Characterisation of alpha 1A Ba2+, Sr2+ and Ca2+ currents recorded with the ancillary beta 1-4 subunits.
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+ 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 time-to-peak of the current. Inactivation kinetics were poorly affected. Changes in gating and voltage-dependence of activation, but not 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.
Mangoni
M E
ME
CRBM-CNRS UPR 9008, INSERM U 249, Montpellier France.
Cens
T
T
Dalle
C
C
Nargeot
J
J
Charnet
P
P
D000818:Animals; D001464:Barium / Q000378:metabolism*; D001665:Binding Sites; D001921:Brain / Q000378:metabolism; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D015220:Calcium Channels / Q000737:chemistry / Q000502:physiology*; D004594:Electrophysiology; D015870:Gene Expression / Q000235:genetics; D015640:Ion Channel Gating / Q000502:physiology; D007700:Kinetics; D008845:Microinjections; D009865:Oocytes; D018408:Patch-Clamp Techniques; D010957:Plasmids; D051381:Rats; D013324:Strontium / Q000378:metabolism*; D014981:Xenopus
D015220:Calcium Channels; D001464:Barium; D002118:Calcium; D013324:Strontium