Does the small conductance Ca2+-activated K+ current (ISK) flow under physiological conditions in rabbit and human atrial isolated cardiomyocytes?

Giommi, Alessandro (2021) Does the small conductance Ca2+-activated K+ current (ISK) flow under physiological conditions in rabbit and human atrial isolated cardiomyocytes? PhD thesis, University of Glasgow.

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Abstract

Background: Small conductance Ca2+-activated K+ current (ISK) may change cardiac atrial action potentials (AP) in response to altered [Ca2+]i; a potential therapeutic target for treating atrial fibrillation (AF). However, the contribution of ISK to atrial APs under physiological conditions is unclear. Furthermore, ISK may be enhanced in ventricles by heart failure, but whether by [Ca2+]i elevation in the non-failing ventricle is unknown. Aims: To test whether ISK flows under normal or increased global Ca2+]i, or with sub-sarcolemmal [Ca2+]i increase from APs in human and rabbit atrial cells. Also, to test an ISK blocker, ICAGEN (ICA), on rabbit left ventricular (LV) ion currents under [Ca2+]i elevation from Na+/Ca2+-exchanger (INa/Ca) stimulation. Methods: Myocytes were isolated enzymatically from hearts removed from anaesthetised rabbits, and from atrial tissues from consenting patients undergoing cardiac surgery. Whole-cell patch clamp (37°C) was used to record ion currents and APs (at 1, 2 or 3Hz), with [Ca2+]i measured using Fura-2. Results: A positive control tested stability/timing of K+ current (IK1) block: Ba2+(0.5 mM) significantly and reversibly decreased inward IK1 (at-115 mV) in 94% of LV cells, from -38.9±5.9 to -12.9±4.5 pA/pF (by 67%), and in 92% of atrial cells, by 43% (P<0.05, mean±SE, t-test, n=16-25 cells, 11-16 rabbits). Atrial ISK was investigated, under increasing [Ca2+]i (100-500 nM; with 5 mM BAPTA), with (100 nM) and ICA (1 µM). Neither drug affected inward or outward current (P>0.05) at any [Ca2+]i, in rabbit or human (5-26 cells, 7-11 rabbits, 3-4 patients). APs recorded at 1 Hz (rabbit) were prolonged by 4-AP (ITO blocker; positive control): action potential depolarization at 30% repolarization (APD30) by 72%, at 70% repolarization (APD70) by 31%. By contrast, ICA (1 µM) had no effect on APD30-90, maximum diastolic potential (MDP), or Vmax, in human or rabbit. ICA at 10 µM (non-specific) increased APD70-90 vs time-matched controls. At 2 or 3 Hz, 1 µM ICA again had no effect on APs. In rabbit LV cells, stimulating INa/Ca increased [Ca2+]i (up to 2.8 µM) and inward /outward currents. ICA (1 µM) had no effect on [Ca2+]i or currents, whereas subsequent NiCl2 (10 mM; INa/Ca blocker) decreased them. By contrast, ICA 10 µM decreased outward (by 35%) and inward (49%) current, and [Ca2+]i (77%), with no effect of subsequent NiCl2. Conclusions: In rabbit and human atrial isolated myocytes, ISK may not flow under physiological conditions, nor during short bursts of supra-physiological stimulation, so atrial ISK activation (and thus its potential pharmacological inhibition during AF) may require changes to cellular electrophysiology or cell signalling systems to develop a sensitivity to ISK iii block. Furthermore, in non-failing LV myocytes, 1 µM ICA-sensitive ISK may not be activated by [Ca2+]i-elevation, and high ICA conc. may inhibit INa/Ca.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: Q Science > QP Physiology
Colleges/Schools: College of Medical Veterinary and Life Sciences > Institute of Cardiovascular and Medical Sciences > Cardiovascular Science
Supervisor's Name: Workman, Dr. Antony J.
Date of Award: 2021
Depositing User: Dr. Alessandro Giommi
Unique ID: glathesis:2021-81927
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 15 Jan 2021 14:24
Last Modified: 15 Jan 2021 14:40
Thesis DOI: 10.5525/gla.thesis.81927
URI: http://theses.gla.ac.uk/id/eprint/81927

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