Abstract
1. 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 microM fluo-3. Acquisition of
line-scan images on a laser scanning confocal microscope was synchronized
with measurement of Cd2+-sensitive Ca$^2+$ currents. In most
cells, individual Ca$^2+$ sparks were observed by reducing Ca$^2+$
current density with nifedipine (0.1-8 microM). 2. 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 microm (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 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. 4. 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.
- 10066927
- algorithms,
- aniline
- animal,
- blockers,
- cadmium,
- calcium
- calcium,
- channel
- channels,
- compounds,
- confocal,
- dyes,
- fluorescent
- gov't,
- heart
- in
- l-type,
- male,
- membrane
- microscopy,
- myocardium,
- nifedipine,
- non-u.s.
- p.h.s.,
- patch-clamp
- potentials,
- rats,
- research
- reticulum,
- s,
- sarcoplasmic
- signaling,
- sprague-dawley,
- support,
- techniques,
- u.s.
- ventricles,
- vitro,
- xanthenes,
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