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

Gender difference in functional properties of Na,K-ATPase in the heart of spontaneously hypertensive rats

The aim of present study was the investigation of functional properties of the cardiac Na,K-ATPase in 16 weeks old male and female spontaneously hypertensive rats (SHR). The Na,K-ATPase activity in the presence of increasing concentrations of ATP, as well as Na(+) was lower in SHR of both genders, as compared to respective normotensive controls. Evaluation of kinetic parameters revealed a significant decrease of the maximum velocity (V(max)) in males (30% for ATP-activation, 40% for Na(+)-activation), as well as in females (24% for ATP, 29% for Na(+)), indicating a hypertension-induced diminution of the number of active enzyme molecules in cardiac sarcolemma. Insignificant changes were observed in the value of Michaelis-Menten constant (K(m)) in both cases. The concentration of sodium that gives half-maximal reaction velocity (K(Na)), increased by 38% in male and by 70% in female SHR. This impairment in the affinity of the Na(+)-binding site together with decreased number of active Na,K-ATPase molecules are probably responsible for the deteriorated enzyme-function in hearts of SHR. Direct comparison of SHR of both genders showed, that the enzyme from female hearts seems to be adapted better to hypertension as documented by its increased activity as a consequence of improved ability to bind and utilize ATP, as suggested by 32% decrease of K(m) value in females. In addition, the enzyme from female hearts is able to increase its activity (by 41%) in the presence of increasing sodium concentration even in the range where the enzyme from male hearts is already saturated.

Neurogenic nitric oxide release increases in mesenteric arteries from ouabain hypertensive rats

OBJECTIVES

We investigated whether chronic ouabain treatment changes the vasoconstrictor responses induced by electrical field stimulation (EFS) in endothelium-denuded rat superior mesenteric arteries and a possible role of neuronal nitric oxide (NO).

METHOD

Mesenteric arteries from untreated and ouabain-treated rats (approximately equal to 8.0 microg/kg per day, for 5 weeks) were used in this study. Vascular reactivity was analyzed by isometric tension recording. Expression of the neuronal NO synthase isoform was analyzed by Western blot. Noradrenaline release was evaluated in segments incubated with [H]noradrenaline.

RESULTS

Systolic (SBP) and diastolic (DBP) blood pressure were higher in ouabain-treated rats than in untreated rats (SBP, untreated: 120 +/- 3.5 mmHg versus ouabain-treated: 150 +/- 4.7 mmHg, P < 0.01; DBP, untreated: 87 +/- 3.0 mmHg versus ouabain-treated: 114 +/- 2.6 mmHg, P < 0.001). EFS-induced vasoconstrictions were smaller in arteries from ouabain-treated rats than in those from untreated animals, while the EFS-induced [H]noradrenaline release and the vasoconstriction induced by exogenous noradrenaline (1 nmol/l-10 micromol/l) remained unmodified. The non-selective NO synthase (NOS) inhibitor, N-nitro-L-arginine methyl ester (100 micromol/l), increased the EFS-induced vasoconstriction in mesenteric arteries from both groups, although the effect was more pronounced in segments from ouabain-treated rats. The selective neuronal NOS inhibitor, 7-nitroindazole (7-NI; 100 micromol/l) increased EFS-induced contraction only in segments from ouabain-treated rats. Neuronal NOS expression was greater in the mesenteric arteries from ouabain-treated rats than in those from untreated animals. Sodium nitroprusside (0.1 nmol/l-10 micromol/l) induced a similar vasodilatation in segments from both groups.

CONCLUSIONS

These results suggest that chronic ouabain treatment is accompanied by an increase in neuronal NO release that reduces EFS-induced vasoconstriction.

Differences in tail vascular bed reactivity in rats with and without heart failure following myocardial infarction

Myocardial infarction (MI) was induced in rats by coronary ligation to compare changes in vascular reactivity from animals that developed heart failure (InfHF) with those that did not (Inf). Infarct size was similar in both groups. In vitro preparations of tail vascular bed were used to investigate the vascular responses to acetylcholine, sodium nitroprusside, and phenylephrine. Acetylcholine-induced relaxation was impaired in the Inf group (53 +/- 2%, n = 6) when compared with Sham (80 +/- 2%, n = 6, P < 0.05). The maximal response (E(max)) to phenylephrine increased in the Inf group (423 +/- 10 mm Hg, n = 9, P < 0.01) and decreased in InfHF (279 +/- 10 mm Hg, n = 7, P < 0.05) when compared with Sham (319 +/- 11 mm Hg, n = 8). Regardless of endothelial integrity, E(max) to phenylephrine increased in the Inf, nitro-l-arginine methyl ester, and indomethacin groups. An increased release of a prostanoid vasodilator was detected in the Inf group. Differently, the InfHF group presented a reduction of the E(max) to phenylephrine and an increment of nitric oxide release. This study demonstrates that MI without heart failure impairs endothelium-dependent relaxation and increases the reactivity to phenylephrine. This increase seems to involve a muscular component. The endothelium participates with an increased release of a vasodilator prostanoid, possibly to compensate the increased smooth muscle response. When heart failure follows MI, the reactivity to phenylephrine decreases, possibly due to an increased nitric oxide release.

Brain Na+,K+-ATPase isozyme activity and protein expression in ouabain-induced hypertension

In normotensive rats, chronic infusion of exogenous ouabain causes hypertension involving central mechanisms. To determine whether ouabain-induced hypertension is associated with specific changes in brain Na+,K+-ATPase activity and expression, we assessed brain Na+,K+-ATPase isozyme activity and protein expression in rats treated with ouabain (50 microg/day s.c. or 10 microg/day i.c.v. for 14 days). Resting mean arterial pressure (MAP) was higher in s.c.- and i.c.v.-ouabain-treated animals vs. control (124+/-2 vs. 105+/-2 and 130+/-2 vs. 109+/-2, respectively, p<0.01). Ouabain infused s.c. or i.c.v. for 14 days had no effect on Na+,K+-ATPase isozyme activity in hypothalamic, pontine/medullary or cortical microsomes. However, the percent increase in total Na+,K+-ATPase activity produced in vitro by antibody Fab fragments that bind ouabain with high affinity (Digibind) was two-fold greater in s.c.- and i.c.v.-ouabain-treated rats vs. control, but only in hypothalamic microsomes. Thus, ouabain infused s.c. or i.c.v. does appear to directly inhibit Na+,K+-ATPase activity in the hypothalamus. On the other hand, in the hypothalamus, s.c.- and i.c.v.-ouabain infusions tended to increase alpha3 (by 30-44%), but had no effect on alpha1 or alpha2 Na+,K+-ATPase isozyme protein expression. In addition, ouabain was found to partially dissociate from the Na+,K+-ATPase enzyme following sample processing. Thus, the inability to detect a decrease in enzyme activity in the hypothalamus in response to ouabain may be due, in part, to an increase in enzyme expression and the dissociation of ouabain during sample processing.

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.

Ouabain-induced hypertension alters the participation of endothelial factors in alpha-adrenergic responses differently in rat resistance and conductance mesenteric arteries

1. This study compares the role of endothelial factors in alpha-adrenoceptor contractile responses in mesenteric resistance (MRA) and superior (SMA) mesenteric arteries from ouabain-treated (8.0 microg day(-1), 5 weeks) and untreated rats. The role of the renin-angiotensin system was also evaluated. 2. Ouabain treatment increased systolic blood pressure. In addition, ouabain reduced the phenylephrine response in SMA but did not alter noradrenaline responses in MRA. 3. Endothelium removal or the nitric oxide synthase (NOS) inhibitor (l-NAME, 100 microm) increased the responses to alpha-adrenergic agonists in both vessels. After ouabain treatment, both endothelial modulation and the l-NAME effect were increased in SMA, while only the l-NAME effect was increased in MRA. Endothelial NOS expression remained unaltered after ouabain treatment. 4. Indomethacin (10 microm) similarly reduced the noradrenaline contraction in MRA from both groups; in contrast, in SMA, indomethacin only reduced phenylephrine-induced contractions in segments from untreated rats. Co-incubation of l-NAME and indomethacin leftward shifted the concentration-response curves for noradrenaline more in MRA from ouabain-treated rats; tetraethylammonium (2 mm) shifted the noradrenaline curves further leftward only in MRA from untreated rats. 5.Losartan treatment prevents the development of hypertension but not all vascular changes observed after ouabain treatment. 6. In conclusion, a rise in endothelial NO and impaired prostanoid participation might explain the reduction in phenylephrine-induced contraction in SMA after ouabain treatment. An increase in the modulatory effect of endothelial NO and impairment of endothelium-dependent hyperpolarizing factor effect might explain why the ouabain treatment had no effect on noradrenaline responses in MRA.

Ca2+-Activated K+ Channels Underlying the Impaired Acetylcholine-Induced Vasodilation in 2K-1C Hypertensive Rats

We tested the hypothesis that an abnormal function of K(+) channels in vascular smooth muscle cells plays a key role in the impaired acetylcholine (ACh) vasodilation in aortas from two kidney-one clip (2K-1C) hypertensive rats and further investigated the K(+) channel subtype involved in this altered response. ACh-induced endothelium-dependent relaxation was assessed in aortic rings from 2K-1C and normotensive two kidney (2K) rats. Glibenclamide, an ATP-sensitive K(+) channel blocker, did not inhibit ACh-induced relaxation in aortic rings from 2K or 2K-1C rats. The voltage-dependent K(+) channels inhibitor 4-aminopyridine attenuated ACh-induced relaxation in both groups. Charybdotoxin and iberiotoxin, blockers of Ca(2+)-sensitive (K(Ca)) and large-conductance K(Ca) (BK(Ca)) channels, respectively, reduced ACh-induced relaxation in aortic rings from 2K rats without affecting this response in those from 2K-1C rats, abolishing the differences between groups. ACh-induced relaxation in vessels from both 2K and 2K-1C rats was unaffected by apamin, a small-conductance K(Ca) blocker. NS1619 [1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl)phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one], an activator of K(Ca), induced a smaller vasodilation in endothelium-denuded aortic rings from 2K-1C rats compared with those from 2K rats. Iberiotoxin reduced sodium nitroprusside-induced relaxation in endothelium-denuded aortic rings from 2K without affecting this response in those from 2K-1C rats. The inhibition of Na(+),K(+)-ATPase with ouabain had no effects on ACh-induced relaxation in aortic rings from 2K-1C or 2K rats. These data indicate that a deficient functional activity of BK(Ca) channels plays a key role in the impaired ACh vasodilation in aortas from 2K-1C rats.

Cyclooxygenase pathway is involved in the vascular reactivity and inhibition of the Na+, K+-ATPase activity in the tail artery from L-NAME-treated rats

L-NAME (LN) induces hypertension by blocking nitric oxide (NO) synthesis. It produces vascular hyperreactivity to phenylephrine (PHE) associated with a reduced vascular Na+, K+-ATPase activity. The aim of this work was to investigate whether products of the cyclooxygenase pathway are involved in alterations of vascular reactivity and Na+-pump activity in the tail artery from LN-induced hypertension rats. Four groups of rats were used: Control (CT, normotensive), LN (50 mg/kg/day, hypertensive), indomethacin (Indo-4 mg/kg/day, normotensive), and LN plus Indo (LN + Indo, partially prevented hypertension). All drugs were administered in drinking water during 7 days. In isolated rat tail vascular beds; the reactivity to PHE, acetylcholine (ACh), sodium nitroprusside (SNP), the functional activity of the Na+, K+-ATPase (K+-induced relaxation) and the modulation of PHE-induced vasoconstriction by constitutively available NO were evaluated. LN increased vascular sensitivity (pD2) and reactivity (Emax) to PHE and Indo blocked the effect of LN on Emax without changing pD2. Emax and pD2 values for ACh were reduced by LN and partially reverted by Indo. SNP-induced vasodilatation was similar in all groups. LN reduced the activity of Na+, K+-ATPase and Indo prevented LN effects. LN also abolished NO ability to modulate PHE-induced contractions. This effect was partially prevented by Indo suggesting that products from the cyclooxygenase pathway might reduce NO actions. Indo itself did not affect vascular reactivity to PHE, ACh or SNP or the Na+,K+-ATPase activity. Results suggested that products from cyclooxygenase pathway are involved in the genesis or maintenance of LN-induced hypertension, playing a role in the increased vascular reactivity, in the reduction of the endothelium-dependent relaxation and in the inhibition of the functional activity of the Na+, K+-ATPase.

Na(+) pump alpha 2-isoform specifically couples to contractility in vascular smooth muscle: evidence from gene-targeted neonatal mice

The relative expression of alpha(1)- and alpha(2)-Na(+)/K(+)-ATPase isoforms found in vascular smooth muscle is developmentally regulated and under hormonal and neurogenic control. The physiological roles of these isoforms in vascular function are not known. It has been postulated that the alpha(1)-isoform serves a "housekeeping" role, whereas the alpha(2)-isoform localizes to a subsarcolemmal compartment and modulates contractility. To test this hypothesis, isoform-specific gene-targeted mice in which the mRNA for either the alpha(1)- or the alpha(2)-Na(+)/K(+)-ATPase isoform was ablated were utilized. Both of these knockouts, alpha(1)(-/-) and alpha(2)(-/-), are lethal; the latter dies at birth, which allows this neonatal aorta to be studied. Isometric force in alpha(2)(-/-)-aorta was more sensitive to contractile agonists and less sensitive to the vasodilators forskolin and sodium nitroprusside (SNP) than wild-type (WT) aorta; alpha(2)(+/-)-aortas had intermediate values. In contrast, neonatal alpha(1)(+/-)-aorta was similar to WT. Western blot analysis indicated a population of 70% alpha(1)- and 30% alpha(2)-isoforms in the WT. Thus in terms of the total Na(+)/K(+)-ATPase protein, the alpha(2)(-/-)-aorta (at 70%) would be similar to the alpha(1)(+/-)-aorta (at 65%) but with a dramatically different phenotype. These data suggest that individual alpha-isoforms of the Na(+)/K(+)-ATPase differ functionally and that the alpha(2)-isoform couples more strongly to activation-relaxation pathways. Three-dimensional image-acquisition and deconvolution analyses suggest that the alpha(2)-isoform is distributed differently than the alpha(1)-isoform. Importantly, these isoforms do not localize to the same regions.

Strain-specific effects of acidic pH on contractile state of aortas from Wistar and Wistar Kyoto rats

The effects of acidosis were investigated on the resting and precontracted aortas from Wistar and Wistar Kyoto (WKY) rats. Decrease in pH from 7.4 to 6.5, having no effect on the resting tension of Wistar aorta, induced a marked contraction of WKY aorta. Acidic pH markedly relaxed the contraction to 300 nM phenylephrine in Wistar aorta, whereas in WKY aorta, it produced a biphasic response, an initial relaxation followed by potentiation of the contraction. In aortas loaded with fura 2-AM, phenylephrine caused an increase in intracellular Ca2+ ([Ca2+]i) and a contraction in both Wistar and WKY rats. pH 6.5 produced a decrease in [Ca2+]i to a near-basal level and almost abolished the phenylephrine-induced contraction in Wistar rat aorta. However, in WKY aorta, a biphasic response, an initial decline and later a recovery of [Ca2+]i level, was observed. Interestingly, at similar sustained [Ca2+]i, the contractile response to phenylephrine in WKY aorta was potentiated under acidic pH conditions. Acidic pH-induced inhibition of the contraction to phenylephrine was unaffected by iberiotoxin, 4-aminopyridine, and glibenclamide (Ca2+-activated, delayed rectifier and ATP-sensitive K+ channel inhibitors, respectively), in aortas from both Wistar and WKY. Decrease in extracellular pH was associated with a rapid fall in intracellular pH (pHi) and the intracellular acidification profile was not different in both strains. All these results show that acidic pH induces strain-specific inhibitory and excitatory effects on the contractile state of aortas from Wistar and WKY rats, respectively. The sustained and transient relaxant responses to acidic pH in Wistar and WKY aortas, respectively, are due to decrease in [Ca2+]i levels, but this decrease in [Ca2+]i is independent of the activation of K+ channels.

Different effect of ouabain on endothelin-1-induced extracellular signal-regulated kinase stimulation in rat heart and tail artery

Endogenous ouabain may play a role in the control of cardiovascular system function. In this study, we investigated the effects of a long-term ouabain treatment on basal and endothelin-1 (ET-1)-induced phosphorylation of cardiac and vascular extracellular signal-regulated kinases 1 and 2 (ERK-1 and ERK-2), which are involved in several cardiac and vascular physiologic and pathologic conditions. Our results show that the hearts from ouabain-treated rats have a higher basal level of ERK-1 and ERK-2 phosphorylation compared with untreated rats. Perfusion of the hearts with ET-1 increased ERK-1 and ERK-2 phosphorylation both in ouabain-treated and in control rats, with a larger stimulatory effect in ouabain-treated animals. On the contrary, exposure of endothelium-free tail artery to ET-1 increased ERKs phosphorylation both in treated and untreated rats, but this effect was blunted in ouabain-treated rats. These findings demonstrate that ouabain treatment has opposite effects on basal and ET-1-induced ERKs phosphorylation in the heart and in the tail artery of the rat.

Ouabain-induced hypertension is accompanied by increases in endothelial vasodilator factors

The involvement of nitric oxide (NO), prostaglandins, and calcium-dependent potassium channel (K(Ca)) activators on the negative modulation of phenylephrine-induced contractions was evaluated on the isolated aorta and caudal (CAU) artery obtained from rats treated with ouabain for 5 wk to induce hypertension. In ouabain-treated rats, the reactivity to phenylephrine was reduced in the endothelium-intact aorta but not the CAU segments. Endothelial modulation of phenylephrine contraction, as demonstrated by endothelium removal, NO synthase (NOS) inhibition with N(omega)-nitro-L-arginine methyl ester and aminoguanidine, as well as K(Ca) inhibition with tetraethylammonium, was more pronounced in segments from ouabain-treated animals, and here greater effects were seen in the aorta than in CAU. An increased expression of endothelial NOS and neuronal NOS was seen in the aorta after ouabain treatment. In CAU, only endothelial NOS was detected and ouabain treatment did not alter its expression. These results suggest that ouabain-induced hypertension is accompanied by increased NO release derived from endothelial NOS and neuronal NOS and increased release of an endothelial hyperpolarizing factor that presumably opens K(Ca), all of which contribute to the increased negative modulation of the phenylephrine contraction.