Objective: 

Along the nephron the paracellular route is the predominant pathway for the absorption of ions. It amounts to 66% in proximal tubule and 50% in thick ascending limb. Salt wasting syndromes in diseases with defective claudins indicate the critical role of tight junction proteins for permeability and selectivity of the paracellular shunt. Since the distribution and expression pattern of claudins along the nephron is highly specific and not necessarily reflected by renal epithelial cells in culture, the functional investigation in isolated perfused tubule segments is the state of the art technique to approach tight junction physiology in the respective animal models.

Methods: 

Renal tubules are microdissected and perfused by a double barreled pipette system. It is mounted on the stage of an inverse microscope and allows the exchange of luminal and basolateral solutions at simultaneous measurements of transepithelial voltage. Transcellular transport has to be restricted by appropriate pharmacological inhibition of the respective membrane transport proteins.

Results: 

Under these conditions paracellular properties are reflected by transepithelial resistance and transepithelial voltage (V_te). V_te is generated by ionic gradients across the epithelium. Permeability (P) ratios for PNa⁺/PCl^- are calculated from diffusion voltages obtained by isoosmotic transepithelial NaCl gradients which determine V_te depending on the preference of the pathway for each of the two ions. A highly selective pathway can be further characterised by biionic diffusion voltages which are generated by confrontation of two cations across the epithelium at the same anion concentration. Examples are given from experiments in mice defective for claudin 16 and claudin 19. The influences of paracellular leaks and of cation dependent claudin function on diffusion voltages are discussed.

Conclusion: 

Electrophysiological analysis in isolated perfused tubules is a powerful tool in the straight forward interpretation of the role of claudins in tight junction function.