Influence of neonatal sympathectomy on proximal renal resistance artery function in spontaneously hypertensive rats

Sympathetic denervation facilitates L-type Ca2+ channel activation in renal but not in mesenteric resistance arteries

OBJECTIVES

Sympathetic denervation enhances agonist-induced vasoconstriction. This effect may involve altered function of signaling mechanisms such as Rho kinase (Rock) and L-type Ca channels downstream from vasoconstrictor receptors. We tested if enhanced Rock and L-type calcium channel activation contribute to exaggerated norepinephrine-induced vasoconstrictions in renal and mesenteric resistance arteries after sympathectomy.

METHODS

Rats underwent neonatal sympathectomy or sham sympathectomy. Resistance arteries were investigated by small vessel myography. Vascular Rock and L-type Ca channel expression as well as Rock activation were investigated by quantitative real-time PCR and Western blot. Vascular smooth muscle cell (VSMC) membrane potential was recorded with microelectrodes.

RESULTS

Sympathetic denervation enhanced norepinephrine sensitivity in renal and mesenteric arteries. Both, Rock inhibition or L-type Ca inhibition shifted the norepinephrine concentration-response curve to the right. This effect was more pronounced in renal than in mesenteric arteries from sympathectomized vs. sham-sympathectomized animals. The L-type Ca channel activator S-(-)-BayK8644 elicited strong vasoconstrictions only in renal arteries from sympathectomized rats. Rock activity and L-type Ca channel α-subunit expression were similar in renal arteries from sympathectomized and sham-sympathectomized animals. VSMC membrane potential was -57.5 ± 2.0 and -64.3 ± 0.3 mV (P < 0.01), respectively, in renal arteries from sympathectomized and from sham-sympathectomized rats. Depolarization enhanced and KATP channel activation abolished S-(-)-BayK8644-induced contractions in renal arteries from sympathectomized rats.

CONCLUSION

Sympathetic denervation enhances L-type Ca channel-dependent signaling in renal but not in mesenteric arteries. This effect may be partly explained by the decreased VSMC membrane potential in denervated renal arteries.

Effects of renal denervation on renal pelvic contractions and connexin expression in rats

AIMS

The renal pelvis shows spontaneous rhythmic contractile activity. We assessed to what extent this activity depends on renal innervation and studied the role of connexins in pelvic contractions.

METHODS

Rats underwent unilateral renal denervation or renal transplantation. Renal pelvic pressure and diuresis were measured in vivo. Spontaneous and agonist-induced contractions of isolated renal pelves were investigated by wire myography. Rat and human renal pelvic connexin mRNA abundances and connexin localization were studied by real-time PCR and immunofluorescence respectively.

RESULTS

Renal denervation or transplantation increased renal pelvic pressure in vivo by about 60 and 150%, respectively, but did not significantly affect pelvic contraction frequency. Under in vitro conditions, isolated pelvic preparations from innervated or denervated kidneys showed spontaneous contractions. Pelves from denervated kidneys showed about 50% higher contraction frequencies than pelves from innervated kidneys, whereas contraction force was similar in pelves from denervated and innervated kidneys. There was no denervation-induced supersensitivity to noradrenaline or endothelin-1. Renal denervation did not increase pelvic connexin37, 40, 43 or 45 mRNA abundances. Gap junction blockade had no effect on spontaneous pelvic contractile activity.

CONCLUSIONS

The denervation-induced effect on pelvic pressure may be the consequence of the enhanced diuresis. The mechanisms underlying the denervation-induced effects on pelvic contraction frequency remain unknown. Our data rule out a major role for two important candidates, by showing that renal denervation neither induced supersensitivity to contractile agonists nor increased connexin mRNA abundance in the pelvic wall.

Intrarenal artery superoxide is mainly NADPH oxidase-derived and modulates endothelium-dependent dilation in elderly patients

AIMS

The present study was performed to investigate the contribution of NADPH oxidases (Nox) to superoxide formation in human renal proximal resistance arteries and to test whether superoxide formation contributes to acute vasoconstrictor responses and endothelium-dependent vasodilation in these vessels.

METHODS AND RESULTS

Arcuate and proximal interlobular artery segments were from patients who underwent nephrectomy because of a renal tumour. Vessels were dissected from tumour-free parts of the kidneys. Additional intrarenal arteries were obtained from rats. Superoxide formation was measured by lucigenin-enhanced chemiluminescence, expression of Nox isoforms was analysed by RT-PCR, and functional studies were performed by small vessel wire myography. Sixty per cent of superoxide formation in human arcuate and proximal interlobular arteries was due to Nox activity. mRNA expression analyses revealed the presence of Nox2 and Nox4 but not Nox1. Phenylephrine and endothelin-1 induced powerful concentration-dependent vasoconstrictions that were unaffected by superoxide scavengers. Vasopressin elicited small and variable vasoconstrictions with signs of tachyphylaxis. Endothelium-dependent vasodilation was blunted by tiron and Nomega-nitro-L-arginine methyl ester but not by superoxide dismutase or catalase. Exogenous hydrogen peroxide elicited vasoconstriction.

CONCLUSION

Nox activity is the major source of superoxide formation in renal proximal resistance arteries from elderly patients. Acute vasoconstrictor responses to alpha1-adrenoreceptor activation and to endothelin-1 do not depend on superoxide formation, while endothelium-dependent vasodilation in intrarenal arteries is reactive oxygen species-dependent.

Apocynin-induced vasodilation involves Rho kinase inhibition but not NADPH oxidase inhibition

AIMS

The present study was designed to test the hypothesis that NADPH oxidase inhibition with apocynin would lower blood pressure and improve endothelial function in spontaneously hypertensive rats (SHRs). Although apocyin effectively dilated arterial segments in vitro, it failed to lower blood pressure or improve endothelial function. Further experiments were performed in normotensive rats and in NADPH oxidase subunit knock-out mice to test if apocynin-induced vasodilation depends on NADPH oxidase inhibition at all.

METHODS AND RESULTS

SHRs were treated with apocynin orally or i.v. Arterial pressure was recorded directly. Rat and mouse arterial function was investigated in vitro by small vessel wire myography. NADPH oxidase activity was measured in human granulocytes and in rat vascular preparations. Rho kinase activity was determined by Western blot analysis. Apocynin did not reduce arterial pressure acutely in SHR when given at 50, 100, or 150 mg kg(-1) day(-1) orally over 1-week intervals or when given i.v. Apocynin potently inhibited granulocyte NADPH oxidase but not vascular NADPH-oxidase-dependent oxygen radical formation unless exogenous peroxidase was added to vascular preparations. Apocynin dilated rat intrarenal and coronary arteries independently of pharmacological interventions that reduce vascular superoxide radical abundance and actions. Aortic rings from p47phox(-/-) mice were more sensitive to apocynin-induced dilation than wild-type aortic rings. Rho kinase inhibition reduced or prevented the inhibitory effect of apocynin on agonist-induced vasoconstriction and apocynin inhibited the phosphorylation of Rho kinase substrates.

CONCLUSION

Apocynin per se does not inhibit vascular NADPH-oxidase-dependent superoxide formation. Its in vitro vasodilator actions are not due to NADPH oxidase inhibition but may be explained at least in part by inhibition of Rho kinase activity. The discrepancy between apocynin-induced vasodilation in vitro and the failure of apocynin to lower arterial pressure in SHR suggests opposing effects on arterial pressure-regulating systems in vivo. Its use as a pharmacological tool to investigate vascular NADPH oxidase should be discontinued.

Low NaCl intake elevates renal medullary endothelin-1 and endothelin A (ETA) receptor mRNA but not the sensitivity of renal Na+ excretion to ETA receptor blockade in rats

AIMS

This study was performed to investigate the effects of NaCl intake on renal mRNA expression of pre-pro-endothelin-1 (ET-1), endothelin A (ET(A)) and endothelin B (ET(B)) receptors as well as on renal ET-1 content in rats. We further tested for NaCl intake-dependent differences in the contribution of the ET system to renal sodium handling.

METHODS

Male Sprague-Dawley rats with telemetric devices were randomized to 0.15%, 0.60% and 1.80% NaCl diets with or without losartan. Renal sodium balance and arterial pressure were monitored. Renal blood flow and fractional sodium excretion (FENa) were measured in response to acute infusion of ET(A) and ET(B) blockers into the inner stripe of the outer renal medulla.

RESULTS

Medullary pre-pro-ET-1, ET(A) and ET(B) receptor mRNA was 50%, 81% and 33% higher in rats on 0.15% vs. 1.80% NaCl. Losartan reduced medullary gene expression in rats on 0.15% NaCl. Medullary ET-1 content was 983 +/- 88 and 479 +/- 42 ng mg(-1) protein in rats on 0.15% and 1.80% NaCl (P < 0.001). Chronic ET(A) receptor blocker treatment reduced arterial pressure by 8-10 mmHg in rats on 0.15% vs. 1.80% NaCl without affecting renal sodium balances. Acute medullary ET(A) or ET(B) receptor blockade did not alter medullary blood flow and FENa in animals on either diet.

CONCLUSION

In rats renal medullary ET-1 content and mRNA expression of three ET system components are inversely related to NaCl intake. Higher expression levels on low NaCl intake are AT(1) receptor dependent but are not associated with increased sensitivity of renal sodium handling to ET(A) receptor blockade.

Impaired coronary function in Wistar Ottawa Karlsburg W rats—a new model of the metabolic syndrome

The metabolic syndrome is associated with an increased risk for coronary heart disease. The underlying mechanisms are not well understood. The present study was designed to investigate coronary function in Wistar Ottawa Karlsburg W (WOKW) rats, a new animal model of the metabolic syndrome. The responses of coronary artery segments from WOKW and Dark Agouti (DA) control rats of different ages to several physiological vasoconstrictors and vasodilators were measured in a small vessel wire myograph, and potential mechanisms involved in the differential responses between the two strains were investigated. WOKW showed increased alpha(1)-adrenoceptor-mediated coronary constriction at 3 and 10 months of age, as well as seriously blunted beta-adrenoceptor-mediated coronary relaxation at 16 months of age. Responses to non-adrenergic agonists were not altered in WOKW compared to DA. The alpha(1)-adrenoceptor-mediated coronary constriction in WOKW was completely blocked by rho-kinase inhibition. Induced hyperinsulinemia did not cause increased alpha(1)-adrenoceptor-mediated coronary constriction or impaired beta-adrenoceptor-mediated coronary relaxation in DA. The association between blunted coronary beta-adrenoceptor responsiveness and the metabolic syndrome was confirmed in Zucker diabetic fatty rats. We conclude that the metabolic syndrome in WOKW rats is associated with impaired coronary function due to altered adrenoceptor sensitivity. The latter may contribute to inappropriately elevated coronary tone in insulin-resistant subjects, especially when sympathetic activity to the heart is increased.

Neonatal sympathectomy reduces NADPH oxidase activity and vascular resistance in spontaneously hypertensive rat kidneys

Neonatal sympathectomy reduces arterial pressure in spontaneously hypertensive rats (SHR). In SHR transplanted with a kidney from sympathectomized SHR, arterial pressure was lower and less Na+ sensitive than in SHR transplanted with a kidney from hydralazine-treated SHR. This study was performed to identify underlying renal mechanisms. Tests for differential renal mRNA expression of nine a priori selected genes revealed robust differences for renal medullary expression of the NADPH oxidase subunit p47phox. Therefore, we investigated the effects of neonatal sympathectomy on renal mRNA expression of NADPH oxidase subunits, NADPH oxidase activity, and renal function. In 10-wk-old sympathectomized SHR fed a 0.6% NaCl diet, medullary p47phox and gp91phox expression was 40% less than in hydralazine-treated SHR. Also, after a 1.8% NaCl diet, medullary p47phox mRNA expression was lower in sympathectomized than in hydralazine-treated SHR. We found lower cortical (-30%, P<0.01) and medullary (-30%, P<0.05) NADPH oxidase activities in sympathectomized than in hydralazine-treated or untreated SHR. Glomerular filtration rate, renal blood flow, medullary blood flow, and fractional Na+ excretion in kidney grafts from sympathectomized and hydralazine-treated donors (n=8 per group) were similar at baseline and in response to a 20-mmHg rise in renal perfusion pressure. Renal vascular resistance was lower in kidneys from sympathectomized than hydralazine-treated donors (25+/-2 vs. 32+/-4 mmHg.min.ml-1, P<0.05). The results indicate that the sympathetic nervous system contributes to the level of renal NADPH oxidase activity and to perinatal programming of alterations in renal vascular function that lead to elevated renal vascular resistance in SHR.

Sympatho-renal interactions in the determination of arterial pressure: role in hypertension

Renal mechanisms and the sympathetic nervous system contribute to the development of arterial hypertension. Renal transplantation experiments in spontaneously hypertensive rats (SHRs) were performed to investigate how the sympathetic nervous system and the kidneys interact during the development and maintenance of hypertension. Our findings indicate that the rise in arterial pressure that occurs after transplantation of a kidney from a SHR into normotensive recipients is not mediated by elevations in sympathetic activity. However, chronic reductions in sympathetic tone reduce the rise in arterial pressure which can be induced by SHR renal grafts in normotensive recipients. Untreated SHRs transplanted with a kidney from sympathectomized donors have lower arterial pressure and reduced sodium sensitivity of arterial pressure compared to SHRs transplanted with a kidney from hydralazine-treated donors. It is concluded that chronic non-adapting changes in sympathetic activity modulate the degree to which renal mechanisms can cause hypertension in SHRs. Severe reduction in sympathetic tone during early ontogeny causes long-term changes in renal function that mitigate hypertension development in SHRs even when the extrarenal neuro-hormonal environment is restored.