Insulin entry into muscle involves a saturable process in the vascular endothelium.

AIMS/HYPOTHESIS: Insulin's rate of entry into skeletal muscle appears to be the rate-limiting step for muscle insulin action and is slowed by insulin resistance. Despite its obvious importance, uncertainty remains as to whether the transport of insulin from plasma to muscle interstitium is a passive diffusional process or a saturable transport process regulated by the insulin receptor. METHODS: To address this, here we directly measured the rate of (125)I-labelled insulin uptake by rat hindlimb muscle and examined how that is affected by adding unlabelled insulin at high concentrations. We used mono-iodinated [(125)I]Tyr(A14)-labelled insulin and short (5 min) exposure times, combined with trichloroacetic acid precipitation, to trace intact bioactive insulin. RESULTS: Compared with saline, high concentrations of unlabelled insulin delivered either continuously (insulin clamp) or as a single bolus, significantly raised plasma (125)I-labelled insulin, slowed the movement of (125)I-labelled insulin from plasma into liver, spleen and heart (p < 0.05, for each) but increased kidney (125)I-labelled insulin uptake. High concentrations of unlabelled insulin delivered either continuously (insulin clamp), or as a single bolus, significantly decreased skeletal muscle (125)I-labelled insulin clearance (p < 0.01 for each). Increasing muscle perfusion by electrical stimulation did not prevent the inhibitory effect of unlabelled insulin on muscle (125)I-labelled insulin clearance. CONCLUSIONS/INTERPRETATION: These results indicate that insulin's trans-endothelial movement within muscle is a saturable process, which is likely to involve the insulin receptor. Current findings, together with other recent reports, suggest that trans-endothelial insulin transport may be an important site at which muscle insulin action is modulated in clinical and pathological settings.

cGMP phosphodiesterase inhibition improves the vascular and metabolic actions of insulin in skeletal muscle.

There is considerable support for the concept that insulin-mediated increases in microvascular blood flow to muscle impact significantly on muscle glucose uptake. Since the microvascular blood flow increases with insulin have been shown to be nitric oxide-dependent inhibition of cGMP-degrading phosphodiesterases (cGMP PDEs) is predicted to enhance insulin-mediated increases in microvascular perfusion and muscle glucose uptake. Therefore, we studied the effects of the pan-cGMP PDE inhibitor zaprinast on the metabolic and vascular actions of insulin in muscle. Hyperinsulinemic euglycemic clamps (3 mU·min(-1)·kg(-1)) were performed in anesthetized rats and changes in microvascular blood flow assessed from rates of 1-methylxanthine metabolism across the muscle bed by capillary xanthine oxidase in response to insulin and zaprinast. We also characterized cGMP PDE isoform expression in muscle by real-time PCR and immunostaining of frozen muscle sections. Zaprinast enhanced insulin-mediated microvascular perfusion by 29% and muscle glucose uptake by 89%, while whole body glucose infusion rate during insulin infusion was increased by 33% at 2 h. PDE2, -9, and -10 were the major isoforms expressed at the mRNA level in muscle, while PDE1B, -9A, -10A, and -11A proteins were expressed in blood vessels. Acute administration of the cGMP PDE inhibitor zaprinast enhances muscle microvascular blood flow and glucose uptake response to insulin. The expression of a number of cGMP PDE isoforms in skeletal muscle suggests that targeting specific cGMP PDE isoforms may provide a promising avenue for development of a novel class of therapeutics for enhancing muscle insulin sensitivity.

Loss of insulin-mediated microvascular perfusion in skeletal muscle is associated with the development of insulin resistance.

AIM: The aetiology of the development of type 2 diabetes remains unresolved. In the present study, we assessed whether an impairment of insulin-mediated microvascular perfusion occurs early in the onset of insulin resistance. MATERIALS AND METHODS: Hooded Wistar rats were fed either a normal diet (ND) or a high-fat diet (HFD) for 4 weeks. Anaesthetized animals were subjected to an isoglycaemic hyperinsulinaemic clamp (3 or 10 mU/min/kg x 2 h), and measurements were made of glucose infusion rate (GIR), hindleg glucose uptake, muscle glucose uptake by 2-deoxy-d-glucose (R'g), glucose appearance (Ra), glucose disappearance (Rd), femoral blood flow (FBF) and hindleg 1-methylxanthine disappearance (1-MXD, an index of microvascular perfusion). RESULTS: Compared with ND-fed animal, HFD feeding led to a mild increase in fasting plasma glucose and plasma insulin, without an increase in total body weight. During the clamps, HFD rats showed an impairment of insulin-mediated action on GIR, hindleg glucose uptake, R'g, Ra, Rd and FBF, with a greater loss of insulin responsiveness at 3 mU/min/kg than at 10 mU/min/kg. The HFD also impaired insulin-mediated microvascular perfusion as assessed by 1-MXD. Interestingly, 1-MXD was the only measurement that remained unresponsive to the higher dose of 10 mU/min/kg insulin. CONCLUSIONS: We conclude that the early stage of insulin resistance is characterized by an impairment of the insulin-mediated microvascular responses in skeletal muscle. This is likely to cause greater whole body insulin resistance by limiting the delivery of hormones and nutrients to muscle.