GROWTH-HORMONE PULSATILITY IN RAM LAMBS OF GENOTYPES SELECTED FOR FATNESS OR LEANNESS

Although it is known that growth hormone (GH) influences body composit ion in ruminants, the precise role of the pattern of GH secretion is n ot known. We have studied the pulsatile release of GH and insulin-like growth factor 1 (IGF 1) secretion in the male progeny of rams from li nes selected either for (fat genotype) or against (lean genotype) fatn ess. Seventy-two lambs (36 each of the fat and lean genotype) were kep t on high-quality pasture and randomly allocated within genotype to tr eatment at 2, 3, 3.5, 4, 5 or 6 months of age. The procedure, which wa s identical for each sampling period, was to sample each lamb through a jugular cannula every 10 min for 6 h, and then, following an overnig ht fast, to slaughter and analyse the carcass for fat. All blood sampl es were analysed for GH and samples taken each hour for total plasma I GF 1. The GH data were further analysed with the pulse detection routi ne PULSAR. Carcass fatness, adjusted for cold carcass weight, was grea ter for fat genotype animals than for the lean genotype. GH was pulsat ile in all profiles but the pattern differed with time and genotype. M ean GH and pulse amplitude decreased with time but did not differ betw een genotype, although the lean genotype had higher mean GH at five of the six sampling periods. In contrast, GH pulse frequency and IGF 1 w ere significantly higher for the fat compared with the lean genotype l ambs. GH mean and amplitude correlated negatively with carcass fatness in both genotypes and GH pulse frequency and total IGF 1 correlated p ositively with fatness for the lean genotype only. When carcass weight and genotype were fitted to these relationships, GH mean and total IG F 1 were found to have independent negative and positive effects, resp ectively, on carcass fatness. Because GH mean had a separate effect on fatness independent of genotype or cold carcass weight, it is likely that GH secretion influences composition by the same basic mechanism i n both genotypes. However, although the slopes of these relationships did not differ significantly between the genotypes, the intercepts wer e significantly different indicating that over and above the basic mec hanism, at any level of GH, the lean genotype lambs were leaner than t he fat genotype lambs. This may indicate a measure of GH resistance in the fat genotype lambs.