Separation of calcium channel current components in mouse chromaffin cells superfused with low- and high-barium solutions.
This study was carried out to characterize the set of voltage-dependent Ca2+ channel subtypes expressed by mouse adrenal chromaffin cells superfused with solutions containing low (2 mM) or high (10 mM) Ba2+ concentrations. Using 50-ms test pulses at 0 mV from a holding potential of -80 mV, averaged peak current in 10 mM Ba2+ was around 1 nA, and in 2 mM Ba2+ 0.36 nA. When using 2 mM Ba2+ as the charge carrier, nifedipine (3 microM) blocked IBa by 40-45%. omega-Conotoxin GVIA (1 microM) caused 26% inhibition, while omega-conotoxin MVIIC (3 microM) produced a 48% blockade. At low concentrations (20 nM), omega-agatoxin IVA caused 5-15% of current inhibition, while 2 microM gave rise to a 35-40% blockade. In 10 mM Ba2+, the blocking effects of nifedipine (40%) and omega-conotoxin GVIA (25%) were similar to those seen in 2 mM Ba2+. In contrast, blockade by omega-conotoxin MVIIC was markedly reduced in 10 mM Ba2+ (20-25%) as compared to 10 mM Ba2+ (48%). The blocking actions of omega-agatoxin IVA (2 microM) were also slowed down in 10 mM Ba2+, though the final blockade was unaffected. In 2 mM Ba2+, IBa was quickly inhibited by over 94% with combined nifedipine + omega-conotoxin MVIIC + omega-conotoxin GVIA; in 10 mM Ba2+, IBa was blocked by 70% with this combination. The data suggest that mouse chromaffin cells express L-type (40%) as well as non-L-type (60%) high-threshold voltage-dependent Ca2+ channels. The current carried by non-L-type Ca2+ channels consists of about 25% N-type and 35% P/Q-type; P-type channels, if anything, are poorly expressed. The data also indicate that the fraction of current blocked by omega-conotoxin MVIIC and omega-agatoxin IVA might considerably change as a function of the Ba2+ concentration of the extracellular solution; taking this fact into consideration, it seems that a residual R-type current is not expressed in mouse chromaffin cells.
Hernández-Guijo
J M
JM
Instituto de Farmacología Teófilo Hernando y Departamento de Farmacología, Facultad de Medicina, Universidad Autónoma de Madrid, Arzobispo Morcillo 4, E-28029 Madrid, Spain.
de Pascual
R
R
García
A G
AG
Gandía
L
L
D000311:Adrenal Glands / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D001464:Barium / Q000008:administration & dosage / Q000494:pharmacology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000145:classification* / Q000502:physiology*; D002413:Cations, Divalent; D002478:Cells, Cultured; D002838:Chromaffin System / Q000187:drug effects / Q000502:physiology*; D004553:Electric Conductivity; D051379:Mice; D009543:Nifedipine / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010455:Peptides / Q000494:pharmacology; D010477:Perfusion; D012996:Solutions; D013111:Spider Venoms / Q000494:pharmacology; D020867:omega-Agatoxin IVA; D020866:omega-Conotoxin GVIA; D020960:omega-Conotoxins
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D002413:Cations, Divalent; D010455:Peptides; D012996:Solutions; D013111:Spider Venoms; D020867:omega-Agatoxin IVA; D020960:omega-Conotoxins; C075851:omega-conotoxin-MVIIC; D001464:Barium; D020866:omega-Conotoxin GVIA; D009543:Nifedipine