title: Dihydropyridine-sensitive skeletal muscle Ca channels in polarized planar bilayers. 1. Kinetics and voltage dependence of gating.
abstract: Rabbit skeletal muscle transverse tubule (T) membranes were fused with planar bilayers. Ca channel activity was studied with a "cellular" approach, using solutions that were closer to physiological than in previous studies, including asymmetric extracellular divalent ions as current carriers. The bilayer was kept polarized at -80 mV and depolarizing pulses were applied under voltage clamp. Upon depolarization the channels opened in a steeply voltage-dependent manner, and closed rapidly at the end of the pulses. The activity was characterized at the single-channel level and on macroscopic ensemble averages of test-minus-control records, using as controls the null sweeps. The open channel events had one predominant current corresponding to a conductance of 9 pS (100 mM Ba2+). The open time histogram was fitted with two exponentials, with time constants of 5.8 and 30 ms (23 degrees C). Both types of events were virtually absent at -80 mV. The average open probability (fractional open time) increased sigmoidally from 0 to a saturation level of 0.08, following a Boltzmann function centered at -25 mV and with a steepness factor of 7 mV. Ensemble averages of test-minus-control currents showed a sigmoidal activation followed by inactivation during the pulse and deactivation (closing) after the pulse. The ON time course was well fitted with "m3h" kinetics, with tau m = 120 ms and tau h = 1.2 s. Deactivation was exponential with tau = 8 ms. This study demonstrates a technique for obtaining Ca channel events in lipid bilayers that are strictly voltage dependent and exhibit most of the features of the macroscopic ICa. The technique provides a useful approach for further characterization of channel properties, as exemplified in the accompanying paper, that describes the consequences on channel properties of phosphorylation by cAMP dependent protein kinase.
authors.affiliation: Department of Physiology, Rush University School of Medicine, Chicago, Illinois 60612.
authors.forename: J
authors.initials: J
authors.lastname: Ma
authors.forename: C
authors.initials: C
authors.lastname: Mundiña-Weilenmann
authors.forename: M M
authors.initials: MM
authors.lastname: Hosey
authors.forename: E
authors.initials: E
authors.lastname: Ríos
mesh_terms: D000818:Animals; D001464:Barium / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D004553:Electric Conductivity / Q000187:drug effects; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects; D007700:Kinetics; D008051:Lipid Bilayers; D008433:Mathematics; D008564:Membrane Potentials; D008954:Models, Biological; D009132:Muscles / Q000502:physiology*; D009568:Nitrendipine / Q000494:pharmacology*; D011336:Probability; D011817:Rabbits
chemical_list: D015220:Calcium Channels; D008051:Lipid Bilayers; D001464:Barium; D009568:Nitrendipine