Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCA135

Poster Communications

Cation selectivity of the thick ascending limb depends on voltage, Ca2+ and ionic strength

C. Merkel1, N. Himmerkus1, J. Hou2, M. Bleich1

1. Institute of Physiology, Kiel, Germany. 2. Department of Internal Medicine, St. Louis, Missouri, United States.

The thick ascending limb of the loop of Henle (TAL) is one of the main reabsorption sites for divalent cations in the mammalian kidney. Transcellular salt transport generates a lumen positive transepithelial voltage (Vte) which serves as a driving force for paracellular transport of the cations Na+, Ca2+ and Mg2+. In addition the luminal fluid becomes more dilute towards the cortical part of the segment because of the water tightness of the TAL. As the paracellular pathway is highly cation selective a lumen positive diffusion potential (DP) can be generated by the Na+ concentration gradient towards the luminal side. This in turn provides a driving force for further reabsorption of Ca2+ and Mg2+ in the cortical parts of the TAL where transcellular transport is already slowed down or even ceased because of reduced luminal Cl- concentrations. The current study was designed to investigate the influence of increased luminal or basolateral Ca2+ concentrations on the paracellular cation selectivity of TAL and its dependence on voltage and ionic strength. Microdissected TAL from the cortex (cTAL) or the inner stripe of outer medulla (ISOM-TAL) of kidneys of 6-12 week old C57/Bl6/J mice were investigated in a double-barreled perfusion system using a standard solution containing 145 mM NaCl and 1 mM Ca2+. Active transcellular transport was inhibited by application of 50 µM furosemide. DPs were generated by either replacing the luminal solution or the basolateral solution by an isosmotic 30 mM NaCl solution to generate lumen positive or lumen negative potentials, respectively, as a measure of paracellular cation selectivity. Ca2+ was increased to 5 mM at one or on both sides of the epithelium. This maneuver was also repeated in the presence of symmetrical 145 mM or 30 mM NaCl solutions. Under lumen negative conditions DPs were higher compared to the lumen positive situation. Basolateral increase of Ca2+ directly and reversibly attenuated the DP if lumen negative, but had less pronounced effects if DP was lumen positive. Luminal increase of Ca2+ directly and reversibly attenuated the lumen positive DP. These effects were not reversed by increasing Ca2+ symmetrically which indicates that the effect is not simply an additional Ca2+ diffusion potential. Generally the effects in the cTAL were much higher than in ISOM-TAL. These findings show voltage dependence of TAL cation selectivity. In addition Ca2+ attenuates cation selectivity from both luminal and basolateral side. The effect of Ca2+ again was voltage dependent and highest at low ionic strength at its site of action.

Where applicable, experiments conform with Society ethical requirements