Electrical conductivity of tubular fluid of the rat nephron

Micropuncture experiments were performed to evaluate the electrolyte concentration of the tubular fluid (TF) in different segments of the rat nephron using the continuous, in situ measurement of electrical conductivity (ϰ) with a recently developed conductivity microprobe. In mid to late portions of proximal convoluted tubules (PCT) and in superficial distal tubules (DT) under free flow conditions the conductivities were 18.3±0.3 and 7.7±0.4 mS · cm−1 at 37°C, respectively. These values correspond to the conductivities of 137 and 58 mmol · l−1 sodium chloride solutions. The conductivity of the fluid in the inaccessible loop of Henle was examined by rapidly propelling its contents towards the conductivity pipette situated in the first accessible distal loop by a perfusion burst from the late proximal tubule. Thus, the free flow concentration profile existing along the ascending and descending limbs could be monitored with little distorsion. The transport functions of the different segments of Henle's loop, particularly of the diluting segment, were recorded using a “stop/pulse flow” method. Following blockade of a PCT the corresponding loop of Henle was perfused with a proximal equilibrium solution. Subsequently perfusion was interrupted for intervals of increasing duration to allow equilibration. The fluid collumn was then rapidly propelled into the distal tubule by suddenly re-establishing perfusion. Dilution of TF was found to proceed exponentially as a function of equilibration time. Minimal conductivity of TF in the thick ascending limb was reached after 25–30 s, the steady-state conductivity minimum being 3.9±0.24 mS · cm−1. This corresponds to the conductivity of a 27 mmol · l−1 NaCl solution. Addition of furosemide to the perfusion fluid (10−4 mol · l−1) resulted in a significant decrease of the dilution capacity, an effect which was enhanced by saline expansion of the animals. TF in contact with more proximal segments of Henle's loop i.e. the descending limb revealed a steady state conductivity of 17.7±0.3 mS · cm−1, which after correction for dilution in the ascending limb is equivalent to a 133 mmol · l−1 NaCl solution. The present technique provides a means to examine some of the transport properties of Henle's loop in vivo, particularly the dilution capacity of the ascending limb, which normally is not accessible to micropuncture.