[Acetylcholine contracture of the fast muscle fibers in the lamprey, Lampetra fluviatilis].

Fast fibers of m. longitudinalis linguae respond to Ach by a transient contracture with a half-decay period of 3-5 sec. The threshold concentration of Ach is approximately 10(-7) g/ml. Ach contracture is based on even depolarization of the whole muscle membrane. Threshold level of the MP for the onset of contracture lies between --50 and --40 mV. In the presence of Ach depolarization decreases twofold within 40-70 sec. Relaxation is not due to the decrease of depolarization. Contraction--Ach concentration curve has a small slope; it reaches maximum at a concentration 10(-4) g/ml, which corresponds to the MP ca. -10mV. Equilibrium Ach potential is significantly shifted to depolarization as compared to that in frog muscles, being equal to +1+/-1.8 mV.