THE MEASUREMENT OF PERMEABILITY IN SINGLE-RAT VENULES USING THE RED-CELL MICROPERFUSION TECHNIQUE

The red cell microperfusion-micro-occlusion technique has been used to measure fluid filtration and reabsorption (J(v)/A) at known microvascular pressures (P-c) in single mesenteric venules of anaesthetized rats. The relation between J(v)/A and P-c is linear over the range of P-o from 15 to 50 cmH(2)O and its slope is the hydraulic permeability, L(p). Reproducible estimates of L(p) can be made in the same venule in separate microperfusions. The value of P-c at J(v)/A = 0 varies linearly with perfusate oncotic pressure and is the effective oncotic pressure of the perfusate, sigma Delta pi, when the hydrostatic pressure in the pericapillary fluid is zero. The mean value for L(p) (+/- S.E.M.) in forty venules was 2.43 (+/-0.2) x 10(-7) cm s(-1) cmH(2)O(-1). Two potential errors of the micro-occlusion technique (vessel distensibility and marker red cell size) were investigated. It was found that the effects of vessel distensibility had little effect on red cell movements at times later than 2 s after a step change in P-c. Red cell size had a potentially large effect on estimates of the absolute values of L(p). Cooling the mesenteric tissues from 37 to 7 degrees C reduced L(p) in proportion to the change in the reciprocal of water viscosity with temperature. Rat venular permeability was shown to be sensitive to histamine, with L(p) increasing and sigma Delta pi falling in a concentration-dependent fashion with histamine concentrations of 1.6 x 10(-5) to 3.3 x 10(-4) mol l(-1) in the perfusate.