A linear relationship between the ex-vivo sodium mediated expression of two sodium regulatory pathways as a surrogate marker of salt sensitivity of blood pressure in exfoliated human renal proximal tubule cells: The virtual renal biopsy

Abstract

Background Salt sensitivity (SS) of blood pressure (BP) affects 25% of adults, shares comorbidity with hypertension, and has no convenient diagnostic test. We tested the hypothesis that urine-derived exfoliated renal proximal tubule cells (RPTCs) could diagnose the degree of an individual's SS of BP. Methods Subjects were selected who had their SS of BP determined 5 y prior to this study (salt-sensitive: ≥7 mm Hg increase in mean arterial pressure (MAP) following transition from a random weekly diet of low (10 mmol/day) to high (300 mmol/day) sodium (Na+) intake, N = 4; inverse salt-sensitive (ISS): ≥7 mm Hg increase in MAP transitioning from a high to low Na+ diet, N = 3, and salt-resistant (SR): <7 mm Hg change in MAP transitioned on either diet, N = 5). RPTC responses to 2 independent Na+ transport pathways were measured. Results There was a negative correlation between the degree of SS and dopamine-1 receptor (D1R) plasma membrane recruitment (y = −0.0107x + 0.68 relative fluorescent units (RFU), R2 = 0.88, N = 12, P < 0.0001) and angiotensin II-stimulated intracellular Ca++ (y = −0.0016x + 0.0336, R2 = 0.7112, P < 0.001, N = 10) concentration over baseline. Conclusions Isolating RPTCs from urine provides a personalized cell-based diagnostic test of SS index that offers advantages over a 2-week controlled diet with respect to cost and patient compliance. Furthermore, the linear relationship between the change in MAP and response to 2 Na+ regulatory pathways suggests that an individual's RPTC response to intracellular Na+ is personalized and predictive.

Salt Sensitivity of Blood Pressure

“…Notably, in this study, as in the index dietary study, 368 the major change in BP was seen after sodium and volume depletion rather than after saline infusion. 1 Although small patient series studied with a dietary approach identified subjects who sustained depressor responses to a salt load, 377,378 in this acute intravenous protocol, none of the 576 subjects exhibited a decrease in BP after the saline load. They sustained an average increase in MAP of <5 mm Hg, which was not different between the normotensive and hypertensive subjects, 1 perhaps reflecting the fact that individuals with an activated renin-angiotensin system were not selected for these studies.…”

“…Hence, assessing the activity of these pathways may allow personalized identification of SS and SR phenotypes. 377 Importantly, characterization of cell pathway activity took place a long time after phenotyping, indicating independence of the marker from the state of salt intake of the subject. In addition, subjects with decreased BP in response to salt (designated inverse SS) had the highest activity of these pathways, suggesting that the marker could serve to identify patients who might be harmed by a reduction in salt intake.…”

“…The surrogates discussed above, derived from 24-hour ABPM, have not been uniformly successful, 367,423,437 and those from the study of urine proximal tubular cells and exosomes are too preliminary. 377,378 In the absence of a marker, demographic and clinical phenotypes associated with SSBP can be used to guide dietary advice. Sodium sensitivity is more common in those with hypertension, 344 in blacks compared with other ethnic groups, 344 in women, 341 in people >45 years of age, 339,341,381 and in those with the metabolic syndrome.…”

The sodium-bicarbonate cotransporter NBCe2 (slc4a5) expressed in human renal proximal tubules shows increased apical expression under high-salt conditions

“…Similar results were obtained, i.e., the increase in intracellular sodium recruited NBCe2 to the apical membrane, paving the way for future studies on receptor-mediated regulation of sodium and bicarbonate transport. In addition, our ability to culture freshly voided RPTCs from human urine will allow us to measure NBCe2 and NBCe1 activity following various dietary perturbations in human subjects (15).…”

Novel round cells in urine sediment and their clinical implications

“…The nuclei degenerate and become eccentric, with a low N/C ratio in contrast rectangular, polygonal or even columnar cells originate from distal tubules or collecting ducts. Also immunostaining for CD13, CD15, epithelial membrane antigen, peanut agglutinin, or segment specific markers can differentiate proximal tubule-derived cells [11]. These characteristics resembles to round cells.…”