Dynamic compliance of single perfused frog mesenteric capillaries and rat venules: a filtration coefficient correction.

The compliance of microvessels produces a potential source of error in the measurement of hydraulic conductance, regardless of which method is used. The compliance characteristics of rat and frog microvessels, in vivo, was studied using a two oil technique capable of measuring changes in radius of 0.01 micron at intervals of 0.02 seconds. Two water insoluble immiscible oils were micro-injected into occluded segments of the vessel. Since the volume of the oil remained constant the position of the interface at different times, following a sudden change in applied pressure, could be used to determine the time course of changes in vessel diameter. It was found that approximately 90% of the compliance response had occurred within 0.4 seconds of applying a step change in pressure. This compliance response coincides with the initial change in fluorescence used in the measurement of fluid with the initial change in fluorescence used in the measurement of fluid filtration by the densitometric technique.