Leptin gene expression, circulating leptin, and luteinizing hormone pulsatility are acutely responsive to short-term fasting in prepubertal heifers: Relationships z to circulating insulin and insulin-like growth factor I

In the present study, we tested the hypothesis that short-term fasting woul d reduce leptin gene expression, circulating leptin, and LH pulsatility in prepubertal heifers in association with a decrease in circulating concentra tions of insulin and insulin-like growth factor I (IGF-I). Twelve prepubert al crossbred heifers (mean +/- SD = 315 +/- 5 kg body weight) were assigned randomly to one of two treatments in two replicates: 1) control; normal fe ed consumption (n = 6) and 2) fasted; 48 h of total feed restriction (n = 6 ), Blood samples were collected at 15-min intervals for 8 h on Days 0 and 2 of the experiment and twice on Day 1. Subcutaneous fat samples were collec ted before treatment onset (Day -1) and at the end of the intensive blood s ampling on Day 2. Acute feed restriction markedly reduced leptin mRNA in ad ipose tissue (P < 0.01) and circulating concentrations of leptin (P < 0.05) , IGF-I (P < 0.01), and insulin (P = 0.05) as compared with controls on Day 2. Moreover, the treatment x day interaction (P < 0.076) and within-day co ntrasts (expressed as a percentage of Day 0 values) revealed that the mean frequency of LH pulses in the fasted group was lower (P < 0.06) than in con trols on Day 2, Neither mean concentrations of growth hormone (CH) nor GH s ecretory dynamics were affected by acute feed restriction. Fasting-mediated decreases in leptin gene expression and circulating leptin, in association with reductions in secretion of ICF-I, insulin, and LH, provide a basis fo r investigating leptin as a hormone signaling energy status to the central reproductive axis in cattle.