Direct Inhibition of Expressed Cardiac L- and T-Type Calcium Channels by IgG From Mothers Whose Children Have Congenital Heart Block

Abstract

Background —Congenital heart block (CHB) is a disease that affects the offspring of mothers with autoimmune diseases. We recently reported that maternal sera containing antibodies against SSA/Ro and SSB/La ribonucleoproteins (positive IgG) inhibited L-type Ca current in isolated cardiac myocytes and induced sinus bradycardia in a murine model of CHB. The direct interaction of positive IgG with L-type Ca channel proteins and the possible inhibition of T-type Ca current that could account for the sinus bradycardia remain unknown. Methods and Results —The 2-electrode voltage-clamp technique was used to record currents via L-type ( I Ba -α 1C or I Ba -α 1C +β 2a +α 2 /δ) and T-type ( I Ba -α 1H ) Ca channels, Na channels ( I Na -hH1), and K channels ( I Ks -minK+KvLQT1) expressed in Xenopus oocytes. Positive IgG (350 μg/mL) inhibited I Ba -α 1C by 50.6±4.7% ( P <0.01) and I Ba -α 1C +β 2a +α 2 /δ by 50.9±4.2% ( P <0.01); I Ba -α 1H was reduced by 18.9±1.0% ( P <0.01). Immunoblot data show cross-reactivity of positive IgG with α 1C subunit. Pretreatment of oocytes with atropine (1 μmol/L) or acetylcholine (10 μmol/L) did not affect the inhibitory effect of IgG on I Ba -α 1C and I Ba -α 1C +β 2a +α 2 /δ ( P <0.05). Positive IgG had no effect, however, on either I Na -hH1 or I Ks -minK+KvLQT1. Conclusions —Positive IgG inhibited expressed L-type I Ba and cross-reacted with the α 1C subunit in Xenopus oocytes, providing strong evidence that maternal antibodies interact directly with the pore-forming α 1 -subunit of Ca channels. In addition, we show for the first time that positive IgG also inhibited T-type I Ba but not I Na -hH1 or I Ks -minK+KvLQT1. This could provide, in part, the ionic basis of sinus bradycardia reported in animal models of CHB and clinically in humans.