We have isolated and characterized lamb liver cDNAs encoding ovine insulin-like growth factor-I (oIGF-I) precursor polypeptide to study IGF-I gene expression in ruminants. Four cDNA clones were sequenced revealing two different exon 1 sequences (designated 1A and 1B) and four different putative poly(A) adenylation sites. cDNAs containing exon 1A or exon 1B encode precursor polypeptides of 138 or 154 amino acids, respectively. A 130-amino-acid peptide is encoded by all cDNAs examined. These precursors include a hydrophobic leader peptide of varying lengths, the 70-amino-acid oIGF-I, and a 35-amino-acid carboxyl terminal extension peptide. The predicted amino acid sequence of the oIGF-I peptide differs from the human, bovine, and porcine IGF-Is at a single amino acid (at position 66, alanine is substituted for proline) and differs from rat and mouse IGF-Is at 4 and 5 positions, respectively. Both the amino- and carboxy-terminal extension peptides showed regions of extensive sequence homology. Ovine IGF-I amino-terminal peptides are 1 amino acid longer than other mammalian IGFs due to the presence of an extra amino acid (glutamine) present at the proposed boundary of exon 1 and exon 2. Northern blot analysis revealed multiple oIGF-I transcripts in a broad band at 800-1,100 nucleotides and other transcripts of higher molecular weight in liver. There was no detectable expression in either spleen or brain.

A human growth hormone-releasing factor analogue, [DesNH2Tyr1,D-Ala2,Ala15]hGRF(1-29)NH2 (GRF-A), was infused s.c. into lambs for 28 d to determine its effects on growth performance and carcass composition. Twenty crossbred wethers weighing 47.0 +/- .5 kg were implanted with 7-d osmotic minipumps at weekly intervals. Minipumps contained either vehicle (dimethyl sulfoxide:H2O, 1:1) or GRF-A, released at a rate of 208 pmol (or .7 micrograms).h-1.kg-1. During the infusion period, plasma GH levels were increased (P less than .01) in GRF-A-treated wethers compared with control wethers (15.0 vs 9.3 ng/ml) and were higher on days that closely followed minipump implantation. Plasma IGF-I and hepatic IGF-I RNA concentrations were similar in lambs of both groups. Analogue treatment improved feed conversion (4.9 vs 5.8 kg dry matter/kg gain, P less than .05), increased average daily gain (.35 vs .30 kg, P = .05) and had no effect on feed intake, wool growth and body, carcass, selected organ and pituitary weights. Carcasses from GRF-A-infused lambs had less adjusted fat depth, a lower percentage of fat and a higher percentage of protein (P less than .05) than carcasses from control lambs. Magnitude of most effects of GRF-A on carcass measurements were correlated with the mean GH level that a lamb had during the infusion period. In conclusion, s.c. infusion of GRF-A improved feed utilization and altered carcass composition of feeder lambs in a relatively short period of time (28 d).

Tissue growth of crossbred fetal beef calves was examined by measuring RNA, DNA, and protein concentrations in liver, heart, and biceps femoris. Furthermore, liver insulin-like growth factor I (IGF-I) mRNA expression and mRNA species size during fetal development was observed. Tissue samples were collected from six fetuses every 42 d of gestation, from d 106 to d 274. In the liver, protein and DNA concentrations decreased, whereas RNA levels remained constant throughout fetal growth. The RNA/DNA and protein/DNA ratios in liver increased with fetal age. Heart DNA and RNA levels decreased, whereas protein concentration and protein/DNA ratios increased with fetal age. Protein and protein/DNA ratios decreased in biceps tissue, whereas DNA and RNA concentrations were constant. IGF-I mRNA was seen at 4.4, 2.5, and 1.2 kb in adult and 4.4, 2.5, and 1.7 kb in fetal bovine liver. Relative expression of liver IGF-I mRNA did not vary during fetal development. The current study shows that during the last 2 and 3 mo of gestation, heart and liver were undergoing hypertrophic growth, whereas biceps tissue did not exhibit the same trend. Elevated ratios of RNA and protein to DNA in liver above that of the heart and biceps suggest extensive hepatic cellular hypertrophy as well as increased transcriptional and translational activity. Insulin-like growth factor I mRNA levels were not related to the changes in RNA, DNA, and protein seen in hepatic tissue.

Osmotic pumps were evaluated for 7-d delivery of growth hormone-releasing factor (GRF). In Exp. 1, 12 steers weighing 253 kg received hGRF(1-29)NH2 in H2O at rates of 0, 3, 30 and 300 pmol.h-1.kg-1. Pumps were implanted s.c. on d 0 and removed at 1200 on d 7. Blood samples were drawn at 20-min intervals from 0800 to 1200 on d -1, 1, 3, 5, 7 and 9. Growth hormone levels were not altered by GRF treatment (P greater than .05). Solubility and volume limitations render hGRF(1-29)NH2 delivery via osmotic pumps problematical. Flow rate and duration of release of dimethyl sulfoxide (DMSO):H2) (1:1) from osmotic pumps incubated in vivo and in vitro were found to be consistent with manufacturer's specifications. Two hGRF(1-29) analogues, Ro23-7863 and 4SG-29, were dissolved in DMSO:H2O. In Exp. 2, six 222-kg steers had pumps implanted and blood samples were taken as in Exp. 1. Three steers received each analogue at a rate of 300 pmol.h-1.kg-1. Analogues had similar GH-releasing ability and GH levels differed (P less than 0.001) among days, being approximately fourfold higher on d 3, 5 and 7 than on d -1, 1 and 9. Residual analogue solutions retained full bioactivity after 7-d implantation, and in vitro biopotencies of Ro23-7863 and 4SG-29 were similar (Exp. 3). In Exp. 4, 15 wethers (means = 31.3 kg) received osmotic pumps delivering 0, 3, 15, 75 and 300 pmol.h-1.kg-1 Ro23-7863 in DMSO:H2O for 7 d. Lambs were bled at 0800 and 1400 from d -1 to 8. The latter two doses increased (P less than .01) mean GH levels 2.7- and 4.3-fold over those in control animals during the treatment period. Results demonstrate that increased GH secretion can be elicited in steers and wethers for 1 wk by continuous s.c. infusion of GRF analogues utilizing osmotic pumps.