Effects of caudal hindbrain lactate infusion on insulin‐induced hypoglycemia and neuronal substrate transporter glucokinase and sulfonylurea receptor‐1 gene expression in the ovariectomized female rat dorsal vagal complex: Impact of estradiol

Abstract

The monocarboxylate, lactate, is produced by astrocytic glycolysis and is trafficked to neurons as a substrate fuel for aerobic respiration. This molecule is a critical monitored metabolic variable in hindbrain detection of cellular energy imbalance, because diminished uptake and/or oxidative catabolism of lactate in this part of the brain activates neural mechanisms that increase systemic glucose availability. Lactate-sensitive chemosensory neurons occur in the hindbrain dorsal vagal complex (DVC). Estradiol (E) enhances expression of the neuronal monocarboxylate transporter MCT2 in the DVC during insulin-induced hypoglycemia (IIH), evidence that this hormone may promote local lactate utilization during systemic glucose shortages. We investigated the hypothesis that E regulates basal and IIH-associated patterns of DVC MCT2 and neuronal glucose transporter gene expression and that caudal fourth ventricular (CV4) lactate infusion exerts divergent effects on blood glucose levels and DVC energy transducer gene profiles in hypoglycemic E- vs. oil (O)-implanted ovariectomized (OVX) rats. Insulin-induced decrements in circulating glucose were significantly augmented by lactate, albeit to a greater extent in the presence of E. DVC MCT2, GLUT3, GLUT4, glucokinase (GCK), and sulfonylurea receptor-1 (SUR1) mRNA levels did not differ between saline-injected OVX + E and OVX + O rats. IIH elevated MCT2 and GLUT3 gene profiles in both E- and O-implanted groups, but up-regulation of MCT2 transcripts was reversed by CV4 lactate infusion during hypoglycemia in E- but not O-implanted animals. DVC GLUT4 and GK mRNA were decreased by insulin alone in OVX + O but not OVX + E, but were suppressed by lactate plus insulin treatment in the latter group. Expression of the SUR1 subunit of the energy-dependent potassium channel K(ATP) was significantly decreased by IIH in both E- and O-treated rats and further suppressed in response to lactate delivery during hypoglycemia in OVX + E. These data reveal that E does not control baseline DVC substrate fuel transporter or energy transducer gene profiles or local MCT2, GLUT3, or SUR1 transcriptional responses to IIH but prevents IIH-associated decreases in GLUT4 and GCK mRNA in this brain site. The results also show that, in the presence of E, intensifying effects of CV4 lactate infusion on hypoglycemia are correlated with reversal of IIH enhancement of DVC MCT2 gene expression, augmented IIH inhibition of SUR1 transcripts, and reductions in GLUT4 and GCK mRNA levels relative to baseline. This work implies that IIH may enhance specific neuronal lactate and glucose transport mechanisms in the female rat DVC and that, in the presence of E, caudal hindbrain lactate repletion may normalize neuronal lactate but not glucose internalization by local neurons. The results also suggest that putative IIH-associated reductions in K(ATP)-mediated regulation of membrane voltage in this brain site may be causally related to diminished glucose availability.