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Dr. Haupert and co-workers have demonstated that human plasma contains a ouabain-binding protein, suggesting a carrier/transport system for EO in human physiology.
Since the discovery of endogenous analogues of opium, endorphins, and encephalins, it has been tempting to postulate that other plant alkaloids, for example, atropine and digitalis, may mimic an intrinsic regulator of vertebrate physiology. Experimental observations suggested that there may be an endogenous analogue of digitalis---a potent inhibitor of membrane Na+, K+ -ATPase ( “sodium pump”). Dr. Haupert’s laboratory and collaborators isolated a steroid derivative from mammalian brain (bovine hypothalamus) with physiological characteristics remarkably parallel to cardiac glycosides, and structural analysis pointed to “endogenous digitalis” as an isomer of the plant glycoside ouabain. The endogenous ouabain (EO) has been linked to the pathogenesis of human essential hypertension (salt-sensitive) and to the development of left ventricular hypertrophy. The differences in stereochemical structure must account for the previously demonstrated differences in biological activity between EO and plant ouabain, as characterized by Dr. Haupert in renal cells, cardiac myocytes, liposomes containing purified Na+,K+ -ATPase, and isolated blood vessels. The physiological studies support the concept that EO acts as a physiological, rather than pharmacological regulator of the mammalian cellular sodium pump. Other studies have shown that EO has potent vasoconstricitive effects in isolated vascular rings in vitro, consistent with the large body of indirect evidence in hypertensive experimental animals and man that excessive activity of an endogenous Na+, K+ - ATPase inhibitor may be involved in the pathogenesis of some forms of hypertensive disease. Dr. Haupert and co-workers have further demonstated that human plasma contains a ouabain-binding protein, suggesting a carrier/transport system for EO in human physiology. They have also raised the first reported ouabain-specific monoclonal antibodies for use as reagents to study the presence of ouabain-like compounds in proposed pathophysiology of hypertension and congestive heart failure. Because of their specificity for ouabain and lack of cross reactivity with digoxin, the cardiac glycoside in prevalent clinical use, these antibodies could make possible standardization in bioassays, and allow clarification of ambiguities in the literature regarding the presence, source, pathogenetic role, and mammalian biosynthetic possibilities for EO. Most recently, using bioinformatics and genomic techniques, a putative steroid biosynthetic pathway was constructed and candidate genes encoding enzymes in this pathway were studied using RNA from hypertensive rat brain. This allowed identification of upregulation of two genes in the pathway compared to RNA from control, normotensive rat brain. Silencing of one of these genes decreased production of EO from neural tissue in vitro. These findings support the possibility that a unique steroid biosynthetic circuit exists in hypertensive rat brain, functioning independently from adrenal.
Research plans underway and for the future are to synthesize the EO candidate compound, and to test the synthetic candidate in biological assays to confirm bioactivity corresponding to that known already for the natural, extracted EO. Success in the synthesis would allow production of large quantities of EO for testing and development of a specific radioimmunoassay for the detection of EO in vivo. In the area of clinical research, correlating amounts of plasma EO and associated clinical consequences of hypertension and left ventricular hypertrophy to mutations of EO pathway genes in humans could identify patients at risk, including prehypertensies. Pharmacological agents that specifically disrupt the EO-sodium pump interaction could provide novel antihypertensive medication for EO-dependent hypertension, illustrating the comprehensive use of functional genomics to direct development of new diagnostics and medical therapeutics.
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