Steroid Biosynthesis and Renal Excretion in Human Essential Hypertension: Association With Blood Pressure and Endogenous Ouabain

Endogenous ouabain (EO), a mammalian counterpart to the plant-derived cardiac glycoside ouabain, is synthesized and released from the adrenal glands and possibly from the hypothalamus. EO is a versatile modulator of the ubiquitously expressed Na+ pump and has been linked to human essential hypertension and cardiac hypertrophy.1–6 Also in genetically hypertensive Milan rats (MHS), Na/K-ATPase activity and blood pressure levels are associated (M. Ferrandi, personal communication) to elevated hypothalamic and plasma levels of EO compared to control rats (MNS).7,8 The molecular basis for the elevated EO in MHS was probed in the hypothalamus and adrenal using bioinformatics and genomic techniques. Elevated transcripts for cholesterol side-chain cleavage (also known as cytochrome P450scc, CYP11A1) and β-hydroxysteroid dehydrogenase/δ5-4 isomerase genes (HSD3B) were detected in hypothalamus but not in the adrenal.9 Other studies have suggested that, as with aldosterone (Aldo), the biosynthesis of cardiac glycosides by the adrenal gland likely involves cholesterol side-chain cleavage to form pregnenolone with further metabolism of progesterone.10–12 Recently, we observed marked increases in circulating EO in uremic patients.13 These data imply that, in addition to secretion, renal clearance is one of the major determinants of plasma EO. The recovery of the cardiac glycosides digoxin and ouabain at the basolateral membrane of the proximal tubular cell is mediated by an organic anion transporting polypeptide (SLCO4C1).14 In addition, the secretion of digoxin at the apical membrane is mediated by the P-glycoprotein (PGP) efflux transporter, a protein encoded by the multidrug resistance 1 gene (MDR1).15 The MDR1 has a broad substrate specificity and is modulated transcriptionally by steroid hormones including ouabain.16–18 Recent work shows that MDR1 variants are associated with blood pressure in humans.19 Accordingly, the CYP11A1, HSD3B, SLCO4C1, and MDR1 genes may modulate blood pressure via changes in circulating EO and Aldo. To address this, we probed for the association of common single-nucleotide polymorphisms (SNPs) and haplotypes of these genes with blood pressure, Aldo, and EO levels in patients with never-treated mild essential hypertension. A total of 26 SNPs were genotyped. These SNPs were selected by tagging or functional criteria and accounted for most of the variants for these genes in the Caucasian population.