Openings in frog microvascular endothelium at different rates of increase in pressure and at different temperatures

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Abstract
Experiments were carried out on single mesenteric capillaries and venules of pithed frogs to determine whether the rate of increase in intravascular pressure (dP/dt) influenced the critical pressure (PB) which increases wall permeability. Vessels, microperfused with frog Ringer solutions containing 0.1 % bovine serum albumin and red cells, were occluded downstream before pressure was raised either as a ramp or in a series of 13.6 cmH2O steps. By varying step duration, the mean dP/dt could be matched to dP/dt applied as a steady ramp. PB was recorded as the pressure at which there was an abrupt increase in filtration with red cells passing to and through one or more sites in the vessel wall. In all vessels, increasing dP/dt raised PB, with no differences between steps and ramps. The relation between PB and dP/dt was linear, consistent with a latent period, T (the slope), between a critical pressure being reached and the abrupt increase in permeability being observed. Direct observation confirmed this latent period. Between 12 and 20 oC, T was 8.5 ± 0.47 s; between 0 and 5 °C, T was 11.5 ± 0.97 s. Tissue cooling did not influence the time constant, τ, describing the rate of stretch of wall following a step increase in pressure and used to measure wall visco‐elastic properties. Nor was the value of τ (1.15 ± 0.06 s, n= 42) consistent with T being accounted for by visco‐elasticity. It is suggested that the latent period may indicate an active response of the endothelium.