Function of phospholipids in axons: depletion of membrane phosphorus by treatment with phospholipase C

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.

Membrane properties of Aplysia neurones intracellularly injected with phospholipases A and C

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.

The effect of phospholipases and proteases on the binding of gamma-aminobutyric acid to junctional complexes of rat cerebellum

A preparation enriched in junctional complexes, as judged by marker enzymes and electron microscopy, was prepared from rat cerebellum. The junctional complexes were incubated with gamma-amino [14C]butyric acid at 25degreesC for 10 min, using [3H]sucrose as a marker for entrapped space, Total binding was determined in the absence of, and non-specific binding in the presence of, and excess of unlabelled gamma-aminobutyric acid. The difference bewteen the two binding values, i.e. the specific binding, was saturable and reversible, and showed positive cooperativity with a Hill number of about 2. The specific binding was inhibited by N-methylbicuculline, picrotoxinine and imidazole-4-acetic acid, but not by curare, strychnine or L-2,4-diaminobutyric acid. The above compounds had little effect on the non-specipic binding, but addition of ethylenediaminetetraacetic acid decreased non-specific binding by 80%. Trypsin, pronase, phospholipase A2 (EC 3.1.1.4), lysolecithin and sodium dodecyl sulfate decreased binding. Phospholipase C (EC 3.1.4.3) increased the specific binding by 260%. Phospholipids competed with gamma-aminobutyric acid for binding, with phosphatidylethanolamine being more potent than phosphatidylcholine. These results lend support for Watkins' hypothesis that phosphatidylethanolamine competes with gamma-aminobutyric acid for binding to the receptor protein.

Penetration of phospholipases A2 and C into the squid (Loligo pealii) giant axon

Following 1 h exposure, the level of phospholipase A2 penetration into the axoplasm of the squid giant axon was 107 to 350% of that in the external media; corresponding values for phospholipase C were 18 to 31%. Phospholipases can therefore be used to study phospholipid function in axons since they can penetrate through connective tissue and Schwann cell to reach the axolemma.

The action of phospholipases on the inner and outer surface of the squid giant axon membrane

1. The effects of phospholipases on resting potential, action potential and membrane ionic conductances of squid giant axons have been studied by means of internal perfusion and voltage clamp techniques. The sample of phospholipases used exhibited both phospholipase A and B activities.2. When applied externally, phospholipases had no effect on the resting and action potentials at a concentration of 100 mug/ml.3. When applied internally, the enzymes effectively suppressed the amplitude and maximum rate of rise of the action potential at a concentration of 100 mug/ml. The resting potential started decreasing appreciably only after the action potential had been suppressed to a considerable extent.4. Under voltage clamp conditions, the peak transient current was suppressed following internal perfusion of phospholipases, 150 mug/ml. The steady-state current began to decrease, and the leakage current to increase only after the transient current had been suppressed to a considerable extent.5. The time course of sodium inactivation was not significantly slowed by internal perfusion of the enzymes.6. The curve relating the peak transient conductance to the membrane potential was shifted in the direction of depolarization after internal perfusion of phospholipases. No appreciable shift of the steady-state conductance curve was observed.7. These experimental results are discussed in connexion with the possible role of membrane phospholipids in the conductance changes associated with excitation.