DUAL EFFECT OF DIGITALIS GLYCOSIDES ON NOREPINEPHRINE RELEASE FROM HUMAN ATRIAL TISSUE AND BOVINE ADRENAL CHROMAFFIN CELLS - DIFFERENTIAL DEPENDENCE ON [NA+](I) AND [CA2+](I)

It was the aim of the present study (1) to characterize the influence of Na+/K+-ATPase inhibition by the digitalis glycoside ouabain on both spontaneous and nicotine-evoked norepinephrine release from the human heart: and (2) to further investigate the role of glycoside-induced c hanges in [Na+](i) and [Ca2+](i) (determined by microfluorimetry) for catecholamine release. The latter experiments were performed in bovine adrenal medullary chromaffin cells (BCC), an established cell culture model for sympathetic nerves. Ouabain (1-1000 mu mol/l) exerted a dua l effect on norepinephrine release (determined by HPLC) from incubated human atrial tissue: (I) Ouabain induced a concentration-dependent in crease in norepinephrine release, that was calcium-independent and alm ost completely prevented by blockade of the uptake(1)-carrier by desip ramine (1 mu mol/l). The characteristics of this release process are c onsistent with a non-exocytotic mechanism. (II) In addition, ouabain a ugmented the nicotine-evoked (1-100 mu mol/l) calcium-dependent norepi nephrine release, which can be considered to be exocytotic. Na+/K+-ATP ase inhibition also reduced the threshold concentration of nicotine fr om 10 to 1 mu mol/l and it delayed the rapid tachyphylaxis of its nore pinephrine releasing effect in human atrial tissue. In BCC, ouabain in creased [Na+](i), [Ca2+](i) and [H-3]-norepinephrine release in parall el. Under calcium-free conditions, not only the ouabain-induced increa se in [Na+](i), but also [H-3]-norepinephrine release were enhanced. T he ouabain-induced [H-3]-norepinephrine release was always closely rel ated to changes in [Na+](i), indicating a key role of [Na+](i) for thi s calcium-independent non-exocytotic norepinephrine release. In additi on, pretreatment with ouabain (1 mmol/l) augmented the nicotine-evoked (0.1-10 mu mol/l) increments in [Na+](i), [Ca2+](i) and [H-3]-norepin ephrine release. As nicotine-induced norepinephrine release depends on an increase in both [Na+](i) and [Ca2+](i), these findings are indica tive of an ouabain-mediated facilitation of exocytosis. In conclusion, increasing [Na+](i) and [Ca2+](i) inhibition of Na+/K+-aTPase by ouab ain triggers non-exocytotic norepinephrine release, and facilitates ni cotine-evoked exocytotic norepinephrine release. (C) 1997 Academic Pre ss Limited.