Plasma insulin and insulin kinetics in growing sheep. Influence of age and diet.

This study was carried out to clarify the nutritional control of insulin metabolism in growing sheep fed a control or an experimental diet low in crude protein and high in propionic acid used as a feed additive. Daily variations in blood insulin and the disappearance from the circulation of unlabelled injected insulin were investigated. These data were used to calculate the metabolic clearance rate of insulin and insulin secretion. At 23 kg of body weight (BW), blood insulin showed an increase in the control group at 10 a.m. (i.e. 2 h after feeding) and a large peak in the experimental group at 3 p.m. At 33-kg BW, blood insulin in both groups showed a peak at 10 a.m. and a moderate increase at 3 p.m. Mean plasma insulin throughout the day (except at 3 p.m.) rose with increasing BW. It was lower in the experimental than in the control group. Injected insulin disappeared rapidly from the circulation; its half-life was constant (13 min). In both groups, the insulin level along the disappearance curve was higher at 33-kg BW than at 23-kg BW, and insulin metabolic clearance rate decreased. Except at 3 p.m., the mean insulin secretion rate over the day was lower in the control group at 33-kg BW than at 23-kg BW. Feeding the experimental diet increased the insulin level along the disappearance curve. In young animals, it decreased insulin metabolic clearance rate and diminished mean insulin secretion rate over the day, except at 3 p.m. when the insulin secretion rate increased. In old animals, the experimental diet did not significantly change the kinetic parameters of insulin metabolism.