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-subunit
1 Departments of Medicine and
Physiology, University of Wisconsin
Madison, Madison, Wisconsin
53792; and 2 Institute of
Molecular Cardiology, Department of Medicine, Johns Hopkins
University, Baltimore, Maryland 21205
The activity of native
L-type Ca channels can be facilitated by strong depolarizations. The
cardiac Ca channel 
1C-subunit was transiently expressed in human embryonic kidney (HEK-293) cells,
but these channels did not exhibit voltage-dependent facilitation. Coexpression of the Ca channel
1a- or
2a-subunit with the
1C-subunit enabled
voltage-dependent facilitation in 40% of cells tested. The onset of
facilitation in 1C + 1a-expressing HEK-293 cells was
rapid after a depolarization to +100 mV (
= 7.0 ms). The kinetic features of the facilitated currents were comparable to those
observed for voltage-dependent relief of G protein inhibition demonstrated for many neuronal Ca channels; however, intracellular dialysis with guanosine
5'-O-(2-thiodiphosphate) and
guanosine 5'-O-(3-thiotriphosphate) in the
patch pipette had no effect on facilitation. Stimulation of G
protein-coupled receptors, either endogenous (somatostatin receptors)
or coexpressed (adenosine A1
receptors), did not affect voltage-dependent facilitation. These
results indicate that the cardiac Ca channel
1C-subunit can exhibit
voltage-dependent facilitation in HEK-293 cells only when coexpressed
with an auxiliary -subunit and that this facilitation is independent
of G protein pathways.
patch clamp; electrophysiology; G protein; somatostatin receptor; adenosine receptor
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