AVP‐mediated regulation of the paracellular permeability in the thick ascending limb (TAL)

Abstract

The TAL drives an important part of the reabsorption of divalent cations. This occurs through the cation selective paracellular pathway. Arginine vasopressin (AVP) stimulates the transepithelial transport in the TAL. We wanted to study how AVP stimulation affects paracellular permeability in the TAL.TALs were mechanically dissected for microperfusion from 8‐10 weeks old mice who were kept for 4 days on water‐restricted (WR) (0.26ml/gBM/d) or water‐loaded (WL) (0.78ml/gBM/d) conditions, and from 5‐10 week old mice to be stimulated acutely with AVP.We measured transepithelial resistance and voltage (Rte, Vte), and calculated the equivalent short circuit current (Isc). From diffusion voltages, in presence of furosemide, we calculated the paracellular permeability (P) for Na+, Cl‐, Mg2+ and Ca2+. Measurements were performed at 37°C with continuous perfusion at a constant pressure.Freshly isolated TAL from the WR group (n=20) were transporting more actively compared to the WL group tubules (n=15). Vte and Isc were a 26% and a 48% higher, respectively, while Rte was 27% lower. Diffusion voltages were also increased in the WR group by 21%, indicating a 30% higher PNa/Cl, and a 27% higher PNa compared to WL.Acute TAL stimulation ex‐vivo for 12 minutes with AVP (n=6), in paired experiments, increased transepithelial transport. Isc and Vte increased by 23% and 25% respectively, while in the time control group Isc and Vte decreased by 28% and 37%. At the same time, AVP induced 20% more pronounced diffusion voltages compared to control, with a 25% increase in PNa/PCl.ConclusionWater restriction and AVP increase paracellular selectivity and permeability for cations in the TAL.