Extremely slow inactivation of the ion channels formed by transfected α1-subunit of L-type Ca2+ channels

Barium currents through ion channels formed by alpha(1)-subunit of L-type Ca2+ channel (I-alpha 1) 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 alpha(1)-subunit. The I-alpha 1 in both cases exhibited similar fast voltage-dependent activation kinetics and slow apparent inactivation kinetics. With 10 mM Ba2+ in the bath solution, I-alpha 1 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 alpha(1)-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 alpha(1)-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 alpha(1)-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 alpha(1)-subunit.