Effect of degree of lipomobilization on results of glucose test in dairy cows in heat stress | Uticaj stepena lipomobilizacije na rezultate testa opterećenja glukozom kod mlečnih krava u toplotnom stresu

Cows exposed to heat stress exhibit a decreased ability to mobilize lipids due to increased sensitivity to insulin, which is expressed in a decreased concentration of NEFA. However, certain cows can preserve the level of lipid mobilization after adapting to heat stress. We assumed that cows that have a preserved ability to mobilize lipids are less sensitive to insulin and that they have a lower tolerance for glucose. The aim of this work was to compare the results of an intravenous glucose tolerance test in cows that exhibited, in prolonged heat stress, a decreased (NEFA is less than 0.20 mmol/l) or preserved (NEFA is greater than 0.30 mmol/l) ability for lipid mobilization. Glucose concentration and NEFA concentration were measured following intravenous application of glucose. The mean glycaemic index value did not differ statistically significantly between the two groups of cows at 10, 15 and 20 minutes after glucose application (p is greater than 0.05), but there was a tendency at 10 and 15 minutes for the glycaemia to be higher in cows with preserved lipomobilization (p is less than 0.1). At 30, 60 and 90 minutes after glucose application, glycaemia was statistically significantly higher (p is less than 0.01; p is less than 0.05 and p is less than 0.05) in the group of cows with preserved lipomobilization. The glycaemic index values (mmol/l) shown in the same order (30, 60 and 90 minutes) were as follows 9.91 +/- 0.21: 9.23 +/- 0.41; 5.41 +/- 0.5: 4.67 +/- 0.33 and 4.31 +/- 0.39: 3.47 +/- 0.37. The mean value for NEFA concentration in samples originating from the two experimental groups of cows did not differ statistically significantly following glucose application. The NEFA concentration showed a tendency to be higher in cows with preserved lipid mobilization in comparison with cows with decreased lipomobilization at 20 and 30 minutes after glucose application (p is less than 0.1). Following the intravenous glucose tolerance test, NEFA and glucose concentrations were in a significant negative correlation, and that correlation was more expressed in cows with decreased lipomobilization. Cows with preserved lipomobilization have a lower tolerance for glucose than cows with a decreased lipomobilization during heat stress.