Soluble phosphatidylinositol phosphodiesterase in normal and denervated fast and slow muscles of the rat

Effect of soluble factors from nerve and muscle on alpha-bungarotoxin binding sites in isolated sarcolemmal membranes of the rat

1. Sarcolemmal membranes were isolated from normal and from denervated muscles and the specific binding of [125I]-alpha-bungarotoxin to the membranes was determined. 2. Cytosol prepared from either slow (soleus) or fast (extensor digitorum longus) muscle increased the toxin binding. Similar effects were seen with cytosol from muscles which had been denervated 5 to 7 days previously and with cytosol from sciatic nerve. A purified preparation of a bacterial phospholipase C also produced this effect. 3. Bathing medium in which normal or denervated muscles had been incubated, increased the toxin binding to normal membranes, but the normal bathing medium decreased the binding to membranes prepared from denervated muscles. 4. The possibility that pre-existing latent acetylcholine receptors can be activated by intracellular factors released from muscles is discussed.

Action of phospholipase C at the neuromuscular junction in rodent skeletal muscles

The isometric tension developed in response to diffuse application of acetylcholine was recorded in intact soleus muscles of the rat. Purified bacterial phospholipases of the C type, which hydrolyse either phosphatidylinositol or phosphatidylcholine, increased the acetylcholine contracture responses of the muscles. Sciatic nerve cytosol which had been purified over 8-fold with respect to phosphatidylinositol phospholipase C activity also increased these responses. The effect of phospholipase C on the miniature endplate potentials and neurally evoked endplate potentials was investigated in mouse diaphragm in vitro. The amplitude of both the miniature endplate potentials and the evoked endplate potentials was increased by the enzyme. The resting membrane potential, the effective input resistance and the frequency of miniature endplate potentials were not significantly altered by concentrations of the enzyme which increased the endplate responses. It is suggested that phospholipase C could have a trophic role at the neuromuscular junction.

Stimulated release of phosphatidylinositol phosphodiesterase in an isolated phrenic nerve-diaphragm preparation

Electrical stimulation of the phrenic nerve in an isolated nerve-diaphragm preparation resulted in the release of phosphatidylinositol phosphodiesterase into the organ bath. The released enzyme was Ca2+-dependent and exhibited two pH optima. The enzyme was released in response to nerve stimulation even in the presence of d-tubocurarine in concentrations that block neuromuscular transmission, and was not therefore released from the muscle as a consequence of its contractile activity. Phosphatidylinositol phosphodiesterase activity was determined in the soluble cytosol fractions prepared from different regions of skeletal muscles and from normal peripheral nerves and nerves that were degenerating after transection. The specific activity of the enzyme in the cytosol from the endplate-rich region of the diaphragm was significantly greater than that in cytosol from either the endplate-free region of the diaphragm or from the phrenic nerve. In degenerating nerve the activity of the enzyme was greater in the distal stump than in the proximal stump at 36 h after nerve section. Possible roles for released phosphatidylinositol phosphodiesterase at the neuromuscular junction are discussed.

Binding of phospholipids to the phosphatidylinositol transfer protein from bovine brain as studied by steady-state and time-resolved fluorescence spectroscopy

The phosphatidylinositol transfer protein isolated from brain, liver, heart and platelets was found to be present in two subforms which could be distinguished on the basis of the isoelectric points. In this study we have demonstrated that the two subforms isolated from bovine brain are due to the presence of either phosphatidylinositol or phosphatidylcholine in the lipid binding site of the protein. The transfer protein accommodates one phosphatidylinositol molecule in the binding site. The binding site for the sn-2 fatty acyl chain was investigated by incorporating in the transfer protein either phosphatidylinositol or phosphatidylcholine carrying a parinaroyl-chain attached at the sn-2 position. Time-resolved fluorescence spectroscopy revealed that the sn-2 fatty acyl chains for both phospholipids in the lipid-protein complex were completely immobilized (i.e., rotational correlation times of 17.4 ns for phosphatidylcholine and 16.3 ns for phosphatidylinositol). The similarity in correlation times suggests that the sn-2 fatty acyl chains of both phospholipids are accommodated in the same hydrophobic binding site of the protein.

Effect of phospholipase C on the binding of alpha-bungarotoxin to isolated plasma membranes of rat skeletal muscle

Sarcolemmal membranes were isolated from hindlimb muscles of the rat and the specific binding of [125I]alpha-bungarotoxin to the membranes was determined. Incubation of the membranes with a purified preparation of phospholipase C (ex Clostridium perfringens) increased the specific binding of the toxin (Bmax). The apparent dissociation constant (Kd) for the binding was unchanged by treatment with the enzyme. The possibility that latent acetylcholine receptors exist in muscle sarcolemma, and that these receptors can be activated by the enzyme, is discussed.

Inhibition of phosphatidylinositol phosphodiesterase activity in skeletal muscle by metal ions and drugs which block neuromuscular transmission

The calcium-dependent phosphatidylinositol phosphodiesterase activity of skeletal muscle cytosol was determined. The enzyme was inhibited by Zn2+, Cu2+ and Pb2+ ions but Mg2+ and Mn2+ were without effect. The antimalarial drugs chloroquine and the quinine and the aminoglycoside antibiotics gentamicin and neomycin all of which, like Zn2+, have been shown to block neuromuscular transmission, also inhibited the enzyme.