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.