Mitochondrial movement during the ascidian sperm reaction

The ascidian sperm reaction, Which involves swelling, migration, and loss of the single large mitochondrion, can be triggered in vitro by raising the seawater pH to 9.3 or lowering Na⁺ to 20 mM, but only if the sperm are allowed to attach to a suitable Substate. Mitochondrial translocation does not usually occur in the absence of sperm attachment. Extracellular Ca²⁺ is necessary for triggering the reaction with low Na⁺ but not high pH; however, the intrecellular Ca²⁺ blocker, TMB-8, inhibits high pH-induced mitochondrial movement in the absence of extracellular Ca²⁺. After swelling, the mitochondrion fluoresces in the presence of chlortetracycline, suggesting that Ca²⁺ becomes membranebound after activation. Elevated cAMP and theophylline both inhibit mitochondrial move ment but not sperm motility. The antiactin drug cytochalasin B(10μM) and the calmodulinblocking drugs TFP (1 μM) and W-13 (10 μM) block mitochondrial movement, suggesting roles for actin and calmodulin in mitochondrial movement. A model is proposed relating intracellular alkalinization, Ca²⁺ influx, actin, myosin, and calmodulin in mitochondrial translocation.