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 Goners magus toxins o mega-conotoxin MVIIC and omega-conotoxin MVIID, as well as their effec ts on K+-evoked Ca-45(2+) entry and whole-cell Ba2+ currents (I-Ba), a nd K+-evoked catecholamine secretion have been studied in bovine adren al chromaffin cells. Binding of [I-125] omega-conotoxin GVIA to bovine adrenal medullary membranes was displaced by omega-conotoxins GVIA, M VIIC and MVIID with IC50 values of around 0.1, 4 and 100 nM, respectiv ely. The reverse was true for the binding of [I-125] omega-conotoxin M VIIC, which was displaced by omega-conotoxins MVIIC, MVIID and GVIA wi th IC50 values of around 30, 80 and 1.200 nM, respectively. The sites recognized by omega-conotoxins MVIIC and MVIID in bovine brain exhibit ed higher affinities (IC50 values of around 1 nM). Both omega-conotoxi n MVIIC and MVIID blocked I-Ba by 70-80%; the higher the [Ba2+](o) of the extracellular solution the lower the blockade induced by omega-con otoxin MVIIC. This was not the case for omega-conotoxin MVIID; high Ba 2+ (10 mM) slowed down the development of blockade but the maximum blo ckade achieved was similar to that obtained in 2 mM Ba2+. A further di fference between the two toxins concerns their reversibility; washout of omega-conotoxin MVIIC did not reverse the blockade of I-Ba while in the case of omega-conotoxin MVIID a partial, quick recovery of curren t was produced. This component was irreversibly blocked by omega-conot oxin GVIA, suggesting that it is associated with N-type Ca2+ channels. Blockade of K+-evoked Ca-45(2+) entry produced results which parallel ed those obtained by measuring I-Ba. Thus, 1 mu M of each of omega-con otoxin GVIA and MVIIA inhibited Ca2+ uptake by 25%, while 1 mu M of ea ch of omega-conotoxin MVIIC and MVIID caused a 70% blockade. K+-evoked catecholamine secretory responses were not reduced by omega-conotoxin GVIA (1 mu M). In contrast, at 1 mu M both omega-conotoxin MVIIC and MVIID reduced the exocytotic response by 70%. These data strengthen th e previously established conclusion that Q-type Ca2+ channels that con tribute to the regulation of secretion and are sensitive to omega-cono toxins MVIIC and MVIID are present in bovine chromaffin cells. These c hannels, however, seem to possess binding sites for omega-conotoxins M VIIC and MVIID whose characteristics differ considerably from those de scribed to occur in the brain; they might represent a subset of Q-type Ca2+ channels or an entirely new subtype of voltage-dependent high-th reshold Ca2+ channel.