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 cy tosolic free Ca2+ ([Ca2+](i)) were studied in single fura-2-loaded bov ine adrenal chromaffin cells. Superfusion of cells with ouabain (10 mu M for 60 min) caused only a delayed mild increase of the [Ca2+](i), f rom around 0.1 mu M to 0.2-0.3 mu M; this increase was Na-o(+)-depende nt. Replacement of all NaCl of the Krebs-Hepes solution by LiCl (144 m M) produced a gradual increase of [Ca2+](i), which remained elevated a t a stable plateau of 0.4-0.5 mu M for 40-50 min. When ouabain (in the presence of normal Na-o(+)) or Li+ (in the absence of Na-o(+)) was gi ven 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 phasi c component which inactivated to a tonic component, omega-Conotoxin MV IIC (3 mu M) and R56865 (10 mu M) inhibited the phasic but not the ton ic component. Veratridine (30 mu M) induced large [Ca2+](i) oscillatio ns. Both ouabain or Li+ abolished such oscillations. These results are compatible with ouabain causing elevation of [Ca2+](i) in bovine chro maffin cells through a dual mechanism, i.e. cell depolarisation and sl owing 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 Ca-i(2+) signals that might be relevant to its known effects on neurosecretory mechanisms.