A Comparative Analysis of Contractures Induced by Acetylcholine and Choline in Twich and Tonic Frog Skeletal F ibres
A comparative analysis of contractile responses evoked by acetylcholine and external sodium removal in twitch and tonic frog skeletal muscle fibers has been performed. To elucidate the possible differences in the participation of various mechanisms of Ca homeostasis in the regulation of these contractile reactions, selective pharmacological agents: dantrolene, 4-<i>m</i>-chlorocresol, tetrodotoxin, and <i>d</i>-tubocurarine were used. It was found that, although Ca<sup>2+</sup> release from sarcoplasmic reticulum is a major source in the acetylcholine contracture for both types of muscle fibers, there are definite differences in the molecular structures of the a-isoform and/or the ratio of α- and β-isoforms of ryanodine-sensitive receptor/channels for twitch and tonic fibers. The substitution of Na<sup>+</sup> by choline in both twitch and tonic fibers evoked significant long-lasting contractile responses, which were sensitive to the inhibitory action of <i>d</i>-tubocurarine or dantrolene. The application of acetylcholine against the background of choline induced contracture with an amplitude closely related to that in normal solution. These results suggest that choline activates acetylcholine receptor/channel insignificantly, but this type of ionic channels is the main mechanism of its penetration through the surface membrane.