Acetylcholine receptor distribution on myotubes in culture correlated to acetylcholine sensitivity.

Abstract

A linear relation, with a slope of 0.9 .+-. 0.2 on a log-log plot, was obtained between acetylcholine (ACh) sensitivity and .alpha.-bungarotoxin (.alpha.-BTX) binding site density in developing L6 and rat primary myotubes. ACh sensitivity was defined as g/Qn where g was conductance, Q was ACh charge and n was the Hill coefficient. For the myotubes n .apprx. 1.7, which was similar in value to that reported for adult systems. The linear relationship was compatible with an organization whereby each ion channel was always complexed with a fixed number of ACh receptors such that the dose-response characteristics of each such complex were independent of average ACh receptor density. Light microscope autoradiography showed that the .alpha.-BTX binding sites on L6 myotubes were uniformly distributed over the surface, while primary rat myotubes exhibited gradients and hot spots. EM autoradiography indicated that about 70% of the 125I-.alpha.-BTX label was on the surface of the myotubes. The .alpha.-BTX site density, after subtracting myoblast background, varied from 5-400 sites/.mu.m2 on different L6 myotubes and from 54-900 sites/.mu.m2 on primary rat myotubes, with occasional hot spots of 3000-4000 sites/.mu.m2. The conductance sensitivities varied from 10-4-2 .times. 10-2M.OMEGA.-1/nC1.7.