Interactive in vitro effect of prolactin, growth hormone and season on leptin secretion by ovine adipose tissue

White adipose tissue is an important endocrine organ. Receptors for several hormones are found in the adipocytes, suggesting that these hormones may directly regulate the activity of the fat cells. The effects of prolactin (PRL), growth hormone (GH), melatonin, insulin and their interaction on the regulation of leptin secretion from ovine peri-renal adipose tissue samples were evaluated. Adipose tissue isolated from 15 ewes slaughtered in May, July (spring/summer), September, October and November (Fall) (3 ewes per month) were used. Adipose tissue was cut into 100 mg samples, which were incubated for 2 h in Eagle's medium or a medium supplemented with melatonin (100 ng/ml) and challenged with or without (control) the following hormones: insulin (100 ng/ml), PRL (100 ng/ml), PRL (300 ng/ml), insulin (100 ng/ml) + PRL (100 ng/ml), insulin (100 ng/ml) + PRL (300 ng/ml), GH (100 ng/ml), or insulin (100 ng/ml) + GH (100 ng/ml). After the addition of the hormones, the incubation continued for another 3 h period. Leptin concentrations in the culture media were determined using RIA. Results showed the basal secretion of leptin from adipose tissue to vary, depending on the month in which the incubation was carried out. Being the lowest in May (end of spring) and the highest during the fall months (P < 0.05). However, no significant individual effects of the added hormones on leptin concentration were recorded. These results suggest that all the examined hormones may affect the activity of the adipocytes. However the hormones' impacts depend on the presence of other hormones. Similarly, there was a substantial seasonality effect recorded regarding the interaction between the hormones.