RESUMO
In cultured nerve cord explants from the crayfish (Procambarus clarkii), the normal impulse activity levels of growing motor axons determine their response to Ca2+ influx. During depolarization or Ca2+ ionophore application, normally active tonic motor axons continue to grow, whereas inactive phasic motor axons retract and often degenerate. To determine the role of Ca2+ regulation in this difference, we measured the intracellular free Ca2+ concentration ([Ca2+]i) with fura-2. Growth cones from tonic axons normally had a higher [Ca2+]i than those from phasic axons. When depolarized with 60 mM K+, growth cones and neurites from phasic axons had a [Ca2+]i three to four times higher than did those from tonic axons. This difference in Ca2+ regulation includes greater Ca2+-handling capacity for growing tonic axons; the increase in [Ca2+]i produced by the Ca2+ ionophore 4-bromo-A23187 (0.25 microM) is four to five times greater in phasic than in tonic axons, and the decline in [Ca2+]i at the end of a depolarizing pulse is three to four times faster in tonic axons than phasic ones. Blocking impulses in growing tonic axons for 2-3 d with tetrodotoxin reduces their capacity to regulate [Ca2+]i. Thus, growing tonic and phasic axons have differences in Ca2+ regulation that develop as a result of their different activity levels. These activity-dependent differences in Ca2+ regulation influence axon growth and degeneration and probably influence other neuronal processes that are mediated by changes in [Ca2+]i.
