Extremely slow inactivation of the ion channels formed by transfected α2 of L-type Ca2+ channelsof L-type Ca2+ channels |
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Authors: | V A Bouryi |
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Institution: | (1) Bogomolets Institute of Physiology, National Academy of Sciences of Ukraine, Kyiv, Ukraine;(2) The Laboratoire de Pharmacologie, Universite Catholique de Louvain, Bruxelles, Belgium |
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Abstract: | Barium currents through ion channels formed by α1-subunit of L-type Ca2+ channel (I
α1) were recorded from cultured chinese hamster ovary (CHO) cells. The cells were stably transfected with either a cardiac or
a smooth muscle (SM) variant of α1-subunit. TheI
α1 in both cases exhibited similar fast voltage-dependent activation kinetics and slow apparent inactivation kinetics. With
10 mM Ba2+ in the bath solution,I
α1 was activated at potentials more positive than −40 mV, peaked between 0 and +10 mV, and reversed at about +50 mV. In addition
to slow apparent inactivation of inward current, both subunits provided an extremely slow voltage-dependent inactivation at
potentials more positive than −100 mV, with half-maximum inactivation at −43.4 mV for cardiac and −41.4 mV for SM α1-subunits.
The onset of inactivation as well as recovery from this process were within a time range of minutes. The voltage dependence
of steady-state inactivation could be fitted by the sum of two Boltzmann's equations with slope factors of about 12 mV and
5 mV. A less sloped component has its midpoints at −75.6 and −63.7 mV, and a steeper component has its midpoints at −42.8
and −37.7 mV for cardiac and SM α1-subunits, respectively. Relative contribution of the steeper component was higher in both
subunits (0.86 and 0.66 for cardiac and SM subunits, respectively). For comparison, the inactivation curves for 5-sec-long
conditioning prepulses could be fitted by single Boltzmann's distribution with a 20 mV more positive midpoint and a slope
factor of about 13 mV. In contrast to the steady-state inactivation curves, they showed considerable overlap with the steady-state
activation curve. Our results reflect functional consequences of known sequence differences between α1-subunits of the cardiac
and SM L-type Ca2+ channels and could be used in structural modeling of Ca2+ channel gating. In addition, they show that depolarization-induced window current has a transient nature and decays with
the development of extremely slow inactivation. This is the first demonstration that slow inactivation of the L-type Ca2+ channel is an intrinsic property of its α1-subunits. |
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