Seasonal variations of capillary hydraulic conductivity and volume status.

The purpose of this study was to determine whether circulating plasma protein concentration, capillary hydraulic conductivity (Lp), and overall volume status in frogs were correlated and whether these correlations varied with season. Individual vessel Lp was thus examined on a monthly basis and correlated with changes in protein content in three body compartments (blood, lymph, and body cavity) over a 10-yr period. The frogs were anesthetized, blood and fluid samples collected, Lp assessed, and samples assayed for overall protein content. Volume status indicators varied cyclically over the course of 1 yr. Protein levels reached their peak in winter and were lowest in summer for all compartments. Lp for venular and arteriolar capillaries reached maximal levels in summer and fell steadily throughout the remainder of the year. Basal true capillary Lp was stable over the course of the year. Lp and total protein concentration were inversely proportional to each other on a seasonal basis but did not correlate on a monthly basis. Therefore, we conclude that the lack of a simple relationship between Lp and volume status suggests an independent driver such as a hormonal signal for the hydraulic conductivity.

Differential action of plasma and albumin on transcapillary exchange of anionic solute.

We tested two hypotheses to account for the reduction in coupling of anionic solute to water flow (solvent drag) in microvessels during perfusion with plasma compared with albumin. Solvent drag is determined by both hydraulic conductivity (Lp) and solute reflection coefficient (sigma). Accordingly, decreased solvent drag during plasma perfusion must be the result of an increase in sigma (hypothesis 1) or reduction of Lp (hypothesis 2) or some combination of both mechanisms. These hypotheses were assessed by measuring Lp, sigma, and diffusive solute permeability (Psd) to the anionic protein alpha-lactalbumin in frog mesenteric exchange microvessels during plasma or albumin perfusion. The solute permeability coefficient to alpha-lactalbumin (Ps alpha-lactalbumin) was lower during exposure to plasma than bovine serum albumin (BSA) [(Ps alpha-lactalbumin)plasma/(Ps alpha-lactalbumin)BSA = 0.31 +/- 0.11 (means +/- SE, n = 9)]. Solute reflection coefficient to alpha-lactalbumin (sigma alpha-lactalbumin) was 0.69 +/- 0.02 in plasma and 0.34 +/- 0.03 in BSA (n = 7). Lp was not significantly influenced by perfusate protein composition (Lp plasma/Lp BSA = 1.02 +/- 0.11; n = 20). These data lead to the conclusion that the actions of plasma are to confer charge selectivity for anionic solute and, to a lesser extent, modify the porous pathways of the microvessel wall. Taken together, these results indicate that porous pathways contribute significantly to macromolecular flux in plasma-perfused vessels.