, with and without D-mannitol, on GHB toxicokinetics/toxicodynamics (TK/TD).

Methods: Rats were administered GHB 600 mg/kg i.v. with L-lactate (low and high doses), D-mannitol, or L-lactate (low dose) with D-mannitol. GHB-induced sleep time and GHB plasma, urine and brain extracellular fluid (ECF) concentrations (by LC/MS/MS) were determined. The effect of L-lactate and D-mannitol on the uptake and efflux of GHB was assessed in rat brain endothelial RBE4 cells.

Results: L-lactate treatment increased GHB renal clearance from 1.4 ± 0.1 ml/min/kg (control) to 2.4 ± 0.2 and 4.7 ± 0.5 ml/min/kg after low and high doses, respectively, and reduced brain ECF AUC values to 65 and 25% of control. Sleep time was decreased from 137 ± 12 min (control) to 91 ± 16 and 55 ± 5 min (low and high L-lactate, respectively). D-mannitol did not alter GHB TK/TD and did not alter L-lactate's effects on GHB TK/TD. L-lactate, but not D-mannitol, inhibited GHB uptake, and increased GHB efflux from RBE4 cells.

Conclusions: L-lactate decreases plasma and brain ECF concentrations of GHB, decreasing sedative/hypnotic effects.

Figure 1

The experimental timeline to study the effects of L-lactate and D-mannitol on GHB TK/TD in rats. All L-lactate and D-mannitol treatments were administered 5 minutes after the GHB dose.

Figure 2

GHB plasma (A) and brain ECF (B) concentration-time profiles after GHB administration in the presence and absence of L-lactate and D-mannitol treatments. L-lactate was administered 5 minutes after GHB dosing either as a low dose (bolus 66 mg/kg, infusion 300 mg/kg/hr) or a high dose (bolus 330 mg/kg, infusion 605 mg/kg/hr). A separate group of rats received D-mannitol 500 mg/kg 5 minutes after GHB dosing with or without the low dose of L-lactate. Data shown are mean ± SD, n = 4 for GHB alone, n = 3 for L-lactate and mannitol treatments.

Figure 3

(A) Effects of L-lactate (1, 5, and 10 mM) on the uptake of GHB (10 mM) in RBE4 cells. L-lactate 5 and 10 mM reduced the uptake of GHB to 74% and 64% of control, respectively (p < 0.05). Data represent mean ± SEM of three sets of studies conducted in triplicate. (B) Effects of D-mannitol (10 and 100 mM) on the uptake of GHB (10 mM) in RBE4 cells. D-mannitol was either applied acutely with GHB (acute) or cells were pre-incubated for 30 minutes at 37°C with mannitol (chronic). No significant differences in the uptake of GHB were observed. Data represent mean ± SEM of three sets of studies conducted in triplicate.

Figure 4

(A) Time-course of GHB efflux in RBE4 cells. Cells were incubated with GHB (10 mM) for 30 minutes at 37°C. Efflux was characterized at room temperature by the removal of GHB and application of uptake buffer for the specified time points. Data represent the mean ± SEM of 3 experiments conducted in triplicate, plotted as intracellular GHB remaining after efflux. (B) Effects of GHB (3 and 15 mM), L-lactate (10 mM), D-mannitol (100 mM), and CHC (2.5 mM) on the efflux of GHB (10 mM) at 0.5 and 2 minutes. At 0.5 minutes, D-mannitol, L-lactate, GHB, and CHC, as well as L-lactate combined with D-mannitol or GHB increased the efflux of GHB compared to control. At 2 minutes, only L-lactate or GHB alone or L-lactate combined with D-mannitol or GHB increased the efflux of GHB. Data represent the mean ± SEM of 3 experiments conducted in triplicate, plotted as intracellular GHB remaining after efflux.