Analogies and differences between omega-conotoxins MVIIC and MVIID: binding sites and functions in bovine chromaffin cells.
The characteristics of the binding sites for the Conus magus toxins omega-conotoxin MVIIC and omega-conotoxin MVIID, as well as their effects on K+-evoked 45Ca2+ entry and whole-cell Ba2+ currents (IBa), and K+-evoked catecholamine secretion have been studied in bovine adrenal chromaffin cells. Binding of [125I] omega-conotoxin GVIA to bovine adrenal medullary membranes was displaced by omega-conotoxins GVIA, MVIIC and MVIID with IC50 values of around 0.1, 4 and 100 nM, respectively. The reverse was true for the binding of [125I] omega-conotoxin MVIIC, which was displaced by omega-conotoxins MVIIC, MVIID and GVIA with IC50 values of around 30, 80 and 1.200 nM, respectively. The sites recognized by omega-conotoxins MVIIC and MVIID in bovine brain exhibited higher affinities (IC50 values of around 1 nM). Both omega-conotoxin MVIIC and MVIID blocked IBa by 70-80%; the higher the [Ba2+]o of the extracellular solution the lower the blockade induced by omega-conotoxin MVIIC. This was not the case for omega-conotoxin MVIID; high Ba2+ (10 mM) slowed down the development of blockade but the maximum blockade achieved was similar to that obtained in 2 mM Ba2+. A further difference between the two toxins concerns their reversibility; washout of omega-conotoxin MVIIC did not reverse the blockade of IBa while in the case of omega-conotoxin MVIID a partial, quick recovery of current was produced. This component was irreversibly blocked by omega-conotoxin GVIA, suggesting that it is associated with N-type Ca2+ channels. Blockade of K+-evoked 45Ca2+ entry produced results which paralleled those obtained by measuring IBa. Thus, 1 microM of each of omega-conotoxin GVIA and MVIIA inhibited Ca2+ uptake by 25%, while 1 microM of each of omega-conotoxin MVIIC and MVIID caused a 70% blockade. K+-evoked catecholamine secretory responses were not reduced by omega-conotoxin GVIA (1 microM). In contrast, at 1 microM both omega-conotoxin MVIIC and MVIID reduced the exocytotic response by 70%. These data strengthen the previously established conclusion that Q-type Ca2+ channels that contribute to the regulation of secretion and are sensitive to omega-conotoxins MVIIC and MVIID are present in bovine chromaffin cells. These channels, however, seem to possess binding sites for omega-conotoxins MVIIC and MVIID whose characteristics differ considerably from those described to occur in the brain; they might represent a subset of Q-type Ca2+ channels or an entirely new subtype of voltage-dependent high-threshold Ca2+ channel.
Gandía
L
L
Departamento de Farmacología, Facultad de Medicina, Universidad Autónoma de Madrid, Arzobispo Morcillo, 4, E-28029 Madrid, Spain.
Lara
B
B
Imperial
J S
JS
Villarroya
M
M
Albillos
A
A
Maroto
R
R
García
A G
AG
Olivera
B M
BM
D000313:Adrenal Medulla / Q000378:metabolism*; D000818:Animals; D001464:Barium / Q000378:metabolism; D001665:Binding Sites; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000378:metabolism* / Q000494:pharmacology; D002395:Catecholamines / Q000378:metabolism; D002417:Cattle; D002462:Cell Membrane / Q000378:metabolism; D002838:Chromaffin System / Q000378:metabolism*; D020916:Conotoxins; D004553:Electric Conductivity; D007700:Kinetics; D010455:Peptides / Q000378:metabolism* / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D020866:omega-Conotoxin GVIA; D020960:omega-Conotoxins
D002121:Calcium Channel Blockers; D002395:Catecholamines; D020916:Conotoxins; D010455:Peptides; D020960:omega-Conotoxins; C075851:omega-conotoxin-MVIIC; C085882:omega-conotoxin MVIID; D001464:Barium; D020866:omega-Conotoxin GVIA; D011188:Potassium; D002118:Calcium