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 mumol/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 (1 mumol/l). The characteristics of this release process are consistent with a non-exocytotic mechanism. (II) In addition, ouabain augmented the nicotine-evoked (1-100 mumol/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 mumol/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 [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+]i for this calcium-independent non-exocytotic norepinephrine release. In addition, pretreatment with ouabain (1 mmol/l) augmented the nicotine-evoked (0.1-10 mumol/l) increments in [Na+]i, [Ca2+]i and [3H]-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.
Haass
M
M
Department of Cardiology, University of Heidelberg, Germany.
Serf
C
C
Gerber
S H
SH
Krüger
C
C
Haunstetter
A
A
Vahl
C F
CF
Nobiling
R
R
Kübler
W
W
D000313:Adrenal Medulla / Q000166:cytology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002395:Catecholamines / Q000378:metabolism; D002417:Cattle; D019439:Chromaffin Cells / Q000187:drug effects* / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005731:Ganglionic Stimulants / Q000494:pharmacology; D006325:Heart Atria / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D009538:Nicotine / Q000494:pharmacology; D009638:Norepinephrine / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology*; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors
D002395:Catecholamines; D004791:Enzyme Inhibitors; D005731:Ganglionic Stimulants; D010042:Ouabain; D009538:Nicotine; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium; D009638:Norepinephrine