Adrenomedullin (ADM) is a 52 amino acid vasodilator peptide isolated, in 1993, from human pheochromocytoma. It has been demonstrated in the adrenal medulla of several mammalian species, including humans and rats. There have been conflicting results of the tissue distribution in the adrenal cortex. Hypertension is a complex trait with multiple genetic and environmental influences. Furthermore, salt-sensitive hypertension is characterized by a cluster of renal, hormonal, and metabolic derangements that might favour the development of cardiovascular and renal complications. Therefore the objective of this study was to investigate the adrenal distribution of ADM as well as to semi-quantitatively assess the adrenomedullin secretory capacity of the adrenal gland in the rat model of salt sensitive hypertension. Fourty-four male weanling rats were divided into 4 experimental groups and placed on a dietary regimen for 6 weeks viz. Dahl salt sensitive (DSS) rats on a high sodium diet (8% NaCl), DSS on a normal sodium diet (1% NaCl) matched with normotensive Dahl salt resistant (DSR) rats on the same dietary treatments. Blood pressure was monitored by tail-cuff readings and by the end of the six weeks, the DSS rats developed hypertension with tachycardia irrespective of the diet they were fed. The normal sodium diet was found to delay the development of hypertension, whilst the high sodium diet exacerbated the development of hypertension. Kidney weights and heart weights were greater in DSS rats than DSR rats probably due to their renal pathology or cardiac hypertrophy. Adrenomedullin immunopositivity was found predominantly in the adrenal medulla, and to varying degrees in the zona glomerulosa and zona reticularis of the adrenal cortex. The semi-quantitative analysis indicate that there was a 6.3 fold increase in ADM content of DSS rats compared to the DSR rats, where both consumed the 1% NaCI supplemented diet (DSR : 5.98 ± 0.3 vs. DSS : 37.85 ± 0.5, P / Thesis (M.Sc.)-University of Durban-Westville, 2003.

High blood pressure or hypertension is a major public health issue that affects more than 50% of adults in the United States. Hypertension is the leading risk factor for multiple cardiovascular events including stroke and myocardial infarction. In general, hypertension is considered a disease of the aged population as 2/3rds of adults over 65 are hypertensive. Critically, a sex dependent component exists as females under age 50 are less likely to develop hypertension than males. Dietary sodium intake significantly influences blood pressure regulation and its importance is underscored by the salt-sensitivity of blood pressure, which is characterized by acute increases in blood pressure in response to dietary salt intake. The salt-sensitivity of blood pressure is prevalent in 25% of normotensive individuals and 50% of hypertensive individuals. Coupled with statistics that show Americans on average consume 2 g of sodium in excess of the recommended daily allowance, the risk for developing salt-sensitive hypertension is drastically higher in salt-sensitive individuals. Moreover with age, there is an increase in the prevalence and severity of salt-sensitivity. Taken together, these findings underscore the need for novel therapeutics to combat hypertension. The pathophysiology of the salt-sensitivity of blood pressure and age dependent hypertension has been attributed in part to excessive sympathetic outflow that can drive increases in sodium reabsorption. Excessive sympathetic outflow via the release of norepinephrine has been linked to increased activity of a key renal sodium transporter, the sodium chloride cotransporter (NCC). This thesis investigates the adrenergic signaling pathway by which excessive sympathetic outflow drives NCC activity and sodium reabsorption as well investigates the mechanisms underlying sex differences in age dependent hypertension. Our findings demonstrate that 1) norepinephrine mediates its influence on NCC activity via an α1-adrenoceptor gated pathway involving WNK/SPAK/OxSR1 kinase signaling, 2) α1-adrenoceptor antagonism can prevent and attenuate the development and maintenance of salt-sensitive hypertension, 3) β-adrencoptor antagonism has no effect on NCC activity, 4) in male rats age dependent salt-sensitivity of blood pressure and hypertension is associated with age dependent- increases in NCC activity and impairments in renal sodium handling, and 5) female rats do not develop age dependent hypertension or salt-sensitivity of blood pressure. Collectively, these results support a sympathetic model of NCC regulation that plays a key role in salt-sensitive hypertension and age dependent hypertension. / 2021-05-26T00:00:00Z

High salt intake contributes to the development of hypertension in salt-sensitive humans and animals and the mechanistic causes are poorly understood. In Dahl salt-sensitive (S) but not salt-resistant (R) rats, high salt diet increases cerebrospinal fluid (CSF) [Na+] and activates an aldosterone-mineralocorticoid receptor-epithelial sodium channel-endogenous ouabain (MR-ENaC-EO) neuromodulatory pathway in the brain that enhances the activity of sympatho-excitatory angiotensinergic and glutamatergic pathways, leading to an increase in sympathetic nerve activity (SNA) and blood pressure (BP). We hypothesize that high salt diet in Dahl S rats enhances Ang II release in the paraventricular nucleus (PVN), causing a decrease in local nitric oxide (NO) action and an increase in local glutamate release thereby elevating SNA, BP and heart rate (HR). The present study evaluated the effects of agonists or blockers of MR, ENaC, EO, nitric oxide synthase (NOS) or glutamate and AT1-receptors on the BP and HR responses to acute infusions of Na+ rich aCSF, intracerebroventricularly (icv), or in the PVN of Dahl S, R or Wistar rats or to high salt diet in Dahl S and R rats. In Wistar rats, aldosterone in the PVN enhanced the BP and HR responses to infusion of Na+ rich aCSF in the PVN, but not in the CSF, and only the enhancement was prevented by blockers of MR, ENaC and EO in the PVN. AT1-receptor blockers in the PVN fully blocked the enhancement by aldosterone and the responses to infusion of Na+ rich aCSF icv, or in the PVN. Na+ rich aCSF in the PVN caused larger increases in BP and HR in Dahl S vs. R rats and the responses to Na+ were fully blocked by an AT1-receptor blocker in the PVN. BP and HR responses to a NOS blocker in the PVN were the same, but L-NAME enhanced Na+ effects more in Dahl R than S rats. High salt diet attenuated increases in BP from L-NAME in the PVN of Dahl S but not R rats. AT1 and glutamate receptor blockers candesartan and kynurenate in the PVN decreased BP in Dahl S but not R rats on high salt diet. At the peak BP response to candesartan, kynurenate in the PVN further decreased BP whereas candesartan did not further decrease BP at the peak BP response to kynurenate. Our findings indicate that both an acute increase in CSF [Na+] and high salt intake in Dahl S rats increases AT1-receptor activation and decreases NO action in the PVN thereby contributing to the pressor responses to Na+ and presumably, to dietary salt-induced hypertension. The increased BP response to AT1-receptor activation in the PVN of Dahl S is mediated by enhanced local glutamate receptor activation. An MR-ENaC-EO pathway in the PVN can be functionally active and further studies need to assess its role in Dahl S rats on high salt intake.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.

Hypertension affects 1 in 3 adults and is the single greatest risk factor for premature death. Salt-sensitive hypertension occurs in approximately 50% of hypertensive patients and results in a 3-fold increase in the risk of adverse cardiovascular events. However, the pathophysiology of salt-sensitive hypertension remains to be fully elucidated. There has been increased interest in the interaction between the sympathetic nervous system and the kidney and how that interaction mediates sodium excretion to drive the development of salt-sensitivity. Previous studies show that sympathetic over-activity increases expression of the sodium chloride cotransporter (NCC) resulting in increased NCC-mediated sodium reabsorption, and the development of salt-sensitive hypertension. In this thesis, I show the effect of increased norepinephrine (NE) and high salt intake in salt-resistant vs. salt-sensitive rat phenotypes on blood pressure regulation, NCC activity, and the adrenoreceptor-mediated regulatory kinase network signal transduction pathway. A high salt diet 1) exacerbates NE-induced hypertension in salt-resistant Sprague-Dawley (SD) rats and 2) results in hypertension in Dahl salt-sensitive (DSS) rats. In contrast to salt-resistant phenotypes (SD & Dahl salt-resistant), dietary sodium-evoked suppression of NCC expression and activity is prevented in salt-sensitive rats (SD-NE infusion & DSS) - I show that this occurs through a failure of a high salt intake to suppress renal OxSR1, SPAK, and WNK1 (NCC regulatory proteins). I demonstrate that α1-adrenoreceptors are responsible for mediating the salt-sensitive component of hypertension and restore dietary sodium-evoked suppression of the NCC via a predominant OxSR1 pathway. Chronic β-adrenoreceptor antagonism significantly reduces blood pressure in NE-mediated hypertension. How the body senses salt remains unknown, but my data show that selective removal of the afferent renal nerves prevents dietary sodium-evoked suppression of NCC expression and activity resulting in salt-sensitive hypertension, suggesting that the afferent renal nerves play an important role as a sodium-sensing mechanism. Overall, these data demonstrate that attenuated afferent renal nerve feedback drives renal efferent nerve release of NE to prevent the downregulation of the NCC via an α1-adrenergic receptor-gated WNK1-OxSR1 signal transduction pathway to evoke the development of salt-sensitive hypertension.

Dysregulation of the sodium-chloride cotransporter (NCC) is believed to significantly impact blood pressure. Recent studies have implicated overactivity of the sympathetic nervous system as a mechanism driving renal NCC dysregulation to evoke the development of salt-sensitive hypertension. It is proposed that the sympathetic nervous system accomplishes this by norepinephrine (NE)-mediated over-activation of the beta2-adrenergic receptors located in the distal tubules of the kidney. Beta2-adrenoreceptor activation is hypothesized to stimulate the protein kinases SPAK and OxSR1 to phosphorylate and thus activate NCC. This beta2-receptor-SPAK/OxSR1-NCC pathway was elucidated in studies that challenged salt-resistant mice with high-salt diets, bilateral adrenalectomies, and NE infusion. To expand the scope of these studies, we investigated the effects of elevated circulating NE on blood pressure, NCC activity, and expression of NCC proteins, SPAK, and OxSR1 in a different salt-resistant animal species (the Sprague-Dawley rat). In this study we implanted male Sprague-Dawley rats with osmotic minipumps delivering a subcutaneous infusion of either saline, NE, a 50:50 solution of DMSO/isotonic saline, a combination of NE and the NCC antagonist hydrochlorothiazide, or a combination of NE and the beta-adrenoreceptor antagonist propranolol. Following implantation of the pumps the rats were randomly assigned to either a standard diet (0.4% NaCl) or a high-salt diet (8% NaCl) for two weeks. After fourteen days all animals underwent acute femoral artery, vein, and bladder cannulation in order to monitor heart rate and blood pressure, administer drugs intravenously, and track renal function, respectively. Following surgical recovery, blood pressure and heart rate were measured continuously, and urine was collected in ten-minute intervals in order to assess peak natriuretic responses to amiloride and hydrochlorothiazide. Following this protocol the rats received an intravenous bolus of hexamethonium (30 mg/kg), and their peak drops in blood pressure were recorded. Afterwards both kidneys were harvested and frozen at -80 °C for measurement of NCC proteins, SPAK, and OxSR1 expression. This study demonstrates that increased circulating NE induces salt-sensitive hypertension in the naturally salt-resistant Sprague-Dawley rat. Chronic infusion of NE raised the blood pressure of the rats, and a high-salt diet exacerbated this effect. Furthermore, NE prevented salt-evoked suppression of NCC activity and NCC, SPAK, and OxSR1 protein expression. Co-infusion of hydrochlorothiazide with NE attenuated NE-mediated hypertension and caused no variance in the blood pressures between the standard salt and high-salt groups. This indicates that chronically antagonizing NCC eliminated the salt-sensitive component of NE-mediated hypertension. Beta-receptor antagonism combined with NE infusion completely eliminated the hypertensive influence of NE and downregulated the expression of NCC proteins, SPAK, and OxSR1. However, NCC activity still remained at a level comparable with that observed in the NE-infused rats, demonstrating dissociation between protein expression and function. These data, the first report in a rat model of an interaction of NE and a high salt intake that impairs NCC function, demonstrate that increased levels of NE in combination with a high dietary salt intake result in NCC dysregulation and the development of NE-mediated salt-sensitive hypertension. To an extent the data also support the proposition that NE activates the beta-adrenergic receptors to influence the activity of NCC and the expression of NCC proteins, SPAK, and OxSR1. Beta-antagonism combined with NE infusion attenuated the effects of NE on blood pressure and the expression of NCC proteins, SPAK, and OxSR1. However, the NE-mediated elevation of NCC activity still remained high. We propose that the beta-receptors are not the only adrenergic receptors that can influence NCC activity. The presence of alpha-adrenergic receptors in the distal tubules suggests that they may be able to keep NCC activity elevated through a pathway independent of the beta-receptors, SPAK, and OxSR1.

No description available.

No description available.

Statement of the problem: The epithelial sodium channels (ENaC) play an important role in regulation of blood pressure (BP). Although the genes are identical in Dahl salt sensitive (S) and Dahl salt resistant (R) rats, expression of ENaC subunits is increased in kidneys of S rats on high salt diet. Intracerebroventricular (icv) infusion of ENaC blocker benzamil prevents Na+ induced hypertension. It was not known whether ENaC subunits are expressed in the brain and whether or not brain ENaC plays a role in regulation of [Na+] in CNS. Hypothesis: 1. Epithelial sodium channels are expressed in the brain. 2. Expression of ENaC is increased in the kidneys and brain of Dahl S rats on high salt diet. 3. ENaC in the brain contributes to regulation of [Na+] in the CSF and brain interstitium. Methods of investigation: We studied expression and distribution of the ENaC subunits and assessed the effects of icv infusion of Na+-rich aCSF in Wistar rats or high salt diet in Dahl S rats in different areas of the brain. Function of ENaC in the choroid plexus was evaluated by studying the effects of benzamil and ouabain on Na+ transport. Major findings: In Wistar rats, both mRNA and protein of all three ENaC subunits are expressed in brain epithelia and magnocellular neurons in the supraoptic (SON) and paraventricular (PVN) nucleus. ENaC abundance is higher on the apical versus basolateral membrane of choroid cells. Benzamil decreases Na+ influx into choroid cells by 20-30% and increases CSF [Na+] by ~8 mmol/L. Na+ rich aCSF increases apical membrane expression of βENaC in the choroid cells and of α and βENaC in basolateral membrane of ependymal cells, but has no effect on neuronal ENaC. Expression of ENaC is higher in choroid cells and SON of Dahl S versus R rats and the higher expression persists on a high salt diet. High salt attenuates the ouabain blockable efflux of Na+ from choroid cells and has no effect on CSF [Na+] in Dahl R rats. In contrast, high salt does not attenuate ouabain blockable efflux of 22Na+ and CSF [Na+] increases in Dahl S. Main Conclusion: ENaC in the brain contributes to Na+ transport into the choroid cells and appear to be involved in reabsorption of Na+ from the CSF. Aberrant regulation of Na+ transport and of Na+K+ATPase activity, might contribute to increases in CSF [Na+] in Dahl S rats on high-salt diet. ENaC in magnocellular neurons may contribute to enhanced secretion of mediators such as ‘ouabain’ leading to sympathetic hyperactivity in Dahl S rats.