The roles of the insulin-like growth factor system and leptin as possible mediators of the effects of nutritional restriction on age at puberty and compensatory growth in dairy heifers

Abstract

The aim of this experiment was to determine to what extent changes in the blood concentrations of insulin-like growth factor-1 (IGF-1), IGF binding proteins (IGFBPs), leptin and glucose are associated with compensatory growth and age at puberty in Friesian heifers. Twelve heifers (6 months old; 179.8 kg body weight) were allocated to one of two dietary treatments for 30 weeks. Treatments were designed to result in two different growth rates during the first 13 weeks of the experiment, viz. 0.3 kg/d (restriction treatment) or 0.6 kg/d (control treatment). From week 14 to 30, the restricted group received the same amount of feed per kg bodyweight as was fed to the control group (compensatory phase). Heifers in the control treatment reached puberty four weeks earlier than the restricted group. Mean body weight at puberty was 256.3 kg and was not affected by treatment. During the compensatory phase, the growth rate of the previously restricted animals was greater, and feed conversion ratio superior, compared to that of the control group. Plasma IGF-1 and IGFBP-3 concentrations were higher in the restricted group than in the control group during the early compensatory phase. Concentrations of IGFBP-3 increased and peaked at puberty for both treatments, even though puberty occurred at different ages. In contrast, concentrations of IGF-1 peaked at the point of maximum growth rate in the previously restricted animals, and in neither treatment was there a clear relationship with the age of puberty. Plasma leptin concentrations increased until puberty in both treatments even though the date of puberty differed. Plasma glucose concentration did not change at puberty for either treatment; however, differences between treatments were found after the restriction period. There were no differences in IGFBP-2 concentrations between treatments or sampling periods. It is concluded that concentrations of glucose, IGF-1 and IGFBP-3 are affected by nutritional restriction and repletion, and may contribute in some measure to the endocrine regulation of compensatory growth. It is suggested that the increased concentrations of leptin and IGFBP-3 during pubertal development may act as physiological signals for the onset of puberty. (South African Journal of Animal Science, 2000, 30(2): 155-163)