Modulation of intracellular Ca2+ via L-type calcium channels in heart cells by the autoantibody directed against the second extracellular loop of the alpha1-adrenoceptors.
The effects of methoxamine, a selective alpha1-adrenergic receptor agonist, and the autoantibody directed against the second extracellular loop of alpha1-adrenoceptors were studied on intracellular free Ca2+ levels using confocal microscopy and ionic currents using the whole-cell patch clamp technique in single cells of 10-day-old embryonic chick and 20-week-old fetal human hearts. We observed that like methoxamine, the autoantibody directed against the second extracellular loop of alpha1-adrenoreceptors significantly increased the L-type calcium current (I(Ca(L))) but had no effect on the T-type calcium current (I(Ca(T))), the delayed outward potassium current, or the fast sodium current. This effect of the autoantibody was prevented by a prestimulation of the receptors with methoxamine and vice versa. Moreover, treating the cells with prazosin, a selective alpha1-adrenergic receptor antagonist blocked the methoxamine and the autoantibody-induced increase in I(Ca(L)), respectively. In absence of prazosin, both methoxamine and the autoantibody showed a substantial enhancement in the frequency of cell contraction and that of the concomitant cytosolic and nuclear free Ca2+ variations. The subsequent addition of nifedipine, a specific L-type Ca2+ channel blocker, reversed not only the methoxamine or the autoantibody-induced effect but also completely abolished cell contraction. These results demonstrated that functional alpha1-adrenoceptors exist in both 10-day-old embryonic chick and 20-week-old human fetal hearts and that the autoantibody directed against the second extracellular loop of this type of receptors plays an important role in stimulating their activity via activation of L-type calcium channels. This loop seems to have a functional significance by being the target of alpha1-receptor agonists like methoxamine.
Department of Anatomy and Cell Biology, Faculty of Medicine, Université de Sherbrooke, Sherbrooke, QC J1H 5N4, Canada. Ghassan.Bkaily@Usherbrooke.ca
Ghassan
G
Bkaily
Nesrine
N
El-Bizri
Michel
M
Bui
Rami
R
Sukarieh
Danielle
D
Jacques
Michael L X
ML
Fu
D058646:Adrenergic alpha-1 Receptor Agonists; D000814:Aniline Compounds; D000818:Animals; D001323:Autoantibodies / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000502:physiology*; D020747:Calcium Channels, T-Type / Q000502:physiology; D002478:Cells, Cultured; D002642:Chick Embryo; D005333:Fetus; D005456:Fluorescent Dyes; D006801:Humans; D008729:Methoxamine / Q000494:pharmacology; D009200:Myocardial Contraction; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology; D018340:Receptors, Adrenergic, alpha-1 / Q000276:immunology / Q000502:physiology*; D015222:Sodium Channels / Q000187:drug effects / Q000502:physiology; D014966:Xanthenes
D058646:Adrenergic alpha-1 Receptor Agonists; D000814:Aniline Compounds; D001323:Autoantibodies; D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D020747:Calcium Channels, T-Type; D005456:Fluorescent Dyes; D015221:Potassium Channels; D018340:Receptors, Adrenergic, alpha-1; D015222:Sodium Channels; D014966:Xanthenes; C059715:Fluo-3; D008729:Methoxamine; D009543:Nifedipine; D002118:Calcium