Electrical characteristics of snake distal tubules: Studies of I-V relationships

Distal tubules of Thamnophis spp. were perfused in vitro with Ringer solution containing either 16 or 150 mM Na and bathed with 150 mM Na Ringer. Current-voltage relationships were obtained by injecting pulses of constant current, I(o), into the tubule lumen and recording changes in voltage, ΔV(o), at the proximal end of the perfused tubule segment. The I(o)-V(o) plots showed a distinct break at a voltage E₁ (~85 mV) that was greater than the open-circuit voltage, VT(oc), and similar to values of E(Na), the transepithelial driving force for Na transport estimated by other methods. The resistance of the shunt pathway, R(s), was estimated from the values of the transepithelial resistance after luminal addition of 10^-5M amiloride, which caused a rapid fall of the VT(oc) to 0 mV with concurrent increases of the transepithelial resistance. These estimates of R(s) were the same as the values of E₁/I₁ obtained from the I(o)-V(o) plots. The VT(oc), RT, and R(s) were independent of the bath [Na] and were not influenced by the addition of amiloride to the bath. As in frog skin and toad urinary bladder, the E(Na) and R(s) of the snake distal tubule can be estimated from studies of their I(o)-V(o) plots, and the E₁ appears to be independent of the transepithelial chemical potential for Na.