Dual Effect of Digitalis Glycosides on Norepinephrine Release from Human Atrial Tissue and Bovine Adrenal Chromaffin Cells: Differential Dependence on [Na+]iand [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+]iand [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 (1–1000μmol/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 uptake1-carrier by desipramine (1μmol/l). The characteristics of this release process are consistent with a non-exocytotic mechanism. (II) In addition, ouabain augmented the nicotine-evoked (1–100μmol/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 1μmol/l and it delayed the rapid tachyphylaxis of its norepinephrine releasing effect in human atrial tissue. In BCC, ouabain increased [Na+]i, [Ca2+]iand [3H] -norepinephrine release in parallel. Under calcium-free conditions, not only the ouabain-induced increase in [Na+]i, but also [3H]-norepinephrine release were enhanced. The ouabain-induced [3H]-norepinephrine release was always closely related to changes in [Na+]i, indicating a key role of [Na+]ifor this calcium-independent non-exocytotic norepinephrine release. In addition, pretreatment with ouabain (1 mmol/l) augmented the nicotine-evoked (0.1–10μmol/l) increments in [Na+]i, [Ca2+]iand [3H]-norepinephrine release. As nicotine-induced norepinephrine release depends on an increase in both [Na+]iand [Ca2+]i, these findings are indicative of an ouabain-mediated facilitation of exocytosis. In conclusion, increasing [Na+]iand [Ca2+]iinhibition of Na+/K+-ATPase by ouabain triggers non-exocytotic norepinephrine release, and facilitates nicotine-evoked exocytotic norepinephrine release.