Evaluating the impact of heat stress and altered glycemic state on plasma ɣ-Aminobutyric Acid (GABA) in lactating Holstein cows

Heat stress (HS) induces hyperinsulinemia and hypoglycemia in lactating dairy cows. We hypothesized that γ-aminobutyric acid (GABA) participates in the regulation of this altered glycemic state as it is produced by the pancreatic beta cells and has a stimulatory effect on pancreatic secretion of insulin. Multiparous lactating Holstein cows (n=6; 63.33±2.35 DIM, 3.17±0.40 lactations) were placed in environmentally controlled rooms for four experimental periods: 1) thermoneutral (TN; d 1-5; 18±4°C), 2) TN + hyperinsulinemic-hypoglycemic clamp (HHC; d 6-10), 3) heat stress (HS; d 16-20; 33±4°C), and 4) HS + euglycemic clamp (EC; d 21-25). Cows were milked twice daily, and blood samples were collected once daily via coccygeal venipuncture into heparinized evacuated tubes. Plasma GABA concentrations were determined using a competitive ELISA. The data were analyzed in two ways. The first analysis included data from all treatment periods and yielded no period-based differences in plasma GABA concentrations. In this analysis, plasma GABA was lowly correlated to plasma insulin concentrations (r = -0.29, P<0.01). The second excluded data from HHC and EC periods so that GABA concentrations during TN were directly compared to concentrations during HS. In this analysis, plasma GABA concentrations tended to be higher in TN than HS (16.31±2.14 vs 13.80±2.15 ng/ml, respectively, P = 0.06). Milk production was moderately correlated with plasma GABA (r=0.42, P<0.01) and the average plasma GABA during TN and HS was moderately correlated to baseline glucose levels for those periods (r=-0.57, P=0.05). Furthermore, the percent change in plasma GABA was strongly correlated with the percent change in plasma glucose from TN to HS (r=-0.95, P<0.01). Plasma GABA was again lowly correlated to plasma insulin concentrations (r = -0.35, P = 0.01). While these analyses are not indicative of causality, the results suggest that GABA is involved in the regulation of the altered glycemic state observed during HS. More research is needed to determine its precise role in heat-stressed lactating dairy cattle.