1. Localized calcium release events (calcium sparks) were studied in voltage-clamped cut twitch fibres of Rana temporaria. 2. A histogram of thousands of spontaneous sparks displayed a monotonically decreasing amplitude distribution from the low to the high limit of > 7 ΔF/F0 units. 3. Several effects of low micromolar concentrations of ryanodine (0.4-2 μM) on spontaneous sparks, reproducing the agent's effects on single ryanodine receptor channel current in bilayers, were observed collectively for the first time in live fibres, namely (a) increases in spark frequency followed by (b) conversions of sparks into steady glows lasting tens of seconds, (c) occasional interruptions of the glows by brief gaps of darkness, and (d) abolition of sparks at the locations of the glows. The glow could reflect the incessant Ca2+ flux through a single (or a few) calcium release channel locked in the semi-open state, which was allowed to make occasional transitions to the closed state but not to the fully open state. 4. Higher concentrations of ryanodine (≥ 20 μM) suppressed the spontaneous sparks effectively and permanently, presumably by deactivating the ryanodine receptors. 5. Depolarization-evoked sparks elicited with small pulses had higher frequencies and larger amplitudes than spontaneous sparks and were abolished by both concentrations of ryanodine. 6. With 1-2 μM ryanodine, however, a uniform non-sparking calcium release persisted during the pulse, with the globally averaged increase in fluorescence intensity being about half that of the control. A possible origin of this non-sparking release may be related to the structural coupling between the voltage sensors and the ryanodine receptors that can exist only in live fibres but not in the bilayer preparation.
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