The role of endogenous inhibition of Na-K-ATPase in human hypertension--sodium pump activity as a determinant of peripheral vascular resistance

The Renin-Angiotensin System in the Development of Salt-Sensitive Hypertension in Animal Models and Humans

Hypertension is still one of the major causes of death from cardiovascular failure. Increased salt intake may aggravate the rise in blood pressure and the development of consequential damage of the heart, the vessels and other organs. The general necessity of restricted salt intake regardless of blood pressure or salt sensitivity has been a matter of debate over the past decades. This review summarizes the main pathogenic mechanisms of hypertension and salt sensitivity in rat models, particularly in the spontaneously hypertensive rat (SHR), and in patients with essential hypertension (EH). Although SHRs are commonly considered to be salt-resistant, there is much evidence that salt loading may deteriorate blood pressure and cardiovascular function even in these animals. Similarly, EH is not a homogenous disorder - some patients, but not all, exhibit pronounced salt sensitivity. The renin-angiotensin system (RAS) plays a key role in the regulation of blood pressure and salt and fluid homeostasis and thus is one of the main targets of antihypertensive therapy. This review focuses on the contribution of the RAS to the pathogenesis of salt-sensitive hypertension in SHRs and patients with EH.

Ouabain-like factors in human urine: identification of a Na-K-ATPase inhibitor as vanadium-diascorbate adduct

We investigated endogenous Na-K-ATPase inhibitors, i.e. ouabain-like factors(OLFs), in the urine of salt-loaded healthy subjects. During an intake of > 30g NaCl/day 24h-urines were collected, lyophilized, redissolved and acidified to pH 3.5. With gelchromatography the inhibitory activity eluted in a post-salt fraction FIV from Sephadex G-25. When this fraction was again passed through Sephadex G-10, one of three OLFs eluted in the early subfractions FIV/1-2 close to H-ouabain and cross-reacted strongly with a ouabain antibody (NEN). Two additional OLFs with Mr around 400 eluted in a late subfraction FIV/8 which resolved after reverse-phase HPLC into a more polar OLF- (water phase) and a more apolar OLF-2 (20% acetonitrile). Only the more apolar OLF-2 cross-reacted with digoxin and ouabain antibodies. OLF-1 and OLF-2 purified to single compounds by preparative thin layer chromatography inhibited Na-K-ATPase with IC50 of around 1.5 x 10(-5) M and 1.5 x 10(-4) M, respectively. Identification of OLF-2 was first attempted because most material was available for further processing. Data from mass-spectroscopy, nuclear magnetic resonance (1H-NMR) and infrared spectroscopy characterized OLF-2 as structurally unrelated to ouabain but resembling ascorbic acid derivatives, i.e. vanadium (V) diascorbates (Mr 403) with similar elution times from RP-HPLC as OLF-2. They inhibited the enzyme in its E2-configuration with IC50 of 9 x 10(-5) M and 2 x 10(-6) M for V(IV)- and V(V)-diascorbate, respectively. OLF-1, OLF-2 and V-diascorbate raise intracellular free calcium in inner medullary collecting duct and vascular smooth muscle cells which also contract in vitro. V-diascorbate was also natriuretic in a bioassay. We suggest that V-diascorbates represent one of several OLFs excreted in human urine.

Modifications induced by plasma of gestational hypertensive women on the Na+/K+-ATPase obtained from human placenta

In order to investigate the molecular mechanisms of the inhibition of Na+/K+-ATPase in Gestational Hypertension (GH), we incubated Na+/K+-ATPase purified from human placenta of 6 healthy normotensive women with plasma from 6 GH women and 6 healthy controls. We determined the enzyme activity by the method of Esman, and the anthroyl-ouabain-binding capacity, dissociation constant (Kd) and average lifetime values (tau) by the static and dynamic fluorescence of anthroyl-ouabain. The lipid annulus of the enzyme was studied by static and dynamic fluorescence of 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5- hexatriene (TMA-DPH). The addition of total and protein-free GH plasma to normal Na+/K+-ATPase significantly inhibited the enzymatic activity even at the lowest concentration studied (1:100), as well as the ouabain-binding capacity, Kd and tau. GH plasma significantly decreased the fluorescence polarization and lifetime values of TMA-DPH. These observations indicate that the inhibition caused by GH plasma on Na+/K+-ATPase might be due to a reduction of the number of active molecules or a modification of the ouabain-binding site suggesting the existence of digitalis-like factor. A link between the modification of the lipid moiety of the enzyme and the Na+/K+-ATPase inhibition might be hypothesized.

Nifedipine antagonizes ouabain-induced ST-segment changes and derangement of epicardial activation pattern in isolated rabbit hearts

Isolated rabbit hearts, perfused according to the Langendorff technique were treated with 0.2 mumol/l ouabain or with a combination of 0.2 mumol/l ouabain and 3 nmol/l nifedipine. In a second series of experiments, a combination of 0.2 mumol/l ouabain with 0.2 mumol/l nitroglycerin was examined. Under this treatment, coronary flow and left ventricular pressure were continuously measured and, furthermore, the epicardial potential distribution was mapped with high temporal (4 kHz/channel) and spatial resolution (256 electrodes, arranged in 4 plates with 64 electrodes each, 8 x 8 matrix with 1 mm mesh size). Ouabain led to the expected positive inotropy of 10%, but also to a decrease in coronary flow of 15%. Besides these changes a nearly generalized ST-segment elevation could be observed. Moreover, the epicardial activation pattern was disturbed by changes in the location of epicardial breakthrough-points. Concomitantly, the epicardial activation vector field was deranged. Under the additional influence of nifedipine, coronary flow was reduced by only 5%, whereas the positive inotropic effect remained unchanged. The epicardial ST-elevation was diminished significantly and there was no derangement in the process of epicardial activation. Nitroglycerin led to an increase in relative coronary flow comparable to that observed under nifedipine but did not antagonize the disturbances of the process of activation by ouabain and only partially inhibited ST-elevation. Hence, it is concluded that ouabain already provokes coronary vasoconstriction in therapeutic concentrations leading to ST-elevation and prearrhythmic changes in the process of excitation. These changes could be diminished by additional treatment with nifedipine, but not with nitroglycerin.