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)