Nutritive blood flow affects microdialysis O/I ratio for [C-14] ethanol and 3H(2)O in perfused rat hindlimb

Changes in the microdialysis outflow-to-inflow (O/I) ratio for [C-14] ethan ol and (H2O)-H-3 were determined in the perfused rat hindlimb after increas es and decreases in nutritive flow mediated by the vasoconstrictors norepin ephrine (NE) and serotonin (5-HT), respectively. Microdialysis probes (cont aining 10 mM [C-14] ethanol and (H2O)-H-3 pumped at 1 or 2 ml/min) were ins erted through the calf of the rat. Hindlimb perfusion flow rate was varied from 6 to 56 ml.min(-1).100 g(-1) in the presence of NE, 5-HT, or saline ve hicle. The O/I ratios for both tracers were determined at each perfusion fl ow rate, as was perfusion pressure, oxygen uptake (a surrogate indicator of nutritive flow), and lactate release. Both tracers showed a decreased O/I ratio as hindlimb perfusion flow was increased, with [C-14] ethanol being h igher than (H2O)-H-3. NE decreased the O/I ratio compared with vehicle, and 5-HT increased it for both tracers and both microdialysis flow rates. We c onclude that the microdialysis O/I ratio, while able to detect changes in t otal flow, is also sensitive to changes in nutritive and nonnutritive flow, where the latter still extracts tracer, but less than the former.