[Absorption and excretion of g-strophanthin after intravenous or sublingual administration (author's transl)]

Evaluation of Ouabain's Tissue Distribution in C57/Black Mice Following Intraperitoneal Injection, Using Chromatography and Mass Spectrometry

Cardiotonic steroids (CTSs), such as digoxin, are used for heart failure treatment. However, digoxin permeates the brain-blood barrier (BBB), affecting central nervous system (CNS) functions. Finding a CTS that does not pass through the BBB would increase CTSs' applicability in the clinic and decrease the risk of side effects on the CNS. This study aimed to investigate the tissue distribution of the CTS ouabain following intraperitoneal injection and whether ouabain passes through the BBB. After intraperitoneal injection (1.25 mg/kg), ouabain concentrations were measured at 5 min, 15 min, 30 min, 1 h, 3 h, 6 h, and 24 h using HPLC-MS in brain, heart, liver, and kidney tissues and blood plasma in C57/black mice. Ouabain was undetectable in the brain tissue. Plasma: Cmax = 882.88 ± 21.82 ng/g; Tmax = 0.08 ± 0.01 h; T1/2 = 0.15 ± 0.02 h; MRT = 0.26 ± 0.01. Cardiac tissue: Cmax = 145.24 ± 44.03 ng/g (undetectable at 60 min); Tmax = 0.08 ± 0.02 h; T1/2 = 0.23 ± 0.09 h; MRT = 0.38 ± 0.14 h. Kidney tissue: Cmax = 1072.3 ± 260.8 ng/g; Tmax = 0.35 ± 0.19 h; T1/2 = 1.32 ± 0.76 h; MRT = 1.41 ± 0.71 h. Liver tissue: Cmax = 2558.0 ± 382.4 ng/g; Tmax = 0.35 ± 0.13 h; T1/2 = 1.24 ± 0.7 h; MRT = 0.98 ± 0.33 h. Unlike digoxin, ouabain does not cross the BBB and is eliminated quicker from all the analyzed tissues, giving it a potential advantage over digoxin in systemic administration. However, the inability of ouabain to pass though the BBB necessitates intracerebral administration when used to investigate its effects on the CNS.

Why Whip the Starving Horse When There Are Oats for the Starving Myocardium?

Digoxin is the oldest drug for treatment of heart failure still in clinical use. Despite over 200 years of clinical experience with this drug, the optimal serum concentration required for both efficacy and safety remains unknown. It has been suggested that low doses have more favorable effects than higher ones. Cardiac glycosides act on the Na/K-ATPase (NKA). They show an inverted U-shaped dose-response curve with inhibition of pumping at high concentrations while increasing NKA activity at low concentrations. The classical sigmoidal dose-response curve describing an inhibition of the NKA by cardiac glycosides cannot explain this stimulatory effect. Cardiac glycosides are prototypical examples of hormetic substances. Biphasic dose-response curves of cardiac glycosides are also found in their neurohormonal effects. In low concentrations, vagomimetic effects are observed, whereas in high concentrations, sympathomimetic effects dominate. Lipophilic Digitalis glycosides have greater sympathomimetic effects; hydrophilic Strophanthus glycosides have greater vagomimetic effects. For digoxin, as a strong inotrope, there is evidence of only weak modulation of the autonomic nervous system. In ouabain, the modulation of the autonomic nervous system prevails over weak inotropic effects. Vagomimetic and sympatholytic effects characterize the therapeutic effects. In contrast to those of digoxin, the therapeutic effects of ouabain follow exactly the measurable serum concentration. Contrary to common prejudice ouabain is suitable for oral administration. Timely adjustments of dosage to patient therapeutic needs are easy to achieve with orally administered ouabain. Ouabain has the potential to crucially improve our arsenal of heart failure medications. Therefore, a clinical re-evaluation of ouabain is warranted. Randomized double-blind prospective clinical studies with ouabain, which meet today's standards, are worthwhile and necessary.

On the differences between ouabain and digitalis glycosides

Digoxin and digitoxin are widely used in the treatment of heart diseases. The exact mechanism of action of these drugs has remained an enigma. Ouabain has become the standard tool to investigate the mode of action of cardiotonic steroids, and results with ouabain are regarded as generally valid for all cardiac glycosides. However, there are marked differences between the effects of ouabain and digitalis glycosides. Ouabain has a different therapeutic profile from digitalis derivatives. Unlike digitalis glycosides, ouabain has a fast onset of action and stimulates myocardial metabolism. The inotropic effect of cardiotonic steroids is not related to inhibition of the Na-K-ATPase. Ouabain and digitalis derivatives develop their effects in different cellular spaces. Digitalis glycosides increase the intracellular calcium concentration by entering the cell interior and acting on the ryanodine receptors and by forming transmembrane calcium channels. Ouabain, by activation of the Na-K-ATPase from the extracellular side, triggers release of calcium from intracellular stores via signal transduction pathways and activates myocardial metabolism. These data no longer support the concept that all cardiotonic steroids exhibit their therapeutic effects by partial inhibition of the ion-pumping function of the Na-K-ATPase. Hence, it is suggested that this deeply rooted dogma be revised.

Pharmacodynamic effects of ouabain following single sublingual and intravenous doses in normal subjects

In this intraindividual, placebo-controlled, double blind study the dynamic effects of single doses of ouabain 0.5 mg i.v. and 12 mg sublingual were compared with those of the vasodilator sublingual nitroglycerin 0.8 mg. In 12 (sublingual) and 6 (i.v.) healthy volunteers, respectively, cardiac performance was assessed for 60 min after administration, using systolic time intervals ( QS2c , PEPc, PEP/LVET), electrical impedance cardiography ( (dZ/dt)/RZ index) and echocardiography (EDD, ESD, FS). After i.v. ouabain the typical positive inotropic glycoside effects appeared (shortening of QS2c , PEPc, and PEP/LVET, increase of (dZ/dt)/RZ and FS, decrease of EDD and ESD). With nitroglycerin preload reduction diminished cardiac performance, as shown by a rise in PEPc and PEP/LVET and depression of (dZ/dt)/RZ. In addition, EDD (not significant) and ESD were somewhat reduced, FS was enhanced, and QS2c tended to shorten. Following sublingual ouabain, QS2c was unchanged, there was an increase in PEPc and PEP/LVET, a decrease in (dZ/dt)/RZ and FS, EDD was unchanged, and ESD rose. By this route the absolute magnitude of the effects was about 1/3 that of the i.v. drug action. The spectrum of effects of sublingual ouabain indicates a reduction in cardiac performance without any detectable inotropic action. The effects seem to be induced by load changes, with an indication of an increase in afterload although an additional preload reduction cannot be excluded. This dose of the drug given by the sublingual route appears, therefore, to alter cardiac function via an effect on the peripheral circulation, although the final mechanism has not yet been elucidated.(ABSTRACT TRUNCATED AT 250 WORDS)