Insulin stimulates phenylalanine uptake across the hind limb in fed lambs

Five lambs (similar to 6 mo of age and 30 kg), with an external iliac arter y and vein catheterized and fed to maintain body weight, were used to exami ne effects of close arterial infusion of insulin and branched-chain amino a cids (BCAA) on net phenylalanine (Phe) uptake across the hind limb. Treatme nts, administered randomly on five consecutive days to each lamb, were 400 min infusions of: i) saline (control); ii) insulin to double iliac artery c oncentration (low insulin); iii) as ii but to quadruple insulin concentrati on thigh insulin); iv) 30 mu mol/min leucine and 22.5 mu mol/min each isole ucine and valine (BCAA) and v) co-infusion of ii and iv. Blood was sampled over the last 200 min from the iliac vein and right ventricle of the heart. High insulin caused a slight decrease (-13%, P < 0.05) in systemic glucose concentration, but did not alter systemic insulin concentration. Insulin, at both doses and in combination with BCAA, resulted in 9-fold greater net Phe uptake (P < 0.05) than the control, as did BCAA alone. Because BCAA alo ne increased net Phe uptake, these may have stimulatory effects directly or may enhance endogenous insulin. Maximum stimulation was achieved with low insulin because there was no increase in net Phe uptake with high insulin o r from cc-infusion with BCAA, Insulin, at low concentrations, may be import ant to growth in animals with marginal nutrition.