Influence of ryanodine receptor agonist and antagonist on development of potassium contracture in phasic (twitch) and tonic fibers of frog skeletal muscle

We compared the influence of external calcium and the inhibitor (dantrolene) and activator (4-chloro-m-cresol) of ryanodine-sensitive Ca channels of the sarcoplasmic reticulum on the characteristics of potassium contracture in phasic and tonic frog skeletal muscle fibers. The duration of contracture in tonic fibers, as contrasted to the phasic ones, is not limited by the presence of Ca super(2+). The tonic contractile response is virtually indifferent to dantrolene and is much less sensitive to chlorocresol than the phasic one (1 mM vs. 0.25 mM). In phasic fibers, the K super(+) contracture on the chlorocresol background is quite similar in amplitude and dynamics to that in control, whereas tonic fibers exhibit response summation without relaxation upon removal of excessive K super(+). One can suggest that in phasic fibers the Ca super(2+) influx can directly create a level sufficient to sustain contraction, while in tonic fibers its effect is mediated by Ca-dependent activation of the beta isoform of the ryanodine-sensitive channel.