Effects of growth hormone administration on the body composition and hormone levels of genetically fat sheep

Coopworth sheep selected for low (lean) or high (fat) backfat have large di fferences in plasma GH profiles. Fat genotype ram lambs (5 months old) were treated with growth hormone (GH) to simulate the plasma GH profiles of lea n sheep and investigate whether exogenous GH could modify carcass fatness. For 77 days, bovine GH was administered at 25 mu g/kg live weight per day e ither as a single, daily subcutaneous bolus (fat bolus) or via portable pul satile infusion pumps (fat pump) which delivered GH solution at 90-min. int ervals into a jugular catheter. Measurements of body composition were made by computed tomography (CT) and ultrasonic scanning during the trial, with linear carcass measurements and proximate analysis undertaken at the end of the experiment. Before treatments began, mean plasma GH levels were lower (P < 0.01) in fat control (0.34 mu g/l) than in lean lambs (1.1 mu g/l). Several weeks after the start of the trial, mean plasma GH had increased in both fat bolus (1. 2 mu g/l) and fat pump (0.45 mu g/l) treatment lambs with major changes in the pulsatility relative to the fat control lambs. Although these changes w ere maintained in the fat bolus lambs, by the end of the trial there was no significant difference in mean plasma GH between fat pump and fat control sheep. Throughout the trial, plasma IGF-1 levels were higher in fat bolus, fat pump and lean lambs than in fat control lambs. Analysis of body composi tion data over the GH treatment period revealed that the slope of the allom etric equation for total fat relative to empty body weight was lower in the fat bolus lambs (1.07) than in the lean lambs (1.50) with fat control and fat pump treatment lambs intermediate (1.30 and 1.36, respectively). Subcut aneous fat was later maturing in lean lambs than in fat control and bolus t reatment lambs when regressed against total fat, with the fat pump treatmen t lambs being intermediate. Linear carcass measurements revealed changes du e to GH administration in the distribution of subcutaneous fat and eye musc le dimensions. It is concluded that sheep from the fat genotype show physiological respons es to exogenous GH. Increasing plasma GH levels of fat sheep increased plas ma IGF-1 and had variable effects on carcass fatness. The change in body co mposition may be affected by the mode of administration of exogenous GH.