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Ca²⁺ Clearance at Growth Cones Produced by Crayfish Motor Axons in an Explant Culture

Department of Biological Sciences, University at Albany, State University of New York, Albany, New York 12222

Submitted 23 October 2002; accepted in final form 9 January 2003

Intracellular free Ca²⁺ concentration ([Ca²⁺]_i) plays an important role in the regulation of growth cone (GC) motility; however, the mechanisms responsible for clearing Ca²⁺ from GCs have not been examined. We studied the Ca²⁺-clearance mechanisms in GCs produced by crayfish tonic and phasic motor axons by measuring the decay of [Ca²⁺]_i after a high [K⁺] depolarizing pulse using fura-2AM. Tonic motor axons regenerating in explant cultures develop GCs with more rapid Ca²⁺ clearance than GCs from phasic axons. When Na/Ca exchange was blocked by replacing external Na⁺ with N-methyl-D-glucamine (NMG), [Ca²⁺]_i decay was delayed in both tonic and phasic GCs. Tonic GCs appear to have higher Na/Ca exchange activity than phasic ones since reversal of Na/Ca exchange by lowering external Na⁺ caused a greater increase in [Ca²⁺]_i for tonic than phasic GCs. Application of the mitochondrial inhibitors, Antimycin A1 (1 µM) and CCCP (10 µM), demonstrated that mitochondrial Ca²⁺ uptake/release was more prominent in phasic than tonic GCs. When both Na/Ca exchange and mitochondria were inhibited, the plasma membrane Ca²⁺ ATPase was effective in extruding Ca²⁺ from tonic, but not phasic GCs. We conclude that Na/Ca exchange plays a prominent role in extruding large Ca²⁺ loads from both tonic and phasic GCs. High Na/Ca exchange activity in tonic GCs contributes to the rapid decay of [Ca²⁺]_i in these GCs; low rates of Ca²⁺ extrusion plus the release of Ca²⁺ from mitochondria prolongs the decay of [Ca²⁺]_i in the phasic GCs.