The effects of propofol and enflurane on single calcium channel currents of guinea‐pig isolated ventricular myocytes

Abstract

The effects of the anaesthetics, propofol (100 μm) and enflurane (3%, 1.46 mm), on single L type calcium channel currents were investigated in single myocytes isolated from guinea‐pig ventricles. Channel activity was recorded from membrane patches by use of the ‘cell‐attached’ patch‐clamp technique (pipette solution containing 110 mm BaCl2, 5 μm Bay K 8644, 5 μm HEPES, pH 7.4; temperature 36°C). Channel conductance was calculated from the slope of the relationship between single channel current and membrane potential during step depolarizations to activate the channel over a range of approximately − 20 to + 20 mV. Neither propofol (6 cells) nor enflurane (7 cells) caused any significant reduction in channel conductance. Both propofol (7 cells) and enflurane (9 cells) decreased the probability of the channel being open during depolarizations to + 10 mV (measured from histograms of the fraction of time spent by the channel at different current levels, taking areas under the Gaussian curves fitted to the open and closed components of the distributions to represent the proportion of time spent in the two states). A fraction of the current traces showed no detectable channel openings in response to step depolarizations to + 10 mV. Both propofol and enflurane significantly increased the fraction of silent traces. Transitions across a threshold halfway between the open and closed levels were used to define periods spent in the open and closed states. Both propofol (7 cells) and enflurane (9 cells) reduced the mean open times and increased the mean closed times of the calcium channel. Histograms were plotted showing the distributions of times spent by the channels in the open and closed states. Two exponentials were fitted to the open and closed time distributions. Both propofol (7 cells) and enflurane (9 cells) shortened both time constants fitted to the open times and lengthened both time constants fitted to the closed times. It is concluded that both propofol and enflurane appear to alter the kinetics of opening and closing of calcium channels to favour shut channels without altering channel conductance. This effect would be expected to result in a reduction of the macroscopic calcium current and thus contribute to the negative inotropic action of these anaesthetics. 1994 British Pharmacological Society