The M. omohyoideus of the mouse as a convenient mammalian muscle preparation. A study of junctional and extrajunctional acetylcholine receptors by noise analysis and cooperativity.

Muscles from cats, rats, guinea pigs and mice have been investigated as preparations for visualizing mammalian neuromuscular junctions with the aid of Nomarski interference optics. The M. omohyoideus of the mouse was found to be 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 degrees 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 degrees C) to 45.6 +/- 3.9 pS (34--39 degrees C) in a very small temperature range around 25.5 degrees C. The mean channel lifetime tau was 0.3 ms at 39 degrees C and 1.0 ms at 23 degrees C.