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

Augmented Ouabain-Induced Vascular Response Reduces Cardiac Efficiency in Mice with Migraine-Associated Mutation in the Na+, K+-ATPase α2-Isoform

Heterozygous mice (α₂^+/G301R mice) for the migraine-associated mutation (G301R) in the Na⁺,K⁺-ATPase α₂-isoform have decreased expression of cardiovascular α₂-isoform. The α₂^+/G301R mice exhibit a pro-contractile vascular phenotype associated with decreased left ventricular ejection fraction. However, the integrated functional cardiovascular consequences of this phenotype remain to be addressed in vivo. We hypothesized that the vascular response to α₂-isoform-specific inhibition of the Na⁺,K⁺-ATPase by ouabain is augmented in α₂^+/G301R mice leading to reduced cardiac efficiency. Thus, we aimed to assess the functional contribution of the α₂-isoform to in vivo cardiovascular function of wild-type (WT) and α₂^+/G301R mice. Blood pressure, stroke volume, heart rate, total peripheral resistance, arterial dP/dt, and systolic time intervals were assessed in anesthetized WT and α₂^+/G301R mice. To address rate-dependent cardiac changes, cardiovascular variables were compared before and after intraperitoneal injection of ouabain (1.5 mg/kg) or vehicle during atrial pacing. The α₂^+/G301R mice showed an enhanced ouabain-induced increase in total peripheral resistance associated with reduced efficiency of systolic development compared to WT. When the hearts were paced, ouabain reduced stroke volume in α₂^+/G301R mice. In conclusion, the ouabain-induced vascular response was augmented in α₂^+/G301R mice with consequent suppression of cardiac function.

Increased constrictor tone induced by ouabain treatment in rats

Ouabain (Oua)-induced hypertension in rodents provides a model to study cardiovascular changes associated with human hypertension. We examined vascular function in rats after a long-term treatment with Oua. Systolic blood pressure was measured by tail-cuff plethysmography in male Sprague-Dawley rats treated with Oua (≈ 25 µg/d) or placebo for 8 weeks. Blood pressure increased in Oua-treated animals, reaching 30% above baseline systolic blood pressure after 7 weeks. At the end of treatment, vascular responses were studied in mesenteric resistance arteries (MRAs) by wire myography. Contraction to potassium chloride in intact and denuded arteries showed greater sensitivity in Oua-treated animals. Contraction to phenylephrine and relaxation to acetylcholine were similar between groups with a lower response to sodium nitroprusside in Oua-treated arteries. Sensitivity to endothelin-1 was higher in Oua-treated arteries. Na⁺-K⁺ ATPase activity was decreased in MRAs from Oua-treated animals, whereas protein expression of the Na⁺-K⁺ ATPase α₂ isoform was increased in heart and unchanged in mesenteric artery. Preincubation with indomethacin (10⁻⁵ M) or Nω-nitro-L-arginine methyl ester (10⁻⁴ M) abolished the differences in potassium chloride response and Na⁺-K⁺ ATPase activity. Changes in MRAs are consistent with enhanced vascular smooth muscle cell reactivity, a contributor to the increased vascular tone observed in this model of hypertension.

Contribution of the endothelin and renin-angiotensin systems to the vascular changes in rats chronically treated with ouabain

Renin-angiotensin and endothelin systems are involved in the cardiovascular effects produced by treatment with ouabain. We recently demonstrated that the contractile response to phenylephrine is decreased in ouabain-treated rats. The present study investigated whether endothelin-1 (ET-1) and angiotensin II (Ang II) contributes to the vascular changes observed in rats chronically treated with ouabain. Wistar rats were treated with ouabain (8.0 microg day(-1), s.c. pellets for 5 weeks) alone or in combination with an endothelin type A receptor (ET(A)) antagonist, BMS182874 (40 mg kg(-1) day(-1), per gavage) or an angiotensin type 1 (AT(1)) receptor antagonist, losartan (15 mg kg(-1) day(-1), p.o.). Treatment with ouabain increased systolic blood pressure and treatment with either losartan or BMS182874 prevented the development of ouabain-induced hypertension. The sensitivity and maximal response for phenylephrine were reduced in aortic rings from ouabain-treated rats. Removal of the endothelium or in vitro exposure to an inhibitor of nitric oxide synthase (NOS), N-nitro-L-arginine methyl ester (L-NAME, 100 microM) increased the responses to phenylephrine, an effect that was more pronounced in aortas from ouabain-treated rats. Endothelial NOS protein (eNOS) expression was increased after ouabain treatment. Treatment with BMS182874, but not with losartan, prevented the effects of ouabain on the reactivity of phenylephrine and in eNOS protein expression. Gene expression of pre-pro-ET-1 and ET(A) receptors was increased in aortic rings from ouabain-treated rats. ET(B) receptor gene expression was not altered by ouabain treatment. In conclusion, our results suggest that endothelin and angiotensin systems play an important role in the development of ouabain-induced hypertension. However, ET-1, by activation of ET(A) receptors, but not Ang II, contributes to changes in vascular reactivity to phenylephrine induced by chronic treatment with ouabain.

The alpha2-isoform of Na-K-ATPase mediates ouabain-induced hypertension in mice and increased vascular contractility in vitro

Although ouabain is known to induce hypertension, the mechanism of how this cardiac glycoside affects blood pressure is uncertain. The present study demonstrates that the alpha2-isoform of the Na-K-ATPase mediates the pressor effects of ouabain in mice. To accomplish this, we analyzed the effect of ouabain on blood pressure in wild-type mice, where the alpha2-isoform is sensitive to ouabain, and genetically engineered mice expressing a ouabain-insensitive alpha2-isoform of the Na-K-ATPase. Thus differences in the response to ouabain between these two genotypes can only be attributed to the alpha2-isoform of Na-K-ATPase. As the alpha1-isoform is naturally resistant to ouabain in rodents, it will not be inhibited by ouabain in either genotype. Whereas prolonged administration of ouabain increased levels of ouabain in serum from both wild-type and targeted animals, hypertension developed only in wild-type mice. In addition, bolus intravenous infusion of ouabain increased the systolic, mean arterial, and left ventricular blood pressure in only wild-type anesthetized mice. In vitro, ouabain increased vascular tone and thereby phenylephrine-induced contraction of the aorta in intact and endothelium-denuded wild-type mice but in alpha2-resistant mice. Ouabain also increased the magnitude of the spontaneous contractions of portal vein and the basal tone of the intact aorta from only wild-type mice. The increase in aortic basal tone was dependent on the presence of endothelium. Our studies also demonstrate that the alpha2-isoform of Na-K-ATPase mediates the ouabain-induced increase in vascular contractility. This could play a role in the development and maintenance of ouabain-induced hypertension.

Alterations in phenylephrine-induced contractions and the vascular expression of Na+,K+-ATPase in ouabain-induced hypertension

Hypertension development, phenylephrine-induced contraction and Na(+),K(+)-ATPase functional activity and protein expression in aorta (AO), tail (TA) and superior mesenteric (SMA) arteries from ouabain- (25 microg day(-1), s.c., 5 weeks) and vehicle-treated rats were evaluated. Ouabain treatment increased systolic blood pressure (127+/-1 vs 160+/-2 mmHg, n=24, 35; P<0.001) while the maximum response to phenylephrine was reduced (P<0.01) in AO (102.8+/-3.9 vs 67.1+/-10.1% of KCl response, n=12, 9) and SMA (82.5+/-7.5 vs 52.2+/-5.8%, n=12, 9). Endothelium removal potentiated the phenylephrine response to a greater extent in segments from ouabain-treated rats. Thus, differences of area under the concentration-response curves (dAUC) in endothelium-denuded and intact segments for control and ouabain-treated rats were, respectively: AO, 56.6+/-9.6 vs 198.3+/-18.3 (n=9, 7); SMA, 85.5+/-15.4 vs 165.4+/-24.8 (n=6, 6); TA, 13.0+/-6.1 vs 39.5+/-10.4% of the corresponding control AUC (n=6, 6); P<0.05. The relaxation to KCl (1 - 10 mM) was similar in segments from both groups. Compared to controls, the inhibition of 0.1 mM ouabain on KCl relaxation was greater in AO (dAUC: 64.8+/-4.6 vs 84.0+/-5.1%, n=11, 14; P<0.05), similar in SMA (dAUC: 39.1+/-3.9 vs 43.3+/-7.8%, n=6, 7; P>0.05) and smaller in TA (dAUC: 62.1+/-5.5 vs 41.4+/-8.2%, n=12, 13; P<0.05) in ouabain-treated rats. Protein expression of both alpha(1) and alpha(2) isoforms of Na(+),K(+)-ATPase was augmented in AO, unmodified in SMA and reduced in TA from ouabain-treated rats. These results suggest that chronic administration of ouabain induces hypertension and regional vascular alterations, the latter possibly as a consequence of the hypertension.

Neuronal Fos-like immunoreactivity in ouabain-induced hypertension

In normotensive Wistar rats, systemic administration of exogenous ouabain for 10 days or more induces hypertension, presumably through central mechanisms. To identify which neuronal populations may be involved, we assessed Fos-like immunoreactivity (FLI) using an antibody that recognizes the protein products of the fos family comprising Fos, Fos B, Fra 1 and Fra 2, thus enabling detection of chronic neuronal activation. Young Wistar rats received s.c. infusions of either ouabain (50 microg/day) or saline for 7 or 14 days. At the end of the experimental period, mean arterial pressure (MAP) was assessed. In a separate set of rats FLI was detected immunohistochemically and quantified in cardiovascular and osmo-regulating centers. Resting MAP in ouabain-treated rats was significantly higher than in control rats at 14 but not at 7 days (125+/-4 vs. 101+/-6, P<0.05 and 102+/-4 vs. 98+/-6 (not significant), respectively). Within the supraoptic nucleus, ouabain induced significant increases in FLI compared with control rats at 14 days (9+/-2 vs. 2+/-2, P<0.05) but not at 7 days. Within the locus ceruleus, FLI was only detectable in rats that received ouabain infusions for 14 days but not in other groups of rats. Ouabain treatment did not induce significant changes in FLI within other areas. These results demonstrate that chronic s.c. ouabain infusion only increases neuronal FLI in the supraoptic nucleus and locus ceruleus where increases in FLI parallel the increase in blood pressure.

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.

Vasoactive effects of potassium in kidneys of hypertensive rats fed a high-potassium diet

DESIGN AND METHODS

Levels of dietary and serum potassium are thought to correlate inversely with vascular resistance and blood pressure. This study examined renal vascular resistance in perfused rat kidneys partially preconstricted with 10 micromol/ phenylephrine, quantifying changes in the resistance when levels of potassium in the perfusate ([K+]o) were varied between 2 and 80 mmol/l.

RESULTS

In kidneys from 17-week-old Wistar-Kyoto rats (WKY strain) fed a normal diet (American Institute of Nutrition AIN-76 diet), the resistance decreased when [K+]o was raised from 4 to 6-20 mmol/l, whereas resistance increased when [K+]o was either lowered to 2 mmol/l or raised above 25 mmol/l. The vasodilation that occurred at 13 mmol/l [K+]o was blocked by 100 micromol/l BaCl2 and 10 micromol/l ouabain in an additive manner, suggesting that both the inward rectifier K+ channel and the Na-K-ATPase underlie the dilation. Kidneys from spontaneously hypertensive rats (SHR strain) fed the AIN-76 diet displayed modestly enhanced vasodilations and vasoconstrictions as compared to WKY. A high-potassium diet (AIN-76 supplemented with 3.5% potassium citrate, provided for 8 weeks) led to exaggerated vasoconstrictive effects of [K+]o, and modestly enhanced vasodilations, in WKY rats. In contrast, the diet led to attenuated vasoconstrictions, and dramatically enhanced vasodilations, in the SHR strain. The diet did not affect the blood pressure increase or weight gain of either strain.

CONCLUSIONS

Changes in the responsiveness of blood vessels to extracellular potassium might underlie some beneficial effects of high-potassium diets in hypertensive individuals.