Endogenous digoxin-like substance as an important factor in the development of hypertension in children

Falsely elevated serum digoxin levels secondary to endogenous digoxin-like immunoreactive substances

Endogenous digoxin-like immunoreactive substances (DLIS) are produced by the human body and can be significantly elevated in specific clinical conditions. Commercially available digoxin assays do not have the specificity to fully distinguish DLIS from exogenous digoxin, though DLIS do not possess the same therapeutic properties as do the drug. The resultant artificial elevation of a reported digoxin level or a completely factitious level in a patient not taking the drug can have clinically significant consequences.

Endogenous digitalis-like factors

The postulate of a natriuretic factor inhibiting the sodium pump in the kidney led to the detection of increased concentrations of endogenous digitalis-like factors in blood after salt loading, in essential hypertension, in pregnancy-induced hypertension and in chronic hypervolaemia. The recent isolation of ouabain or a close isomer thereof from human plasma and the demonstration of a compound similar if not identical to digoxin in adrenals and human urine shows that mammals like non-vertebrates and toads may synthesize cardiac glycosides in their adrenals and possibly in hypothalamus. The hypothalamus also forms other compounds of unknown structure which bind to the cardiac glycoside receptor site. The differential functions of endogenously formed ouabain and of a digoxin-like substance are unclear. The detailed knowledge of the physiological role of both endogenously formed cardiac glycosides in the regulation of blood pressure has still to be worked out.

Digoxin-like immunoreactivity: is it still worth measuring?

On the assumption that digoxin-like immunoreactivity may represent digitalis-like sodium pump inhibitors in the mammalian body, many investigators have used radioimmunoassay for digoxin to monitor such factors during the past decade. The presence of digoxin-like immunoreactivity has been confirmed by numerous studies using biochemical, immunological or morphological methods. Very recently, ouabain or a very similar substance, which did not cross-react with antidigoxin antibodies, was identified from the human plasma as the long-sought sodium pump inhibitor. However, it is yet to be determined whether sodium pump inhibitory activity in the circulation results from one substance or several. Some researchers still insist on the possible physiological roles of digoxin-like immunoreactivity which may or may not be related to the regulation of sodium pump. These issues are critically reviewed in this article.