Ionic mechanisms mediating the acetylcholine-elicited contraction in Aplysia buccal muscle

Ion channel mechanisms mediating acetylcholine (ACh)-elicited contractions in Aplysia buccal muscles were investigated using conventional force and membrane potential recording techniques and whole-cell voltage clamping of dissociated buccal muscle fibers. Previous work on buccal muscles of Aplysia showed that acetylcholine (ACh) depolarizes muscle fibers to, at most, -30 mV and that contraction is dependent upon extracellular calcium. In intact muscle, removal of either calcium or sodium from the extracellular medium reduced ACh-elicited depolarization. Absence of each ion reduced the depolarization approximately 50% at low [ACh], but at high [ACh], absence of sodium had a greater effect, while absence of calcium had a proportionally smaller effect on depolarization. Nifedipine (3 microM) reduced contraction by 50% with little accompanying change in depolarization. In whole cell voltage-clamping of isolated muscle fibers, ACh elicited an inward current at resting potential. The reversal potential for ACh-elicited current was -24 +/- 6 mV. The current-voltage curve was rectified, with a bend in the inward direction near -35 mV. In zero calcium medium, ACh-elicited current averaged 54% of control, and in zero sodium medium, 42% of control. Nifedipine reduced ACh-elicited current by less than 20%. Amiloride, an inhibitor of Aplysia buccal muscle contraction, reduced ACh-elicited current by 30-70%. A model based on the above results is that ACh depolarizes muscle fibers by opening channels with a reversal potential of about -30 mV that are permeable to calcium, sodium, and at least one other ion. Depolarization activates voltage-dependent calcium channels, enabling activator calcium to enter the fibers through both ACh-operated and voltage-dependent channels.