Luminal Na concentration and the electrical properties of the snake distal tubule

In previous studies of isolated perfused distal tubules of Thamnophis spp., elevation of luminal [Na] from 16 to 150 mM resulted in a transient hyperpolarization of the open-circuit voltage, VT(oc). To characterize further this response, studies were done to examine the concurrent changes of the transepithelial resistance, RT. After elevation of luminal [Na] from 16 to 150 mM, the VT(oc) increased sharply from a mean of 38.5 to 61.2 mV and the RT decreased from a mean of 22.3 to 15.8 kΩ^.cm. Thereafter, the VT(oc) declined slowly below control values, and the RT increased well above control values. The short-circuit current calculated as VT(oc)/RT changed in parallel with the VT(oc), increasing at first (from 1.8 to 4.1 μA/cm) and then falling to about 0.2 μA/cm. Luminal addition of 10^-5 M amiloride caused the VT(oc) → 0 and the RT to increase during control and all phases of the transient. On the assumption that RT during amiloride perfusion is the same as the shunt resistance, R(s), the values of the transepithelial driving force, E(Na), and its series resistance, R(Na), were calculated. An analysis of the data in this way indicated that the principal changes in the epithelium could be attributed to alterations of the R(Na) and not the E(Na) or R(s).