Direct inhibition of expressed cardiac L- and T-type calcium channels by IgG from mothers whose children have congenital heart block

Background-Congenital heart block (CHB) is a disease that affects the offsp ring of mothers with autoimmune diseases. We recently reported that materna l 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 u nknown. Methods and Results-The 2-electrode voltage-clamp technique was used to rec ord currents via L-type.(I-Ba-alpha (1C) or I-Ba-alpha (1C)+beta (2a)+alpha (2)/delta) and T-type (I-Ba-alpha (1H)) Ca channels, Na channels (I-Na-hH1 ), and K channels (I-Ks-minK+KvLQT1) expressed in Xenopus oocytes. Positive IgG (350 mug/mL) inhibited I-Ba-alpha (1C) by 50.6 +/-4.7% (P<0.01) and I- Ba-<alpha>(1C)+beta (2a)+alpha (2)/delta by 50.9 +/-4.2% (P<0.01); I-Ba-<al pha>(1H) was reduced by 18.9 +/-1.0% (P<0.01). Immunoblot data show cross-r eactivity of positive IgG with <alpha>(1C) subunit. Pretreatment of oocytes with atropine (1 mu mol/L) or acetylcholine (10 mu mol/L) did not affect t he inhibitory effect of IgG on I-Ba-alpha (1C) and I-Ba-alpha (1C)+beta (2a )+alpha (2)/delta (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 <alpha>(1C) subunit in Xenopus oocytes, providing strong evidence that maternal antibodies interact directly with the pore-forming alpha (1)- subunit of Ca channels, In addition, we show for the first time that positi ve IgG also inhibited T-type I-Ba but not I-Na-hH1 or I-Ks-minK+KvLQT1. Thi s could provide, in part, the ionic basis of sinus bradycardia reported in animal models of CHB and clinically in humans.