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

Rumpal, Nidhi and Gregory A. Lnenicka. Ca clearance at growth cones produced by crayfish motor axons in an explant culture. J Neurophysiol 89: 3225–3234, 2003; 10.1152/jn.00952.2002. Intracellular free Ca concentration ([Ca ]i) plays an important role in the regulation of growth cone (GC) motility; however, the mechanisms responsible for clearing Ca from GCs have not been examined. We studied the Ca -clearance mechanisms in GCs produced by crayfish tonic and phasic motor axons by measuring the decay of [Ca ]i after a high [K ] depolarizing pulse using fura-2AM. Tonic motor axons regenerating in explant cultures develop GCs with more rapid Ca clearance than GCs from phasic axons. When Na/Ca exchange was blocked by replacing external Na with N-methyl-Dglucamine (NMG), [Ca ]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 [Ca ]i for tonic than phasic GCs. Application of the mitochondrial inhibitors, Antimycin A1 (1 M) and CCCP (10 M), demonstrated that mitochondrial Ca uptake/release was more prominent in phasic than tonic GCs. When both Na/Ca exchange and mitochondria were inhibited, the plasma membrane Ca ATPase was effective in extruding Ca from tonic, but not phasic GCs. We conclude that Na/Ca exchange plays a prominent role in extruding large Ca loads from both tonic and phasic GCs. High Na/Ca exchange activity in tonic GCs contributes to the rapid decay of [Ca ]i in these GCs; low rates of Ca 2 extrusion plus the release of Ca from mitochondria prolongs the decay of [Ca ]i in the phasic GCs.