Permeability of the fenestrated capillaries in the cat submandibular gland to lipid‐insoluble molecules.

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Abstract
Permeability-surface area (PS) products for the fenestrated capillaries in the perfused cat submandibular gland were measured for graded lipid-insoluble molecules using the single-passage multiple-tracer dilution technique. The PS for [57Co]cyanocobalamin (MW 1353) increased as the perfusion flow was increased but reached a constant value of 4.11 .+-. 0.25 ml/min per g (mean .+-. SE n [number] = 9) at flows > 8 ml/min per g. For [125I]insulin (MW .apprxeq. 6000) PS was 1.80 .+-. 0.13 ml/min per g (mean .+-. SE, n = 9) and apparently diffusion-limited at all high flow rates studied. A similar permeability-surface area product was measured for [14C]inulin (MW 5500): 1.76 .+-. 0.10 (mean .+-. SE, n = 4). PS values for cyanocobalamin and insulin in the salivary gland are, respectively, .apprxeq. 20 and 200 times larger than the estimates reported for the continuous capillaries of cardiac and skeletal muscle. The PS ratio [57Co]cyanocobalamin/[125I]insulin (2.33 .+-. 0.15, mean .+-. SE, n = 9) was significantly greater than the apparent ratio of their free diffusion coefficients (1.76), suggesting restricted diffusion of insulin relative to cyanocobalamin across the capillary endothelium. Permeability-surface area products for the smaller MW tracers (22Na, 86Rb and 51Cr-EDTA, MW 357) increased continuously with perfusion rate, indicating flow-limited solute exchange. The PS ratio of Rb/EDTA was close to unity; the corresponding free diffusion ratio was 3.85. The high permeability-surface area values measured apparently were associated with the fenestrae which appeared to act as high concentrations of small pores rather than as large pores.