Central Sodium Sensing: Implications for Blood Pressure Regulation

University of Delaware29 enrolled

Overview

The ability of the brain to sense changing sodium levels in the blood is critical in mediating the neurohumoral responses to hypernatremia, however, the mechanisms underlying sodium sensing in humans is poorly understood. The purpose of this study is to identify key sodium-sensing regions of the human brain in older adults and determine if the Na-K-2Cl co-transporter mediates the neurohumoral response to acute hypernatremia. Completion of this project will increase our understanding of blood pressure regulation, which has major public health implications.

The prevalence of hypertension is very high in older adults, and a major factor in hypertension is salt sensitivity of blood pressure (BP) and elevated sympathetic nerve activity (SNA). However, we know very little about how the human brain 'senses' sodium, and what molecular mechanisms are involved. Rodent studies have identified specialized sodium chloride (NaCl)-sensing neurons in the circumventricular organs (CVOs), which mediate NaCl-induced changes in SNA, arginine vasopressin (AVP), and BP. Recent data suggest the Na-K-2Cl co-transporter (NKCC2) is not kidney specific but is also expressed in brain regions that regulate whole body NaCl and water homeostasis. In addition, NKCC2 is accessible by drugs in the circulation since the CVOs lack a complete blood brain barrier. The objective of this R21 is to identify key NaCl-sensing regions of the brain in older adults and determine if NKCC2 mediates the neurohumoral response to acute hypernatremia. We seek to translate the prior rodent findings to humans by assessing neuronal activation (using blood oxygen level dependent functional magnetic resonance imaging, BOLD fMRI) as well as thirst, AVP, SNA and BP during an acute hypernatremic stimulus, with and without an NKCC2 antagonist (furosemide). This will enable us to assess the role of NKCC2 in NaCl sensing. The overall hypothesis is that acute hypernatremia will elicit detectable changes in the BOLD fMRI signal and increase thirst, AVP, SNA, and BP largely through NKCC2 in healthy older adults. Accordingly, the first specific aim is to identify the areas of the human brain that respond to acute hypernatremia and determine the role of NKCC2 in central NaCl- sensing. Acute hypernatremia will be induced with a 30-minute infusion of 3% NaCl delivered intravenously. Brain activity during the hypertonic saline infusion will be measured in regions such as the organum vasculosum laminae terminalis, subfornical organ, anterior cingulate cortex, hypothalamus, and insular cortex. The second specific aim is to determine the effect of acute hypernatremia on thirst, AVP, SNA, and BP, and determine the role of NKCC2 in mediating these responses. Salt sensitivity of BP will be individually assessed and comparisons will be made between those with a salt resistant and salt sensitive phenotype; we anticipate that acute hypernatremia will elicit changes in the BOLD fMRI signal and SNA \& AVP in all subjects, but the responses will be greater in those who are classified as salt sensitive. This would represent the first trial in healthy human subjects to identify a putative brain NaCl-sensing co-transporter, and we think the scope and innovative approaches are ideal for the R21 funding mechanism. Older adults are prone to hypertension, so it is critically important to understand how normotensive older adults centrally sense sodium, to provide a needed foundation for exploring the mechanistic underpinning of salt sensitive hypertension.

Conditions

Salt Sensitivity of Blood Pressure

Interventions

Hypertonic Saline with furosemideOTHER

Subjects will undergo MRI with a hypertonic saline infusion with NKCC2 antagonism (furosemide). The hypertonic saline will be a 3% NaCl solution infused intravenously at a rate of 0.15 ml/kg/min for 30 minutes; the furosemide will be infused intravenously as a 40 mg bolus in 4mL of isotonic saline (0.9% NaCl) immediately prior to the hypertonic saline infusion.

Hypertonic Saline without furosemideOTHER

Subjects will undergo MRI with a hypertonic saline infusion. The hypertonic saline will be a 3% NaCl solution infused intravenously at a rate of 0.15 ml/kg/min for 30 minutes.

Eligibility

Sex: ALLMin age: 18 YearsMax age: 45 YearsHealthy volunteers:

Medical Language ↔ Plain English

Inclusion Criteria: * Age: 18 - 45 years * Blood pressure: \>100/60 mmHg and \<130/80 mmHg * BMI: 18.5 kg/m2 - 30 kg/m2 * Serum potassium: 3.5 mmol/L - 5.5 mmol/L Exclusion Criteria: * Age: \< 18 years or \> 45 years * Blood pressure: \< 100/60 mmHg or \> 130/80 mmHg * BMI: \< 18.5 kg/m2 or \> 30 kg/m2 * Serum potassium: \< 3.5 mmol/L or \> 5.5 mmol/L * Abnormal ECG * History of - cardiovascular, cancer, metabolic, respiratory, renal disease * Hormone replacement therapy * Current tobacco or nicotine use * Pregnant or nursing mothers * Major brain injury (concussions do not count) * Clinically diagnosed psychiatric or neurological disorder * Clinically diagnosed anxiety or depression * Psychiatric, neurological, anxiety or depression medications * Hypertension medications * Sulfonamide drug allergy

Locations (1)

William B Farquhar

Newark, Delaware, United States

Outcomes

Primary Outcomes

Functional Connectivity Between the Subfornical Organ and Organum Vasculosum of the Lamina Terminalis (Z-score)

Functional connectivity between sodium sensing circumventricular organs (subfornical organ (SFO) and organum vasculosum of the lamina terminalis(OVLT)) was calculated (expressed as the z-score). Functional connectivity is a measure of the the correlation (or synchronization) of the blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) signal time course between two brain regions. Pearson correlations were computed between the BOLD fMRI signal in the SFO and OVLT in a seed-to-seed functional connectivity analysis. Pearson correlations were converted to Z-scores using a Fisher's transform. A Z-score of 0 indicates no correlation between the BOLD fMRI signal between these 2 brain regions; a higher score indicates a greater, positive correlation between the BOLD fMRI signal in these 2 brain regions; a lower score indicates a greater, negative correlation between the fMRI signal in these 2 brain regions. This data does not have any clinical thresholds.

Time frame: Functional connectivity (FC) was calculated at baseline (~10 min). Then, participants received a 30-minute hypertonic saline infusion (HSI) with or without furosemide before. FC was calculated during the early (0-15 min) and late phase (15-30 min) of HSI.

Data from ClinicalTrials.gov

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