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 changes 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 dual effect on norepinephrine release (determined by HPLC) from incubated human atrial tissue: (I) Ouabain induced a concentration-dependent increase in norepinephrine release, that was calcium-independent and almost completely prevented by blockade of the uptake(1)-carrier by desipramine (1 mu mol/l). The characteristics of this release process are consistent with a non-exocytotic mechanism. (II) In addition, ouabain augmented the nicotine-evoked (1-100 mu mol/l) calcium-dependent norepinephrine release, which can be considered to be exocytotic. Na+/K+-ATPase inhibition also reduced the threshold concentration of nicotine from 10 to 1 mu mol/l and it delayed the rapid tachyphylaxis of its norepinephrine releasing effect in human atrial tissue. In BCC, ouabain increased [Na+](i), [Ca2+](i) and [H-3]-norepinephrine release in parallel. Under calcium-free conditions, not only the ouabain-induced increase in [Na+](i), but also [H-3]-norepinephrine release were enhanced. The ouabain-induced [H-3]-norepinephrine release was always closely related to changes in [Na+](i), indicating a key role of [Na+](i) for this calcium-independent non-exocytotic norepinephrine release. In addition, 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]-norepinephrine release. As nicotine-induced norepinephrine release depends on an increase in both [Na+](i) and [Ca2+](i), these findings are indicative of an ouabain-mediated facilitation of exocytosis. In conclusion, increasing [Na+](i) and [Ca2+](i) inhibition of Na+/K+-aTPase by ouabain triggers non-exocytotic norepinephrine release, and facilitates nicotine-evoked exocytotic norepinephrine release. (C) 1997 Academic Press Limited.