Claudio Cobelli

Glucose Transport in Human Skeletal Muscle: The In Vivo Response to Insulin

Transmembrane glucose transport plays a key role in determining insulin sensitivity. We have measured in vivo WBGU, FGU, and KIn and Kout of 3-O-methyl-D-glucose in forearm skeletal muscle by combining the euglycemic clamp technique, the forearm-balance technique, and a novel dual-tracer (1-[3H]-L-glucose and 3-O-[4C]-methyl-D-glucose) technique for measuring in vivo transmembrane transport. Twenty-seven healthy, lean subjects were studied. During saline infusion, insulin concentration, FGU (n = 6), KIn, and Kout (n = 4) were similar to baseline. During SRIF-induced hypoinsulinemia (insulin <15 pM, n = 4) WBGU was close to 0, and FGU, KIn, and Kout were unchanged from basal (insulin = 48 pM) values. During insulin clamps at plasma insulin levels of ∼180 (n = 4), ∼420 (n = 5), ∼3000 (n = 4), and ∼9500 pM (n = 4), WBGU was 14.2 ± 1.3, 34.2 ± 4.1 (P < 0.05 vs. previous step), 55.8 ± 1.8 (P < 0.05 vs. previous step), and 56.1 ± 6.3 ixmol · min−1 · kg−1 of body weight (NS vs. previous step), respectively. Graded hyperinsulinemia concomitantly increased FGU from a basal value of 4.7 ± 0.5 μmol · min−1 · kg−1 up to 10.9 ± 2.3 (P < 0.05 vs. basal value), 26.6 ± 4.5 (P < 0.05 vs. previous step), 54.8 ± 4.3 (P < 0.05 vs. previous step), and 61.1 ± 10.8 μmol · min−1 · kg−1 of forearm tissues (NS vs. previous step), respectively. KIn of 3-O-methyl-D-glucose in forearm skeletal muscle was increased by hyperinsulinemia from a basal value of 6.6 · 10−2 ± 0.38 · 10−2 to 10.0 · 10−2 ± 1.4 · 10(p < 0.05 vs. baseline), 17.2 · 10−2 ± 2.2 · 10−2 (P < 0.05 vs. previous step), 26.3 · 10−2 ± 1.8 · 10−2 (P < 0.05 vs. previous step), and 29.8 · 10−2 ± 5.3 · 10−2 · min−1 (NS vs. previous step), respectively. FGU and Kln were positively correlated (r = 0.88, P < 0.01). Kout of 3-O-methyl-D-glucose did not change from the basal value at the lowest insulin dose (3.9 · 10−2 ± 1.1 · 10−2 vs. 3.8 ·10−2 ± 0.33 · 10−2 · 10−2 · min−1, NS), but rose significantly at the following insulin steps to 6.1 · 10−2 ± 0.8 · 10−2, 6.9 ·10−2 ± 0.5 · 10−2, and 11.9 · 10−2 ± 0.3 · 10−2 · min−1 (P < 0.05 for all three vs. baseline). Thus, in human skeletal muscle, in vivo, insulin stimulates K, n and uptake of glucose in a parallel fashion, whereas SRIF-induced acute hypoinsulinemia does not seem to affect transmembrane transport or uptake of glucose.

Effect of insulin on system A amino acid transport in human skeletal muscle.

Transmembrane transport of neutral amino acids in skeletal muscle is mediated by at least four different systems (system A, ASC, L, and Nm), and may be an important target for insulins effects on amino acid and protein metabolism. We have measured net amino acid exchanges and fractional rates of inward (k(in), min-1) and outward (kout, min-1) transmembrane transport of 2-methylaminoisobutyric acid (MeAIB, a nonmetabolizable amino acid analogue, specific for system A amino acid transport) in forearm deep tissues (skeletal muscle), by combining the forearm perfusion technique and a novel dual tracer ([1-H3]-D-mannitol and 2-[1-14C]-methylaminoisobutyric acid) approach for measuring in vivo the activity of system A amino acid transport. Seven healthy lean subjects were studied. After a baseline period, insulin was infused into the brachial artery to achieve local physiologic hyperinsulinemia (76 +/- 8 microU/ml vs 6.4 +/- 1.6 microU/ml in the basal period, P < 0.01) without affecting systemic hormone and substrate concentrations. Insulin switched forearm amino acid exchange from a net output (-2,630 +/- 1,100 nmol/min per kig of forearm tissue) to a net uptake (1,610 +/- 600 nmol/min per kg, P < 0.01 vs baseline). Phenylalanine and tyrosine balances simultaneously shifted from a net output (-146 +/- 47 and -173 +/- 34 nmol/min per kg, respectively) to a zero balance (16.3 +/- 51 for phenylalanine and 15.5 +/- 14.3 nmol/min per kg for tyrosine, P < 0.01 vs baseline for both), showing that protein synthesis and breakdown were in equilibrium during hyperinsulinemia. Net negative balances of alanine, methionine, glycine, threonine and asparagine (typical substrates for system A amino acid transport) also were decreased by insulin, whereas serine (another substrate for system A transport) shifted from a zero balance to net uptake. Insulin increased k(in) of MeAIB from a basal value of 11.8.10(-2) +/- 1.7.10(-2).min-1 to 13.7.10(-2) +/- 2.2.10(-2).min-1 (P < 0.02 vs the postabsorptive value), whereas kout was unchanged. We conclude that physiologic hyperinsulinemia stimulates the activity of system A amino acid transport in human skeletal muscle, and that this effect may play a role in determining the overall concomitant response of muscle amino acid/protein metabolism to insulin.