| Summary: | 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. === Master of Science === Heat stress causes large annual financial losses for the dairy industry and presents potential welfare issues for dairy cows when they can no longer cope appropriately with their environment. As climate change continues, the intensity and duration of heat stress experienced by dairy cows will increase which will cause the effects of heat stress on the dairy industry, as well as on the wellbeing of dairy cows, to become more significant. For this reason, it is important to understand the physiological processes underlying a cow's adaptive response to heat stress to improve future farm management in a changing climate. It is well documented that heat stressed dairy cows experience an increase in plasma insulin concentrations, as well as a concurrent decrease in plasma glucose concentrations. It is not well understood how or why these changes occur, but ɣ-aminobutyric acid (GABA) is known to be secreted from the same cells that secrete insulin and to have effects on the secretion of insulin, as well as on concentrations of glucose in the blood through effects on pancreatic glucagon secretion. This work began the process of determining the physiological mechanism behind these changed concentrations by determining how plasma GABA changes during heat stress in lactating dairy cows and how those changes are related to other physiological changes observed during heat stress. It was determined that plasma GABA tends to decline during heat stress and is significantly related to milk production, as well as blood glucose concentrations. While these results cannot be taken without more research to imply cause, they do support the idea that GABA plays a role in coordinating the altered glycemic state observed in heat stressed dairy cattle and would be an interesting research target in the future.
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