Depolarization‐Induced Increase in Surface Binding and Internalization of 125I‐Nerve Growth Factor by PC 12 Pheochromocytoma Cells

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Abstract
The binding and internalization of 125I-nerve growth factor (NGF) by PC 12 pheochromocytoma cells was studied as a function of extracellular potassium concentration. Both surface-bound and internalized fractions of 125I-NGF associated with the cells under depolarizing conditions (50 mM K+) increased to 144 ± 28% (average ± SEM, six different cell preparations) and to 176 ± 12% (n = 6), respectively, of those observed at 6.0 mM K+. Scat-chard-type analysis of the data indicates increased sites for the binding and internalization of iodinated NGF by the cells. Similar enhancement was observed for cells treated with NGF as well. This voltage-dependent phenomenon was reversible, and also observed in the presence of veratri-dine. Moreover, withdrawal of extracellular Ca2+ abolished high K+-induced modulation of 125I-NGF binding and internalization, indicating that this effect may be mediated by Ca2+.