The effects of flow on the transport of potassium ions through the walls of single perfused frog mesenteric capillaries.

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