The actions of ouabain and lithium chloride on cytosolic Ca2+ in single chromaffin cells.

The effects of ouabain, Li+ and veratridine on the concentration of cytosolic free Ca2+ ([Ca2+]i) were studied in single fura-2-loaded bovine adrenal chromaffin cells. Superfusion of cells with ouabain (10 microM for 60 min) caused only a delayed mild increase of the Ca2+]i, from around 0.1 microM to 0.2-0.3 microM; this increase was Nao(+)-dependent. Replacement of all NaCl of the Krebs-Hepes solution by LiCl (144 mM) produced a gradual increase of [Ca2+]i, which remained elevated at a stable plateau of 0.4-0.5 microM for 40-50 min. When ouabain (in the presence of normal Nao+) or Li+ (in the absence of Nao+) was given in Krebs-Hepes solution containing no Ca2+, the reintroduction of 2.5 mM Ca2+ produced a fast elevation of the [Ca2+]i. In the case of ouabain-treated cells, the [Ca2+]i curve exhibited an initial phasic component which inactivated to a tonic component. omega-Conotoxin MVIIC (3 microM) and R56865 (10 microM) inhibited the phasic but not the tonic component. Veratridine (30 microM) induced large [Ca2+]i oscillations. Both ouabain or Li+ abolished such oscillations. These results are compatible with ouabain causing elevation of [Ca2+]i in bovine chromaffin cells through a dual mechanism, i.e. cell depolarisation and slowing down of the Na(+)-Ca2+ exchanger of their plasmalemma. Through its binding to the Na+ site on the Na(+)-Ca2+ exchanger, Li+ ions generate powerful Cai2+ signals that might be relevant to its known effects on neurosecretory mechanisms.