Abstract
Cattle exposed to shifts in light-dark phases during late pregnancy develop hypoglycemia and insulin resistance. Our objective was to investigate if differences in liver carbon flux for gluconeogenesis were driving circadian-disrupted metabolic alterations in glucose homeostasis, and relate changes in carbon flux to hepatic gene expression. We hypothesized circadian disruption would decrease hepatic carbon flux for glucose synthesis. Milking was ceased in late-gestation Holstein cows (n = 8) at 60 d before expected calving (BEC), and animals were assigned to either a control (n = 4) or a phase-shifted (PS; n = 4) group. From d 35 to 21 BEC both groups of cows were exposed to 16 h of light and 8 h of dark, but for the PS, light was shifted forward 6 h every 3 d. On d 21 BEC, liver biopsies were collected, subdivided, and incubated in 1.0 mM [U-13C] propionate for 2 h. Total RNA was isolated from a separate liver sample and used for RNA-sequencing analysis. Postincubation 13C mass isotopologue distribution was determined for aspartate, serine, alanine, and glutamate and used to calculate metabolic flux ratios. Enrichment of serine to enrichment of aspartate ratio (eSer:eAsp) was lower for PS (0.75 ± 0.02) cows compared with control (0.81 ± 0.04), indicating a reduction in carbon flux toward glucose for PS animals. eSer:eAsp ratio was negatively correlated to propionyl-CoA carboxylase (PCCB; r = -0.79) and succinate dehydrogenase subunit D (SDHD; r = -0.82). These relationships indicate that when dairy cattle are exposed to circadian disruption during late gestation, propionate carbon is preferentially used for energy rather than gluconeogenesis.