Normal pregnancy: mechanisms underlying the paradox of a ouabain-resistant state with elevated endogenous ouabain, suppressed arterial sodium calcium exchange, and low blood pressure

Abstract

Endogenous cardiotonic steroids (CTS) raise blood pressure (BP) via vascular sodium calcium exchange (NCX1.3) and transient receptor-operated channels (TRPCs). Circulating CTS are superelevated in pregnancy-induced hypertension and preeclampsia. However, their significance in normal pregnancy, where BP is low, is paradoxical. Here we test the hypothesis that vascular resistance to endogenous ouabain (EO) develops in normal pregnancy and is mediated by reduced expression of NCX1.3 and TRPCs. We determined plasma and adrenal levels of EO and the impact of exogenous ouabain in pregnancy on arterial expression of Na(+) pumps, NCX1.3, TRPC3, and TRPC6 and BP. Pregnant (embryonic day 4) and nonpregnant rats received infusions of ouabain or vehicle. At 14-16 days, tissues and plasma were collected for blotting and EO assay by radioimmunoassay (RIA), liquid chromatography (LC)-RIA, and LC-multidimensional mass spectrometry (MS3). BP (-8 mmHg; P < 0.05) and NCX1.3 expression fell (aorta -60% and mesenteric artery -30%; P < 0.001) in pregnancy while TRPC expression was unchanged. Circulating EO increased (1.14 ± 0.13 nM) vs. nonpregnant (0.6 ± 0.08 nM; P < 0.05) and was confirmed by LC-MS3 and LC-RIA. LC-MS3 revealed two previously unknown isomers of EO; one increased ∼90-fold in pregnancy. Adrenal EO but not isomers were increased in pregnancy. In nonpregnant rats, similar infusions of ouabain raised BP (+24 ± 3 mmHg; P < 0.001). In ouabain-infused rats, impaired fetal and placental growth occurred with no BP increase. In summary, normal pregnancy is an ouabain-resistant state associated with low BP, elevated circulating levels of EO, two novel steroidal EO isomers, and increased adrenal mass and EO content. Ouabain raises BP only in nonpregnant animals. Vascular resistance to the chronic pressor activity of endogenous and exogenous ouabain is mediated by suppressed NCX1.3 and reduced sensitivity of events downstream of Ca(2+) entry. The mechanisms of EO resistance and the impaired fetal and placental growth due to elevated ouabain may be important in pregnancy-induced hypertension (PIH) and preeclampsia (PE).

Reduction in Maternal Circulating Ouabain Impairs Offspring Growth and Kidney Development

“…In agreement with previous studies, 15,21 we found higher levels of circulating ouabain in pregnant women ( Figure 10A). However, no significant differences in the steroid concentration were found at the different times tested.…”

“…First, circulating ouabain levels were determined in nonpregnant and pregnant mice. As shown in other studies, 15 the levels of the steroid are increased by .3-fold during pregnancy (Figure 2). Notably, because the determination of ouabain is based on the interaction with antibodies, immunoreactive material is actually being measured.…”

“…In addition, Jacobs et al recently demonstrated that the administration of ouabain to rats under normal conditions impaired fetal and placental growth. 15 We also examined the effect of an elevation in maternal circulating ouabain on offspring development using the experimental system described in this study. The administration of ouabain (15 mg/kg per day) to pregnant mice resulted in similar effects to that of a reduction in the steroid levels-namely, a reduction in offspring body weight (Supplemental Figure 1) and a decrease in kidney and liver weights, without changes in heart and brain weights ( Supplemental Figure 2).…”

Arterial α2-Na+pump expression influences blood pressure: lessons from novel, genetically engineered smooth muscle-specific α2mice

“…In contrast, mice with one null ␣ 1 -allele (␣ 1 ϩ/Ϫ ) express many fewer total Na ϩ pumps in ASMCs (ϳ60% of normal) but have normal basal BP (57). In addition, plasma EO is elevated in many humans with essential hypertension or mineralocorticoid hypertension (33,39,49), in several rodent hypertension models (14,15,24,27), and in pregnant rats (19). Indeed, prolonged ouabain administration induces hypertension in rodents (11,12,34).…”

2011 Homer Smith Award

“…Circulating levels of ouabain are increased during pregnancy. 82 Patients with renal failure have increased levels of endogenous CTS, and there is evidence that the high levels of CTS may be deleterious for the cardiovascular system. This would particularly be the case for patients who have also been given CTS drugs.…”