A comparative study of anaesthetic agents on high voltage activated calcium channel currents in identified molluscan neurons
Summary (2 min read)
INTRODUCTION
- General anaesthetics form a chemically diverse group of agents which have in common the property of inducing narcosis and analgesia to varying degrees.
- In vertebrate tissues, calcium channels have been classified by their electrophysiological and biochemical properties.
METHODS
- Briefly, intact central ganglia were pinned out in the perfusion chamber, ventral surface uppermost, and the sheath of connective tissue removed from the right parietal ganglion with fine ground forceps.
- The whole cell currents measured were Ba2+ currents rather than Ca2+ currents per se.
- CC-BY-NC-ND 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
- The anaesthetic-air mixture was bubbled at 1 litre per minute into a small volume (about 150 ml) of recording solution in a 250 ml flask and vented though a closed extraction system.
- BioRxiv preprint Optimum results were achieved when the preparation was allowed to equilibrate in recording solution 4−6 minutes before inserting microelectrodes into the selected cell.
1) The properties of whole cell calcium channel currents recorded from LYCs
- Two electrode voltage clamp recordings from LYCs in Lymnaea revealed large inward currents elicited by depolarizing the cells from a holding potential of −70 mV in the recording solution with Ba2+ as the charge carrier.
- The copyright holder for this preprintthis version posted December 18, 2020.
- .CC-BY-NC-ND 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
- The plots of IBa(max) for Itrans and Isus against holding potential fit quite well to a single sigmoidal shaped curve and there were no significant differences between the level of steady state inactivation of any of these currents at each holding potential.
2) The effect of anaesthetics on IBa
- Calcium channel currents recorded from LYCs were reversibly depressed in the presence of all the anaesthetics tested.
- There was also a tendency for activation rates to decrease and inactivation to increase instep with increasing anaesthetic concentration.
- At the highest concentrations used, activation rates were significantly decreased for halothane, isoflurane and pentobarbitone and inactivation significantly decreased for isoflurane sevoflurane and pentobarbitone (Table 2).
- Mean maximum slope of IBa traces, measured before and after (in) the slope turning point at peak amplitude.
- Significant statistical differences of slopes compared to control values (marked with an asterisk) were evaluated using Student’s two tailed, paired t-test at the 5% level, n = 5 for each data set.
3) The effect of anaesthetics on the current-voltage relationship of IBa
- To determine the effect of drugs on the current-voltage relationship, I−V plots were constructed for mean peak and end-pulse current recorded from several cells and normalized to control values of IBa(max).
- Similar families of I−V curves were produced for isoflurane, sevoflurane and pentobarbitone, which differed principally in their dose-response profile.
- The slope coefficients for peak currents show wider variation compared to those of end-pulse currents, where the regression slopes are divergent.
- Hence, the relative ability of the volatile anaesthetics to depress IBa are different when comparing peak and end-pulse IBa(max) regressions of dose-response.
- CC-BY-NC-ND 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
DISCUSSION
- The aim of these experiments was to compare the effects of a number of general anaesthetic agents on whole cell calcium channel currents recorded from identified light yellow cells, located in the central ganglia of Lymnaea stagnalis.
- The results show that these agents alter calcium channel function, represented as the dose-dependent depression of a calcium channel current, often associated with a decreased rate of activation and accelerated inactivation.
- IBa appears to have two HVA components, Isus and Itrans, which were indistinguishable by their steady state inactivation kinetics and the pharmacological probes used in this study.
- The copyright holder for this preprintthis version posted December 18, 2020.
- The authors results show that a variety of general anaesthetics, including the non-volatile pentobarbitone, produce dose-dependent depression of whole cell HVA Ca2+ channel current, IBa.
HIROTA, K., KUDO, M., KUDO, T., KITAYAMA, M. KUSHIKATA, T., LAMBERT, D.G. & MTSUKI, A. (2000).
- Barbiturates inhibit K+-evoked noradrenaline and dopamine release from rat striatal slices – involvement of voltage sensitive Ca2+ channels.
- Halothane blocks low-voltage-activated calcium current in rat sensory neurons.
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"A comparative study of anaesthetic ..." refers background in this paper
...They have become the prototypes that form the basis of a widely accepted system of calcium channel classification [11-13]....
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Q2. What have the authors stated for future works in "A comparative study of anaesthetic agents on high voltage activated calcium channel currents in identified molluscan neurons" ?
( Haung et al, 2010 ), but further work will be required to demonstrate this with certainty. It has been shown, using the same model system, that a calcium-dependent pseudoplateau of action potentials is abbreviated in the presence of halothane ( Winlow et al., 1982 ; Winlow et al., 1989 ; Winlow et al., 1992 ). Contrasting with the findings of this study and those of Yar and Winlow ( 2016 ), in which volatile anaesthetics produce little or no displacement of the I−V curve along the voltage axis, DHPs tend to shift calcium channel inactivation to more negative potentials. Conversely, volatile anaesthetic induced depression of Ca2+ influx through L-type and DHP insensitive voltage operated channels is not relieved by increasing [ Ca2+ ] out ( Gross & Macdonald, 1988 ), indicating that anaesthetic action is independent of changes in surface potential.