The transmural potential of the newt ureter: evidence for amiloride-sensitive active sodium transport

Transmural electrical potential difference (PD) of the newt ureter was measured with glass microelectrodes and its nature was studied pharmacologically or by ion replacement experiments. The PD in the presence of standard Ringer solution on both sides averaged -76.0 +/- 1.3 mV, lumen negative. The magnitude of the PD depended on the Na+ concentration of the luminal fluid; the PD increased in a saturable way with increase in the Na+ concentration. Amiloride added to the luminal fluid at 10(-4) M greatly reduced the PD and increased the transmural resistance. The half maximum inhibition was seen at 2.6 x 10(-6) M and the maximum inhibition at 10(-4) M where the PD reduced to 3.8% of control. Amiloride added to the external medium at 10(-4) M had no effect on both the PD and the resistance. Ouabain added to the external medium at 10(-3) M caused a gradual decrease in the PD, which fell to 36% of its initial value after 60 min. The specific transmural resistance (Rt) was estimated by recording spatially decaying electrotonic potentials along the ureter and applying cable analysis. The short-circuit current (Isc) was calculated from the PD and the Rt. The Rt and the Isc averaged 31.7 +/- 7.7 k omega X cm2 and 3.9 +/- 1.5 microA X cm-2, respectively. The results indicate that the epithelial cells of the newt ureter have amiloride-sensitive Na+ channels at the luminal membrane and are transporting Na+ actively, like epithelia of the urinary bladder and the skin of amphibia.