Role of Voltage-gated Ca2+ Channels and Intracellular Ca2+ in Rat Sympathetic Neuron Survival and Function Promoted by High K+ and Cyclic AMP in the Presence or Absence of NGF

We have examined how NGF-dependent rat sympathetic neurons maintain Ca²⁺ homeostasis when challenged with high K⁺ or 8-(4-chlorophenylthio)cyclic AMP (CPTcAMP), two survival factors. In the presence of NGF, high K⁺ (55 mM) caused a stable, 65% reduction in the density of cell soma voltage-sensitive Ca²⁺ channels within 2 days. Although resting Ca²⁺]_i was elevated by 1.6-fold, this was 50% less than the rise in Ca²⁺]_i measured before down-regulation occurred, suggesting that down-regulation may help prevent the toxic effects of persistently elevated Ca²⁺]_i. Inhibition of protein synthesis by cycloheximide blocked recovery from down-regulation. Moreover, treatment with cycloheximide or actinomycin-D caused a 2-fold rise in the peak Ca²⁺ current, suggesting that voltage-sensitive Ca²⁺ channel activity may be tonically attenuated during normal growth. In the absence of NGF, neurons survived for several days in high K⁺ medium with no significant rise in resting Ca²⁺]_i, although neurites did not grow. Neither Ca²⁺ channel density nor resting Ca²⁺]_i were altered in neurons surviving with CPTcAMP. Moreover, CPTcAMP lowered the dependence on extracellular Ca²⁺. However, the dihydropyridine antagonist nitrendipine blocked both high K⁺- and CPTcAMP-dependent survival although it had no effect in the presence of NGF. Thus, in the absence of NGF, sympathetic neurons do not require elevation of Ca²⁺]_i above resting levels to survive with either high K⁺ or CPTcAMP, but dihydropyridine-sensitive Ca²⁺ channel activity may be essential for their survival promoting actions.