TRANSPORT OF INSULIN AND ALBUMIN BY THE MICROVASCULAR ENDOTHELIUM OF THE RETE-MIRABILE

Vascular permeability for albumin and insulin in the continuous capill ary network of the rete mirabile of the eel swimbladder was evaluated by ultrastructural immunocytochemistry and countercurrent perfusion ex periments, Upon perfusion of the rete capillaries with a buffer soluti on containing albumin and insulin, these serum proteins were revealed at the electron microscope level, by the Protein A-gold immunocytochem ical technique on a post-embedding step. For the simultaneous detectio n of both proteins, the double labeling technique with different sized gold particles was used, Furthermore, labeling was performed with the mixture of anti-albumin and anti-insulin antibodies, The labelings ob tained were morphometrically evaluated and demonstrate that: (1) serum proteins such as albumin and insulin are transported by the endotheli al cells through their plasmalemmal vesicular system; (2) insulin is t ransported preferentially to albumin; and (3) this transport involves different populations of plasmalemmal vesicles, Measurements of diffus ion. permeability coefficients have confirmed the preferential transpo rt of insulin, its coefficient being higher than that of albumin, Conv ersely, when compared to that of inulin or sucrose, which are assumed to be markers of the paracellular diffusion, it was found to be much l ower, indicating that transcytosis through the vesicular system is les s efficient than diffusion along the intercellular junctions, These re sults indicate that transcytosis of insulin and albumin occurs via dif ferent sets of plasmalemmal vesicles, probably through receptor-mediat ed mechanisms, and that the overall rate of transport across the rete capillaries, with respect to paracellular diffusion, is higher for ins ulin than for albumin.