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Potassium Channels of the Heart

Kirstine Callø 

Summary

Every heart beat is governed by an electrical impulse that spreads throughout the cardiac muscle and results in a coordinated contraction. The initiation and propagation of the electrical impulse are dependent on activation of ion channels, and even slight changes in ion channel function may cause arrhythmias. These changes may be either inherited or acquired
Mutations in the genes encoding delayed rectifier potassium channels were identified in two families with hereditary long QT syndrome (LQTS) and the mutations characterized using electrophysiological techniques and confocal microscopy.
In a German family, a C-terminal mutation in the KCNQ1 gene, M520R, was identified. Co-expression experiments in both CHO cells and Xenopus laevis oocytes revealed a 50% reduction in current compared to wild-type KCNQ1 channel alone. Immunofluorescent studies of transfected COS-1 cells indicated that the M520R mutant channel was retained in the endoplasmic reticulum (ER).
Compound mutations in the KCNQ1 and HERG genes were found in a British family and characterized. Voltage-clamp experiments in Xenopus laevis oocytes did not reveal any functional deviation of the HERG_R328C channel phenotype, whereas the KCNQ1_R591H mutation resulted in a severe reduction of current. Immunofluorescent studies indicated that this was due to retention of the mutant in the ER. Co-expression of KCNQ1_R591H with wild-type KCNQ1 channel resulted in a reduction of current by approximately 50% compared to wild-type alone. Co-expression of KCNQ1 and HERG wild-type or mutant channels indicated that the two channels did not affect each other functionally.
Even though both KCNQ mutations resulted in a 50% reduction of current when co-expressed with wild-type KCNQ1 channels in oocytes or cell lines, carriers of the KCNQ1_R591H mutation in general have a more severe phenotype than carriers of the KCNQ1_M520R mutation.
Acquired changes in ion channel function may be a result of pharmaceutical treatment, disease or changes in the environment surrounding the cell.
Two studies on how cell volume changes affect cardiac ion channels are included in this thesis.
It has been shown that KCNQ1 current is increased upon hypoosmotic cell swelling of cardiomyocytes (Rees et al., 1995). The increase in current during hypoosmotic cell swelling results in a reduced action potential duration in isolated cardiomyocytes (Kocic et al., 2001). We wanted to establish whether the increase in KCNQ1 current during hypoosmotic cell swelling is involved in the regulatory volume decrease (RVD) in isolated rat cardiomyocytes. In the presence of the KCNQ1 blocker XE-991, the rate of the RVD response after cell swelling in isolated neonatal rat cardiomyocytes was strongly inhibited. The regulatory volume decrease response was found to be abolished by a disruption of the F-actin cytoskeleton.
In the final study, the effect of hypoosmotic cell swelling on cloned human HCN2 channels expressed in Xenopus oocytes was investigated. We found that HCN2 currents were increased during cell swelling, resulting in an increased inward current. The physiological relevance of this phenomenon is unclear; however it can be speculated if there is a pathophysiological relevance, for instance, during ischemia, cell swelling may activate ion channels and thereby cause ectopic activity.