Regulation of the tight junction permeabilities in the TAL
The thick ascending limb of Henle's loop (TAL) is a key nephron tubule for the reabsorption of divalent cations. This reabsorption occurs entirely through the cation selective paracellular pathway, driven by a lumen-positive transepithelial voltage created by the active transcellular transport. The discovery of the claudin (Cld) family of proteins and their properties has put into focus the study of the regulation in this paracellular pathway. To functionally study the regulation of the tight junction permeabilities, TALs were dissected and microperfused ex vivo. The recording of transepithelial voltages (Vte) and transepithelial resistances (Rte) allowed the calculation of equivalent short circuit currents (Isc), permeability ratios, and absolute paracellular permeabilities to ions (Px) in the TAL. TALs from mice on high calcium diet reached lower values of Vte, PMg and PCa than TALs from the control group, indicating clearly that a regulatory process has taken place. Low calcium intake for 8 days had no verifiable effects in the TAL of those mice. TALs were studied after challenging mice water homeostasis or by directly stimulating isolated TALs with vasopressin (AVP). TALs from water-restricted mice had lower Rte, with higher Vte, Isc, and PNa than tubules from water-loaded mice. Acute ex vivo stimulation of TALs with AVP, showed that AVP increase the paracellular selectivity for cations. Thus, water restriction and AVP induce regulation in the paracellular pathway in the TAL. Transgenic knock-out mouse models were used to characterize the functionality of the tight-junction in the TAL when Cld10, Cld16, or both claudins simultaneously were absent. The results suggest that both claudins are complementary, and their deficiency largely affects the paracellular permeability to cations. In summary, the data presented here are a further step in characterizing the paracellular pathway in the TAL and its regulation as a physiologically relevant mechanism in ion homeostasis.