Age-related changes in tissue metabolic rates and sensitivity to insulin in the chicken.

In vitro metabolic studies were conducted with tissues from broiler pullets at 4, 8, and 12 weeks of age. In order to accurately assess plasma insulin at each of these ages, blood samples were withdrawn via indwelling venous catheters at 30-min intervals for 6 hr, and plasma was assayed by a homologous chicken insulin (cIns) radioimmunoassay. Birds (n = 6) were sacrificed 24 hr following blood sampling and livers, hearts, and abdominal fat pads removed for metabolic studies. Tissue slices of removed organs were incubated in Krebs-Ringer Bicarbonate buffer containing 13.9 mM glucose, .25 mM leucine, and .35 mM palmitate with either 0.0, 0.1, 1.0, or 100 ng/ml cIns. Carbon-14 labelled substrates were added to appropriate slice incubations and oxidation and incorporation into endproducts assessed. Plasma insulin decreased with age from 4 to 12 weeks (area under 6-hr concentration curve = 172.0 vs 56.9 ng X min X ml-1) to a degree approaching significance (P less than .10), in conjunction with decreased feed intake per unit of body weight. Palmitate oxidation and incorporation into cell lipid by adipose tissue, glucose oxidation and incorporation into lipid by liver and incorporation of leucine into trichloroacetic acid (TCA)-precipitable protein by cardiac muscle decreased substantially with age from 4 to 8 weeks (P less than .05). Oxidation of the branched-chain amino acid by cardiac muscle did not change significantly with age. Insulin enhanced the rate of leucine oxidation by cardiac muscle approximately 100% at 12 weeks but had no significant effect at earlier ages.(ABSTRACT TRUNCATED AT 250 WORDS)