Transforming growth factor-β1 decreases cardiac muscle L-type Ca2+ current and charge movement by acting on the Cav1.2 mRNA

Transforming growth factors-beta (TGF-betas) are essential to the structural remodeling seen in cardiac disease and development; however, little is known about potential electrophysiological effects. We hypothesized that chronic exposure (6-48 h) of primary cultured neonatal rat cardiomyocytes to the type 1 TGF-beta (TGF-beta1, 5 ng/ml) may affect voltage-dependent Ca(2+) channels. Thus we investigated T- (I(CaT)) and L-type (I(CaL)) Ca(2+) currents, as well as dihydropyridine-sensitive charge movement using the whole cell patch-clamp technique and quantified Ca(V)1.2 mRNA levels by real-time PCR assay. In ventricular myocytes, TGF-beta1 did not exert significant electrophysiological effects. However, in atrial myocytes, TGF-beta1 reduced both I(CaL) and charge movement (55% at 24-48 h) without significantly altering I(CaT), cell membrane capacitance, or channel kinetics (voltage dependence of activation and inactivation, as well as the activation and inactivation rates). Reductions of I(CaL) and charge movement were explained by concomitant effects on the maximal values of L-channels conductance (G(max)) and charge movement (Q(max)). Thus TGF-beta1 selectively reduces the number of functional L-channels on the surface of the plasma membrane in atrial but not ventricular myocytes. The TGF-beta1-induced I(CaL) reduction was unaffected by supplementing intracellular recording solutions with okadaic acid (2 microM) or cAMP (100 microM), two compounds that promote L-channel phosphorylation. This suggests that the decreased number of functional L-channels cannot be explained by a possible regulation in the L-channels phosphorylation state. Instead, we found that TGF-beta1 decreases the expression levels of atrial Ca(V)1.2 mRNA (70%). Thus TGF-beta1 downregulates atrial L-channel expression and may be therefore contributing to the in vivo cardiac electrical remodeling.