Nerve extract induces increase and redistribution of acetylcholine receptors on cloned muscle cells

Abstract

The effect of rat spinal cord explants and cell-free nerve extract on acetylcholine receptor site density and distribution was studied using 125I- and rhodamine-labeled .alpha.-bungarotoxin on L6, a cloned rat muscle cell line. Control L6 myotubes have a low and uniform distribution of acetylcholine receptors (20 .+-. 3 sites/.mu.m2). Addition of spinal cord explants caused an increase in average receptor site density of about 6 times on myotubes within 2 mm of the explant, while a smaller increase of 3 times was observed at distances greater than 5 mm. Formation of high-density patches of receptors was also stimulated. A diffusible substance originating from the explant was responsible for these changes. Cell-free homogenates of the CNS were prepared and apparently produced the same effects. The effect of the homogenate was not strongly dependent on age of the fetus from which the tissue was isolated and fetal liver had little or no effect. Active component(s) may be a protein(s) with a MW of about 100,000. Because the nerve homogenates make the L6 cells resemble primary muscle cultures, a common factor may be responsible for regulating the acetylcholine receptor in the 2 types of muscle culture. The normally acetylcholine receptor-poor L6 cells may provide a more sensitive assay for these factors than do primary muscle cultures.