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Laboratoire de Neurobiologie Cellulaire du Centre National de la Recherche Scientifique, 91190 Gif sur Yvette, France

1. The effects of phospholipases A from bee venom and from porcine pancreas and of phospholipases C from Clostridium welchii and Bacillus cereus on active and passive membrane properties of Aplysia neurones have been studied. Consistent alterations in electrical membrane properties were found following intracellular application of three of these enzymes.

2. Bee venom phospholipase A produced a rapid decrease of membrane potential and resistance. Voltage clamping revealed a marked depression of peak transient current with little or no effect in the late outward current.

3. Mammalian phospholipase A was found ineffective in changing either the resting or active membrane properties.

4. Phospholipase C from Bacillus cereus led to a strong hyperpolarization and a fall in membrane resistance. Voltage clamping revealed a marked increase in the late outward current.

5. Neurones injected with Clostridium welchii phospholipase C manifested a several-fold rise in resting membrane resistance as well as a tendency to slight hyperpolarization.

6. All enzymes were ineffective when externally applied.

7. It is tentatively concluded that the internally applied phospholipases affect specific ionic permeabilities both in the resting and active excitable membrane. Various mechanisms by which the differing actions of enzymes of the same type could be explained are discussed.