Abstract
BACKGROUND: Increased activity of single ventricular L-type Ca(2+)-channels
(L-VDCC) is a hallmark in human heart failure. Recent findings suggest
differential modulation by several auxiliary beta-subunits as a possible
explanation. METHODS AND RESULTS: By molecular and functional analyses
of human and murine ventricles, we find that enhanced L-VDCC activity
is accompanied by altered expression pattern of auxiliary L-VDCC
beta-subunit gene products. In HEK293-cells we show differential
modulation of single L-VDCC activity by coexpression of several human
cardiac beta-subunits: Unlike beta(1) or beta(3) isoforms, beta(2a)
and beta(2b) induce a high-activity channel behavior typical of failing
myocytes. In accordance, beta(2)-subunit mRNA and protein are up-regulated
in failing human myocardium. In a model of heart failure we find
that mice overexpressing the human cardiac Ca(V)1.2 also reveal increased
single-channel activity and sarcolemmal beta(2) expression when entering
into the maladaptive stage of heart failure. Interestingly, these
animals, when still young and non-failing (Ädaptive Phase"), reveal
the opposite phenotype, viz: reduced single-channel activity accompanied
by lowered beta(2) expression. Additional evidence for the cause-effect
relationship between beta(2)-subunit expression and single L-VDCC
activity is provided by newly engineered, double-transgenic mice
bearing both constitutive Ca(V)1.2 and inducible beta(2) cardiac
overexpression. Here in non-failing hearts induction of beta(2)-subunit
overexpression mimicked the increase of single L-VDCC activity observed
in murine and human chronic heart failure. CONCLUSIONS: Our study
presents evidence of the pathobiochemical relevance of beta(2)-subunits
for the electrophysiological phenotype of cardiac L-VDCC and thus
provides an explanation for the single L-VDCC gating observed in
human and murine heart failure.
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