NH4+ conductance in Xenopus laevis oocytes - II. Effect of hypoosmolality

Superfusing Xenopus laevis oocytes with NH4Cl (10 mmol/l, pH 7.5) resulted in an inward current at a clamp potential of -70 mV. In paired experiments (n=22), the NH4Cl-induced peak current was -293+/-94 nA, under control cond itions (osmolality: 240 mosmol/kg), and rose to -523+/-196 nA when osmolali ty was reduced to 144 mosmol/kg. In parallel with the rise in NH4Cl-induced inward current, membrane conductance at -70 mV doubled and the zero-curren t potential changed from +3.3+/-9.4 mV to -22.0+/-8.0 mV (n=22) in the pres ence of NH4Cl during exposure to a hypoosmolar solution. In the absence of NH4Cl, oocytes responded to hypoosmolality with a shift in zero-current pot ential to more negative values and an increased conductance which became pa rtially sensitive to isosorbiddinitrate (ISDN), suggesting the activation o f a volume-sensitive K+ channel. Membrane conductance in the presence of NH 4Cl was decreased by ISDN to similar extents under isoosmolal and hypoosmol al conditions, indicating that NH4+ enters the oocytes through a volume-sen sitive conductance separate from the ISDN-sensitive K+ channel.