Anabolic effects of insulin in growing lambs

Romney cross-bred wether lambs (aged 16-56 weeks and weighing 16-36 kg) were fed a concentrate diet ad lib. and surgically prepared with catheters in the left and right external iliac arteries and veins, inserted via the ipsilateral medial saphenous artery and vein. Sterile saline was continuously infused through all catheters and after appetite had fully recovered, bovine insulin was infused into one arterial catheter at rates of 5, 10 or 20 mU min-1 for periods of 15, 30 or 45 days in different lambs. At the end of the infusion the lambs were slaughtered to measure the composition of each hindquarter by tissue dissection and chemical analysis. As a result of the insulin infusions, plasma insulin concentrations were increased in venous blood sampled downstream from the infusion site and the arteriovenous concentration differences of glucose were greater across the treated than the contralateral limb. At the 10 and 20 mU min-1 infusion rates, systemic plasma insulin concentrations were increased and glucose concentrations decreased. Pooling results from all the treated lambs showed that intra-arterial insulin infusion increased weight of the hindquarter by 7%, weight of muscle by 5%, weight of dissected fat by 12% and weight of chemical fat by 11% when compared with the contralateral limb. No significant differences were detected among infusion rates or times of infusion. Much of the variation in response between lambs was negatively correlated to their age and/or liveweight when they entered the study. The results demonstrate that tissues in the hindlimb respond to local concentrations of a circulating hormone, provide strong evidence that insulin is an anabolic hormone in the growing postnatal lamb and suggest that such responses diminish with age.