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 microvasc ular pressures (P-c) in single mesenteric venules of anaesthetized rat s. 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 s eparate microperfusions. The value of P-c at J(v)/A = 0 varies linearl y with perfusate oncotic pressure and is the effective oncotic pressur e of the perfusate, sigma Delta pi, when the hydrostatic pressure in t he pericapillary fluid is zero. The mean value for L(p) (+/- S.E.M.) i n forty venules was 2.43 (+/-0.2) x 10(-7) cm s(-1) cmH(2)O(-1). Two p otential errors of the micro-occlusion technique (vessel distensibilit y and marker red cell size) were investigated. It was found that the e ffects of vessel distensibility had little effect on red cell movement s at times later than 2 s after a step change in P-c. Red cell size ha d 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 hi stamine, with L(p) increasing and sigma Delta pi falling in a concentr ation-dependent fashion with histamine concentrations of 1.6 x 10(-5) to 3.3 x 10(-4) mol l(-1) in the perfusate.