Depolarization-induced changes in neurite elongation and intracellular Ca2+ in isolated Helisoma neurons.

This study focuses on the effects of K+ depolarization on neurite elongation of identified Helisoma neurons isolated into culture. Application of K+ to the external medium caused a dose-dependent suppression of neurite elongation. Lower concentrations of K+ were associated with a slowing in the rate of neurite elongation, whereas higher concentrations produced neurite retraction. Surprisingly, the effects of K+ depolarization were transient, and neurite elongation rates recovered towards control levels within 90 min even though the neurons remained in high-K+ solution. Identified neurons differed in the magnitude of their response to K+ depolarization; neurite elongation of buccal neuron B4 was inhibited at 5 mM K+, but elongation in B5 and B19 was not affected until concentrations of 25 mM. Electrophysiologically, K+ application evoked a brief period (5-10 s) of action potential activity that was followed by a steady-state membrane depolarization lasting 2 h or more. The changes in neurite elongation induced by K+ depolarization occurred in isolated growth cones severed from their neurites and were blocked by application of calcium antagonists. Intracellular free Ca2+ levels in growth cones of B4 and B19 increased and then decreased during the 90-min depolarization, corresponding to the changes in elongation. B4 and B19 showed differences in the magnitude, time course, and spatial distribution of the Ca2+ change during depolarization, reflecting their different sensitivities to depolarization.