Endogenous sodium pump inhibition: current status and therapeutic opportunities

Association of cord blood digitalis-like factor and necrotizing enterocolitis

OBJECTIVE

Endogenous digoxin-like factor (EDLF) has been linked to vasoconstriction, altered membrane transport, and apoptosis. Our objective was to determine whether increased EDLF in the cord sera of preterm infants was associated with an increased incidence of necrotizing enterocolitis (NEC).

STUDY DESIGN

Cord sera from pregnant women enrolled in a randomized trial of MgSO4 for fetal neuroprotection were analyzed for EDLF using a red cell Rb(+) uptake assay in which the inhibition of sodium pump-mediated Rb(+) transport was used as a functional assay of EDLF. Specimens were assayed blinded to neonatal outcome. Cases (NEC, n = 25) and controls (neonates not developing stage 2 or 3 NEC, n = 24) were matched by study center and gestational age. None of the women had preeclampsia. Cases and controls were compared using the Wilcoxon test for continuous and the Fisher exact test for categorical variables. A conditional logistic regression analysis was used to assess the odds of case vs control by EDLF level.

RESULTS

Cases and controls were not significantly different for gestational age, race, maternal steroid use, premature rupture of membranes, or MgSO4 treatment. In logistic models adjusted for treatment group, race, premature rupture of membranes, and gestational age, cord sera EDLF was significantly associated with development of NEC (P = .023).

CONCLUSION

These data demonstrated an association between cord sera EDLF and NEC.

Spherical Oligo-Silicic Acid SOSA Disclosed as Possible Endogenous Digitalis-Like Factor

The Na(+)/K(+)-ATPase is a membrane ion-transporter protein, specifically inhibited by digitalis glycosides used in cardiac therapy. The existence in mammals of some endogenous digitalis-like factors (EDLFs) as presumed ATPase ligands is generally accepted. But the chemical structure of these factors remained elusive because no weighable amounts of pure EDLFs have been isolated. Recent high-resolution crystal structure data of Na(+)/K(+)-ATPase have located the hydrophobic binding pocket of the steroid glycoside ouabain. It remained uncertain if the EDLF are targeting this steroid-receptor or another specific binding site(s). Our recently disclosed spherical oligo-silicic acids (SOSA) fulfill the main criteria to be identified with the presumed EDL factors. SOSA was found as a very potent inhibitor of the Na(+)/K(+)-ATPase, Ca(2+)-ATPase, H(+)/K(+)-ATPase, and of K-dp-ATPase, with IC50 values between 0.2 and 0.5 μg/mL. These findings are even more astonishing while so far, neither monosilicic acid nor its poly-condensed forms have been remarked biologically active. With the diameter ϕ between 1 and 3 nm, SOSA still belong to molecular species definitely smaller than silica nano-particles with ϕ > 5 nm. In SOSA molecules, almost all Si-OH bonds are displayed on the external shell, which facilitates the binding to hydrophilic ATPase domains. SOSA is stable for long term in solution but is sensitive to freeze-drying, which could explain the failure of countless attempts to isolate pure EDLF. There is a strong resemblance between SOSA and vanadates, the previously known general inhibitors of P-type ATPases. SOSA may be generated endogenously by spherical oligomerization of the ubiquitously present monosilicic acid in animal fluids. The structure of SOSA is sensitive to the concentration of Na(+), K(+), Ca(2+), Mg(2+), and other ions suggesting a presumably archaic mechanism for the regulation of the ATPase pumps.

Application of supercritical fluid chromatography to characterize a labile digitalis-like factor

A sodium pump inhibitor (digitalis-like factor), isolated from the peritoneal dialysate of volume-expanded, hypertensive patients with kidney failure who were treated with this dialysis modality, was further purified and characterized by means of supercritical fluid chromatography, a separation technique whose application to very-low-concentration biomolecules is new. Previous studies suggested that after high-performance liquid chromatography (HPLC) purification, this inhibitor was the only factor correlated with volume status and blood pressure in these patients. When this same HPLC fraction was furthered purified on 2-dimensional supercritical fluid chromatography, a single peak coeluted with [Na,K]ATPase inhibitory activity. When split specimens were used, there was a strict correlation between the peak area, measured by flame ionization detection, and activity (n=10, R=0.98, P=0.00001). Inhibitory activity after supercritical fluid chromatography was still correlated with the degree of volume expansion of donor patients (P=0.01). After HPLC purification, this volume-sensitive inhibitor was chemically labile. With further purification on supercritical fluid chromatography, the active peak was still labile with comparable half-life. Supercritical fluid chromatography coupled with flame ionization detection provided an estimate of the amount of the inhibitor present. Again using split specimens, we determined that the labile digitalis-like factor was approximately 30-fold more effective than ouabain in inhibiting renal [Na,K]ATPase activity and >/=500 times more effective than ouabain in causing vascular smooth muscle contraction. The data suggest that we have purified to homogeneity a labile digitalis-like factor that is readily distinguished from ouabain or bufalin, based on chromatographic characteristics, chemical lability, and a much lower effective concentration for its biological activity.