Fei Hua PhD, David C. Johns PhD, Robert F. Gilmour Jr.
PhD,
Cornell University, Ithaca, NY; Johns Hopkins University.
Suppression of electrical alternans may be antiarrhythmic. Computer simulations
have shown that increasing IKr may be one means of suppressing alternans. We tested this hypothesis experimentally
using HERG overexpression to increase IKr in cultured adult canine ventricular
myocytes. Myocytes were infected with AdHERG, a recombinant adenovirus
containing the HERG gene driven by a Rous sarcoma virus, and IKr was measured
on day 2 or 3 in culture using perforated or whole cell patch-clamp. Action
potentials recorded at different pacing cycle lengths (CLs) were applied to
myocytes as the command waveforms. HERG infection markedly increased peak IKr
(from 0.54 +- 0.03 pA/pF in control (freshly isolated) myocytes to 3.60 +- 0.81
pA/pF in HERG-infected myocytes, n = 26 and 16, respectively). Rate-dependent
alterations of peak IKr were similar for freshly isolated and HERG-infected
myocytes: IKr increased when CL was reduced from 1000 to 500 ms, then decreased
as CL was reduced further. During alternans at
CL = 170 ms, normalized peak IKr was smaller for the long than for the short
action potential for HERG-infected (0.73 +- 0.02 vs. 0.85 +/- 0.02) and control
(0.81 +- 0.02 vs. 0.86 +- 0.01) myocytes, but the difference in peak IKr was
larger for HERG-infected myocytes. Peak IKr occurred at less negative voltages
in HERG-infected than in control myocytes (-34.8 +- 1.4 vs. -55.0 +- 0.9 mV; n
= 15 and 26, respectively), in association with shifts of the steady-state
voltage-dependent activation and inactivation curves to less negative
potentials. alternans did not occur in HERG-infected
myocytes at any CL. Although ICa decreased with time in culture, ICa was
reduced to the same extent in cultured myocytes with or without HERG
overexpression (from 1.12 +- 0.1 pA/pF in control (n = 10), to 0.88 +- 0.05
pA/pF in HERG-infected myocytes (n = 5) and to 0.74 +- 0.08 pA/pF in uninfected
cultured myocytes (n = 7). Moreover, uninfected cultured myocytes still
exhibited alternans. Therefore, the lack of alternans in HERG-infected myocytes was not caused by
reduced ICa. This study is the first to verify experimentally the idea that
increasing IKr might be a viable approach to suppressing electrical alternans.