Nonexcitable cells do not express voltage-activated Na+ channels. Instead, selective Na+ influx is accomplished through GTP-activated Na+ channels, the best characterized of which are found in renal epithelia. We have described recently a GTP-dependent Na+ current in rat basophilic leukemia (RBL) cells that differs from previous reported Na+ channels in several ways including selectivity, pharmacology and mechanism of activation. In this report, we have investigated the biophysical properties of the RBL cell Na+ current using the whole cell patch-clamp technique. Following activation by 250–500 μm GTPγS, hyperpolarizing steps to a fixed potential (−100 mV) from a holding potential of 0 mV evoked transient inward Na+ currents that declined during the pulse. If the holding potential was made more positive (range 0 to +100 mV), then the amplitude of the transient inward current evoked by the hyperpolarization increased steeply, demonstrating that the conductance of the channels was voltage-dependent. Using a paired pulse protocol (500 msec pulses to −100 mV from a holding potential of 0 mV), it was found that the peak amplitude of the current during the second pulse became larger as the interpulse potential became more positive. In addition, increasing the time at which the cells were held at positive potentials also resulted in larger currents, indicating a time-dependent conductance change. With symmetrical Na+ solutions, outward currents were recorded at positive potentials and these demonstrated both a time- and voltage-dependent increase in conductance. The results show that a nonvoltage activated Na+ channel in an electrically nonexcitable cell undergoes prominent voltage-dependent transitions. Possible mechanisms underlying this voltage dependency are discussed.
The Journal of Membrane Biology – Springer Journals
Published: Sep 15, 1998
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