title: Cardiac calcium channels in planar lipid bilayers. L-type channels and calcium-permeable channels open at negative membrane potentials.
abstract: Planar lipid bilayer recordings were used to study Ca channels from bovine cardiac sarcolemmal membranes. Ca channel activity was recorded in the absence of nucleotides or soluble enzymes, over a range of membrane potentials and ionic conditions that cannot be achieved in intact cells. The dihydropyridine-sensitive L-type Ca channel, studied in the presence of Bay K 8644, was identified by a detailed comparison of its properties in artificial membranes and in intact cells. L-type Ca channels in bilayers showed voltage dependence of channel activation and inactivation, open and closed times, and single-channel conductances in Ba2+ and Ca2+ very similar to those found in cell-attached patch recordings. Open channels were blocked by micromolar concentrations of external Cd2+. In this cell-free system, channel activity tended to decrease during the course of an experiment, reminiscent of Ca2+ channel "rundown" in whole-cell and excised-patch recordings. A purely voltage-dependent component of inactivation was observed in the absence of Ca2+ stores or changes in intracellular Ca2+. Millimolar internal Ca2+ reduced unitary Ba2+ influx but did not greatly increase the rate or extent of inactivation or the rate of channel rundown. In symmetrical Ba2+ solutions, unitary conductance saturated as the Ba2+ concentration was increased up to 500 mM. The bilayer recordings also revealed activity of a novel Ca2+-permeable channel, termed "B-type" because it may contribute a steady background current at negative membrane potentials, which is distinct from L-type or T-type Ca channels previously reported. Unlike L-type channels, B-type channels have a small unitary Ba2+ conductance (7 pS), but do not discriminate between Ba2+ and Ca2+, show no obvious sensitivity to Bay K 8644, and do not run down. Unlike either L- or T-type channels, B-type channels did not require a depolarization for activation and displayed mean open times of greater than 100 ms.
authors.affiliation: Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06510.
authors.forename: R L
authors.initials: RL
authors.lastname: Rosenberg
authors.forename: P
authors.initials: P
authors.lastname: Hess
authors.forename: R W
authors.initials: RW
authors.lastname: Tsien
mesh_terms: D000818:Animals; D001464:Barium / Q000378:metabolism / Q000502:physiology; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D015220:Calcium Channels / Q000145:classification / Q000378:metabolism* / Q000502:physiology; D002417:Cattle; D004553:Electric Conductivity; D006706:Homeostasis; D007700:Kinetics; D008051:Lipid Bilayers; D008564:Membrane Potentials; D009206:Myocardium / Q000378:metabolism*; D010539:Permeability
chemical_list: D015220:Calcium Channels; D008051:Lipid Bilayers; D002104:Cadmium; D001464:Barium; D002118:Calcium