Skin glucose metabolism and microvascular blood flow during local insulin delivery and after an oral glucose load, Microcirculation, 2016. 23 (7) Grants:The study has been financially supported by ALF grants, Region Östergötland, by Sinnescentrum, Region Östergötland, and by The Grönberg Foundation. Keywords:Insulin, Skin, Metabolism, Microcirculation, Microdialysis AbstractInsulin causes capillary recruitment in muscle and adipose tissue, but the metabolic and microvascular effects of insulin in the skin have not been studied in detail. The aim of this study was to measure glucose metabolism and microvascular blood flow in the skin during local insulin delivery and after an oral glucose load using intracutaneous microdialysis and laser speckle contrast imaging (LSCI).Within 15 minutes of local insulin delivery, microvascular blood flow in the skin increased (urea clearance: p=0.047, LSCI: p=0.002) paralleled by increases in pyruvate (p=0.01) and lactate (p=0.04), indicating an increase in glucose uptake. Thus, local delivery of insulin to the skin via microdialysis resulted in rapid vasodilatation, paralleled by an increased glucose uptake. An oral glucose load increased urea clearance from the catheters, indicating an increase in skin perfusion, although no perfusion changes were detected with LSCI. The time course of the effects of insulin on glucose uptake in the skin differed markedly depending on whether insulin is delivered directly to the interstitium via microdialysis, or via a systemic route. These results show that insulin has specific metabolic and vasodilatory effects in the skin both when given locally and after systemic delivery through an oral glucose load.

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 (L-NMMA) using microdialysis. MethodsMicrodialysis catheters, perfused with L-NMMA 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 (LSCI), and glucose metabolites were measured. ResultsAfter oral glucose intake,microvascular blood flow and glucose metabolism were both significantly suppressed in the L-NMMA catheter compared to the control catheter (urea clearance: p<0.006, glucose dialysate concentration: p<0.035). No significant effect of L-NMMA on microvascular blood flow was observed with LSCI (p=0.81). ConclusionLocal delivery of L-NMMA to the skin by microdialysis reduces microvascular blood flow and glucose delivery in the skin after oral glucose intake, presumably by decreasing local insulinmediated vasodilation.

Sampling the concentration of insulin in human skin using microdialysis is challenging because of low intracutaneous concentrations and low recovery, presumably due to adsorption of insulin to the microdialysis system. In this study, we aimed to (1) measure how the concentration of insulin varies in three different tissue compartments (intracutaneous, subcutaneous and intravenous) and (2) to study how much insulin is adsorbed to the microdialysis catheter membranes and tubing during a typical microdialysis experiment, both in vivo and in vitro. We hypothesized that (1) the concentration of insulin decreases from the intravenous compartment to the intracutaneous and subcutaneous tissue, and that (2) adsorption of insulin to the microdialysis membrane and tubing impairs the recovery of insulin from the tissue. In this experimental study, microdialysis catheters were inserted intracutaneously, subcutaneously and intravenously in 11 healthy subjects. Systemic endogenous hyperinsulinemia was induced by intake of an oral glucose load. Insulin concentration was measured in the dialysate and in the extracted samples from the catheter membrane and tubings. In vitro microdialysis was performed to investigate the temporal resolution of the adsorption. After an oral glucose load insulin concentration increased intravenously, but not in the intracutaneous or subcutaneous compartments, while glucose, lactate and pyruvate concentrations increased in all compartments. The adsorption of insulin to the microdialysis membrane in vivo was highest in the intravenous compartment (p = 0.01), compared to the intracutaneous and subcutaneous compartments. In vitro, the adsorption to the microdialysis membrane was highest one hour after sampling, then the concentration gradually decreased after three and five hours of sampling. The concentration of insulin in peripheral tissues is low, probably due to decreasing tissue vascularity. Adsorption of insulin to the microdialysis membrane is modest but time-dependent. This finding highlights the importance of a stabilization time for the microdialysis system before sampling tissue analytes.

Insulin has metabolic and vascular effects in the human body. What mechanisms that orchestrate the effects in the microcirculation, and how the responds differ in different tissues, is however not fully understood. It is therefore of interest to search for markers in microdialysate that may be related to the microcirculation. This study aims to identify proteins related to microvascular changes in different tissue compartments after glucose provocation using in vivo microdialysis. Microdialysis was conducted in three different tissue compartments (intracutaneous, subcutaneous and intravenous) from healthy subjects. Microdialysate was collected during three time periods; recovery after catheter insertion, baseline and glucose provocation, and analyzed using proteomics. Altogether, 126 proteins were detected. Multivariate data analysis showed that the differences in protein expression levels during the three time periods, including comparison before and after glucose provocation, were most pronounced in the intracutaneous and subcutaneous compartments. Four proteins with vascular effects were identified (angiotensinogen, kininogen-1, alpha-2-HS-glycoprotein and hemoglobin subunit beta), all upregulated after glucose provocation compared to baseline in all three compartments. Glucose provocation is known to cause insulin-induced vasodilation through the nitric oxide pathway, and this study indicates that this is facilitated through the interactions of the RAS (angiotensinogen) and kallikrein-kinin (kininogen-1) systems.

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