Effect of age, body condition, pregnancy and lactation on circulating leptin concentrations in beef cattle

A series of experiments were conducted to evaluate the potential role of leptin in bovine reproduction. In Experiment 1, mean circulating leptin concentrations of postpartum cows were not affected by exogenous dexamethasone treatments. In Experiment 2, mean leptin concentrations were not correlated with female age or body weight but were positively correlated with body condition scores of beef cattle. Leptin concentrations were higher in 1 year old heifers (8.9 ng/ml) compared with 2 year old cows (6.0 ng/ml), but heifer leptin concentrations were not different than 4 to 6 year old cows (8.0 ng/ml) and cows ≥7 years of age (10.5 ng/ml). Mean leptin concentrations were negatively correlated with age in heifers and cows ≤2 years of age and positively correlated with age in cows \u3e3 years of age. In Experiment 3, there were no differences in mean leptin concentrations for 56 days starting 14 days following AI among 2-year old and 3-year old cows pregnant to AI (1.2 ng/ml), the clean-up bulls (1.2 ng/ml) and nonpregnant females (2.2 ng/ml) after a 60-day breeding season. Plasma leptin concentrations were lower for lactating cows (1.0 ng/ml) compared with nonlactating cows (2.1 ng/ml). Female age did not affect circulating leptin concentrations. In Experiment 4, oviduct and uterine epithelial cells from mid-luteal phase females stained positive for the long form of the leptin receptor, and uterine biopsies revealed intense staining for the long form of the leptin receptor on the luminal side of the uterine endometrium. Bovine blastocysts stained positive for the long form of the leptin receptor in the trophoblast cells. In Experiment 5, addition of leptin to culture medium at 0, 100 and 1,000 ng/ml did not affect the percentage embryos developing to the blastocyst stage. Also, leptin did not affect the ratio of blastocysts:8- to 16-cell embryos among the 0 ng/ml treatment group, the 100 ng/ml treatment and the 1,000 ng/ml treatment groups. Results indicate that in the beef cow, the release of leptin and subsequent role(s) of leptin in reproductive processes are likely different than those that have been reported for mice, rats and humans