In vitro insulin sensitivity of subcutaneous and omental adipocytes of precalving dairy cows across a range of BCS

The adipose tissue of dairy cows plays an important role during the transition period. Recent research indicates a selective mobilization of adipose depots during negative energy balance. In the present study, we investigated the effect of the size of the adipocytes and type of adipose depot (omental versus subcutaneous) on insulin sensitivity of the lipolytic activity of adipocytes from precalving dairy cows. Ten pregnant dairy cows (BCS 2.75–5) were euthanized and samples of subcutaneous (at the tail base) and omental adipose tissue were transferred to sterile falcon tubes containing medium (Krebs ringer bicarbonate hepes + 3% BSA). Adipose tissue fragments were minced and approximately 100 mg adipose tissue explants were incubated in 3 mL medium at 38°C on a shaker. Five different culture conditions were tested in duplicate: lipolytic activity in all the plates was stimulated with 10-6 mol/l isoproterenol and five different insulin concentrations (0; 1; 10; 200; 1000 μU/ml) were added. After 3 h of incubation, media were sampled for glycerol analysis. Results are expressed as nmol glycerol release per 3 h per million adipocytes. The number and volume of adipocytes were determined as described by DiGirolamo et al. (1971). Insulin decreased glycerol release and insulin sensitivity was expressed as percentage decrease of the maximal glycerol release (10-6 mol/l isoproterenol; 0 μU/ml insulin). Dose response curves were created using PROC NLIN in SAS to determine maximal effect and insulin dose needed to elicit halfmaximal effect (logED50). Effects on both parameters were analyzed using PROC MIXED in SAS with cow as random factor, adipose depot and volume of adipocytes as fixed factors. One cow was excluded because insulin failed to inhibit lipolytic activity. The maximal effect and logED50 for the insulin sensitivity were 0.32 ± 0.1279% and -0.22 ± 0.415 μU/ml, respectively in subcutaneous adipocytes and 0.23 ± 0.148% and 0.15 ± 0.849 μU/ml in omental adipocytes (mean ± stdev), respectively. Statistical analysis revealed negative effects of volume of the adipocytes (β = -0.2569; P < 0.05) and adipose depot (β = -0.2145; 0.1 < P < 0.05, subcutaneous depot as reference) on the maximal effect while no significant effects were found on the logED50. Insulin sensitivity of the lipolytic activity of adipocytes in precalving dairy cows is influenced by the size of the adipocytes and the adipose depot with a lower maximal effect in omental adipocytes and lower maximal effect in larger adipocytes. The dose to elicit halfmaximal effect is not influenced by adipocyte size or adipose depot.