Protein reabsorption in the kidney proximal tubules occurs simultaneously with the transport of ions and water, but little is known about the dependence of receptor-mediated protein endocytosis on water-salt balance changes. The aim of the study was to investigate tubular reabsorption and intracellular vesicular transport of various proteins in a model of hypernatremia in lake frogs (Pelophylax ridibundus). Frogs were injected with hypertonic sodium chloride solution (0.75 M NaCl) 1 hour before injection of green or yellow fluorescent proteins (GFP or YFP), as well as lysozyme. The method of fluorescent immunohistochemistry was used for detection of lysozyme and endocytic receptor megalin in kidney sections. Specimens were investigated using laser scanning confocal microscopy. The intensity of fluorescent signals of proteins and megalin in proximal tubular cells was determined on the images obtained. To study the dynamics of endocytosis, an automated method for quantifying colocalized protein and megalin signals was used. A statistically significant decrease in the reabsorption of GFP, YFP and lysozyme in the proximal tubules after 0.75 M of NaCl injection was found. The accumulation of proteins in the early endocytic compartment and decrease in their entry into late endosomes and lysosomes are shown, that is considered as evidence of a delay in intracellular vesicular transport in hypernatremia. The data obtained were analyzed in connection with changes in blood parameters and kidney activity during osmoregulation, and also with the role of chloride channels in receptor-mediated protein endocytosis. It can be assumed that increased ion transport in the proximal tubules cells in hypernatremia leads to decreased reabsorption capacity of epitheliocytes and delayed intracellular transport of proteins.
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