Single capillary permeability to proteins having similar size but different charge

We investigated the hypothesis that solute charge modulates transcapillary exchange in microvessels with continuous endothelium. Two globular proteins, alpha-lactalbumin and ribonuclease, having approximately the same size (mol wt 14,176 and 13,683, respectively) but different net charge (-10 and +4, respectively) were test solutes. Each solute was labeled with the fluorescent probe tetramethylrhodamine isothiocyanate. Labeling did not significantly change solute size, but increased negative charge on each solute by one valency unit. An in vivo fluorescent microscope technique [Huxley et. al., Am. J. Physiol. 252 (Heart Circ. Physiol. 21): H188-H197, 1987] was used to measure solute permeability coefficients (P) in single microvessels of frog mesentery at 14-16 degrees C. The mean P for alpha-lactalbumin, measured when capillary pressure was 10 cmH2O, was 2.1 X 10(-6) cm/s and the mean P for ribonuclease was 4.3 X 10(-6) cm/s. Our results conform to the hypothesis that the transcapillary pathways of frog mesenteric microvessels are negatively charged. With the use of a Donnan-type model for electrostatic partitioning, charge density in the pathway is estimated as 11.4 meq/l. Comparison of measured Ps with those for small solutes in frog mesenteric microvessels indicates that molecular size is a proportionally more significant determinant of solute permeability in continuous capillaries than is solute charge.