The single K-channel current reported in a previous note was also studied in "outside-out" conditions. The electrode filling solutions used for the "cell-attached" experiments faced in this case the intracellular side of the membrane patches, the extracellular side facing the bath saline, i.e. Ringer standard. The most significant observations were obtained with filling solutions with varying proportions in K/Na concentrations solutions. In the absence of Na+ ([K+] = 110 mM), the elementary conductance was still around 90 pS and the I/V diagram was again somewhat bell shaped, though the distinctive reduction of the elementary conductance began at more positive potentials (+110 mV). No inward current could be detected upon membrane repolarization also in this case. The rectification became less evident and conductance increased with increasing Na+ concentration in the filling solution, until the I/V curve became a linear one and conductance was 270 pS with standard Ringer. Distinct inward elementary currents were evident upon repolarization in these conditions. Thus a complex interaction between Na+ and K+ takes place for conduction through the outward K channel in the frog oocyte, both cations probably competing for at least one active site inside. Another interesting observation concerns the process of gating of the OPC: the open times of the elementary currents were in fact much greater in outside out experiments as compared to cell-attached experiments, probably due to the presence of Ca++ in contact with the inner membrane side. Even increasing Na+ concentration prolonged the open time duration. The gating of the OPC in the membrane was not only voltage dependent, but also Ca++ and Na+ dependent.
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