Phosphate binding reduces aortic angiotensin-converting enzyme and enhances nitric oxide bioactivity in experimental renal insufficiency.

BACKGROUND: Disturbed calcium-phosphorus metabolism is associated with increased kidney angiotensin-converting enzyme (ACE) in experimental chronic renal insufficiency (CRI). However, information about the effects of phosphate binding and loading on vascular ACE is lacking. METHODS: Fifteen weeks after 5/6 nephrectomy (NX), rats were placed on a phosphate-binding (NX+Ca, 3.0% Ca), phosphate-loading (NX+Pi, 1.5% Pi), or control diet for 12 weeks (NX and sham). RESULTS: Aortic ACE, blood pressure, plasma phosphate, and parathyroid hormone were increased in the NX and NX+Pi groups, but were reduced with phosphate binding. Endothelium-mediated relaxations of isolated mesenteric conduit artery rings to acetylcholine were impaired in the NX and NX+Pi groups, but did not differ from sham in NX+Ca rats. Experiments with nitric oxide (NO) synthase inhibition in vitro suggested that the NO-mediated component of acetylcholine response was lower in the NX and NX+Pi groups, but did not differ from sham in NX+Ca rats. In all NX groups, aortic endothelial NO synthase (eNOS) was reduced, while plasma and urine concentrations of NO metabolites were increased. Aortic nitrated proteins and calcification were increased in the NX and NX+Pi groups when compared with the NX+Ca and sham groups. CONCLUSION: Hypertension in the NX model of CRI was associated with reduced vasorelaxation, decreased eNOS, and increased ACE and nitrated proteins in the aorta. Phosphate binding with calcium carbonate enhanced vasorelaxation via endogenous NO and suppressed elevation of ACE and nitrated proteins, suggesting reduced vascular oxidative stress. Our findings support the view that correction of the calcium-phosphorus balance prevents CRI-induced vascular pathophysiology.

Hyperuricemia, oxidative stress, and carotid artery tone in experimental renal insufficiency.

BACKGROUND: Hyperuricemia may play a role in the pathogenesis of cardiovascular disease, but uric acid is also a significant antioxidant. We investigated the effects of oxonic acid-induced hyperuricemia on carotid artery tone in experimental renal insufficiency. METHODS: Three weeks after 5/6 nephrectomy (NX) or Sham operation, male Sprague-Dawley rats were allocated to 2.0% oxonic acid or control diet for 9 weeks. Blood pressure was monitored using tail cuff, isolated arterial rings were examined using myographs, and blood and urine samples were taken, as appropriate. Oxidative stress and antioxidant status were evaluated by measuring urinary 8-isoprostaglandin F(2 alpha) (8-iso-PGF(2 alpha)) excretion and plasma total peroxyl radical-trapping capacity (TRAP), respectively. RESULTS: Plasma creatinine was elevated twofold in NX rats, but neither NX nor oxonic acid diet influenced blood pressure. Urinary 8-iso-PGF(2 alpha) excretion was increased over 2.5-fold in NX rats on control diet. Oxonic acid diet increased plasma uric acid 2-3-fold, TRAP 1.5-fold, and reduced urinary 8-iso-PGF(2 alpha) excretion by 60-90%. Carotid vasorelaxation to acetylcholine in vitro, which could be abolished by nitric oxide (NO) synthase inhibition, was reduced following NX, whereas maximal response to acetylcholine was augmented in hyperuricemic NX rats. Vasorelaxation to nitroprusside was impaired in NX rats, whereas oxonic acid diet increased sensitivity also to nitroprusside in NX rats. CONCLUSIONS: Oxonic acid-induced hyperuricemia reduced oxidative stress in vivo, as evaluated using urinary 8-iso-PGF(2 alpha) excretion, increased plasma TRAP, and improved NO-mediated vasorelaxation in the carotid artery in experimental renal insufficiency.

Paricalcitol treatment and arterial tone in experimental renal insufficiency.

AIM: To examine whether treatment of secondary hyperparathyroidism with paricalcitol provides benefits to arteries in uremic rats. METHODS: 5/6-nephrectomized rats were treated (NX+Pari) or not treated (NX) with paricalcitol (200 ng/kg, thrice weekly) for 12 weeks. Aortic histology and isolated segments of the main and 2nd-order mesenteric arterial branches were studied. RESULTS: Creatinine clearance was reduced by 54-61%, plasma phosphate increased 2.1- to 2.5-fold, and blood pressure by 40 mm Hg in both NX groups. PTH increased 13-fold in NX and 5-fold in NX+Pari rats. Calcification in aortic cross-sections increased from 2.1 to 7.1% after paricalcitol. In the large mesenteric artery, vasoconstriction to noradrenaline was reduced in NX+Pari rats. In the large and small arteries, vasorelaxation to acetylcholine was impaired in NX rats and unaffected by paricalcitol. In the small artery, paricalcitol increased nitric oxide synthase inhibition-resistant relaxation to acetylcholine, and maximal relaxation to levcromakalim. The small arteries of NX rats featured increased wall cross-sectional area, while paricalcitol further increased wall thickness and the wall:lumen ratio. CONCLUSION: Paricalcitol treatment showed both benefits and harmful effects in uremic rats: in the large artery vasoconstriction was reduced but calcification increased, while in the small artery vasorelaxation via potassium channels was moderately improved but hypertrophic remodeling was aggravated.

Oxonic acid-induced hyperuricemia elevates plasma aldosterone in experimental renal insufficiency.

BACKGROUND: Hyperuricemia is associated with renal insufficiency and may predispose to Na retention and hypertension. Whether hyperuricemia plays a causal role in the pathogenesis of cardiovascular disease remains controversial. OBJECTIVE: We examined the effects of hyperuricemia on circulating and renal components of the renin-angiotensin-aldosterone system in experimental renal insufficiency. METHODS: Three weeks after 5/6 nephrectomy or sham-operation, rats were put on 2.0% oxonic acid diet for 9 weeks. Blood pressure was monitored using tail-cuff, and blood, urine, and kidney samples were taken, as appropriate. Kidney angiotensin-converting enzyme, angiotensin-converting enzyme 2 and angiotensin II receptors (AT1R, AT2R) were examined using real-time reverse transcriptase-PCR and autoradiography. RESULTS: Oxonic acid diet increased plasma uric acid by 80-90 micromol/l, while blood pressure was elevated only in hyperuricemic 5/6 nephrectomy rats (18 mmHg). Creatinine clearance was reduced by 60% in both 5/6 nephrectomy groups and by 25% in hyperuricemic Sham rats. The 5/6 nephrectomy group showed over 90% suppression of plasma renin activity, whereas the Sham + oxonic acid diet group showed 1.2 and 1.4-fold, and 5/6 nephrectomy + oxonic acid diet group 2.5 and 2.3-fold increases in plasma renin activity and plasma aldosterone, respectively. Hyperuricemia increased K and decreased Na excretion in Sham and 5/6 nephrectomy rats, leading to a more than 1.6-fold increase in urine K to Na ratio. No changes in kidney angiotensin-converting enzyme, angiotensin-converting enzyme 2, AT1R or AT2R were detected that could explain the hyperuricemia-induced alteration in Na-K balance. CONCLUSION: As oxonic acid diet increased plasma renin activity, plasma aldosterone, and urine K to Na ratio, these changes may play a significant role in the harmful cardiovascular actions of hyperuricemia.

High-calcium vs high-phosphate intake and small artery tone in advanced experimental renal insufficiency.

BACKGROUND: Disturbed calcium-phosphorus balance significantly contributes to uraemic changes in large arteries. We examined the influences of high-calcium and high-phosphate intake on small artery tone in experimental renal insufficiency. METHODS: Sixty-five rats were assigned to 5/6 nephrectomy (NTX) or sham operation. After 15 week disease progression, NTX rats were given high-calcium (3%), high-phosphate (1.5%) or control diet (0.3% calcium, 0.5% phosphate) for 12 weeks. Then isolated segments of small mesenteric arteries were studied using wire and pressure myographs. RESULTS: Subtotal nephrectomy reduced creatinine clearance by 60% and increased parathyroid hormone (PTH) and phosphate 12-fold and 2.7-fold, respectively. High-phosphate intake further elevated PTH and phosphate (33-fold and 5.5-fold, respectively), while the calcium diet suppressed them (to 3.5 and 62% vs sham, respectively). Ventricular B-type natriuretic peptide synthesis was increased, and blood pressure was 27 and 18 mmHg higher in NTX rats on control and phosphate diet, respectively, than in calcium-fed rats. Vasorelaxation to acetylcholine was impaired by approximately 50% in uraemic rats, and was further deteriorated by high-phosphate intake, whereas the calcium diet improved endothelium-mediated relaxation via nitric oxide and potassium channels. Small arteries of all NTX groups featured eutrophic inward remodelling: wall-to-lumen ratio was increased 1.3-fold without change in cross-sectional area. CONCLUSION: High-phosphate intake had a detrimental influence on secondary hyperparathyroidism and vasodilatation, whereas high-calcium intake reduced blood pressure and PTH, alleviated volume overload and improved vasorelaxation in experimental renal insufficiency. Therefore, alterations in the calcium-phosphorus balance can significantly modulate small artery tone during impaired kidney function.

High calcium diet down-regulates kidney angiotensin-converting enzyme in experimental renal failure.

BACKGROUND: Calcium salts are used as phosphate binders in renal failure, while high calcium diet also improves vasorelaxation and enhances natriuresis. The influences of calcium intake on renal renin-angiotensin system (RAS) are largely unknown. METHODS: Four weeks after NTX, rats were put on 3.0% or 0.3% calcium diet for 8 weeks (12-week study). In additional experiments, 15 weeks after NTX, rats were put on similar diets for 12 weeks (27-week study). Appropriate blood, urine, and kidney samples were taken. Renal angiotensin-converting enzyme (ACE) and angiotensin II receptors (AT1, AT2) were examined using autoradiography, ACE also using Western blotting, and connective tissue growth factor (CTGF) using immunohistochemistry. RESULTS: In the 12-week study, albuminuria increased 5-fold in NTX rats, but only 2-fold in calcium NTX rats on 3.0% calcium. In the 27-week study, high calcium intake decreased blood pressure, retarded progression of renal failure, reduced glomerulosclerosis, interstitial damage, and aortic calcifications, and improved survival from 50% to 92% in NTX rats. In both experiments plasma parathyroid hormone and phosphate were elevated after NTX, and suppressed by high calcium diet, while kidney ACE was down-regulated by 40% or more after increased calcium intake. In the 27-week study renal CTGF was decreased and cortical AT1 receptor density reduced after high calcium diet. CONCLUSION: High calcium diet down-regulated kidney ACE, reduced albuminuria and blood pressure, and favorably influenced kidney morphology in experimental renal failure. These findings suggest a link between calcium metabolism and kidney ACE expression, which may play a role in the progression of renal damage.