L-type channel inactivation balances the increased peak calcium current due to absence of Rad in cardiomyocytes

The L-type Ca(2+) channel (LTCC) provides trigger calcium to initiate cardiac contraction in a graded fashion that is regulated by L-type calcium current (I(Ca,L)) amplitude and kinetics. Inactivation of LTCC is controlled to fine-tune calcium flux and is governed by voltage-dependent inactivation (VDI) and calcium-dependent inactivation (CDI). Rad is a monomeric G protein that regulates I(Ca,L) and has recently been shown to be critical to β-adrenergic receptor (β-AR) modulation of I(Ca,L). Our previous work showed that cardiomyocyte-specific Rad knockout (cRadKO) resulted in elevated systolic function, underpinned by an increase in peak I(Ca,L), but without pathological remodeling. Here, we sought to test whether Rad-depleted LTCC contributes to the fight-or-flight response independently of β-AR function, resulting in I(Ca,L) kinetic modifications to homeostatically balance cardiomyocyte function. We recorded whole-cell I(Ca,L) from ventricular cardiomyocytes from inducible cRadKO and control (CTRL) mice. The kinetics of I(Ca,L) stimulated with isoproterenol in CTRL cardiomyocytes were indistinguishable from those of unstimulated cRadKO cardiomyocytes. CDI and VDI are both enhanced in cRadKO cardiomyocytes without differences in action potential duration or QT interval. To confirm that Rad loss modulates LTCC independently of β-AR stimulation, we crossed a β(1),β(2)-AR double-knockout mouse with cRadKO, resulting in a Rad-inducible triple-knockout mouse. Deletion of Rad in cardiomyocytes that do not express β(1),β(2)-AR still yielded modulated I(Ca,L) and elevated basal heart function. Thus, in the absence of Rad, increased Ca(2+) influx is homeostatically balanced by accelerated CDI and VDI. Our results indicate that the absence of Rad can modulate the LTCC without contribution of β(1),β(2)-AR signaling and that Rad deletion supersedes β-AR signaling to the LTCC to enhance in vivo heart function.