Amiloride-sensitive Na + conductance in native Xenopus oocytes

Abstract

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 μM) 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 μM), 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.