Amiloride-sensitive Na+ Conductance in Native Xenopus Oocytes

Endogenous Na+ conductances in the plasma membrane of oocytes of the South African clawed toad Xenopus laevis were investigated by microelectrode techniques and influx measurements. Removal of Na+ from the bath solution under voltage clamp conditions led to a decrease in the clamp current indicating the existence of native Na+ conductances. The observed current was voltage dependent but showed no marked rectification. Amiloride (10 microM) blocked this Na+ current reversibly. However, amiloride analogues such as benzamil and phenamil had no effect on this Na+ conductance. The Na+/H(+)-exchanger blocker EIPA (5-(N-ethyl-N-isopropyl)amiloride), another amiloride analogue, also had no effect thereby excluding a possible involvement of the Na+/H+ exchanger. The Na+ mediated current had a reversal potential of about 50 mV suggesting high selectivity of these Na+ conductances for Na+ over other monovalent cations. When Na+ was replaced by K+ in the bath solution, amiloride had no effect on the clamp current over the whole potential range demonstrating that only Na+ but not K+ can enter the cell via the investigated conductances. In radio tracer experiments 22Na+ influx into oocytes was nearly halved in presence of amiloride (10 microM), whereas benzamil and phenamil again failed to influence 22Na+ influx. These results suggest that the endogenous amiloride-sensitive Na+ conductance belongs to a new class of channels which is quite different from amiloride-sensitive epithelial Na+ channels.