Body fluid variations and endogenous digitalis-like compounds during chronic NaCl loading in Wistar rats

Effect of carbidopa on the excretion of sodium, dopamine, and ouabain-like substance in the rat

The effect of carbidopa, a competitive inhibitor of dopamine synthesis on the excretion of sodium, dopamine (DA), and endogenous sodium transport inhibitor (ESTI), was examined in rats on normal and high salt diets for 7 days. ESTI activity was measured (1) as ouabain-like substance (OLS) by radioimmunoassay using an antibody raised against ouabain, (2) by inhibition of purified Na+,K+-ATPase activity (ATPI), and (3) as digoxin-like substance (DLS) using a commercial digoxin assay. The OLS immunoassay was validated against rubidium transport studies. High salt diet caused an increase in OLS and ATPI but not DLS. Sodium excretion in rats on normal sodium intake was not affected by carbidopa treatment but was significantly lower in the high sodium diet group during the first 5 days of the study. In the low salt group, carbidopa treatment caused significant increases in OLS. We conclude that ESTI (measured as OLS) and DA are important in the natriuresis of salt loading.

Drugs and endogenous ligands compete for receptor occupancy

Dietary and endogenous ligands compete with drugs for receptor occupancy and therefore should be considered during therapeutic interventions and during pharmacokinetic/pharmacodynamic modeling. When disease is the result of an overabundance of these natural ligands, antibodies and/or their Fab fragments may be useful as therapeutic agents to reverse the effects of the natural ligands.

The effect of high salt intake on Na+, K(+)-ATPase activity of tissues in the rat

1. The effect of chronic feeding of high salt diet on Na+, K(+)-ATPase activity of heart, liver, skeletal muscle, kidney and aorta was studied in the rat. 2. Groups of rats were either given tap water or 18 g/L saline to drink. After 7 days, 3 months or 12 months, the control group and salt loaded groups were sacrificed and Na+, K(+)-ATPase activity of heart, liver, skeletal muscle and kidney was determined by a coupled enzyme assay and that of the aorta by the K(+)-stimulated hydrolysis of 3-0-methylfluorescein phosphate. 3. Na+, K(+)-ATPase activity of heart, liver, skeletal muscle and aorta were not different between the experimental and control groups at 7 days. After 3 months, Na+, K(+)-ATPase activity of liver in salt-loaded group was higher than the control group. After 12 months of salt loading all tissues examined showed higher Na+, K(+)-ATPase activity compared to control groups. The activity of renal medulla of salt-loaded group was higher than that of control group as early as 7 days. 4. We conclude that long term salt loading causes an increase in the activity of Na+, K(+)-ATPase of kidney, heart, liver, muscle and aorta.

The effect of salt loading on the urinary excretion of dopamine and sodium transport inhibitor in the rat

1. The temporal relationship between the excretion of dopamine and sodium transport inhibitor (STI) during salt loading was examined in the rat. 2. Urine samples were collected before and during salt loading (given as 18 g/l NaCl solution to replace drinking water) for the measurement of sodium, creatinine, dopamine and STI in 6 female rats. Dopamine was measured by HPLC and STI was extracted and measured by its ability to inhibit purified Na+, K(+)-ATPase enzyme. 3. Urinary sodium and STI (expressed in relation to creatinine) on day 1 of salt loading were 4.6 and 4.2 times respectively of the control values. Urinary excretion of dopamine did not increase significantly until day 2 when it was 21% higher. 4. The excretion of STI paralleled that of sodium excretion whereas the excretion of dopamine lagged behind. 5. We conclude that salt loading increases STI and dopamine and that the increase in STI precedes that of dopamine.

Alterations of membrane properties in erythrocytes of salt hypertensive Sabra rats

This study was designed to investigate the effects of a hypertensive stimulus, high salt intake, in hypertension-prone (SBH) and -resistant (SBN) Sabra rats on erythrocyte Na+ content (Na+i), Ca2+ influx and cytosolic Ca2+ concentration ([Ca2+]i). The relationships of these parameters to plasma lipids, circulating digoxin-like immunoreactivity and membrane microviscosity, determined by the fluorescence anisotropy of trimethylamino-diphenylhexatriene (TMA-DPH) and diphenylhexatriene (DPH), were also evaluated. Erythrocytes of SBH rats were characterized by increased [Ca2+]i, unchanged Ca2+ influx and reduced Na+i. There were no significant differences in the plasma digoxin-like immunoreactivity between the two strains. High-salt intake decreased membrane microviscosity (DPH anisotropy) in SBH rats but did not alter the above parameters. Erythrocyte [Ca2+]i correlated positively with diastolic blood pressure and negatively with erythrocyte Na+i. Membrane dynamics evaluated by the two fluorescent probes did not correlate with [Ca2+]i, Ca2+ influx or Na+i whereas DPH anisotropy was inversely related to blood pressure. These relationships were independent of plasma cholesterol or triglycerides. It can be concluded that 1) similarly to earlier observations in essential hypertension and spontaneously hypertensive rats, erythrocyte [Ca2+]i correlates positively with blood pressure in salt-dependent hypertension, and 2) increased erythrocyte Na+ content need not be a hallmark of hypertension.

Red cell sodium in DOCA-salt hypertension: a Brattleboro study

The alteration of red cell Na+ content (Na+i), its causes and the possible relationship to the development of DOCA-salt hypertension were studied in Brattleboro rats. A pronounced hypertension developed in heterozygous (non-DI) animals that synthesize vasopressin (VP) although no substantial Na+i elevation was observed in their erythrocytes. In contrast, Na+i rose progressively in red cells of homozygous VP-deficient (DI) rats in which only marginal increase of systolic blood pressure was found after six weeks of DOCA-salt regimen. DOCA-salt treatment of non-DI rats did not cause major alterations in ouabain-resistant (OR) net Na+ uptake or ouabain-sensitive (OS) net Na+ extrusion but moderately increased furosemide-sensitive (FS) Rb+ uptake. The same treatment of DI rats doubled Na+i by an increased OR net Na+ uptake (due to a major elevation in both Na(+)-K+ cotransport and Na+ leak). Consequently, OS net Na+ extrusion was augmented in red cells of these animals. This was accompanied by an about threefold elevated FS Rb+ uptake. It can be concluded that a) the alterations of OR and/or OS Na+ or K+ transport observed in erythrocytes of Brattleboro DI rats are not essential for the development of severe DOCA-salt hypertension, b) red cell ion transport abnormalities revealed in DOCA-salt treated DI rats might be rather ascribed to cell potassium depletion, and c) increased inward Na(+)-K+ cotransport and Na+ leak causes red cell Na+i elevation that stimulates Na(+)-K+ pump activity.

Digoxin-like immunoreactivity: is it still worth measuring?

On the assumption that digoxin-like immunoreactivity may represent digitalis-like sodium pump inhibitors in the mammalian body, many investigators have used radioimmunoassay for digoxin to monitor such factors during the past decade. The presence of digoxin-like immunoreactivity has been confirmed by numerous studies using biochemical, immunological or morphological methods. Very recently, ouabain or a very similar substance, which did not cross-react with antidigoxin antibodies, was identified from the human plasma as the long-sought sodium pump inhibitor. However, it is yet to be determined whether sodium pump inhibitory activity in the circulation results from one substance or several. Some researchers still insist on the possible physiological roles of digoxin-like immunoreactivity which may or may not be related to the regulation of sodium pump. These issues are critically reviewed in this article.

Age-dependent salt-induced hypertension in the rat: prevention with DSP-4, a selective noradrenergic neurotoxin

The present study was performed to determine if chronic administration of a high salt diet induces hypertension similarly in young and adult rats and if treatment with DSP-4 alters the development of the hypertension. Three- (young) and ten- (adult) week-old male Sprague-Dawley rats were fed either a standard rat chow diet (0.71% NaCl), a 4% NaCl diet or an 8% NaCl diet for 12 weeks. Systolic blood pressure, using a standard tail-cuff technique, and body weight were recorded weekly during the dietary treatment period. Direct mean arterial pressure, heart rate, heart weight and kidney weight were determined after 12 weeks. Body weight was slightly reduced in young rats on the 8% NaCl diet. A significant increase in blood pressure as well as heart weight was observed only in young rats on the 8% NaCl diet. An increase in kidney weight was observed in both young and adult rats on the 8% NaCl diet. DSP-4 treatment prevented the development of hypertension as well as cardiac hypertrophy in rats fed the high salt diet but had no effect on rats receiving the normal diet. Body and kidney weights were similar in vehicle- and DSP-4-treated rats on the 8% NaCl diet. These results demonstrate that a critical developmental/maturational period exists during which the young rat is susceptible to the hypertensinogenic effects of a high salt diet. An intact central noradrenergic system appears to be necessary for the expression of this enhanced susceptibility and the subsequent development of hypertension.