Ryanodine as a trigger of tension oscillations in rat ventricular muscle

In rat ventricular muscles, ryanodine (10-30 nM) evoked tension oscillation in the relaxation phase of an isometric twitch (relaxation oscillations) and an increase of tonic tension (ryanodine contracture). Both events were more pronounced in Ca2+-loaded rat muscles due to the addition of 0.5 microM adrenaline, an increase in the Ca2+ concentration in the solution and high muscle activity. The ryanodine-induced tension oscillations were comparable to those triggered by caffeine in this species. In both cases, blockers of the release of Ca2+ from the sarcoplasmic reticulum, namely tetracaine and dantrolene, abolished the relaxation oscillations and the ryanodine contracture. The results suggest that the ability of low concentrations of ryanodine to facilitate the release of Ca2+ from the sarcoplasmic reticulum, as shown recently in biochemical experiments, makes a direct contribution to triggering the relaxation oscillations and the ryanodine contracture in intact ventricular muscles. The Ca2+ load of the sarcoplasmic reticulum appears to be essential for the manifestation of the ryanodine Ca2+-releasing activity in rat ventricular muscles.