Abstract
The time courses of Ca
2+
current and Ca
2+
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 Cd
2+
-sensitive Ca
2+
currents. In most cells, individual Ca
2+
sparks were observed by reducing Ca
2+
current density with nifedipine (0.1-8 μM).
Ca
2+
sparks elicited by depolarizing voltage-clamp pulses had a peak [Ca
2+
] 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.
The time between the beginning of a depolarization and the initiation of each Ca
2+
spark was calculated and data were pooled to construct waiting time histograms. Exponential functions were fitted to these histograms and to the decaying phase of the Ca
2+
current. This analysis showed that the time constants describing Ca
2+
current and Ca
2+
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 Ca
2+
spark occurrence was significantly larger than the time constant of the Ca
2+
current.
A simple model is developed using Poisson statistics to relate macroscopic Ca
2+
current to the opening of single L-type Ca
2+
channels at the dyad junction and to the time course of Ca
2+
spark occurrence. The model suggests that the time courses of macroscopic Ca
2+
current and Ca
2+
spark occurrence should be closely related when opening of a single L-type Ca
2+
channel initiates a Ca
2+
spark. By comparison with the data, the model suggests that Ca
2+
sparks are initiated by the opening of a single L-type Ca
2+
channel at all membrane potentials encountered during an action potential.