Crude brain extract (100,000 g supernate from newborn or fetal @entity35 brain homogenate) was studied for its effects on the number and distribution of @entity934 receptors (AChRs) on myotubes of the L5 cloned myogenic cell line and compared to that of @entity35 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 @entity9176 site density (2-5-fold) and with an increase in @entity9176 clustering. Clustering was manifested by both an increase in the number of @entity9176 clusters and in the ratio of receptor site density within clusters relative to that between clusters. The increase in average @entity9176 site density was shown to be due to an increase in the rate of @entity9176 insertion into the surface membrane with little change in the rate of receptor degradation. As also previously reported, the @entity35 myotubes had a similar clustering response but only a very slight (approximately 1.2-fold) increase in average @entity9176 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 @entity35 primary cells are sensitive primarily to the high molecular weight factors.

Differential responses of L5 and XXXX primary muscle cells to factors in @entity35 brain extract.

@entity35 :: ('10116', 'Species') :: ['rat'] @entity9176 :: ('170945(Tax:10116)', 'Gene') :: ['AChR'] @entity934 :: ('MESH:D000109', 'Chemical') :: ['acetylcholine']
@entity35 :: (10116,Species) :: ['rat']
