Increased Na+/H+ exchange activity in vascular smooth muscle cells of spontaneously hypertensive rats and possible involvement of protein kinase C

The effects of amiloride, a Na+-H+ exchange inhibitor, on iliac artery stenosis after balloon injury in rabbits

This study was designed to explore the effects of amiloride, a Na+-H+ exchange (NHE) inhibitor, on vessel stenosis by observing the expression of NHE-1 protein in vascular smooth muscle (VSM) after balloon injury and the effects of amiloride on VSM cell proliferation, migration, and excretion of extracellular matrices (ECMs). A total of 32 adult male New Zealand white rabbits were randomly divided into a balloon injury group (BG), an amiloride-treated group (AG), and a sham-operated group (SG). The left iliac artery was injured by inflating a 2.5 mm x 20 mm Foley catheter in BG and AG rabbits; in SG rabbits, the Foley catheter was inserted but not inflated. Amiloride (5 mg x kg(-1) x d(-1)) was injected intraperitoneally in AG and the same volume of distilled water was used in BG 3 days before balloon injury and for 28 days after the injury. The left iliac artery was stained by hematoxylin-eosin, alpha-actin, and Masson's trichrome to observe the vessel cava, neointima, media layer, and ECMs. NHE-1 proteins of the VSM were detected by Western blotting. A narrowing of the arterial cava, neointima formation, and thickened VSM layer were observed 28 days after balloon injury in BG and AG. However, in AG, the vessel cava was not as narrowed as that of BG and the intimal areas were to a lesser extent than in BG. In AG, the alpha-actin-positive areas and the ECM areas in the neointima were increased compared with SG, but to a lesser extent than in BG. The expression of NHE-1 protein in VSM was increased in BG and AG after balloon injury; however, the levels in AG were significantly less than in BG. In conclusion, VSM cell proliferation, migration, and excretion of ECMs contributed to vessel stenosis in the BG and AG rabbits. The expression of NHE-1 protein in VSM increased after balloon injury. Amiloride, an inhibitor of NHE-1, can limit the development of vessel stenosis through inhibition of VSM cell proliferation, migration, and excretion of ECMs.

Effects of chronic treatment with simvastatin on endothelial dysfunction in spontaneously hypertensive rats

OBJECTIVE

To investigate the effects of chronic treatment with simvastatin (SV) on endothelium-dependent relaxation and ouabain-induced contractions in aortic rings from spontaneously hypertensive rats (SHR), comparing with normotensive Wistar-Kyoto rats (WKY).

METHODS

After a 12-week period of administration of 1 or 2 mg/kg SV to SHR and WKY, systolic blood pressure (SBP) and vascular reactivity in endothelium-intact aortic rings were assessed.

RESULTS

Relaxation in response to acetylcholine (ACh) in WKY remained unaltered, but in SHR treated with 1 mg/kg SV, enhanced ACh-induced relaxation (P<0.05 versus untreated SHR) reached values observed in untreated WKY. The 2 mg/kg treatment also improved ACh relaxation (P<0.01 and P<0.05 versus untreated SHR and WKY respectively). Inhibiting cyclo-oxygenase (COX) with indomethacin (INDO) improved ACh relaxation in SHR (P<0.05) but not in WKY, independent of treatment with SV. Inhibition of nitric oxide synthase (NOS) with N(G)-nitro-L-arginine (L-NOARG) abolished ACh relaxations in all cases (P<0.001). The result was unaltered when combining INDO plus L-NOARG. SV treatment also decreased ouabain-induced contractions in endothelium-intact aortic rings from SHR, diminishing the percentage effect of contraction from 64.56+/-2.95 (untreated SHR) to 26.98+/-7.06 and 38.10+/-8.21 (1 and 2 mg/kg treated SHR respectively). Response to ouabain in WKY was not significantly affected by SV treatment

CONCLUSIONS

Chronic treatment of SHR with SV improves endothelium-dependent ACh relaxation of the aortic rings, probably by an NO-involving mechanism more than by inhibiting contractile COX-derived factors. An improvement in endothelial modulation of ouabain-induced contractions was also observed after treatment with SV in SHR, which might be due to an inhibition of a calcium-sodium exchanger.

Genetic and biochemical determinants of abnormal monovalent ion transport in primary hypertension

Data obtained during the last two decades show that spontaneously hypertensive rats, an acceptable experimental model of primary human hypertension, possess increased activity of both ubiquitous and renal cell-specific isoforms of the Na+/H+ exchanger (NHE) and Na+-K+-2Cl- cotransporter. Abnormalities of these ion transporters have been found in patients suffering from essential hypertension. Recent genetic studies demonstrate that genes encoding the beta- and gamma-subunits of ENaC, a renal cell-specific isoform of the Na+-K+-2Cl- cotransporter, and alpha3-, alpha1-, and beta2-subunits of the Na+-K+ pump are localized within quantitative trait loci (QTL) for elevated blood pressure as well as for enhanced heart-to-body weight ratio, proteinuria, phosphate excretion, and stroke latency. On the basis of the homology of genome maps, several other genes encoding these transporters, as well as the Na+/H+ exchanger and Na+-K+-2Cl- cotransporter, can be predicted in QTL related to the pathogenesis of hypertension. However, despite their location within QTL, analysis of cDNA structure did not reveal any mutation in the coding region of the above-listed transporters in primary hypertension, with the exception of G276L substitution in the alpha1-Na+-K+ pump from Dahl salt-sensitive rats and a higher occurrence of T594M mutation of beta-ENaC in the black population with essential hypertension. These results suggest that, in contrast to Mendelian forms of hypertension, the altered activity of monovalent ion transporters in primary hypertension is caused by abnormalities of systems involved in the regulation of their expression and/or function. Further analysis of QTL in F2 hybrids of normotensive and hypertensive rats and in affected sibling pairs will allow mapping of genes causing abnormalities of these regulatory pathways.

Can we use erythrocytes for the study of the activity of the ubiquitous Na+/H+ exchanger (NHE-1) in essential hypertension?

Both Na+/Li+ countertransport and electrochemical proton gradient (delta mu(H+))-induced Na+ and H+ fluxes are increased in erythrocytes from patients with essential hypertension. It was assumed that these abnormalities are related to ubiquitous (housekeeping) forms of the Na+/H+ exchanger (NHE-1). To examine this hypothesis, we compared kinetic and regulatory properties of erythrocyte Na+/Li+ countertransport and delta mu(H+)-induced Na+ and H+ fluxes with data obtained for cloned isoforms of the Na+/H+ exchanger. In human erythrocytes, Na+/Li+ countertransport exhibited a hyperbolic dependence on [Na+]0 with a K0.5 of approximately 30 to 40 mmol/L. The activity of this carrier was increased by two-fold in the fraction of erythrocytes enriched with the old cells, was inhibited by 0.1 mmol/L phloretin, and was insensitive to both 1 mmol/L amiloride and ATP depletion. In contrast, delta mu(H+)-induced 22Na influx was exponentially increased at [Na+]0 > 60 mmol/L, was insensitive to phloretin, was partly decreased by both 1 mmol/L amiloride and ATP depletion, and was the same in total erythrocytes and in the old cells. The values of Na+/Li+ countertransport and delta mu(H+)-induced Na+ influx in erythrocytes from different species were not correlating and their ratio in human, rat, and rabbit erythrocytes was 10:1:170 and 1:5:1 for Na+/ Li+ countertransport and delta mu(H+)-induced Na+ influx, respectively. In contrast to the majority of nonepithelial cells and cells transfected with an ubiquitous isoform of Na+/H+ exchanger, both delta mu(H+)-induced Na+ influx and Na+/Li+ countertransport in human erythrocytes were completely insensitive to ethylisopropyl amiloride (20 micromol/L) and cell shrinkage. Thus, our data strongly suggest that human erythrocyte Na+/Li+ countertransport and delta mu(H+)-induced Na+/H+ exchange are mediated by the distinct transporters. Moreover, because the properties of these erythrocyte transporters and NHE-1 are different, it complicates the use of erythrocytes for the identification of the mechanism for activating the ubiquitous form of Na+/H+ exchanger in primary hypertension.

Kinetics of Na+/H+ exchange in vascular smooth muscle cells from WKY and SHR: effects of phorbol ester

The kinetic properties of Na+/H+ exchange were investigated in vascular smooth muscle cells (VSMC) in culture from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Antiport activity was measured in 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein-loaded cells after nigericin-induced cytosolic acidification. Studies were performed without (control) and with pretreatment of the cells with phorbol 12-myristate 13-acetate (PMA; 200 nM). Na+/H+ exchange markedly differed between the two strains with lower Hill coefficients [1.56 +/- 0.17 (SE) vs. 2.62 +/- 0.36] and higher maximal activity (Vmax) values (55.85 +/- 5.24 vs. 31.11 +/- 2.38 mmol H+.l-1.min-1) in SHR compared with WKY cell lines. PMA markedly altered the antiport kinetics in WKY VSMC with a decrease in the Hill coefficient (1.75 +/- 0.14) without affecting Vmax (31.88 +/- 1.55 mmol H+.l-1.min-1). In VSMC from SHR, PMA had no effect on the kinetic variables investigated. Thus two kinetic abnormalities are present with respect to Na+/H+ antiport activity in VSMC from SHR compared with WKY, i.e., increased Vmax and decreased Hill coefficient. The observation that PMA does not affect the kinetics of the Na+/H+ antiport in VSMC from SHR suggests a marked degree of antiporter prestimulation in this animal model of genetic hypertension.