Association of laminin and other basement membrane components with regions of high acetylcholine receptor density on cultured myotubes.

The distribution of immunoreactivity to basement membrane components in cultures of rat skeletal myotubes was compared to acetylcholine receptor distribution by fluorescence microscopy. Laminin occurred in patches on the myotube surface, and most laminin patches coincided or overlapped with acetylcholine receptor aggregates. Almost all receptor aggregates coincided with laminin patches. Most of the heparan sulfate proteoglycan and fibronectin was associated with non-muscle cells, but some patches coincided with receptor aggregates on myotubes. In cultures treated with l-ascorbate, collagen types IV and V covered much of the myotube surface and receptor aggregates often coincided with intense collagen patches. When receptor aggregation was induced by treatment of cultures with soluble neural factors, the newly formed receptor aggregates coincided with laminin patches. The results suggest that each of the basement membrane components studied has a distinct distribution pattern in primary rat muscle cell cultures. The striking association of laminin immunoreactivity with receptor aggregates, together with the enhancement of receptor aggregation by laminin reported previously (Vogel et al., J. Neurosci. 3, 1058-1068, 1983) suggests that laminin could have a role in the organization of acetylcholine receptors on developing muscle fibers.