Acute Na+,K+-ATPase inhibition with bufalin impairs pressure natriuresis in the rat

Natriuretic hormones, endogenous ouabain, and related sodium transport inhibitors

The work of deWardener and colleagues stimulated longstanding interest in natriuretic hormones (NHs). In addition to the atrial peptides (APs), the circulation contains unidentified physiologically relevant NHs. One NH is controlled by the central nervous system (CNS) and likely secreted by the pituitary. Its circulating activity is modulated by salt intake and the prevailing sodium concentration of the blood and intracerebroventricular fluid, and contributes to postprandial and dehydration natriuresis. The other NH, mobilized by atrial stretch, promotes natriuresis by increasing the production of intrarenal dopamine and/or nitric oxide (NO). Both NHs have short (<35 min) circulating half lives, depress renotubular sodium transport, and neither requires the renal nerves. The search for NHs led to endogenous cardiotonic steroids (CTS) including ouabain-, digoxin-, and bufadienolide-like materials. These CTS, given acutely in high nanomole to micromole amounts into the general or renal circulations, inhibit sodium pumps and are natriuretic. Among these CTS, only bufalin is cleared sufficiently rapidly to qualify for an NH-like role. Ouabain-like CTS are cleared slowly, and when given chronically in low daily nanomole amounts, promote sodium retention, augment arterial myogenic tone, reduce renal blood flow and glomerular filtration, suppress NO in the renal vasa recta, and increase sympathetic nerve activity and blood pressure. Moreover, lowering total body sodium raises circulating endogenous ouabain. Thus, ouabain-like CTS have physiological actions that, like aldosterone, support renal sodium retention and blood pressure. In conclusion, the mammalian circulation contains two non-AP NHs. Identification of the CNS NH should be a priority.

Natriuretic effect of bufalin in isolated rat kidneys involves activation of the Na+-K+-ATPase-Src kinase pathway

Bufadienolides are structurally related to the clinically relevant cardenolides (e.g., digoxin) and are now considered as endogenous steroid hormones. Binding of ouabain to Na(+)-K(+)-ATPase has been associated, in kidney cells, to the activation of the Src kinase pathway and Na(+)-K(+)-ATPase internalization. Nevertheless, whether the activation of this cascade also occurs with other cardiotonic steroids and leads to diuresis and natriuresis in the isolated intact kidney is still unknown. In the present work, we perfused rat kidneys for 120 min with bufalin (1, 3, or 10 μM) and measured its vascular and tubular effects. Thereafter, we probed the effect of 10 μM 3-(4-chlorophenyl)1-(1,1-dimethylethyl)-1H-pyrazolo[3,4-d]pyrimidin-4amine (PP2), a Src family kinase inhibitor, and 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio] butadiene (UO126), a highly selective inhibitor of both MEK1 and MEK2, on bufalin-induced renal alterations. Bufalin at 3 and 10 μM profoundly increased several parameters of renal function in a time- and/or concentration-dependent fashion. At a concentration that produced similar inhibition of the rat kidney Na(+)-K(+)-ATPase, ouabain had a much smaller diuretic and natriuretic effect. Although bufalin fully inhibited the rat kidney Na(+)-K(+)-ATPase in vitro, its IC(50) (33 ± 1 μM) was threefold higher than the concentration used ex vivo and all its renal effects were blunted by PP2 and UO126. Furthermore, the phosphorylated (activated) ERK1/2 expression was increased after bufalin perfusion and this effect was totally prevented after PP2 pretreatment. The present study shows for the first time the direct diuretic, natriuretic, and kaliuretic effects of bufalin in isolated rat kidney and the relevance of Na(+)-K(+)-ATPase-mediated signal transduction.

Developmental programming of aortic and renal structure in offspring of rats fed fat-rich diets in pregnancy

Evidence from human and animal studies suggests that maternal nutrition can induce developmental programming of adult hypertension in offspring. We have previously described a model of maternal dietary imbalance in Sprague-Dawley rats whereby administration of a maternal diet rich in animal lard programmes the development of increased blood pressure, insulin resistance, dyslipidaemia, obesity and mesenteric artery endothelial dysfunction in adult offspring. To further characterize the mechanism of hypertension in this model we have examined vascular and renal structure in adult offspring of Sprague-Dawley rats fed a control diet (OC) or lard-rich diet (OHF) during pregnancy and suckling followed by a control diet post-weaning. To gain further insight, we assessed aortic reactivity and elasticity in an organ bath preparation and renal renin and Na+,K+-ATPase activity. Plasma aldosterone concentration was also measured. Stereological examination of the aorta in OHF demonstrated reduced endothelial cell volume and smooth muscle cell number compared with OC. Adult OHF animals showed increased aortic stiffness and reduced endothelium-dependent relaxation. Renal stereology showed no differences in kidney weight, glomerular number or volume in OHF compared with OC, but renin and Na+,K+-ATPase activity were significantly reduced in OHF compared with controls. Programmed alterations to aortic structure and function are consistent with previous observations that exposure to maternal high fat diets produces systemic vascular changes in the offspring. Despite normal renal stereology, altered renal Na+,K+-ATPase and renin activity offers further insight into the mechanism underlying the increased blood pressure characteristic of this model.

Linkage of hypertension to chromosome 2q14-q23 in Chinese families

OBJECTIVES

To identify chromosome regions containing hypertension susceptibility genes in Chinese.

SUBJECTS AND METHODS

A three-stage study was carried out in Chinese siblings ascertained through outpatient clinics. In the first stage, 283 affected sib-pairs from 79 nuclear families were subjected to a genome-wide scan with 240 microsatellite marker loci. The second stage focused on chromosome 2 with additional markers resulting in an average distance of 5 cM and used an independent sample of 637 affected sib-pairs from 161 families. In the third stage, a fine-scale mapping study on the suggestive region was performed in an independent set of 777 affected sib-pairs from 106 families. Fourteen markers were used with an average distance less than 2 cM. Non-parametric linkage analyses (NPL), parametric linkage analyses and transmission-disequilibrium tests were used to assess evidence for linkage and association.

RESULTS

Three markers (D2S168 at 27.06 cM, D2S151 at 152.04 cM and D2S142 at 161.26 cM) on chromosome 2 with suggestive linkage to hypertension susceptibility genes were identified in the genome-wide scan. In stage II, the suggestive region around D2S151 and D2S142 was replicated, while the linkage around D2S168 was not. In the stage III fine-scale mapping study, multipoint linkage analyses showed LOD scores greater than 2.0 throughout a region between 157.16 cM and 162.46 cM (all P < 0.001) with a maximum peak of 2.24 (P= 0.00067) at 160.52 cM. We also observed a NPL Z-score peak of 3.27 at 157.55 cM (P= 0.00086).

CONCLUSIONS

The results of a suggestive region on chromosome 2q14-q23 (D2S112-D2S2370) were consistent between each of the three studies. Interestingly, this region overlaps a syntenic region that contains blood pressure quantitative trait loci identified in rat models of hypertension. These data suggest that the region near D2S142 and D2S151 deserves to be further screened for hypertension susceptibility genes.

Natriuresis and inhibition of Na+/K(+)-ATPase: modulation of response by physiological manipulation

1. There is considerable evidence for the existence of an endogenous inhibitor of Na+/K(+)-ATPase. The exact physiological nature and role of this postulated agent remains unclear, although it would be predicted that one of its actions would be stimulation of renal sodium excretion. 2. The natriuretic effect of renal arterial infusion of ouabain is relatively slow in onset and is sustained. 3. The natriuresis is not modified by changes in sodium status, unlike the natriuretic effect of atrial natriuretic peptide. 4. The natriuretic action of ouabain is enhanced dramatically by acute volume expansion or chronic mineralocorticoid treatment, which both result in hypokalaemia, hypertension and hypervolaemia. 5. The natriuretic response to small increments in blood pressure is markedly enhanced by treatment with ouabain. 6. We hypothesize that the interaction between the inhibition of Na+/K(+)-ATPase and elevated blood pressure could result in the shedding of sodium in conditions where there are increased levels of circulating endogenous digitalis-like factors.