1. The time courses of Ca2+ current and Ca2+ spark occurrence were determined in single rat ventricular myocytes voltage clamped with patch pipettes containing 0.1 μM fluo-3. Acquisition of line-scan images on a laser scanning confocal microscope was synchronized with measurement of Cd2+-sensitive Ca2+ currents. In most cells, individual Ca2+ sparks were observed by reducing Ca2+ current density with nifedipine (0.1-8 μM). 2. Ca2+ sparks elicited by depolarizing voltage-clamp pulses had a peak [Ca2+] amplitude of 289 ± 3 nM with a decay half-time of 20.8 ± 0.2 ms and a full width at half-maximum of 1.40 ± 0.03 μm (mean ± S.E.M., n = 345), independent of the membrane potential. 3. The time between the beginning of a depolarization and the initiation of each Ca2+ spark was calculated and data were pooled to construct waiting time histograms. Exponential functions n ere fitted to these histograms and to the decaying phase of the Ca2+ current. This analysis showed that the time constants describing Ca2+ current and Ca2+ spark occurrence at membrane potentials between -30 mV and +30 mV were not significantly different. At +50 mV, in the absence of nifedipine, the time constant describing Ca2+ spark occurrence was significantly larger than the time constant of the Ca2+ current. 4. A simple model is developed using Poisson statistics to relate macroscopic Ca2+ current to the opening of single L-type Ca2+ channels at the dyad junction and to the time course of Ca2+ spark occurrence. The model suggests that the time courses of macroscopic Ca2+ current and Ca2+ spark occurrence should be closely related when opening of a single L-type Ca2+ channel initiates a Ca2+ spark. By comparison with the data, the model suggests that Ca2+ sparks are initiated by the opening of a single L-type Ca2+ channel at all membrane potentials encountered during an action potential.
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