Effects of second messengers on the permeability and morphology of eel rete capillaries

Sequential morphological and permeability changes in the rete capillaries during hyperglycaemia

With the rete model of the eel swimbladder, we have studied the appearance and development of a microangiopathy during a 2-year period of hyperglycaemia. Hyperglycaemia was induced in the eel by chronic exposure to cold water. At 3-5 months, basement membrane thickness was twice the normal value and increased only slightly thereafter. Diffusion coefficients of permeability were measured in counter-current perfusion experiments for a variety of tracers that are believed to use different pathways of transcapillary transport. The permeability to sucrose was the first to significantly increase, at 6-8 months, followed by that of albumin, insulin, and inulin, at 9-11 months and that of sodium, at 18-24 months. The permeability to water and antipyrine remained stable throughout the study. The results indicate that in the rete model, chronic hyperglycaemia induces a rapid thickening of the capillary basement membrane and selective permeability increments in the various paths of transcapillary transport.

Transport of insulin and albumin by the microvascular endothelium of the rete mirabile

Vascular permeability for albumin and insulin in the continuous capillary network of the rete mirabile of the eel swimbladder was evaluated by ultrastructural immunocytochemistry and countercurrent perfusion experiments. Upon perfusion of the rete capillaries with a buffer solution containing albumin and insulin, these serum proteins were revealed at the electron microscope level, by the Protein A-gold immunocytochemical technique on a post-embedding step. For the simultaneous detection 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 anti-bodies. The labelings obtained were morphometrically evaluated and demonstrate that: (1) serum proteins such as albumin and insulin are transported by the endothelial cells through their plasmalemmal vesicular system; (2) insulin is transported preferentially to albumin; and (3) this transport involves different populations of plasmalemmal vesicles. Measurements of diffusion permeability coefficients have confirmed the preferential transport of insulin, its coefficient being higher than that of albumin. Conversely, when compared to that of insulin or sucrose, which are assumed to be markers of the paracellular diffusion, it was found to be much lower, indicating that transcytosis through the vesicular system is less efficient than diffusion along the intercellular junctions. These results indicate that transcytosis of insulin and albumin occurs via different sets of plasmalemmal vesicles, probably through receptor-mediated mechanisms, and that the overall rate of transport across the rete capillaries, with respect to paracellular diffusion, is higher for insulin than for albumin.