Abnormalities of a key repolarizing cardiac potassium current, the fast transient outward potassium current,
I
to,f
, are associated with both heart failure and congenital arrhythmia syndromes. However, the precise role of
I
to,f
in shaping action potential waveforms remains unclear. This study was designed to define the functional role of the fast transient outward potassium current,
I
to,f
, in shaping action potentials in human iPSC-derived cardiomyocytes (iPSC-CMs). Most iPSC-CMs (29 of 43 cells) demonstrated spontaneous electrical activity, slow upstroke velocity (63±71 V/s), a wide range of action potential durations (APD90 = 860±722 ms) and heterogeneous action potential waveforms. Using dynamic current clamp, a modeled human ventricular inwardly rectifying K
+
current,
I
K1
, was introduced into iPSC-CMs, resulting in silencing of spontaneous activity, hyperpolarization of the resting membrane potential (RMP = -90±3 mV), increased peak upstroke velocity (dV/dt = 346±71 V/s) and decreased APD90 (420±211 ms) to values similar to those recorded in isolated adult human ventricular myocytes (RMP = -84±3 mV, dV/dt = 348±101 V/s and APD90 = 468±133 ms, all p>0.05). Importantly, a ventricular-like action potential waveform was observed in 25 of the 26 cells studied following the dynamic clamp addition of
I
K1
. Using these cells as a model of human ventricular myocytes, further dynamic current clamp experiments introduced a modeled human fast transient outward K
+
current,
I
to,f
, and revealed that increasing in the amplitude of
I
to,f
results in an increase in the phase 1 notch and a progressive shortening of the action potential duration in iPSC-CMs. Together, the experiments here demonstrate that combining human iPSC-CMs with the power of the dynamic current clamp technique to modulate directly and precisely the “expression” of individual ionic currents provides a novel and quantitative approach to defining the roles of specific ionic conductances in regulating the excitability of human cardiomyocytes.
α1-adrenergic stimulation increases ventricular action potential duration in the intact mouse heart
