Muscle metabolism and control of capillary blood flow: insulin and exercise

Effect of NG -monomethyl l-arginine on microvascular blood flow and glucose metabolism after an oral glucose load

OBJECTIVE

The aim of this study was to investigate whether the effects on local blood flow and metabolic changes observed in the skin after an endogenous systemic increase in insulin are mediated by the endothelial nitric oxide pathway, by administering the nitric oxide synthase inhibitor N^G -monomethyl l-arginine using microdialysis.

METHODS

Microdialysis catheters, perfused with N^G -monomethyl l-arginine and with a control solution, were inserted intracutaneously in 12 human subjects, who received an oral glucose load to induce a systemic hyperinsulinemia. During microdialysis, the local blood flow was measured by urea clearance and by laser speckle contrast imaging, and glucose metabolites were measured.

RESULTS

After oral glucose intake, microvascular blood flow and glucose metabolism were both significantly suppressed in the N^G -monomethyl l-arginine catheter compared to the control catheter (urea clearance: P < .006, glucose dialysate concentration: P < .035). No significant effect of N^G -monomethyl l-arginine on microvascular blood flow was observed with laser speckle contrast imaging (P = .81).

CONCLUSION

Local delivery of N^G -monomethyl l-arginine to the skin by microdialysis reduces microvascular blood flow and glucose delivery in the skin after oral glucose intake, presumably by decreasing local insulin-mediated vasodilation.

Increases in muscle blood flow after a mixed meal are impaired at all stages of type 2 diabetes

OBJECTIVE

In type 2 diabetes, although the impairment of postprandial muscle blood flow response is well established, information on the effect of this impairment on glucose uptake and lipid metabolism is controversial.

DESIGN

Postprandial forearm blood flow responses and metabolic parameters were assessed in a cross-sectional study of subjects at various stages of insulin resistance.

PATIENTS

Eleven healthy subjects (CONTROLS), 11 first-degree relatives of type-2 diabetics (RELATIVES), 10 patients with impaired glucose tolerance (IGT), 10 diabetic patients with postprandial hyperglycaemia (DMA), and 13 diabetic patients with both fasting and postprandial hyperglycaemia (DMB).

MEASUREMENTS

All subjects received a meal. Blood was drawn from a forearm deep vein and the radial artery at specific time-points during a period of 360 min for measurements of glucose, insulin, triglycerides and nonesterified-fatty acids. Forearm muscle blood flow was measured with strain-gauge plethysmography. Glucose uptake and ISI Index were calculated.

RESULTS

Peak-baseline muscle blood flow was higher in CONTROLS (3.32 ± 0.4) than in RELATIVES (0.53 ± 0.29), IGT (0.82 ± 0.2), DMA (1.44 ± 0.34), DMB (1.23 ± 0.35 ml/min/100 ml tissue), P < 0.001. Glucose uptake (AUC(0-360,) μmol/100 ml tissue) was higher in CONTROLS (1023 ± 132) than in RELATIVES (488 ± 42), IGT (458 ± 43), DMA (347 ± 63), DMB (543 ± 53), P < 0.001. ISI index, postprandial triglycerides and nonesterified-fatty acids behaved in a similar way. Peak-baseline muscle blood flow correlated positively with glucose uptake (r = 0.440, P = 0.001) and ISI index (r = 0.397, P = 0.003), and negatively with postprandial triglycerides (r = -0.434, P = 0.001) and nonesterified-fatty acids (r = -0.370, P = 0.005).

CONCLUSIONS

These results suggest that increase in muscle blood flow after a meal is impaired at all stages of type-2 diabetes. This defect influences glucose uptake and is associated with impaired lipid metabolism in the postprandial state.

Dynamics of muscle microcirculatory and blood-myocyte O(2) flux during contractions

The O(2) requirements of contracting skeletal muscle may increase 100-fold above rest. In 1919, August Krogh's brilliant insights recognized the capillary as the principal site for this increased blood-myocyte O(2) flux. Based on the premise that most capillaries did not sustain RBC flux at rest, Krogh proposed that capillary recruitment [i.e. initiation of red blood cell (RBC) flux in previously non-flowing capillaries] increased the capillary surface area available for O(2) flux and reduced mean capillary-to-mitochondrial diffusion distances. More modern experimental approaches reveal that most muscle capillaries may support RBC flux at rest. Thus, rather than contraction-induced capillary recruitment per se, increased RBC flux and haematocrit within already-flowing capillaries probably elevate perfusive and diffusive O(2) conductances and hence blood-myocyte O(2) flux. Additional surface area for O(2) exchange is recruited but, crucially, this may occur along the length of already-flowing capillaries (i.e. longitudinal recruitment). Today, the capillary is still considered the principal site for O(2) and substrate delivery to contracting skeletal muscle. Indeed, the presence of very low intramyocyte O(2) partial pressures (PO(2)s) and the absence of intramyocyte PO(2) gradients, whilst refuting the relevance of diffusion distances, place an even greater importance on capillary hemodynamics. This emergent picture calls for a paradigm-shift in our understanding of the function of capillaries by de-emphasizing de novo'capillary recruitment'. Diseases such as heart failure impair blood-myocyte O(2) flux, in part, by decreasing the proportion of RBC-flowing capillaries. Knowledge of capillary function in healthy muscle is requisite for identification of pathology and efficient design of therapeutic treatments.

Interleukin-6 attenuates insulin-mediated increases in endothelial cell signaling but augments skeletal muscle insulin action via differential effects on tumor necrosis factor-alpha expression

OBJECTIVE

The cytokine interleukin-6 (IL-6) stimulates AMP-activated protein kinase (AMPK) and insulin signaling in skeletal muscle, both of which result in the activation of endothelial nitric oxide synthase (eNOS). We hypothesized that IL-6 promotes endothelial cell signaling and capillary recruitment in vivo, contributing to increased glucose uptake.

RESEARCH DESIGN AND METHODS

The effect of IL-6 with and without insulin on AMPK, insulin, and eNOS signaling in and nitric oxide (NO) release from human aortic endothelial cells (HAECs) was examined. The physiological significance of these in vitro signaling events was assessed by measuring capillary recruitment in rats during control and euglycemic-hyperinsulinemic clamps with or without IL-6 infusion.

RESULTS

IL-6 blunted increases in insulin signaling, eNOS phosphorylation (Ser1177), and NO production and reduced phosphorylation of AMPK in HAEC in vitro and capillary recruitment in vivo. In contrast, IL-6 increased Akt phosphorylation (Ser473) in hindlimb skeletal muscle and enhanced whole-body glucose disappearance and glucose uptake during the clamp. The differences in endothelial cell and skeletal muscle signaling were mediated by the cell-specific, additive effects of IL-6 and insulin because this treatment markedly increased tumor necrosis factor (TNF)-alpha protein expression in HAECs without any effect on TNF-alpha in skeletal muscle. When HAECs were incubated with a TNF-alpha-neutralizing antibody, the negative effects of IL-6 on eNOS signaling were abolished.

CONCLUSIONS

In the presence of insulin, IL-6 contributes to aberrant endothelial cell signaling because of increased TNF-alpha expression.

Impaired endothelial function and insulin action in first-degree relatives of patients with type 2 diabetes mellitus

First-degree relatives (FDR) of patients with type 2 diabetes mellitus are at increased risk of developing type 2 diabetes mellitus. We studied if endothelial dysfunction of the resistance vessels is present and may coexist with metabolic insulin resistance in FDR. Male FDR (n = 13; 26 +/- 1 years; body mass index, 25 +/- 1 kg m(2) [mean +/- SEM]) and matched control subjects (CON) (n = 22; 25 +/- 1 years; body mass index, 24 +/- 1 kg m(2)) were studied by hyperinsulinemic (40 mU min(-1)m(-2)) isoglycemic clamp combined with brachial arterial and deep venous catheterization of the forearm. Forearm blood flow (FBF) was measured by venous occlusion plethysmography upon stimulation with systemic hyperinsulinemia (291 +/- 11 pmol/L, pooled data from both groups) and upon intraarterial infusion of adenosine (ADN) and acetylcholine (ACH) +/- hyperinsulinemia. Forearm blood flow response to ADN and ACH was less in FDR vs CON (P < .05); systemic hyperinsulinemia added to the FBF effect of ADN in CON (P < .05) but not in FDR. In addition, FDR demonstrated impaired FBF to hyperinsulinemia (2.1 +/- 0.2 vs 4.0 +/- 0.6 mL 100 mL(-1) min(-1)) in FDR and CON, respectively (P < .05). Both M-value (5.0 +/- 0.7 vs 7.0 +/- 0.5 mg min(-1) kg(-1)) and forearm glucose clearance (0.6 +/- 0.1 vs 1.4 +/- 0.4 mL 100 mL(-1)min(-1)) were diminished in FDR compared with CON (all P < .05). FDR demonstrated endothelial dysfunction of the resistance vessels in addition to impaired insulin-stimulated increase in bulk flow. Moreover, FDR demonstrated whole-body insulin resistance as well as decreased basal and insulin-stimulated forearm glucose uptake. It remains to be established whether FDR also demonstrate impaired insulin-stimulated microvascular function.

Local nitric oxide synthase inhibition reduces skeletal muscle glucose uptake but not capillary blood flow during in situ muscle contraction in rats

OBJECTIVE

We have previously shown in humans that local infusion of a nitric oxide synthase (NOS) inhibitor into the femoral artery attenuates the increase in leg glucose uptake during exercise without influencing total leg blood flow. However, rodent studies examining the effect of NOS inhibition on contraction-stimulated skeletal muscle glucose uptake have yielded contradictory results. This study examined the effect of local infusion of an NOS inhibitor on skeletal muscle glucose uptake (2-deoxyglucose) and capillary blood flow (contrast-enhanced ultrasound) during in situ contractions in rats.

RESEARCH DESIGN AND METHODS

Male hooded Wistar rats were anesthetized and one hindleg electrically stimulated to contract (2 Hz, 0.1 ms) for 30 min while the other leg rested. After 10 min, the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) (arterial concentration of 5 micromol/l) or saline was infused into the epigastric artery of the contracting leg.

RESULTS

Local NOS inhibition had no effect on blood pressure, heart rate, or muscle contraction force. Contractions increased (P < 0.05) skeletal muscle NOS activity, and this was prevented by L-NAME infusion. NOS inhibition caused a modest significant (P < 0.05) attenuation of the increase in femoral blood flow during contractions, but importantly there was no effect on capillary recruitment. NOS inhibition attenuated (P < 0.05) the increase in contraction-stimulated skeletal muscle glucose uptake by approximately 35%, without affecting AMP-activated protein kinase (AMPK) activation.

CONCLUSIONS

NOS inhibition attenuated increases in skeletal muscle glucose uptake during contraction without influencing capillary recruitment, suggesting that NO is critical for part of the normal increase in skeletal muscle fiber glucose uptake during contraction.