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Research Communication

Insulin Stimulates Phenylalanine Uptake across the Hind Limb in Fed Lambs,¹

Timothy J. Wester^*, Gerald E. Lobley², Linda M. Birnie^* and Michael A. Lomax^*

^* Department of Agriculture, University of Aberdeen, 581 King St., Aberdeen AB24 5UA, Scotland, Rowett Research Institute, Bucksburn, Aberdeen AB21 9SB, Scotland

²To whom correspondence should be addressed.

Five lambs (~6 mo of age and 30 kg), with an external iliac artery and vein catheterized and fed to maintain body weight, were used to examine effects of close arterial infusion of insulin and branched-chain amino acids (BCAA) on net phenylalanine (Phe) uptake across the hind limb. Treatments, administered randomly on five consecutive days to each lamb, were 400 min infusions of: i) saline (control); ii) insulin to double iliac artery concentration (low insulin); iii) as ii but to quadruple insulin concentration (high insulin); iv) 30 µmol/min leucine and 22.5 µmol/min each isoleucine 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 alone 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 or from co-infusion with BCAA. Insulin, at low concentrations, may be important to growth in animals with marginal nutrition.

KEY WORDS: • protein metabolism • sheep • branched-chain amino acids • insulin • muscle protein

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