Microdialysis sampling from skeletal muscle and adipose tissue with special reference to the effects of insulin on tissue blood flow and glucose metabolism

Abstract

The general aim of the present thesis was to explore the possibility of using rnicrodialysis in vivo, for studies of the effects of insulin on glucose metabolism and blood flow in skeletal muscle and adipose tissue. With microdialysis sampling, a probe is perfused with a physiological solution. Water soluble substances in the interstitial fluid (lSF) will diffuse across a semipermeable dialysis membrane and enter the perfusate, which finally will be collected for analysis. With this technique, substances in the ISF can be monitored and the effects of locally delivered drugs can be studied.

Studies at non-equilibrium microdialysis conditions (dialysate concentration < interstitial concentration) It was found that changes in blood flow, induced by local perfusion with vasoactive agents, markedly influenced the concentration of glucose in dialysate, both in rat and in human experiments. In addition, the diffusion of ethanol from the microdialysis perfusate was increased with vasodilatation and decreased with vasoconstriction, indicating that ethanol diffusion may be used as a marker of changes in tissue blood flow (study I and II). If changes in blood flow are considered, the release of amino acids can be indirectly monitored from rat skeletal muscle, by measurements of their concentrations in the interstitial space.

Using this approach the present data support that the effects of adrenaline on amino acid metabolism originate from a direct hormonal interaction in skeletal muscle via b-adrenergic stimulation (study III). During insulin-induced hypoglycaemia in the rat, the decrease in the concentration of glucose in dialysate from adipose tissue was significantly greater than that from skeletal muscle. These decreases were accompanied by an increased blood flow (ethanol diffusion) in skeletal muscle, but not in adipose tissue (study IV). With physiological hyperinsulinaemia (at euglycaemia) in humans, an increased blood flow was achieved in the calf and forearm (plethysmography).

As determined by ethanol diffusion, the blood flow was increased in the subcutaneous adipose tissue but not in the gastrocnemius or brachioradialis muscles. The increase in the estimated arterial-interstitial glucose difference in the gastrocnemius and brachioradialis muscles was greater than the increase in the arterio-venous glucose difference over the forearm (study V). The result of experiments in skeletal muscle using dextran-70 and dialysis membranes with a large pore size (100 kDa), indicate that when insulin is added to the perfusate, it will diffuse into the interstitial space and exert local effects on tissue glucose metabolism (study IX).

Studies with equilibrium microdialysis conditions (dialysate concentration = interstitial concentration. A complete equilibration between the perfusate and ISF was achieved for glucose, lactate, glycerol and urea using very low perfusion flows in human skeletal muscle and adipose tissue. The concentration of glucose was found to be equal in adipose tissue and plasma, while the concentration in skeletal muscle was clearly lower. The concentration of lactate was higher in skeletal muscle than in adipose tissue, whereas both tissue concentrations of lactate were higher than in blood. The concentration of glycerol in skeletal muscle was slightly lower than in plasma, while the concentration of glycerol in adipose tissue was markedly higher than both the skeletal muscle and plasma levels (study VIII). With a low perfusion flow in human skeletal muscle and adipose tissue, a substantial perfusate loss was observed which, however, could be prevented by adding dextran-70 to the perfusate (study VI). Overall, the present investigations indicate several possibilities of using microdialysis sampling to obtain tissue-specific information about insulin-induced changes in metabolism and blood flow.