ROLE OF VOLTAGE-GATED CA2-AMP IN THE PRESENCE OR ABSENCE OF NGF( CHANNELS AND INTRACELLULAR CA2+ IN RAT SYMPATHETIC NEURON SURVIVAL AND FUNCTION PROMOTED BY HIGH K+ AND CYCLIC)

We have examined how NGF-dependent rat sympathetic neurons maintain Ca 2+ homeostasis when challenged with high K+ or 8-(4-chlorophenylthio)c yclic AMP (CPTcAMP), two survival factors. In the presence of NGF, hig h K+ (55 mM) caused a stable, 65% reduction in the density of cell som a 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-regu lation may help prevent the toxic effects of persistently elevated [Ca 2+]i. Inhibition of protein synthesis by cycloheximide blocked recover y from down-regulation. Moreover, treatment with cycloheximide or acti nomycin-D caused a 2-fold rise in the peak Ca2+ current, suggesting th at voltage-sensitive Ca2+ channel activity may be tonically attenuated during normal growth. In the absence of NGF, neurons survived for sev eral days in high K+ medium with no significant rise in resting (Ca2+] i, although neurites did not grow. Neither Ca2+ channel density nor re sting [Ca2+]i were altered in neurons surviving with CPTcAMP. Moreover , CPTcAMP lowered the dependence on extracellular Ca2+. However, the d ihydropyridine antagonist nitrendipine blocked both high K+- and CPTcA MP-dependent survival although it had no effect in the presence of NGF . Thus, in the absence of NGF, sympathetic neurons do not require elev ation of [Ca2+]i above resting levels to survive with either high K+ o r CPTcAMP, but dihydropyridine-sensitive Ca2+ channel activity may be essential for their survival promoting actions.