1. We have investigated the effects of varying flow velocity upon permeability to potassium ions (P(K)) of single perfused mesenteric microvessels in pithed frogs. 2. P(K) was estimated using a development of the single bolus microperfusion technique at chosen flow velocities (U) in the range of 150-7000 μm s-1. 3. In thirty-seven out of forty-three vessels, there was a strong positive correlation between P(K) and U. Average values (median) for P(K)(μm s-1) were related to U (μm s-1) by the expression: P(K) = 0.0043U + 4.05 (n = 43). 4. The correlation between P(K) and U was independent of microvascular pressure (and hence fluid filtration) over the range of 5-70 cmH2O. 5. The correlation between P(K) and U was independent of the potassium concentration in the bolus over the range of 2-40 mmol l-1 and of the direction of the potassium flux through the capillary walls.
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