VOLTAGE-DEPENDENT CONDUCTANCE CHANGES IN A NONVOLTAGE-ACTIVATED SODIUM CURRENT FROM A MAST-CELL LINE

Nonexcitable cells do not express voltage-activated Na+ channels. Inst ead, selective Na+ influx is accomplished through GTP-activated Na+ ch annels, the best characterized of which are found in renal epithelia. We have described recently a GTP-dependent Na+ current in rat basophil ic leukemia (RBL) cells that differs from previous reported Na+ channe ls in several ways including selectivity, pharmacology and mechanism o f activation. In this report, we have investigated the biophysical pro perties of the RBL cell Na+ current using the whole cell patch-clamp t echnique. Following activation by 250-500 mu M GTP gamma S, hyperpolar izing steps to a fixed potential (-100 mV) from a holding potential of 0 mV evoked transient inward Na+ currents that declined during the pu lse. 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 conductanc e 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 add ition, increasing the time at which the cells were held at positive po tentials 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 tim e- and voltage-dependent increase in conductance. The results show tha t a nonvoltage activated Na+ channel in an electrically nonexcitable c ell undergoes prominent voltage-dependent transitions. Possible mechan isms underlying this voltage dependency are discussed.