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Regulation of hydraulic conductivity in response to sustained changes in pressure

¹Department of Molecular and Cellular Physiology, Louisiana State University Health Sciences Center, Shreveport, Louisiana; ²Department of Bioengineering, Pennsylvania State University, University Park, Pennsylvania; and ³Department of Biomedical Engineering, City College of New York, City University of New York, New York, New York

Submitted 6 June 2005 ; accepted in final form 12 August 2005

The present study addresses the effect of a sustained change in pressure on microvascular permeability assessed by hydraulic conductivity (L_p) measurements from microvessels of the rat mesentery. With a microperfusion technique, transvascular filtration (normalized to surface area; J_v/S) and L_p were measured in small arterioles (baseline L_p = 0.26 x 10^–7 cm·s^–1·cmH₂O^–1) and venules (baseline L_p = 2.88 x 10^–7 cm·s^–1·cmH₂O^–1). The main finding of this study is that step increases in microvascular pressure led to time-dependent alterations of L_p. Immediately after a twofold step increase in pressure, J_v/S increased in proportion to the pressure change. This observation is consistent with Starling's law that predicts filtration proportional to the overall pressure gradient when L_p is constant. However, when J_v/S measurements continued for 60–90 min past the step in pressure, there was an initial decrease in J_v/S for 30 min ("sealing effect") followed by a substantial increase in J_v/S out to 90 min. The sustained increase in J_v/S suggests an increase in L_p of 36 ± 7% for small arterioles and 42 ± 5% for small venules (P < 0.05 for both). In addition, the increase in L_p in response to an increase in pressure was attenuated significantly by nitric oxide synthase inhibition. These results indicate that a pressure-induced mechanical stimulus (possibly J_v) activates a NO-dependent biochemical response that leads to an increase in hydraulic conductivity.

hydrostatic pressure; nitric oxide; transvascular filtration

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