Abstract
Ca(2+) current (I(Ca)) recovery from inactivation is necessary for
normal cardiac excitation-contraction coupling. In normal hearts,
increased stimulation frequency increases force, but in heart failure
(HF) this force-frequency relationship (FFR) is often flattened or
reversed. Although reduced sarcoplasmic reticulum Ca(2+)-ATPase function
may be involved, decreased I(Ca) availability may also contribute.
Longer action potential duration (APD), slower intracellular Ca(2+)
concentration (Ca(2+)(i)) decline, and higher diastolic Ca(2+)(i)
in HF could all slow I(Ca) recovery from inactivation, thereby decreasing
I(Ca) availability. We measured the effect of different diastolic
Ca(2+)(i) on I(Ca) inactivation and recovery from inactivation
in rabbit cardiac myocytes. Both I(Ca) and Ba(2+) current (I(Ba))
were measured. I(Ca) decay was accelerated only at high diastolic
Ca(2+)(i) (600 nM). I(Ba) inactivation was slower but insensitive
to Ca(2+)(i). Membrane potential dependence of I(Ca) or I(Ba) availability
was not affected by Ca(2+)(i) <600 nM. Recovery from inactivation
was slowed by both depolarization and high Ca(2+)(i). We also used
perforated patch with action potential (AP)-clamp and normal Ca(2+)
transients, using various APDs as conditioning pulses for different
frequencies (and to simulate HF APD). Recovery of I(Ca) following
longer APD was increasingly incomplete, decreasing I(Ca) availability.
Trains of long APs caused a larger I(Ca) decrease than short APD
at the same frequency. This effect on I(Ca) availability was exacerbated
by slowing twitch Ca(2+)(i) decline by approximately 50\%. We conclude
that long APD and slower Ca(2+)(i) decline lead to cumulative inactivation
limiting I(Ca) at high heart rates and might contribute to the negative
FFR in HF, independent of altered Ca(2+) channel properties.
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