TEMPORAL CHANGE IN SKELETAL-MUSCLE IGF-I MESSENGER-RNA ABUNDANCE AND NITROGEN-METABOLISM RESPONSES TO ABOMASAL CASEIN INFUSION IN STEERS

Skeletal muscle IGF-I and alpha-actin mRNA responses to increased amin o acid availability were investigated iu young, rapidly growing steers . Four Holstein steers (208 kg BW) mere surgically implanted with an a bomasal cannula and jugalar catheters and allowed 2 wk to recover. Ste ers were offered hourly a 43:57 forage:concentrate diet at 95% of ad l ibitum intake supplemented with continuous abomasal infusion of glucos e (to replace 12.5% of metabolizable ad libitum energy intake) for 13 d before the start of abomasal infusion of 67 g of casein N/d. Biopsie s of the liver and both semimembranosus muscles were removed and froze n in liquid N, and casein infusion was begun. Muscle biopsies were col lected at 8, 16, 24, and 48 h, and on d 7 and 14. Nitrogen balance inc reased from 23.6 to 71.5 g/d (P < .001) within 24 h and remained eleva ted (mean = 58.4 g/d) during the 14 d of casein infusion. Plasma urea N increased from 4 to 9.5 mg/dL at 24 h and remained unchanged to d 14 . Muscle IGF-I mRNA abundance increased to 215% of basal values at 16 h (P < .01), 244% of basal values at 24 h, and 222% of basal values at 48 h after initiation of casein infusion. Values reached a maximum of 274% of basal values on d 7 and then declined to near preinfusion lev els on d 14. The IGF-I mRNA abundance was approximately 100 times high er in liver than in skeletal muscle and was not different on d 0 and 1 4. Although plasma IGF-I concentrations were numerically higher during the first 24 h of abomasal casein infusion, they mere not significant ly higher during the chronic phase of treatment. Plasma IGF binding pr otein (BP)-2 concentrations were higher at 16, 24, and 48 h after case in infusion was begun, but IGFBP-3 concentrations were not altered at these sampling times. Neither acute (first 24 h) nor chronic (daily) p lasma insulin concentrations were altered by abomasal casein infusion. Plasma somatotropin concentrations were lower (P = .008) at 24 h of c asein infusion and beyond. Results suggest that enhanced amino acid av ailability may modulate skeletal muscle protein synthesis and accretio n through an autocrine or paracrine IGF-I influence.