Pharmacological vasodilation improves insulin-stimulated muscle protein anabolism but not glucose utilization in older adults

Abstract

Objective: Skeletal muscle protein metabolism is resistant to the anabolic action of insulin in healthy, nondiabetic older adults. This defect is associated with impaired insulin-induced vasodilation and mTORC1 signaling. We hypothesized that, in older subjects, pharmacological restoration of insulin-induced capillary recruitment would improve the response of muscle protein synthesis and anabolism to insulin.

Research design and methods: Twelve healthy, nondiabetic older subjects (71 ± 2 years) were randomized to two groups. Subjects were studied at baseline and during local infusion in one leg of insulin alone (Control) or insulin plus sodium nitroprusside (SNP) at variable rate to double leg blood flow. We measured leg blood flow by dye dilution; muscle microvascular perfusion with contrast enhanced ultrasound; Akt/mTORC1 signaling by Western blotting; and muscle protein synthesis, amino acid, and glucose kinetics using stable isotope methodologies.

Results: There were no baseline differences between groups. Blood flow, muscle perfusion, phenylalanine delivery to the leg, and intracellular availability of phenylalanine increased significantly (P < 0.05) in SNP only. Akt phosphorylation increased in both groups but increased more in SNP (P < 0.05). Muscle protein synthesis and net balance (nmol · min(-1) · 100 ml · leg(-1)) increased significantly (P < 0.05) in SNP (synthesis, 43 ± 6 to 129 ± 25; net balance, -16 ± 3 to 26 ± 12) but not in Control (synthesis, 41 ± 10 to 53 ± 8; net balance, -17 ± 3 to -2 ± 3).

Conclusions: Pharmacological enhancement of muscle perfusion and amino acid availability during hyperinsulinemia improves the muscle protein anabolic effect of insulin in older adults.

FIG. 1.

Study design. Blood and muscle samples are indicated by arrows. ICG indicates indocyanine green infusion for blood flow measurement. CEU indicates contrast enhanced ultrasound measurement of muscle perfusion using Definity microspheres.

FIG. 2.

Representative contrast enhanced ultrasound images of Definity microsphere infusion into the vastus lateralis of one control and one SNP subject during the basal period and during insulin infusion with or without concomitant SNP infusion. Each image is captured during the 10-s pulsing interval.

FIG. 3.

Microvascular blood perfusion (A) measured by CEU and fractional synthetic rate (B) in the basal state and during local insulin infusion with (SNP) or without (Control) concomitant infusion with the vasodilator, sodium nitroprusside. To provide normal reference values, previously published data (16) from a group of younger individuals (n = 7, age = 32 ± 2) undergoing the same insulin infusion protocol as the Control group are also shown. Data are the mean ± SE. *P < 0.05 versus baseline, †P < 0.05 versus Control.

FIG. 4.

Phenylalanine delivery to the leg (A), phenylalanine utilization for protein synthesis (B), and phenylalanine net balance (C) measured in the basal state and during local insulin infusion with (SNP) or without (Control) concomitant infusion with the vasodilator, sodium nitroprusside. Data are the mean ± SE. *P < 0.05 versus baseline, †P < 0.05 versus Control.

FIG. 5.

Phosphorylation of Akt (A), mTOR (B), S6K1 (C), and 4E-BP1 (D) in the skeletal muscle of two groups of healthy elderly subjects in the basal state and during local insulin infusion (biopsy 3, Bx 3; and biopsy 4, Bx 4) in one leg with (SNP) or without (Control) concomitant infusion of the vasodilator sodium nitroprusside. Representative phosphorylated (e.g., P-Protein) and total protein blots for each panel are from a single control and SNP subject. The basal, Bx3, and Bx4 samples were run on the same gel for each subject. Data are the mean ± SE. *P < 0.05 versus baseline, †P < 0.05 versus Control.