Differential responses of L5 and rat primary muscle cells to factors in rat brain extract.

Crude brain extract (100,000 g supernate from newborn or fetal rat brain homogenate) was studied for its effects on the number and distribution of acetylcholine receptors (AChRs) on myotubes of the L5 cloned myogenic cell line and compared to that of rat primary cultures. Gamma counting, light autoradiography and scanning electron microscopic autoradiography were used. We found that the L5 cells responded to the brain extract with an increase in the average AChR site density (2-5-fold) and with an increase in AChR clustering. Clustering was manifested by both an increase in the number of AChR clusters and in the ratio of receptor site density within clusters relative to that between clusters. The increase in average AChR site density was shown to be due to an increase in the rate of AChR insertion into the surface membrane with little change in the rate of receptor degradation. As also previously reported, the rat myotubes had a similar clustering response but only a very slight (approximately 1.2-fold) increase in average AChR site density. The surface area of myotubes was also increased slightly (approximately 1.2-1.3-fold) by the brain extract. Autoradiography viewed by scanning EM was found to be very useful in illustrating the shape and distribution of the receptor clusters. After the brain extract was fractionated on Sephadex G-200, the fractions with greatest clustering activity could be separated from those causing predominantly an increase in receptor site density. Increased receptor site density was primarily produced by the low molecular weight fractions (less than 12 kD), whereas the strongest (but not exclusive) effect on clustering was produced by the high molecular weight fractions (greater than 140 kD). Furthermore, the two cell types assayed had different sensitivity to the different factors. L5 cells responded to both the high and low molecular weight factors while rat primary cells are sensitive primarily to the high molecular weight factors.