Digitalis glycosides--recent advances in clinical pharmacology and treatment.
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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.
The authors compare and discuss sensitivity and reproducibility of commercial RIA kits of digoxin and digitoxin commonly used in routine hospital laboratories and in emergency. Cross reactivity and recovery of these kits with different digitalis preparations currently prescribed in medical practice are studied with a view to practical application in patients receiving digitalis glycosides and derivatives.
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