Introduction
In microdialysis, various calibration method are being used. A different approach is to use a perfusion flow low enough to ensure complete equilibration between interstitial and perfusion fluids, Hagström-Toft et al. (1997). To make sampling at these low perfusion flows possible, without a substantial loss of perfusate into the tissue, we have previously found that a colloid needs to be included in the perfusion fluid, Rosdahl et al. (1997) or alternatively that the hydrostatic pressure on the dialysis membrane is decreased by lowering the position of the outlet tubing.
Methods
This study evaluated the effect of changes in the colloid osmotic and hydrostatic pressures of the microdialysis perfusion solution on net fluid transport across the dialysis membrane and on metabolite concentrations in the dialysate. Microdialysis catheters were either immersed in an external medium (Krebs-Henseleit buffer with 5 mM glucose, 2.5 mM lactate, 40 µM glycerol and 5 mM urea), or were inserted in the quadriceps femoris muscles of ten healthy volunteers. The study was approved by the ethics committee of the Karolinska Institute. Changes in osmotic pressure were induced by using perfusate with or without the inclusion of 40 g/l dextran-70. Changes in hydrostatic pressure were induced by varying the position of the outflow tubing. The perfusion flow rates were; in vitro: 0.33, 1.33 and 2.66 µl/min and in vivo: 0.33, 0.66 and 1.33 µl/min.
Results
The sample volumes were significantly smaller in catheters perfused without a colloid compared with those with a colloid (40 g/l dextran-70). The sample volumes were also significantly smaller when the outflow tubing was placed 8 cm above the microdialysis catheter (maximal hydrostatic pressure) compared with when the outflow tubing was placed 7 cm below the catheter (minimal hydrostatic pressure). This was true both in vivo and in vitro.
There was no effect of varying the position of the outflow tubing on metabolite concentrations in dialysate, neither in vitro or in vivo. In vitro, the metabolite concentrations did not differ when perfused with or without a colloid. In vivo, glucose and urea concentrations were significantly lower when the catheters were perfused with a colloid, (26 ± 2 % for glucose, p<0.05 and 4 ± 2% for urea p<0.05). Lactate and glycerol concentrations did not differ between the different perfusion fluids.
Conclusions
In conclusion, both hydrostatic and osmotic forces determine the net fluid balance across a dialysis membrane of a microdialysis catheter. When the catheters are inserted in human skeletal muscle dialysate concentrations of glucose and urea are significantly lower when perfused with a colloid while lactate and glycerol concentrations do not differ.
References
Hagström-Toft, E., Enoksson, S., Moberg, E., Bolinder, J.& Arner, P. 1997. Am J Physiol 273, E584-E592
Rosdahl, H., Ungerstedt, U. & Henriksson, J. 1997. Acta Physiol Scand 159, 261-262