Abstract
Abstract only
The L-type Ca
2+
channel current (I
Ca,L
) provides trigger Ca
2+
to contribute to cardiac contraction. Rad GTPase associates with the L-type Ca
2+
channel (LTCC) and serves as an endogenous inhibitor of LTCC activity. Overexpression of Rad blocks I
Ca,L
; absence of Rad increases I
Ca,L
. Rad attenuates β-adrenergic receptor (β-AR) signaling. Chronic β-AR stimulation associates with Ca
2+
mishandling and can promote signaling that progresses towards heart failure. Early studies of global, constitutive Rad-knockout mice (gRadKO) suggested that elevated Ca
2+
dynamics leads to pathological cardiac hypertrophy; however, Rad is also expressed in non-cardiac tissues.
Our objective is to test the hypothesis that increased myocardial I
Ca,L
via Rad deletion safely enhances cardiac function without driving pathological remodeling.
We created a cardiac-restricted inducible Rad knockout mouse (Rad
Δ/Δ
).
In vivo
function was measured with echocardiography. We examined I
Ca,L
through whole cell configuration of the patch clamp technique, assessed Ca
2+
handling, and sarcomere dynamics.
Unlike gRadKO, Rad
Δ/Δ
showed no elevation of fetal gene program, nor fibrosis, and no change to aortic pressure. Rad
Δ/Δ
had a sustained increase of inotropy without structural or functional remodeling (EF: Rad
Δ/Δ
=76 ± 2%, n=16; Rad
fl/fl
=59 ± 4%, n=7; p=0.001.) I
Ca,L
was significantly increased, with Rad loss mirroring a β-AR modulated phenotype on basal I
Ca,L
(max. conductance: Rad
Δ/Δ
=254 ± 19 pS/pF, n=15; Rad
fl/fl
=144 ± 12 pS/pF, n=18; p<10
-4
). Contrary to models of chronic β-AR stimulation, Rad
Δ/Δ
retained β-AR signaling shown
in vivo
using isoproterenol, and by preserved phosphorylation of protein regulators of Ca
2+
reuptake and contractility. Rad
Δ/Δ
cardiomyocytes show enhanced cytosolic Ca
2+
handling (Decay of Ca
2+
transient: Rad
Δ/Δ
= 0.07 ± 0.003 (F
340
/F
380
)/s, n=67, Rad
fl/fl
= 0.10 ± 0.005, n=69; p<10
-4
), increased contractile function, and elevated SERCA2a expression.
These new findings challenge the canonical assumption that increased myocardial Ca
2+
necessarily promotes pathology. We conclude that cardiac hypertrophy in gRadKO was caused by non-cardiac tissue effects, and myocardial Rad deletion is a promising cardiac inotropic therapeutic direction.