Activity-dependent plasticity of calcium clearance from crayfish motor axons

Previous studies of a crayfish explant culture demonstrated that regenerating motor axons with high impulse activity develop more rapid clearance of cytoplasmic free Ca(2+) than those with low impulse activity. We examined whether Ca(2+) clearance in mature axons also showed activity-dependent plasticity. We studied the phasic and tonic axons of the motor bundle innervating the crayfish closer muscle that display large differences in impulse activity. To compare their Ca(2+) regulation, we applied the Ca(2+) ionophore Br-23187 (1 microM) and measured the increase in intracellular free Ca(2+) concentration ([Ca(2+)](i)) with fura-2. After 55 min of ionophore application, the increase in [Ca(2+)](i) in the phasic axons (1,326 +/- 192 nM) was significantly greater than in the tonic axons (359 +/- 148 nM). This resulted from stronger Ca(2+) clearance in the tonic axon rather than less Ca(2+) influx because blocking Ca(2+) clearance by Na/Ca exchange and mitochondria eliminated these differences in [Ca(2+)](i). Next we determined whether Ca(2+) clearance from the phasic axon could be strengthened by a prolonged increase in impulse activity. The phasic axon was stimulated in vivo at 5 Hz for 1 h/day for 5 days, and 1-3 days after stimulation, Ca(2+) clearance was again examined. After 55 min of Br-23187 (1 microM) exposure, the increase in [Ca(2+)](i) in the stimulated phasic axon was only 232 plus minus 123 nM, which was much less than in the control phasic axons and similar to that in the tonic axons. Thus Ca(2+)-clearance mechanisms adapt to changes in impulse activity both in growing and mature axons.