Effects of intra- and extracellularly applied phospholipases C on excitability of squid giant axons.

The effects of phospholipases C on the membrane excitability of the squid giant axon were investigated using phosphatidylcholine-hydrolyzing phospholipase C and sphingomyelinase C of Bacillus cereus, and phosphatidylinositol-specific phospholipase C of Bacillus thuringiensis. When the squid axon was perfused internally with phosphatidylcholine-hydrolyzing phospholipase C in KF or K-glutamate solution, the action potential was blocked in 4-7 min and membrane resistance decreased with time to a level less than one-tenth that of control. These effects were irreversible. When the axon was perfused internally with sphingomyelinase C in KF solution, the action potential was decreased to 30% in 3 min. Perfusion with enzyme-free KF solution fully restored the action potential. When the axon was perfused internally with phosphatidylinositol-specific phospholipase C in K - glutamate solution, the action potential was gradually decreased and blocked after 10 min. Perfusion with enzyme-free KF solution restored the action potential by 70%. When phosphatidylcholine-hydrolyzing phospholipase C was applied externally to the squid axon, the action potential and the membrane resistance were slowly but irreversibly decreased. These results suggest that membrane phospholipids, such as phosphatidylcholine and phosphatidylinositol, may be associated with the excitability of the membrane of the squid axon.