Acetylcholine induces voltage-independent increase of cytosolic calcium in mouse myotubes

Electrophysiological, biochemical, and Ca²⁺ imaging studies of cultured mouse myotubes were used to investigate whether the neurotransmitter acetylcholine causes an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i through activation of a second messenger system. Bath applications of acetylcholine to myotubes (i) elicited a significant membrane current even in a Na⁺-free Ca²⁺ medium, when the current was carried mainly by calcium ions; (ii) caused a rapid and transient cytosolic accumulation of inositol 1,4,5-triphosphate; (iii) evoked a conspicuous α-bungarotoxin-sensitive long-lasting [Ca²⁺]_i enhancement even in the presence of Cd²⁺; and (iv) transiently increased [Ca²⁺]_i when cells were equilibrated in a Ca²⁺-free atropine-containmg medium. We propose that, in addition to opening ion channels, the nicotinic action of acetylcholine on the muscle cell membrane increases [Ca²⁺]_i through activation of the inositol 1,4,5-trisphosphate second messenger system and mobilization of Ca²⁺ from intracellular stores. (.