Digoxin-like immunoreactivity of a circulating (Na+ + K+)ATPase inhibitor

Endogenous digoxin-like factor in neonates: effect of age and relation to serum bilirubin levels

Endogenous digoxin-like immunoreactive factor(s) (DLIF) have been found in serum and urine of newborn infants, including those born prematurely. We assessed the effect of age on serum levels of DLIF in 73 samples obtained from 66 healthy full term newborn infants at birth and during the first two months of life. DLIF concentrations were highest at birth and fell progressively with age. In cord blood, DLIF levels were 0.73 +/- 0.35 ng/ml (mean +/- SD). DLIF concentrations were 0.45 +/- 0.11 ng/ml on day 1, 0.26 +/- 0.08 ng/ml on day 3, 0.19 +/- 0.07 ng/ml on day 5, 0.17 +/- 0.09 ng/ml on day 11, 0.11 +/- 0.02 ng/ml on days 15-30, and not detectable after 45 days of life. We also studied the relation between serum levels of DLIF and bilirubin in 23 jaundiced newborns between 3-5 days of life. We found a highly significant positive correlation between serum bilirubin concentrations and DLIF. These findings support the assumption that DLIF plays a role in impeding bilirubin excretion in the neonatal period, perhaps by inhibiting the activity of (Na-K)ATPase.

Effects of K+-canrenoate on the development of DOCA-salt hypertension

The effects of K+-canrenoate, a digoxin antagonist, on the role of digoxin-like factor in the development of DOCA-salt hypertension has been examined. DOCA-salt rats treated with 66 mg kg-1 day-1 of K+-canrenoate (s.c.) presented a lower increase in blood pressure (P less than 0.01), less cardiac hypertrophy (P less than 0.05) and hypokalaemia (P less than 0.05) than non-treated DOCA-salt rats. K+-canrenoate treatment did not lead to significant changes in urinary volume, Na+ and K+ urinary excretion or suppression of plasma renin activity in DOCA-salt rats. None of the parameters were significantly different between uninephrectomized-salt rats treated or non-treated with K+-canrenoate. These data suggest a role for digoxin-like factor in DOCA-salt hypertension. However, the non-normalization of blood pressure observed in K+-canrenoate DOCA-salt treated rats indicates that other factors contribute to the initiating mechanisms in this type of hypertension. Moreover, these data suggest that digoxin-like factor plays no role in the suppression of plasma renin activity induced by DOCA and salt treatment.

Endogenous cardiac glycosidelike compounds

The possibility that endogenous inhibitors of the sodium pump exist and bind to the cardiac glycoside binding site on Na+,K+-adenosine triphosphatase (ATPase) has been a source of much controversy. Although numerous hormones and inorganic ions that modulate Na+,K+-ATPase activity have been described, most of these affect the sodium pump indirectly by varying the intracellular sodium concentration or by increasing the number of enzyme units. None of these endogenous compounds has been shown conclusively to modulate sodium pump activity by binding to the cardiac glycoside binding site on Na+,K+-ATPase. However, the near-universal presence of three high-affinity binding sites on the alpha-subunit of the enzyme has engendered much speculation that endogenous ligands for these receptors must exist. In addition, none of the hormones known to indirectly affect sodium pump activity in vivo has been shown to modulate Na+,K+-ATPase activity in response to extracellular volume expansion or to play a role in the pathogenesis of hypertension or chronic renal failure, conditions in which a circulating inhibitor of Na+,K+-ATPase has been implicated. This report presents a condensed history of the search for endogenous inhibitors of Na+,K+-ATPase and describes recent advances in the field. Despite progress in identifying and characterizing compounds that could affect Na+,K+-ATPase activity in vivo, definitive proof for the existence of endogenous ligands for the cardiac glycoside binding site remains elusive.

Activatory effect of two cardioglycosides on Cavia cobaya kidney Na+/K+-ATPase activity

Ouabain and K-strophanthoside promote an enhancement of Na+/K+-ATPase activity in a range of cardioglycoside concentrations from 100 nM to 100 pM, with a maximum (+30%) between 10 and 4 nM. Binding experiments with [3H]ouabain show upward-curved Scatchard plots and evidence two intrinsic affinity constants for the ligand: (a) High-affinity constant: 350 nM (microsomes) and 15 nM (purified enzyme). (b) Low-affinity constant: 2100 nM (microsomes) and 890 nM (purified enzyme). The reaction velocity trend indicates that at ouabain concentrations higher than 20 nM but lower than the minimal inhibiting level, the enhanced reaction velocity is tending towards the control values.

Endogenous digitalis-like factors: in vitro comparison of biological and immunological activities of peptide and steroid candidates

Endogenous substances that modulate the activity of (Na+ + K+)-ATPase through interaction at the cardiac glycoside site have been postulated. Reports of digitalis-like biological and immunological activity exhibited by certain ACTH/MSH peptides and 14-OH steroids make these compounds potential candidates as endogenous digitalis-like factors. We tested several ACTH/MSH peptides and 14 alpha-OH steroids in four in vitro assays and detected no significant cardiac glycoside-like activity. On the other hand, chlormadinone acetate, a progesterone derivative shown to bind with high affinity to the digitalis receptor, was nearly equipotent to digoxigenin in a [3H]ouabain radioreceptor assay. In a [3H]digoxin radioimmunoassay, however, digoxigenin and digoxin were equipotent but chlormadinone acetate was inactive. A clear dissociation between radioreceptor assay and radioimmunoassay activity was also observed using 15 beta-OH-progesterone. Our findings indicate that (a) ACTH/MSH peptides and 14 alpha-OH steroids are not viable candidates as endogenous digitalis-like factors, (b) digoxin antibodies are not necessarily directed at molecular determinants critical for biological activity, and (c) among the compounds reported to exhibit digitalis-like activity and postulated to share structural features with an endogenous steroidal digitalis-like factor, only chlormadinone acetate and its congeners appear to constitute tenable models.