no long-term use
The pharmacokinetics of cardiac glycosides (A) are dictated by their polarity, i.e., the number of hydroxyl groups. Membrane penetrability is virtually nil in ouabain, high in digoxin, and very high in digitoxin. Ouabain (g-strophanthin) does not penetrate into cells, be they intestinal epithelium, renal tubular, or hepatic cells. At best, it is suitable for acute intravenous induction of glycoside therapy.
The absorption of digoxin depends on the kind of galenical preparation used and on absorptive conditions in the intestine. Preparations are now of such quality that the derivatives methyl-digoxin and acetyldigoxin no longer offer any advantage. Renal reabsorption is incomplete; approx. 30% of the total amount present in the body (s.c. full "digitalizing" dose) is eliminated per day. When renal function is impaired, there is a risk of accumulation. Digi-toxin undergoes virtually complete reabsorption in gut and kidneys. There is active hepatic biotransformation: cleavage of sugar moieties, hydroxylation at C12 (yielding digoxin), and conjugation to glucuronic acid. Conjugates secreted with bile are subject to enterohepatic cycling (p. 38); conjugates reaching the blood are renally eliminated. In renal insufficiency, there is no appreciable accumulation. When digitoxin is withdrawn following overdosage, its effect decays more slowly than does that of di-goxin.
Other positive inotropic drugs. The phosphodiesterase inhibitor am-rinone (cAMP elevation, p. 66) can be administered only parenterally for a maximum of 14 d because it is poorly tolerated. A closely related compound is milrinone. In terms of their positive inotropic effect, p-sympathomimetics, unlike dopamine (p. 114), are of little therapeutic use; they are also arrhyth-mogenic and the sensitivity of the p-re-ceptor system declines during continuous stimulation.
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