Ap. Moloney et al., TEMPORAL CHANGE IN SKELETAL-MUSCLE IGF-I MESSENGER-RNA ABUNDANCE AND NITROGEN-METABOLISM RESPONSES TO ABOMASAL CASEIN INFUSION IN STEERS, Journal of animal science, 76(5), 1998, pp. 1380-1388
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.