The general aim of this project was to develop experimental in vivo models that allow for minimally invasive investigations of responses in the skin to microvascular and metabolic provocations. The cutaneous microvasculature has emerged as a valuable model and been proposed to mirror the microcirculation in other organs. Dysfunction in the cutaneous microcirculation has thus been linked to systemic diseases such as hypertension and diabetes mellitus. Models for investigating skin responses could facilitate the understanding of pathophysiological mechanisms as well as effects of drugs.

In the first study, three optical measurement techniques (laser Doppler flowmetry (LDF), laser speckle contrast imaging (LSCI) and tissue viability imaging (TiVi)) were compared against each other and showed differences in their ability to detect microvascular responses to provocations in the skin. TiVi was found more sensitive for measurement of noradrenaline-induced vasoconstriction, while LSCI was more sensitive for measurement of vascular occlusion. In the second study, microvascular responses in the skin to iontophoresis of vasoactive drugs were found to depend on the drug delivery protocol. Perfusion half-life was defined and used to describe the decay in the microvascular response to a drug after iontophoresis. In the third study, the role of nitric oxide (NO) was assessed during iontophoresis of insulin. The results showed a NO-dependent vasodilation in the skin by insulin. In the fourth study the vasoactive and metabolic effects of insulin were studied after both local and endogenous administration. Local delivery of insulin increased skin blood flow, paralleled by increased skin concentrations of interstitial pyruvate and lactate, although no change in glucose concentration was observed. An oral glucose load resulted in an increased insulin concentration in the skin paralleled by an increase in blood flow, as measured using the microdialysis urea clearance technique, although no changes in perfusion was measured by LSCI.

The thesis concludes that when studying skin microvascular responses, the choice of measurement technique and the drug delivery protocol has an impact on the measurement results, and should therefore be carefully considered. The thesis also concludes that insulin has metabolic and vasodilatory effects in the skin both when administered locally and as an endogenous response to an oral glucose load. The vasodilatory effect of insulin in the skin is mediated by nitric oxide.

The microcirculation in the skin is essential for skin homeostasis. In instances of altered microvascular function, that may be the result of insulin resistance, tissue morbidity may ensue. The underlying mechanisms are however complex and not fully understood. By studying the physiological effects of insulin in the skin, the understanding of the complex interplay between glucose metabolism and skin microcirculation can be improved. 

The general aim of this thesis was to develop an experimental in vivo model to study metabolic and microvascular responses to insulin in the skin in healthy subjects. Microdialysis is a suiting technique as it allows for both local delivery of drugs and simultaneous monitoring of the local metabolic and vascular effects in the very same tissue compartment. 

The effects of local and systemic insulin provocation on skin blood flow and metabolism were investigated using microdialysis urea clearance and laser speckle contrast imaging (paper I). An insulin dependent increase in skin blood flow was observed, presumably induced through the nitric oxide pathway (paper II). Investigating the protein expression during an oral glucose provocation using proteomic approaches however indicates interactions with other pathways, such as the renin-angiotensin system and the kallikrein-kinin system (paper IV). Paper III also investigated methodological concerns regarding the sampling of insulin using microdialysis. 

This in vivo model can, in the future, be applied to assess the microvascular effects of insulin in the skin in different patient groups, including those with micro-vascular dysfunction due to, for instance, insulin resistance.  

Insulin är ett hormon som produceras i bukspottskörteln som svar på höga blodsockernivåer efter matintag. Via blodet når insulinet alla kroppens vävnader och möjliggör upptag av socker till cellerna. Dessa effekter på ämnesomsättning är välkända. Senaste årtionden har dock intresset för att kartlägga insulinets effekter på blodcirkulationen ökat.  

Målet med denna avhandling var att skapa en modell för att studera insulinets effekt på blodcirkulationen i huden på friska försökspersoner. Detta har gjorts med mikrodialysteknik. En mikrodialyskateter är en tunn plastslang som spolas igenom med en vätska i mycket långsam takt (µL/minut). I toppen av katetern sitter ett genomsläppligt membran med många små hål, genom vilket molekyler kan vandra ut och in från kateterslangen tills jämvikt uppstår. Funktionen liknar ett konstgjort blodkärl. På så sätt fås ett mer vävnadsspecifikt prov från just den vävnad man sätter in mikrodialyskatetern i, jämfört med ett blodprov.  

Genom att tillföra insulin lokalt till huden (via mikrodialyskatetern) och systemiskt via kroppsegen insulinproduktion (stimulerat av sockerintag) sågs att insulin ökar både blodflödet och ämnesomsättningen av socker lokalt i huden (delarbete I). Detta regleras av kväveoxid (delarbete II), som produceras lokalt i kärlväggen vid insulinstimulering. I delarbete III utfördes olika experiment för att undersöka svårigheter till att få ut insulin via mikrodialyskatetern, såsom att insulin klibbar fast mot själva membranet. Till sist analyserades mikrodialysvätskan för att identifiera proteiner involverade i andra signalvägar som också är vik-tiga för blodcirkulationen vid en insulinstimulering (delarbete IV).

I framtiden kan denna modell appliceras på olika patientgrupper för att se hur effekterna av insulin på blodflöde och ämnesomsättning skiljer sig hos vid insulinresistens, såsom vid övervikt eller diabetes.