The purpose of this study was to examine muscle protein metabolism in healthy young and elderly subjects during pharmacologic vasodilation of the lower limb vasculature in combination with increased amino acids delivery. We obtained femoral arteriovenous blood samples and v. lateralis muscle biopsies during a primed continuous infusion of L-[ring-13C6] Phenylalanine. Muscle protein kinetics were measured in the basal state and during a 180 min infusion of sodium nitroprusside (SNP) and amino acids (Premsol 10%) (SNP+AA). Skeletal muscle microvascular recruitment and flow were measured during the basal state and SNP+AA using contrast-enhanced ultrasound (CEU).