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

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.

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.

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.

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.

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.

No description available.