1. We investigated the currents through L-type Ca$^2+$ channels
when Ca$^2+$ (1-10 mM) was the charge carrier, as is the case
physiologically. 2. Na$^+$ was removed from both the external
and internal solutions to eliminate currents through Na$^+$ channels
and Na$^+$-Ca$^2+$ exchange. 3. From a holding potential
of -50 mV only L-type channels were available to open with depolarization.
Macroscopic L-type currents were maximal during depolarizing pulses
to +10 mV (peak current density of 4.7 +/- 0.3 nA nF-1). 4. During
depolarizing steps as long as 180 ms, the decay of current through
L-type channels was incomplete, in contrast to that of T-type current.
5. Unitary currents recorded with comparable ionic conditions and
voltage protocols exhibited a single-channel conductance of 6.9 pS
in 10 mM Ca$^2+$. Ensemble average currents reproduced accurately
the features of whole-cell L-type current, including the maintained
component. 6. Convolution analysis was employed to clarify the single-channel
basis of the complex current waveform of L-type channels. First openings
underlie the peak, while the maintained pedestal is generated by
multiple re-openings. As with T-type channels, single openings are
brief and contribute little to the time course of the average current.
7. The prominent maintained component of macroscopic and ensemble
average L-type current cannot be explained by simple Markov models
in which current decay reflects the progressive entry of channels
into an absorbing inactivated state. 8. We considered the possibility
that the maintained component of current arises from the existence
of multiple distinct gating patterns, one of which lacks inactivation.
Individual sweeps were sorted among three patterns of gating (no
openings, active-early and active-late). Patterns of activity are
not randomly distributed; instead, they tend to cluster over time.
9. Most of the maintained current is attributable to the 'active-late'
pattern of gating. Considered separately, this pattern can be well
described by a simple Markov chain lacking an inactivated state.
The 'active-early' gating pattern accounts entirely for the initial
current transient, and for about one-third of the maintained component;
thus, inactivation, even when present, must be reversible rather
than absorbing. 10. The unitary current amplitudes and peak open
probabilities measured for single L-type channels, when compared
to the average macroscopic L-type current density, predict 170 functional
channels per picofarad, or 28,000 L-type channels per typical ventricular
myocyte.(ABSTRACT TRUNCATED AT 400 WORDS)