Overexpression of the Large-Conductance, Ca2+-Activated K+ (BK) Channel Shortens Action Potential Duration in HL-1 Cardiomyocytes

Abstract

Long QT syndrome is characterized by a prolongation of the interval between the Q wave and the T wave on the electrocardiogram. This abnormality reflects a prolongation of the ventricular action potential caused by a number of genetic mutations or a variety of drugs. Since effective treatments are unavailable, we explored the possibility of using cardiac expression of the large-conductance, Ca2+-activated K+ (BK) channel to shorten action potential duration (APD). We hypothesized that expression of the pore-forming α subunit of human BK channels (hBKα) in HL-1 cells would shorten action potential duration in this mouse atrial cell line. Expression of hBKα had minimal effects on expression levels of other ion channels with the exception of a small but significant reduction in Kv11.1. Patch-clamped hBKα expressing HL-1 cells exhibited an outward voltage- and Ca2+-sensitive K+ current, which was inhibited by the BK channel blocker iberiotoxin (100 nM). This BK current phenotype was not detected in untransfected HL-1 cells or in HL-1 null cells sham-transfected with an empty vector. Importantly, APD in hBKα-expressing HL-1 cells averaged 14.3 ± 2.8 ms (n = 10), which represented a 53% reduction in APD compared to HL-1 null cells lacking BKα expression. APD in the latter cells averaged 31.0 ± 5.1 ms (n = 13). The shortened APD in hBKα-expressing cells was restored to normal duration by 100 nM iberiotoxin, suggesting that a repolarizing K+ current attributed to BK channels accounted for action potential shortening. These findings provide initial proof-of-concept that the introduction of hBKα channels into a cardiac cell line can shorten APD, and raise the possibility that gene-based interventions to increase hBKα channels in cardiac cells may hold promise as a therapeutic strategy for long QT syndrome.

Case Report on: Very Early Afterdepolarizations in HiPSC-Cardiomyocytes—An Artifact by Big Conductance Calcium Activated Potassium Current (Ibk,Ca)

“…Support for this idea came from a study describing the electrophysiological function of BK Ca in HL-1 cells by viral overexpression, a cell line derived from a murine atrial tumor. BK Ca overexpression reduced the very short AP of this murine model by 50% (APD 90 from 30 to 14 ms) and was proposed as a potential genetic therapy to reduce AP duration (APD) of the LQT syndrome [13]. In contrast to the experiments in HL-1 cells, we observed that in hiPSC-CMs, the presence of BK Ca induces oscillations in the early plateau phase and no speed-up in the final repolarization [13].…”

“…Recently, overexpression of non-cardiac BK Ca in CMs was proposed as a treatment for LQTS [13], since BK Ca is a hyperpolarizing channel, which might shorten human AP. Support for this idea came from a study describing the electrophysiological function of BK Ca in HL-1 cells by viral overexpression, a cell line derived from a murine atrial tumor.…”

“…BK Ca is widely expressed in the human body, mainly in neural cells, blood vessels, kidney, but not in in cardiomyocytes. Recently, artificial expression of BK Ca was shown to be able to shorten action potentials (APs) in murine CMs [13]. Consequently, expression of the non-cardiac BK Ca current in human CMs was proposed as a genetic therapy for the long QT syndrome.…”

Conserved Role of the Large Conductance Calcium-Activated Potassium Channel, K Ca 1.1, in Sinus Node Function and Arrhythmia Risk

“…This may be a contributing factor to the substantial reductions (∼70%) in atrial I CaL that are typically seen in patients with persistent AF. 42 While initially adaptive, increased expression of human K Ca 1.1 has been shown to shorten the action potential duration in HL-1 atrial cardiomyocytes, 43 and this could increase the propensity for initiation and maintenance of AF via re-entry mechanisms. The phosphodiesterase-III inhibitor, milrinone, is a heart failure therapy that has recently been shown to cause K Ca 1.1dependent dilatation of the pulmonary veins.…”

Conserved Role of the Large Conductance Calcium-Activated Potassium Channel, KCa1.1, in Sinus Node Function and Arrhythmia Risk

“…Taken together, these findings indicate that K Ca 1.1 excess, as well as K Ca 1.1 deficiency can have deleterious effects on cardiac function and have implications for administration of K Ca 1.1 activating drugs, which have been proposed as potential therapies for long QT syndrome and ischemiareperfusion injury. 43,46 Pharmacological manipulation of K Ca 1.1 activity warrants investigation as a new treatment option for cardiac arrhythmias and myopathies, but as for many other cardiac ion channels, there may be a narrow beneficial therapeutic range. lines showed increased peaks per event and higher event duration compared to controls (see Table VIII in the Data Supplement); Y axis: voltage (mV), X axis: time (ms).…”