Correlated release of acetylcholine and protein from the neuromuscular junction

Transmitter release at adrenergic synapses is accompanied by release of chromogranin proteins, which are contained in synaptic vesicles. To determine if a similar phenomenon occurs at the neuromuscular junction, correlated release of acetylcholine (ACh) and protein was investigated using in vitro neuromuscular preparations (phrenic nerve-diaphragm muscle of the rat and mouse, sciatic nerve-sartorius muscle of Rana pipiens and R. catesbeiana). Nerve stimulation of curare-paralyzed preparations increased the rate of efflux of Lowry-reactive material relative to control values. Stimulus-specific responses outlasted the period of neural stimulation. Stimulus-induced release of Lowry-reactive material was correlated with ACh release since it was Ca2+ dependent and Mg2+ antagonized. Conditions that potentiate spontaneous ACh release also significantly increased the rate of efflux of Lowry-reactive material. Most of the Lowry-reactive material released with ACh is not a secretory product of synaptic vesicles because the amount released exceeds the contents of synaptic vesicles that undergo exocytosis. It is concluded that ACh release from the neuromuscular junction is accompanied by release of proteinaceous material that is not entirely derived from synaptic vesicles.