Endogenous natriuretic factors 1: sodium pump inhibition does not correlate with natriuretic or pressor activities from uremic urine

Has the elusive "natriuretic factor" been discovered, and if so, is it a hormone?

This review summarizes the interesting and significant papers reported at the International Conference on Natriuretic and Digitalis-like Factors held at the ASH meeting in San Francisco, June 1-2, 1997. This area of investigation has been rejuvenated as of late with near structural determination of two ouabain-like isolates from human plasma (OLF) and bovine hypothalamus (HIF) which are apparently the same compound and the isolation and structural elucidation of a natriuretic metabolite of gamma-tocopherol. Spectroscopic information has also been obtained for two other compounds, an ouabain-like factor from bovine adrenals and HHIF from the hypothalamus. An explanation was offered for how low concentrations of digitalis-like factors can regulate vascular reactivity when the predominant isoform of the sodium pump has a low affinity for these compounds. Various groups are examining possible in vivo synthetic pathways that could lead to the production digitalis-like factors. The natriuretic metabolite of gamma-tocopherol, LLU-alpha, fits deWardener's postulates for a natriuretic hormone and is being examined for its involvement in ECF control. Once the structures for some of these ouabain-like compounds are determined and they are synthesized, these compounds will also be able to be studied employing classical pharmacologic methods.

A new endogenous natriuretic factor: LLU-alpha

For over three decades, renal physiology has sought a putative natriuretic hormone (third factor) that might control the body's pool of extracellular fluid, an important determinant in hypertension, congestive heart failure, and cirrhosis. In our search for this hormone, we have isolated several pure natriuretic factors from human uremic urine that would appear, alone or in combination, to mark a cluster of phenomena previously presumed to be that of a single "natriuretic hormone." This paper reports the purification, chemical structure, and total synthesis of the first of these compounds, LLU-alpha, which proved to be 2,7,8-trimethyl-2-(beta-carboxyethyl)-6-hydroxychroman, presumably a metabolite of gamma-tocopherol. Both natural LLU-alpha and synthetic material are identical (except for optical activity) with respect to structure and biological activity. It appears that the natriuretic activity of LLU-alpha is mediated by inhibition of the 70 pS K+ channel in the apical membrane of the thick ascending limb of the kidney.

Endogenous natriuretic factors. 5. Synthesis and biological activity of a natriuretic metabolite of diltiazem and its derivatives

In our search for endogenous natriuretic factors from human uremic urine, we have previously identified a new metabolite of the drug diltiazem (Murray et al. Life Sci. 1995, 57, 2145-2161). The structure of this metabolite, (+)-(2S,3S)-3-hydroxy-5-(2-hydroxyethyl)-2,3-dihydro-2-(4-methoxyphenyl) -1,5-benzothiazepin-4(5H)-one (LLU-beta1; 2), was proved by unequivocal synthesis from a diltiazem synthon. The synthetic material also proved to be natriuretic as had the urinary isolate. Given the acetylation at C-3 in diltiazem, the 3-monoacetate (8) and diacetate (3) derivatives of 2 were prepared. The 4-nor-keto (6) derivative of 2 was also synthesized. Only the parent 2 induced natriuresis over a range of doses without accompanying kaliuretic activity at some doses.

Endogenous natriuretic factors 3: isolation and characterization of human natriuretic factors LLU-alpha, LLU-beta 1, and LLU-gamma

A low molecular weight endogenous substance believed to be responsible for extracellular fluid homeostasis in mammals has been sought for many years. Our goal is to isolate and structurally characterize this putative "natriuretic hormone." We have developed an assay using the conscious rat to measure prolonged natriuresis (Benaksas et al (1993) Life Sciences, 52, 1045-1054), the activity originally described for this putative substance. Using this assay we have identified a number of natriuretic compounds isolated from human uremic urine. The collected urine is processed by ultrafiltration (< or = 3 kDa), gel filtration chromatography (G-25) and extraction with isopropanol and diethyl ether. The organic soluble material is then subjected to sequential high-performance liquid chromatography. We report here the initial characterization of two pure isolates (LLU-alpha and LLU-gamma) obtained by this method, and the structural elucidation of a third pure compound, LLU-beta 1, a natriuretic and previously unreported metabolite of the drug diltiazem.