Seasonal Change of Membrane Potential across the Proximal Tubular Epithelium in Bullfrog Kidneys

Abstract

In order to examine the seasonal changes in the relationship between the membrane potential and potassium activity of proximal tubular epithelium, a micropuncture study was performed with potassium selective microelectrodes on the kidney of the bullfrog (Rana catesbeiana) in two different seasons: winter (7°) and summer (20°).
The potassium activity in winter animals (7°) was 2.92±0.33 (mean± S.D., n=20), 63.2±12.7 (n=26), and 2.68±0.19 (n=26) mM for the tubular fluid, cell, and plasma, whereas that in summer animals (20°) was 2.84±0.05 (n=22), 61.8± 11.2 (n=24), 2.63±0.24 (n=24) mM, respectively, indicating no seasonal difference. On the other hand, the mean values of the membrane PD in winter animals were 59.4±1.8 (n=26) and 71.7±7.2 (n=26) mV for the luminal and peritubular borders, whereas those in summer animals were 55.1 ± 1.7 (n=24) and 63.9 ± 6.9 (n=24) mV, respectively, indicating that there was a significant seasonal difference (p<0.05 and p<0.001). Hence, compared to winter animals, the changes in the electrochemical profile for potassium in summer animals were: 1) the peritubular membrane PD is lower and 2) the transtubular electrochemical gradient is less steep. The sodium permeability calculated as the best fitting for a modified Goldman equation was 0.01 and 0.03 for winter and summer animals, respectively.
In view of the fact that potassium in the cell and luminal fluid of the proximal tubule is kept at similar levels, potassium homeostasis is maintained in both groups of animals. The seasonal changes in electrical potentials are probably be due to an increase of cellular membrane permeability to ions other than potassium and to increased paracellular shunt conductance through the epithelium.