Contribution of two types of calcium channels to membrane conductance of single myocytes from guinea-pig coronary artery

Abstract

1. Whole-cell and single-channel current recordings were used to study calcium channels in single smooth muscle cells isolated from guinea-pig coronary artery. Potassium currents were blocked by intracellular Cs+ ions. 2. Whole-cell currents were recorded with 10 mM-barium in the bath. Step pulses of 200 ms from a holding potential of -90 mV activated calcium channel current when the depolarization reached -55 to -50 mV. All cells showed a current component which inactivated slowly and incompletely. About half of the cells showed an additional current component with a rapid inactivation time course. Both components were abolished by Cd2+ ions (1 mM) and were reduced by changing the holding membrane potential to -40 mV or by addition of 0.1 mM-Ni2+. 3. Single calcium channel currents were measured in cell-attached patches with 110 or 10 mM-Ba2+ as a current carrier. Two different types of single calcium channel activity were observed. 4. A high-conductance calcium channel was activated near -30 mV with 110 mM-Ba2+ and this threshold was changed to about -60 mV with 10 mM-Ba2+ in the patch pipette. The conductance was 28.0 +/- 1.5 pS (mean +/- S.D.) in 110 mM-Ba2+ and 16.0 +/- 1.0 pS in 10 mM-Ba2+. Dependence of the conductance on the concentration of Ba2+ in the patch pipette followed a Langmuir curve: the apparent dissociation constant of Ba2+ was 8 mM. It was concluded that this channel type corresponds to L-type calcium channels. 5. Another calcium channel was found in these experiments. It had a low conductance and was activated at around -50 mV with 110 mM-Ba2+, and this threshold was shifted to about -70 mV when 10 mM-Ba2+ was the charge carrier. The conductance of this calcium channel was 7.5 +/- 0.6 pS in 110 mM-Ba2+ and 5.5 +/- 1.0 pS in 10 mM-Ba2+. With 10 mM-Ba2+, inactivation of the mean current was slow at potentials -70 to -50 mV, but fast and complete (within 100 ms) at more positive potentials. It was concluded that this type of calcium channel corresponds to T-type calcium channels. 6. With the membrane potential continuously held at -50 to -40 mV (with 10 mM-Ba2+ in the patch pipette), i.e. close to the usual resting potential of these cells, T-type calcium channels were completely inactivated whereas rare openings of L-type calcium channels could be detected.(ABSTRACT TRUNCATED AT 400 WORDS)