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 (IBa-α1C or IBa-α1C+β2a+α2 /δ) and T-type (IBa-α1H) Ca channels (INa-hH1), and K channels (IKs-minK+KvLQT1) expressed in Xenopus oocytes. Positive IgG (350 μg/mL) inhibited IBa-α1C by 50.6±4.7% (P<0.01) and IBa-α1C+β2a+α2 /δ by 50.9±4.2% (P<0.01); IBa-α1H was reduced by 18.9±1.0% (P<0.01). Immunoblot data show cross-reactivity of positive IgG with α1C subunit. Pretreatment of oocytes with atropine (1 μmol/L) or acetylcholine (10 μmol/L) did not affect the inhibitory effect of IgG on IBa-α1C and IBa-α1C+β2a+α2 /δ (P<0.05). Positive IgG had no effect, however, on either INa-hH1 or IKs-minK+KvLQT1. Conclusions - Positive IgG inhibited expressed L-type IBa and cross-reacted with the α1C subunit in Xenopus oocytes, providing strong evidence that maternal antibodies interact directly with the pore-forming α1-subunit of Ca channels. In addition, we show for the first time that positive IgG also inhibited T-type IBa but not INa-hH1 or IKs-minK+KvLQT1. This could provide, in part, the ionic basis of sinus bradycardia reported in animal models of CHB and clinically in humans.