Effect of Hypernatremia on Protein Reabsorption in Renal Proximal Tubules of the Lake Frog Pelophylax ridibundus

Although protein reabsorption in the renal proximal tubulesis concurrent with ion and water transport, little is known aboutthe dependence of receptor-mediated protein endocytosis on changesin water and salt balance. The aim of the study was to investigatetubular reabsorption and intracellular vesicular transport of variousproteins in a lake frog (Pelophylax ridibundus)model of hypernatremia. The frogs were injected with hypertonicsodium chloride solution (0.75 M NaCl) 1 h before the administrationof green or yellow fluorescent proteins (GFP or YFP), as well aslysozyme. Fluorescent immunohistochemistry and confocal microscopywere used to detect lysozyme and the endocytic receptor megalinin the epithelial cells of proximal tubules. The endocytosis dynamicswas assessed by automated quantification of colocalized fluorescentsignals of megalin and injected proteins. We found a statisticallysignificant decrease in the reabsorption of GFP, YFP, and lysozymein proximal tubules after NaCl injection. The accumulation of proteinsin early endosomes and the decrease in their entry into late endosomesand lysosomes suggest a delay in the intracellular vesicular transportof proteins under hypernatremia. The data obtained are analyzedas related to changes in blood parameters and kidney activity duringosmoregulation, as well as to the role of chloride channels in receptor-mediatedprotein endocytosis. We assume that increased ion transport in proximaltubular epitheliocytes under hypernatremia conditions leads to adecreased protein reabsorption capacity of these cells and delayed intracellulartransport of proteins.