Endogenous inhibitors of the Na, K-ATPase in preeclampsia

Abstract

Objectives: Previously we have shown, that mammalian tissues contain a steroidal inhibitor of Na, K-ATPase which is similar to Amphibian vasoconstrictor hormone, marinobufagenin (MBG). The mammalian MBG is implicated in plasma volume dependent forms of hypertension. We compared plasma levels of MBG, in normotensive pregnancy and in preeclampsia with that of ouabain-like compound (OLC), and characterized the partially purified MBG immunoreactive factor from preeclamptic plasma.

Full Text

Methods: Plasma MBG and OLC were measured by solid phase fluoroimmunoassays. MBG- and ouabain immunoreactive materials were partially purified from preeclamptic plasma via reverse-phase HPLC and studied for their ability to react with MBG and ouabain antibodies, and to inhibit the Na, K-ATPase from human mesenteric arteries. Vasoconstrictor effect of authentic MBG was studied in isolated rings of human umbilical arteries.

Results: In 11 nonpregnant controls plasma concentrations of MBG and OLC were 0.19 ± 0.04 nmol/L and 0.297 ± 0.037 nmol/L, respectively. In the third trimester of noncomplicated pregnancy (n = 6), plasma MBG increased (0.625 ± 0.067nmol/L, P< 0.05), and OLC did not (0.32 ± 0.07 nmol/L). In 15patients with preeclampsia plasma levels of both MBG and OLC increased dramatically (2.63 ± 0.10 nmol/L and 0.697 ± 0.16 nmol/L, respectively, P<0.01 vs. both control groups). When fractionated by reverse phase HPLC, OLC and MBG were eluted by 18% and 48% acetonitrile, respectively. Serially diluted samples of MBG and OLC immunoreactive material from HPLC fractions reacted with MBG and ouabain antibody in a concentration dependent fashion. Authentic MBG constricted isolated rings of human mesenteric arteries in a concentration-dependent manner. HPLC purified MBG immunoreactive material from preeclamptic plasma inhibited Na, K-ATPase purified from human mesenteric artery similarly to the authentic MBG.

Conclusions: Our observations demonstrate the coexistence of a more polar OLC and a less polar MBG-like compound in human plasma. Substantial increases in plasma OLC and MBG immunoreactivity in preeclampsia, along with the vasoconstrictor properties of authentic MBG and Na, K-ATPase inhibitory activity of human MBG immunoreactive factor, suggest, that in preeclampsia, plasma concentrations of MBG are elevated enough to substantially inhibit the sodium pump in cardiovascular tissues. These findings attribute MBG a pathogenic role in the preeclamptic hypertension.