A Three‐Pathway Pore Model Describes Extensive Transport Data from Mammalian Microvascular Beds and Frog Microvessels

ABSTRACT Objective : To show that a three‐pathway pore model can describe extensive transport data in cat and rat skeletal muscle microvascular beds and in frog mesenteric microvessels. Methods : A three‐pathway pore model was used to predict transport data measured in various microcirculatory preparations. The pathways consist of 4‐ and 24‐nm radii pore systems with a 2.5:1 ratio of hydraulic conductivities and a water‐only pathway of variable conductivity. The pore sizes and relative hydraulic conductivities of the small‐ and large‐pore systems were derived from a model fit to reflection coefficient (σ) data in the cat hindlimb. The fraction (α w ) of total hydraulic conductivity ( L p ) or hydraulic capacity ( L p S ) contributed by the water‐only pathway was uniquely determined for each preparation by a fit of the three‐pathway model (parameters fixed as above) to σ data measured in that preparation. These parameter values were unchanged when the model was used to predict diffusion capacity (permeability‐surface area product, P d S ) data in the cat or rat preparations or diffusional permeability ( P d ) data in frog microvessels. The values for L p or L p S used to predict diffusional data in each preparation were taken from the literature. Predictions of P d ratios for solute pairs were also compared with experimental data. Results : The three‐pathway model closely predicted the trend of P d S or P d experimental data in all three preparations; in general, predicted P d ratios for paired solutes were quite similar to experimental data. For these comparisons, the only parameter varied between these preparations was α w .It varied considerably, from 7 to 16 to 41% of total in frog, rat, and cat preparations. Individual P d S or P d experimental data were closely predicted in the cat but somewhat overestimated in the frog and rat. This result could be due the use of L p or L p S values in the model that were affected by methodological problems. Calculated hydraulic conductivities of the water‐only pathway in the three preparations were quite similar. Conclusions : These results support the hypothesis of a common structure of the transmembrane pathways in these three, very different, microcirculatory preparations. What varies considerably between them is the total number of soluteconducting pathways, but not their dimensions, nor the hydraulic conductivities of their water‐only pathways. Because of the wide variation of α w among these preparations, the ratio of P d to L p for any solute is not constant, but the deviation from constancy may not be detectable because of errors in the experimental data.