Insulin status and carcass composition of lambs : effects of weaning age and treatment with exogenous insulin

Abstract

Groups of lambs with different insulin profiles were created, either nutritionally or by treatment with exogenous insulin. Lambs were either weaned at 5 weeks of age or remained suckling with access to grass. A subset of weaned and suckled lambs received exogenous insulin by intravenous injection or by continuous infusion. The lambs were treated with insulin for 8 or 18 weeks. Body composition and insulin status were determined in lambs slaughtered at 12 or 23 weeks.

Suckled lambs were insulin resistant compared to weaned lambs on the basis of slower clearance of injected glucose despite increased pancreatic insulin release, slower removal of glucose from the plasma after an injection of insulin and lower metabolic clearance rates of glucose, as measured by the euglycaemic clamp procedure. Increases in basal plasma insulin concentration were also observed. The absolute rate of incorporation of glucose carbon into glyceride glycerol carbon, as determined by infusing [U-¹⁴C] glucose, was higher in the suckled than the weaned lambs. Suckling was associated with an increase in body fatness and a decrease in body protein. However, the observed changes in insulin status and the increased fatness were not accompanied by any differences in insulin binding to membranes isolated from fat and muscle tissues.

The effect of exogenous insulin was to suppress the release of insulin from the pancreas, lower the responsiveness of plasma glucose to injected insulin and to increase the metabolic clearance rates of glucose as measured by euglycaemic clamp. Treatment with exogenous insulin decreased carcass fatness and increased carcass protein. An age related decline in glucose tolerance, as shown by increased t-half values and decreased pancreatic insulin release, was recorded. The response to injected insulin also decreased with age.

In this work there is evidence that nutritionally induced obesity in sheep is associated with changes in insulin sensitivity, although there were no apparent alterations in the binding characteristics of insulin receptors in muscle and adipose tissue. Imposing a change in insulin status, by treatment with exogenous insulin, does not necessarily cause increased fatness.