Résumé
Xenopus oocytes have been injected with different combinations of expression plasmids carrying the rat brain alpha(1A) and different beta (beta(1-4)) Ca2+ channel subunit cDNAs. Whole-cell Ba2+ and Ca2+ currents were recorded up to seven days after injection. Intra-oocyte injection of BAPTA allowed us to record uncontaminated Ba2+, Sr2+ and Ca2+ currents. The alpha(1A) calcium channel showed relative current amplitudes according to the sequence: IBa2+ > ISr2+ > ICa2+. The ratio ICa2+/IBa2+ was significantly larger when compared to the class C L-type Ca2+ channel (alpha(1C)). However, currents flowing through alpha(1A) and alpha(1C) subunits saturate for similar Ba2+ concentrations and display the anomalous mole fraction effect in the presence of mixtures of Ba2+ and Ca2+ ions in the external medium. In oocytes expressing the alpha(1A) Ca2+ channel subunit, switching from extracellular Ba2+ to Ca2+ also induced a depolarising shift of current-to-voltage relation and the steady-state inactivation curve, and increased the rime-to-peak of the current. Inactivation kinetics were poorly affected. Changes in gating and voltage-dependence of activation, but nor in the voltage-dependent inactivation, were independent from the coexpressed beta subunit (except with the beta(4) subunit). Our data constitute strong evidence for the existence of differences in intra-pore Ca2+ binding sites between the alpha(1C) and alpha(1A) subunits, and emphasise the influence of the charge carrier on the modulation of alpha(1A) properties by the beta subunits.