Body reserves affect the reproductive endocrine responses to an acute change in nutrition in mature male sheep

Metabolic status is a powerful regulator of reproductive activity, but the metabolic mediators involved and the relationships between fat reserves, food intake and the systems that control reproduction are not fully understood. In this study with mature male Merino sheep, we tested whether the effect of an acute nutritional stimulus on pulsatile LH secretion depended on body condition. Two groups of rams ("Fat" and "Lean") were fed differentially for 4 months to achieve high or low levels of body mass and body condition score. Half of each group was then assigned to be fed either their maintenance requirement or twice their maintenance requirement and, 7 days later, plasma samples were collected every 20 min for 24 h. All samples were used for the analysis of LH pulses and pooled samples were used for the measurement of metabolic hormone concentrations. In the rams that were fed the maintenance diet, the frequency of LH pulses was similar for the Fat and Lean groups, but plasma concentrations of leptin and insulin were significantly higher in the Fat group than in the Lean group. Following an acute increase in food intake, plasma concentrations of insulin were significantly increased in both Fat and Lean rams, but plasma leptin concentrations were increased only in Fat rams and LH pulse frequency was increased only in Lean rams. We concluded that the secretion of LH and leptin, but not insulin, is differentially influenced by nutritional status and body condition and that the role of leptin in the central regulation of the GnRH-LH system is probably permissive.