Distribution and density of α-bungarotoxin binding sites on innervated and noninnervated Xenopus muscle cells in culture

Abstract

The distribution and density of α-bungarotoxin (α-BT) binding sites on Xenopus muscle cells in culture by autoradiography using 125I-α-BT were examined. In muscle cells grown alone α-BT binding sites were fairly uniformly distributed over the entire surface with a mean density of 104/μm 2 (background density). Occasionally, spots of higher density were observed (“hot spots”) where the mean density was 890/μm 2. The addition of neural tube cells did not change the background density. Similarly in the majority of cases medium contained with neural tube cells did not affect the density of α-BT binding sites. Previous findings that the background acetylcholine sensitivity of muscle cells increased in the presence of neural tube cells (by approximately 50%) or in conditioned medium (by approximately 70%), therefore, are not likely due primarily to an increase in the acetylcholine receptor (AChR) density. In cocultures of nerve and muscle cells regions of high α-BT binding sites were occasionally associated with the path of neurites. In such regions the density of α-BT binding sites was estimated to be approximately 1000/μm 2. However, even in these cells the density at non-nerve contacted regions was not different from that in muscle cells cultured alone. Whether the increase in AChR density at the junctional area is sufficient to explain a previous observation of a fivefold increase in the amplitude of spontaneous synaptic potentials during the process of AChR accumulation is discussed.