Olga Schmidlin

Chloride-Dominant Salt Sensitivity in the Stroke-Prone Spontaneously Hypertensive Rat

We tested the hypothesis that in the stroke-prone spontaneously hypertensive rat (SHRSP), the Cl− component of dietary NaCl dominantly determines its pressor effect (salt-sensitivity). We telemetrically measured systolic aortic blood pressure (SBP) in SHRSP loaded with: nothing (CTL); NaCl alone (NaCl) (44 mmol/100 grams chow); KCl (KCl) alone (44 mmol); NaCl (44 mmol) combined with KHCO3 (77 mmol) (NaCl/KBC) or with KCl (77 mmol) (NaCl/KCl). Across all groups, from age 10 to 15 or 16 weeks, SBP increased linearly (mm Hg/week) (dp/dt, change in SBP as a function of time): CTL, 5.6; NaCl, 9.5; KCl, 8.8; NaCl/KBC, 9.1; and NaCl/KCl, 14.6. Thus, the value of dp/dt in KCl matched that in NaCl. The value of dp/dt in NaCl/KCl exceeded that in NaCl in direct proportion to the greater Cl− load. Across all groups, only Cl− load bore a direct, highly linear relationship with dp/dt. Strokes occurred only, but always with SBP >250 mm Hg, a value observed almost exclusively in NaCl/KCl. Thus, Cl− dominantly determined the pressor effect induced with dietary NaCl, both with NaCl loaded alone and combined with either KCl or KHCO3, and thereby likely determined the occurrence of stroke with NaCl loading. Over the initial 3-day period of NaCl loading and exacerbating hypertension, external balance of Na+ increased similarly among all groups. However, within 24 hours of initiating NaCl loading, urinary creatinine excretion decreased in direct proportion to dp/dt and urinary Cl− excretion. We conclude that in the SHRSP, the Cl− component of a dietary NaCl dominantly determines salt sensitivity and thereby phenotypic expression. We suggest that Cl− might do so by inducing renal vasoconstriction.

Relationship and Interaction between Sodium and Potassium

Compared with the Stone Age diet, the modern human diet is both excessive in NaCl and deficient in fruits and vegetables which are rich in K+ and HCO3−-yielding organates like citrate. With the modern diet, the K+/Na+ ratio and the HCO3−/Cl− ratio have both become reversed. Yet, the biologic machinery that evolved to process these dietary electrolytes remains largely unchanged, genetically fixed in Paleolithic time. Thus, the electrolytic mix of the modern diet is profoundly mismatched to its processing machinery. Dietary potassium modulates both the pressor and hypercalciuric effects of the modern dietary excess of NaCl. A marginally deficient dietary intake of potassium amplifies both of these effects, and both effects are dose-dependently attenuated and may be abolished either with dietary potassium or supplemental KHCO3. The pathogenic effects of a dietary deficiency of potassium amplify, and are amplified by, those of a dietary excess of NaCl and in some instances a dietary deficiency of bicarbonate precursors. Thus, in those ingesting the modern diet, it may not be possible to discern which of these dietary electrolytic dislocations is most determining of salt-sensitive blood pressure and hypercalciuria, and the hypertension, kidney stones, and osteoporosis they may engender. Obviously abnormal plasma electrolyte concentrations rarely characterize these dietary electrolytic dislocations, and when either dietary potassium or supplemental KHCO3 corrects the pressor and hypercalciuric effects of these dislocations, the plasma concentrations of sodium, potassium, bicarbonate and chloride change little and remain well within the normal range.

NaCl-Induced Renal Vasoconstriction in Salt-Sensitive African Americans Antipressor and Hemodynamic Effects of Potassium Bicarbonate

In 16 African Americans (blacks, 14 men, 2 women) with average admission mean arterial pressure (MAP, mm Hg) 99.9+/-3.5 (mean+/-SEM), we investigated whether NaCl-induced renal vasoconstriction attends salt sensitivity and, if so, whether supplemental KHCO3 ameliorates both conditions. Throughout a 3-week period under controlled metabolic conditions, all subjects ate diets containing 15 mmol NaCl and 30 mmol potassium (K+) (per 70 kg body wt [BW] per day). Throughout weeks 2 and 3, NaCl was loaded to 250 mmol/d; throughout week 3, dietary K+ was supplemented to 170 mmol/d (KHCO3). On the last day of each study week, we measured renal blood flow (RBF) and glomerular filtration rate (GFR) using renal clearances of PAH and inulin. Ten subjects were salt sensitive (SS) (DeltaMAP >+5%) and 6 salt resistant (SR). In NaCl-loaded SS but not SR subjects, RBF (mL/min/1.73 m2) decreased from 920+/-75 to 828+/-46 (P<0.05); filtration fraction (FF, %) increased from 19. 4+/- to 21.4 (P<0.001); and renal vascular resistance (RVR) (10(3)xmm Hg/[mL/min]) increased from 101+/-8 to 131+/-10 (P<0.001). In all subjects combined, DeltaMAP varied inversely with DeltaRBF (r =-0.57, P=0.02) and directly with DeltaRVR (r = 0.65, P=0.006) and DeltaFF (r = 0.59, P=0.03), but not with MAP before NaCl loading. When supplemental KHCO3 abolished the pressor effect of NaCl in SS subjects, RBF was unaffected but GFR and FF decreased. The results show that in marginally K+-deficient blacks (1) NaCl-induced renal vasoconstrictive dysfunction attends salt sensitivity; (2) the dysfunction varies in extent directly with the NaCl-induced increase in blood pressure (BP); and (3) is complexly affected by supplemented KHCO3, GFR and FF decreasing but RBF not changing. In blacks, NaCl-induced renal vasoconstriction may be a pathogenetic event in salt sensitivity.

NMR spectroscopy and MRI investigation of a potential bioartificial liver

NMR feasibility was established for a coaxial hydrophobic‐membrane bioreactor containing isolated rat hepatocytes with features designed to mimic the human liver. A novel triple‐tuned NMR probe and a perfusion system controlling temperature, gas concentrations, flow‐rate, and pH were used. We determined the optimum coaxial interfiber distance (i.e. diffusion distance) for maintaining hepatocyte viability in two bioreactor prototypes. Prototype no. 1 and no. 2 had diffusion distances of 500 μm and 200 μm, respectively. Cell viability was established by 31P NMR and trypan blue exclusion. Only prototype no. 2 maintained cell viability for more than 6 h, indicating the importance of diffusion distance. 31P spectra obtained over this 6 h time period were similar to in vivo spectra of rat liver. The 31P spectra were found to be more sensitive to subacute cell viability than trypan blue exclusion. In the 1H and 31P spectra, 1H2O and inorganic phosphate signals were split in two at all flow‐rates, probably due to bulk magnetic susceptibility effects originating from the three bioreactor compartments. MRI was useful for quality control and determining flow dynamics, fiber integrity, and cell inoculate distribution. MRI revealed that the inner fibers were not centered in either prototype. Although an increased flow‐rate did not influence spectral resolution or chemical shifts, significant degradation of MRI quality occurred above 50 mL/min. NMR spectroscopy and imaging provide valuable, real‐time information on cell biochemistry and flow dynamics which can be used in development and monitoring of bioreactors designed as artificial livers.

What Initiates the Pressor Effect of Salt in Salt-Sensitive Humans?: Observations in Normotensive Blacks

We tested the traditional hypothesis that an abnormally enhanced renal reclamation of dietary NaCl alone initiates its pressor effect (“salt sensitivity”). Under metabolically controlled conditions, we grouped 23 normotensive blacks as either salt-sensitive (SS) or salt-resistant (SR), depending on whether or not dietary NaCl loading did or did not increase mean arterial blood pressure (MAP) by ≥5 mm Hg. We determined whether dietary NaCl loading induces greater increases in external Na+ balance, plasma volume, and cardiac output in SS, compared with any in SR subjects, and differential changes in systemic vascular resistance (SVR) that could account for the pressor differences between SS and SR subjects. Using impedance cardiography, we measured cardiac output and SVR daily at 4-hour intervals throughout the last 3 days of a 7-day period of low NaCl intake (30 mmol per day) and throughout a subsequent 7-day period of NaCl loading (250 mmol per day). In the 11 SS subjects, compared with the 12 SR subjects, NaCl loading induced no greater increases in Na+ balance, body weight, plasma volume, and cardiac output. Yet, from days 2 to 7 of NaCl loading, changes of MAP in SS diverged progressively from those in SR. From days 2 to 4, progressive increases of MAP in SS subjects reflected importantly impaired decreases of SVR, as judged from “normal” decreases of SVR in SR subjects. In SS and SR subjects combined, changes in both MAP and SVR on day 2 strongly predicted changes in MAP on day 7. In many normotensive blacks, vascular dysfunction is critical to the initiation of a pressor response to dietary NaCl.

Vasodysfunction That Involves Renal Vasodysfunction, Not Abnormally Increased Renal Retention of Sodium, Accounts for the Initiation of Salt-Induced Hypertension.

It has long been recognized that in some people substantially increasing dietary intake of salt (NaCl) increases blood pressure, whereas in others, “salt loading” has little or no effect on blood pressure.1 Blood pressure so affected by salt has been called salt sensitive and salt resistant, respectively. Although the blood pressure response to salt is a continuous variable and the trait of salt sensitivity, like that of hypertension, is arbitrarily defined,2 it has been estimated that 30% to 50% of hypertensive humans are salt sensitive and ≈25% of normotensive humans are salt sensitive.3,4 Salt sensitivity confers an increased risk for the occurrence of hypertension and cardiovascular disease.5–7 Furthermore, pathophysiological mechanisms mediating salt sensitivity may contribute to the risk for cardiovascular disease beyond their effects on blood pressure.5 Accordingly, the mechanisms of salt sensitivity continue to be studied intensively with the hope that better understanding of those mechanisms could lead to improved approaches to the prevention and treatment of salt-induced increases in blood pressure and cardiovascular disease. Response by Hall on p 893 Prevailing theory holds that an abnormally large increase in renal retention of salt8–16 is an early pathophysiological event in the causation of salt sensitivity and salt-induced hypertension. In accord with this theory, it is held that a substantial increase in dietary salt does not induce a pressor effect in salt-resistant subjects because they excrete a salt load more rapidly and retain less sodium than salt-sensitive individuals.12,17 In the present analysis, we challenge this conventional view of salt sensitivity/salt resistance and make the case for a “vasodysfunction” theory for the initiation of salt-induced hypertension: An abnormal vascular resistance response to increases in salt intake, in the absence of an abnormally large increase in renal …

Sodium-Selective Salt Sensitivity Its Occurrence in Blacks

We tested the hypothesis that the Na+ component of dietary NaCl can have a pressor effect apart from its capacity to complement the extracellular osmotic activity of Cl− and, thus, expand plasma volume. We studied 35 mostly normotensive blacks who ingested a low-NaCl diet, 30 mmol/d, for 3 weeks, in the first and third of which Na+ was loaded orally with either NaHCO3 or NaCl, in random order (250 mmol/d). In subjects adjudged to be salt sensitive (n=18; &Dgr; mean arterial pressure: ≥5 mm Hg with NaCl load), but not in salt-resistant subjects (n=17), loading with NaHCO3 was also pressor. The pressor effect of NaHCO3 was half that of NaCl: mean arterial pressure (millimeters of mercury) increased significantly from 90 on low NaCl to 95 with NaHCO3 and to 101 with NaCl. The pressor effect of NaCl strongly predicted that of NaHCO3. As judged by hematocrit decrease, plasma volume expansion with NaCl was the same in salt-resistant and salt-sensitive subjects and twice that with NaHCO3, irrespective of the pressor effect. In salt-sensitive subjects, mean arterial pressure varied directly with plasma Na+ concentration attained with all Na+ loading. In salt-sensitive but not salt-resistant subjects, NaHCO3 and NaCl induced decreases in renal blood flow and increases in renal vascular resistance; changes in renal blood flow were not different with the 2 salts. Responses of renal blood flow and renal vascular resistance to NaHCO3 were strongly predicted by those to NaCl. In establishing the fact of “sodium-selective” salt sensitivity, the current observations demonstrate that the Na+ component of NaCl can have pressor and renal vasoconstrictive properties apart from its capacity to complement Cl− in plasma volume expansion.

Reduced Dietary Potassium Reversibly Enhances Vasopressor Response to Stress in African Americans

Acute vasopressor responses to stress are adrenergically mediated and hence potentially subject to differential modulation by dietary potassium and sodium. The greater vasopressor responsiveness in blacks compared with whites might then be consequent not only to a high dietary salt intake but also to a marginally reduced dietary potassium intake. Under controlled metabolic conditions, we compared acute vasopressor responses to cold and mental stress in black and white normotensive men during three successive dietary periods: (1) while dietary potassium was reduced (30 mmol K+/70 kg per day) and salt was restricted (10 to 14 days); (2) while salt was loaded (15 to 250 mmol Na+/70 kg per day) (7 days); and (3) while salt loading was continued and potassium was either supplemented (70 mmol K+/70 kg per day) (7 to 21 days) in 9 blacks and 6 whites or continued reduced (30 mmol K+/70 kg per day) (28 days) in 4 blacks (time controls). At the lower potassium intake, cold-induced increase in forearm vascular resistance in blacks was twice that in whites during both salt restriction and salt loading. Normalization of dietary potassium attenuated cold-induced increases in both forearm vascular resistance and systolic and diastolic blood pressures in blacks but only in systolic pressure in whites. In blacks but not in whites, normalization of dietary potassium attenuated mental stress-induced increases in systolic and diastolic pressures. In normotensive blacks but not whites, a marginally reduced dietary intake of potassium reversibly enhances adrenergically mediated vasopressor responsiveness to stress. That responsiveness so enhanced over time might contribute to the pathogenesis of hypertension in blacks.

In vivo monitoring of hepatic glutathione in anesthetized rats by 13C NMR.

A method for in vivo 13C NMR monitoring of hepatic glutathione (GSH) in intact, anesthetized rats has been developed. Studies were conducted using a triple‐tuned, surgically implanted surface coil designed for this animal model. The coil permitted complete decoupling and sufficient resolution in the 13C NMR spectrum to monitor the time course of hepatic 13C‐metabolites of intravenously administered 2‐13C‐glycine, particularly GSH at 44.2 ppm and serine signals at 61.1 and 57.2 ppm, respectively. It further allowed concomitant monitoring of high‐energy phosphagens and intracellular pH by 31P NMR. To confirm in vivo NMR peak assignments, we compared high‐resolution 2D 1H{13C} heteronuclear multiple quantum coherence and 1D 13C spectra of hepatic perchloric acid extracts to those of authentic standards. The fractional isotopic enrichment of hepatic 13C‐glycine increased exponentially at a rate of 1.68 h−1 and reached its plateau level of 81% in 2 h. The 13C fractional isotopic enrichment of GSH increased exponentially at a rate of 0.316 h−1 and reached 55% after 4 h of 2‐13C‐glycine infusion, but without achieving a plateau. To confirm that the resonance at 44.2 ppm resulted from GSH, a rat was given an intravenous dose of 2‐oxothiazolidine‐4‐carboxylic acid (OTC), a cysteine precursor that increases intracellular GSH. As expected, with OTC administration the hepatic 13C GSH‐to‐glycine peak area increased more than sevenfold. Magn Reson Med 48:430–439, 2002.

An alternative hypothesis to the widely held view that renal excretion of sodium accounts for resistance to salt-induced hypertension

It is widely held that in response to high salt diets, normal individuals are acutely and chronically resistant to salt-induced hypertension because they rapidly excrete salt and retain little of it so that their blood volume, and therefore blood pressure, does not increase. Conversely, it is also widely held that salt-sensitive individuals develop salt-induced hypertension because of an impaired renal capacity to excrete salt that causes greater salt retention and blood volume expansion than that which occurs in normal salt-resistant individuals. Here we review results of both acute and chronic salt-loading studies that have compared salt-induced changes in sodium retention and blood volume between normal subjects (salt-resistant normotensive control subjects) and salt-sensitive subjects. The results of properly controlled studies strongly support an alternative view: during acute or chronic increases in salt intake, normal salt-resistant subjects undergo substantial salt retention and do not excrete salt more rapidly, retain less sodium, or undergo lesser blood volume expansion than do salt-sensitive subjects. These observations: (i) directly conflict with the widely held view that renal excretion of sodium accounts for resistance to salt-induced hypertension, and (ii) have implications for contemporary understanding of how various genetic, immunologic, and other factors determine acute and chronic blood pressure responses to high salt diets.