M-OMOHYOIDEUS OF MOUSE AS A CONVENIENT MAMMALIAN MUSCLE PREPARATION - STUDY OF JUNCTIONAL AND EXTRAJUNCTIONAL ACETYLCHOLINE RECEPTORS BY NOISE-ANALYSIS AND COOPERATIVITY

Muscles from cats, rats, guinea pigs and mice were investigated as preparations for visualizing mammalian neuromuscular junctions with the aid of Nomarski interference optics. The M [Musculus] omohyoideus of the mouse was most convenient. Electrophysiological investigations showed that an endplate is normally surrounded by a population of perijunctional receptors. For junctional receptors in the endplate, a Hill coefficient of nH = 2.6 for acetylcholine was determined at 38.degree. C, decreasing to a value of 2.3 at room temperature. For both perijunctional and extrajunctional receptors (the latter occurring after denervation), the coefficient nH was 1.9. Noise analysis revealed a channel conductance .gamma. which changed abruptly from 22.4 .+-. 1.0 pS (10-23.degree. C) to 45.6 .+-. 0.9 pS (34-39.degree. C) in a very small temperature range around 25.5.degree. C. The mean channel lifetime .tau. was 0.3 ms at 39.degree. C and 1.0 ms at 23.degree. C.