In ability of Na+,K+-ATPase inhibitor to cause hypertension in sodium-loaded or deoxycorticosterone-treated one kidney rats

Effect of long-term ouabain treatment on contractile responses of rat aortae

Male Sprague-Dawley rats were infused with 50 microg/kg/day of ouabain for 4 weeks to address the question whether prolonged exposure to the drug affects blood pressure, the in vitro contractile responses to agonists and high K+ of their aortae, and the influence of endothelium on these responses. Systolic blood pressure was not affected by ouabain treatment. The responsiveness of endothelium-intact aortae from ouabain-treated rats to endothelin-1 increased, that to phenylephrine decreased, and that to high K+ was unchanged, as compared with control. The responses of endothelium-free aortae to endothelin-1, phenylephrine, and high K+ were lower in ouabain-treated than in control rats. The removal of endothelium increased the response to phenylephrine and decreased that to high K+ in either control or ouabain-treated rat aortae, whereas it did not affect the response to endothelin-1 in control rat aortae and decreased it in ouabain-treated rat aortae. The response to caffeine was unaffected by either ouabain treatment or endothelium removal. Thus rat ouabain long-term treatment induces opposing effects on the responsiveness of their intact aortae to an alpha-adrenergic agonist and endothelin-1. If these effects observed in the ex vivo experiments occur also in vivo on rat microvasculature, they could balance out and contribute to the lack of effect on systolic blood pressure of prolonged ouabain treatment.

Long-term ouabain administration does not alter blood pressure in conscious Sprague-Dawley rats

1. We tested the ability of ouabain to cause chronic hypertension by continuously infusing ouabain for 28 days (miniosmotic pump implantation; i.p.). The blood pressure and metabolic effects of sham (150 mmol/L NaCl; n = 12) or ouabain infusion (10 micrograms/kg per day; n = 14; 100 micrograms/kg per day; n = 14) were examined in conscious Sprague-Dawley rats. 2. Plasma ouabain concentrations measured after 28 days of ouabain infusion were as follows: sham, not detectable (n = 11); ouabain 10 micrograms/kg per day, 0.60 +/- 0.07 nmol/L (n = 14); and ouabain 100 micrograms/kg per day, 7.17 +/- 0.57 nmol/L (n = 14; P < 0.001). 3. Sham or ouabain infusion did not alter food intake, bodyweight, water intake or urine output in conscious rats. 4. Blood pressure was not altered by sham treatment. Ouabain at 10 micrograms/kg per day or 100 micrograms/kg per day did not produce consistent rises in blood pressure. Ouabain at 10 micrograms/kg per day increased blood pressure on treatment day 12 only (+6 mmHg; P < 0.05), while at 100 micrograms/kg per day blood pressure increased on treatment days 16 (+9 mmHg; P < 0.05) and day 18 (+8 mmHg; P < 0.05) only. There was no significant difference in blood pressure between sham and ouabain groups. 5. Renal blood flow was decreased in rats infused with ouabain at 10 micrograms/kg per day (2.0 +/- 0.3 mL/min per 100 g bodyweight; n = 5; P < 0.01) and 100 micrograms/kg per day (2.2 +/- 0.4 mL/min per 100 g bodyweight; n = 7; P < 0.05) compared with sham treatment (3.5 +/- 0.2 mL/min per 100 g bodyweight; n = 6). Renal vascular resistance was increased in rats treated with ouabain at 10 micrograms/kg per day (65.5 +/- 12.6 mmHg/mL per min per 100 g bodyweight; n = 5; P < 0.01) and 100 micrograms/kg per day (66.0 +/- 15.6 mmHg/mL per min per 100 g bodyweight; n = 7; P < 0.05) compared with sham treatment (32.6 +/- 2.5 mmHg/mL per min per 100 g bodyweight; n = 6). 6. High plasma concentrations of ouabain do not cause consistent increases in blood pressure in conscious Sprague-Dawley rats.

Endogenous digitalis: reality or myth?

Endogenous digitalis is defined as a natural ligand for the digitalis-binding site of the Na+, K(+)-ATPase and is a specific, high-affinity reversible inhibitor of the enzyme activity. Such endogenous digitalis is thought to be involved in sodium homeostasis and blood pressure regulation as a vasoactive and natriuretic substance. The search for endogenous digitalis goes back to the early 1960s. Since then large efforts have been exerted by numerous laboratories worldwide, but little advance has been made until recently except for the identification of nonspecific Na+, K(+)-ATPase inhibitors. Some researchers even doubt the existence of endogenous digitalis. The recognition that assay methodology is associated with many pitfalls and problems has accelerated the rate of recent progress. Chemical identification of endogenous digitalis will be forthcoming in the very near future. In this article, important issues surrounding endogenous digitalis are critically reviewed.

Digoxin enhances the pressor response to aldosterone administration in conscious sheep

1. This study examined the hypothesis that inhibition of Na,K ATPase with digoxin would enhance the pressor response to aldosterone infusion in conscious sheep. 2. While intravenous infusion of digoxin (10 micrograms/kg per day for 5 days) had no effect on blood pressure and aldosterone infusion (6 micrograms/kg per day for 5 days) increased blood pressure by 7 mmHg, combined infusion of digoxin and aldosterone increased blood pressure by 17 mmHg. 3. The metabolic effects of the combined digoxin and aldosterone infusion were similar to those for aldosterone alone, suggesting that digoxin did not enhance the mineralocorticoid action of aldosterone. 4. The results of this study suggest that changes in Na influx (aldosterone-dependent) and efflux (digoxin-dependent) are important in the genesis of aldosterone-induced hypertension.