Renal acid-base excretion in normotensive salt-sensitive humans

Dietary acid load, kidney function and risk of chronic kidney disease: A systematic review and meta-analysis of observational studies

Aim: Study findings examining the association between dietary acid load (DAL), kidney function and risk of chronic kidney disease (CKD) are inconsistent and there has been no meta-analysis on the relationship between DAL, kidney function and risk of CKD, hence we investigated this association in this paper. Methods: PubMed, ISI web of science and Scopus were searched up to January 2018 to identify all relevant articles. Effect sizes of eligible studies were pooled in random- effect model using the Der Simonian-Laird method. The I² index was used to assess the amount of heterogeneity. Result: Twenty three studies with 200092 subjects were included. Meta-analysis of 9 observational studies showed that DAL had a positive significant association with risk of CKD (1.31; 95% CI: 1.06, 1.62; P = 0.011). Furthermore, increased DAL can decrease urine pH (-0.47; 95% CI: -0.85, -0.08; P = 0.017) significantly. Subgroup analysis could not identify the sources of heterogeneity about the association of DAL and risk of CKD. However, it showed the method of measurement was the source of heterogeneity about the association of DAL and urine pH (24 h urine pH: -0.62; 95% CI: -0.70, -0.54; P < 0.0001; Fasting urine pH: -0.08; 95% CI: -0.18, 0.02; P = 0.111). Conclusion: Our study showed that DAL can increase the risk of CKD and have an inverse association with urine pH.

Sodium bicarbonate cotransporter NBCe2 gene variants increase sodium and bicarbonate transport in human renal proximal tubule cells

RATIONALE

Salt sensitivity of blood pressure affects >30% of the hypertensive and >15% of the normotensive population. Variants of the electrogenic sodium bicarbonate cotransporter NBCe2 gene, SLC4A5, are associated with increased blood pressure in several ethnic groups. SLC4A5 variants are also highly associated with salt sensitivity, independent of hypertension. However, little is known about how NBCe2 contributes to salt sensitivity, although NBCe2 regulates renal tubular sodium bicarbonate transport. We hypothesized that SLC4A5 rs10177833 and rs7571842 increase NBCe2 expression and human renal proximal tubule cell (hRPTC) sodium transport and may be a cause of salt sensitivity of blood pressure.

OBJECTIVE

To characterize the hRPTC ion transport of wild-type (WT) and homozygous variants (HV) of SLC4A5.

METHODS AND RESULTS

The expressions of NBCe2 mRNA and protein were not different between hRPTCs carrying WT or HV SLC4A5 before or after dopaminergic or angiotensin (II and III) stimulation. However, luminal to basolateral sodium transport, NHE3 protein, and Cl-/HCO3- exchanger activity in hRPTCs were higher in HV than WT SLC4A5. Increasing intracellular sodium enhanced the apical location of NBCe2 in HV hRPTCs (4.24±0.35% to 11.06±1.72% (P<0.05, N = 3, 2-way ANOVA, Holm-Sidak test)) as determined by Total Internal Reflection Fluorescence Microscopy (TIRFM). In hRPTCs isolated from kidney tissue, increasing intracellular sodium enhanced bicarbonate-dependent pH recovery rate and increased NBCe2 mRNA and protein expressions to a greater extent in HV than WT SLC4A5 (+38.00±6.23% vs HV normal salt (P<0.01, N = 4, 2-way ANOVA, Holm-Sidak test)). In hRPTCs isolated from freshly voided urine, bicarbonate-dependent pH recovery was also faster in those from salt-sensitive and carriers of HV SLC4A5 than from salt-resistant and carriers of WT SLC4A5. The faster NBCe2-specific bicarbonate-dependent pH recovery rate in HV SCL4A5 was normalized by SLC4A5- but not SLC4A4-shRNA. The binding of purified hepatocyte nuclear factor type 4A (HNF4A) to DNA was increased in hRPTCs carrying HV SLC4A5 rs7571842 but not rs10177833. The faster NBCe2-specific bicarbonate-dependent pH recovery rate in HV SCL4A5 was abolished by HNF4A antagonists.

CONCLUSION

NBCe2 activity is stimulated by an increase in intracellular sodium and is hyper-responsive in hRPTCs carrying HV SLC4A5 rs7571842 through an aberrant HNF4A-mediated mechanism.

[Dietary acid load: A novel target for the nephrologist?]

The acid production of endogenous origin depends mainly on the metabolism of the food and varies with the nature of these. Of the order of 1mEq/kg/day for contemporary food in industrialized countries, it is reduced by more than one third among vegetarians and close to neutrality among vegans. The dietary acid load is eliminated by the normal kidneys, thus maintaining the acid-base equilibrium. In the setting of CKD, it will overflow the capacities of the nephrons, generating a retention of H+ ions, promoting subclinical acidosis. This tissue retention of H+ ions was confirmed by direct techniques in animal models and indirect techniques in humans. The systemic retention of H+ ions and the accompanying compensatory mechanisms have negative consequences on bone tissue, skeletal muscle, cardiovascular risk and renal function. In the animal, the substitution of casein (acid) by soy (alkaline) prevents metabolic acidosis and slows the progression of renal insufficiency. In man, various prospective studies have confirmed that the risk of renal insufficiency was positively correlated with the dietary acid load. Conversely, bicarbonate supplementation and/or a diet enriched with fruits and vegetables, have a favorable effect on renal insufficiency, including in subjects with normal bicarbonate. These results lead to reconsider the K/DOQI recommendations to correct acidosis when the bicarbonate level falls below 22mEq/L, since tissue retention of H+ ions and its negative consequences appear at higher or even normal levels of bicarbonates.

The Renal Sodium Bicarbonate Cotransporter NBCe2: Is It a Major Contributor to Sodium and pH Homeostasis?

The sodium bicarbonate cotransporter (NBCe2, aka NBC4) was originally isolated from the human testis and heart (Pushkin et al. IUBMB Life 50:13-19, 2000). Subsequently, NBCe2 was found in diverse locations where it plays a role in regulating sodium and bicarbonate transport, influencing intracellular, extracellular, interstitial, and ultimately plasma pH (Boron et al. J Exp Biol. 212:1697-1706, 2009; Parker and Boron, Physiol Rev. 93:803-959, 2013; Romero et al. Mol Asp Med. 34:159-182, 2013). NBCe2 is located in human and rodent renal-collecting duct and proximal tubule. While much is known about the two electrogenic sodium bicarbonate cotransporters, NBCe1 and NBCe2, in the regulation of sodium homeostasis and pH balance in the rodent kidney, little is known about their roles in human renal physiology. NBCe2 is located in the proximal tubule Golgi apparatus under basal conditions and then disperses throughout the cell, but particularly into the apical membrane microvilli, during various maneuvers that increase intracellular sodium. This review will summarize our current understanding of the distribution and function of NBCe2 in the human kidney and how genetic variants of its gene, SLC4A5, contribute to salt sensitivity of blood pressure.

Urinary pH reflects dietary acid load in patients with type 2 diabetes

Dietary acid load is important information, however, survey of food intake needs time and skill. Therefore, it is difficult to survey food intake from all patients. It remains to be elucidated the association between dietary acid load and urinary pH in patients with type 2 diabetes. In this cross-sectional study of 173 patients, we investigated the relationship between urinary pH and dietary acid load, assessed with potential renal acid load. Habitual food and nutrient intake was assessed by a self-administered diet history questionnaire. Urinary pH was negatively correlated with potential renal acid load (r = –0.24, p = 0.002). Multivariate regression analysis revealed that potential renal acid load (standardized regression coefficient = –0.21, p = 0.036) was associated with urinary pH after adjusting for covariates. In addition, according to the receiver operator characteristic analysis, the optimal cut-off point of urinary pH for high dietary acid load, defined as potential renal acid load over 7.0 mEq/day was 5.7 (area under the receiver operator characteristic curve 0.63 (95% CI 0.54–0.71), sensitivity = 0.56, specificity = 0.70, p = 0.004). Urinary pH was associated with dietary acid load in patients with type 2 diabetes. We suggest that urinary pH can be a practical screening marker for dietary acid load in patients with type 2 diabetes.

Higher dietary acid load is weakly associated with higher adiposity measures and blood pressure in Japanese adults: The National Health and Nutrition Survey

Epidemiologic evidence on the associations between dietary acid load and metabolic risk factors are limited and inconsistent. We investigated the hypothesis that dietary acid load is associated with adverse profiles of metabolic risk factors in Japanese adults. This cross-sectional study included 15 618 Japanese adults aged ≥20 years from the 2012 National Health and Nutrition Survey, Japan. Dietary acid load was characterized as potential renal acid load (PRAL) and net endogenous acid production (NEAP) based on information on nutrient intake, which was derived from a 1-day semi-weighed dietary record. After adjustment for potential confounding factors, both PRAL and NEAP were positively associated with body mass index (BMI) and waist circumference in both sexes (P for trend ≤.01). PRAL and NEAP also showed positive associations with systolic and diastolic blood pressure in men, independently of BMI (P for trend ≤.005), while they showed positive associations with systolic (but not diastolic) blood pressure in women (P for trend ≤.03). For other metabolic risk factors, there were positive associations between PRAL and NEAP and total and LDL-cholesterol concentrations only in men (P for trend ≤.04). After excluding participants taking medication for hypertension, dyslipidemia, and diabetes, similar associations were observed for adiposity measures and blood pressure (P for trend ≤.01), with there being positive associations between NEAP and HDL-cholesterol and between PRAL and NEAP and glycated hemoglobin in men (P for trend ≤.04). In conclusion, higher dietary acid load was significantly but weakly associated with higher adiposity measures and blood pressure in Japanese adults.

Short-term effects of espresso coffee on heart rate variability and blood pressure in habitual and non-habitual coffee consumers--a randomized crossover study

OBJECTIVE

Coffee is one of the most widely consumed beverages worldwide. Aim of this study was to investigate short-term effects of espresso coffee on heart rate variability (HRV), a marker of vagal activity, in healthy habitual and non-habitual coffee consumers.

METHODS

Seventy-seven healthy subjects (38 habitual and 39 non-habitual coffee consumers, 74% women, mean age 26.97 ± 6.88 years) took part in three laboratory sessions in a randomized order. In condition 1, subjects consumed espresso; in condition 2, subjects consumed decaffeinated espresso; and in condition 3, subjects consumed warm water. HRV and blood pressure were assessed at rest before and after ingestion of the respective beverage.

RESULTS

HRV was significantly increased after consumption of caffeinated espresso, decaffeinated espresso, or water, indicating increased vagal activity in the course of the experiments. In the habitual coffee consumers, the increase in vagally mediated HRV was significantly lower after consumption of decaffeinated espresso compared to caffeinated espresso. Increases of systolic blood pressure were only found in the non-habitual consumers.

CONCLUSION

We found no evidence for specific short-term effects of caffeinated espresso on vagal activity in healthy subjects. Instead, consumption of decaffeinated espresso inhibited vagal activity in habitual consumers. This may be explained by an attempt of the organism to establish a sympathovagal equilibrium comparable to that after caffeine consumption. In the absence of caffeine-induced sympathetic activation, this may have been achieved by relative vagal withdrawal.

Lower vegetable protein intake and higher dietary acid load associated with lower carbohydrate intake are risk factors for metabolic syndrome in patients with type 2 diabetes: Post-hoc analysis of a cross-sectional study

Aims/Introduction

A low-carbohydrate diet based on animal sources is associated with higher all-cause mortality, whereas a vegetable-based low-carbohydrate diet is associated with lower cardiovascular disease mortality. It has been suggested that acid/base imbalance might play an important role in some cardiometabolic abnormalities. The aims of the present study were to evaluate whether carbohydrate intake is associated with quality of dietary protein and acid load, and whether these are related to metabolic syndrome in patients with type 2 diabetes.

Materials and Methods

The present cross-sectional study involved 149 patients with type 2 diabetes. Dietary intake was assessed using a validated self-administered diet history questionnaire. Dietary acid load was assessed by potential renal acid load and net endogenous acid production.

Results

Mean daily total energy intake, carbohydrate intake, animal protein intake and vegetable protein intake were 1821.5 kcal, 248.8 g, 36.1 g and 31.1 g, respectively. Carbohydrate energy/total energy was negatively correlated with animal protein energy/total energy, potential renal acid load or net endogenous acid production score, and was positively correlated with vegetable protein energy/total energy. Logistic regression analyses showed that the subgroup of patients with a lower vegetable protein energy/total energy or higher potential renal acid load or net endogenous acid production score was significantly associated with the prevalence of metabolic syndrome.

Conclusions

The present study showed that carbohydrate intake was associated with the quality of dietary protein and dietary acid load. Furthermore, decreased vegetable protein intake and increased dietary acid load were associated with the prevalence of metabolic syndrome.

Estimated dietary acid load is not associated with blood pressure or hypertension incidence in men who are approximately 70 years old

BACKGROUND

Dietary acid load affects acid-base homeostasis, which may be associated with blood pressure (BP). Previous research on dietary acid load and BP in the community has provided conflicting results, which may be confounded by underlying kidney function with inability to eliminate acid excess.

OBJECTIVE

The objective of this study was to determine whether dietary acid load is associated with blood pressure or the incidence of hypertension in older men taking into account each individual's kidney function.

METHODS

We included 673 men aged 70-71 y and not receiving antihypertensive medication from the Uppsala Longitudinal Study of Adult Men. Of those, 378 men were re-examined after 7 y. Dietary acid load was estimated at baseline by potential renal acid load (PRAL) and net endogenous acid production (NEAP), based on nutrient intake assessed by 7-d food records at baseline. Ambulatory blood pressure monitoring (ABPM) was performed at both visits. Cystatin C-estimated kidney function allowed identification of underlying chronic kidney disease.

RESULTS

Median estimated PRAL and NEAP were 3.3 and 40.7 mEq/d, respectively. In cross-section, PRAL was in general not associated with ABPM measurements (all P > 0.05, except for the 24-h diastolic BP). During follow-up, PRAL did not predict ABPM changes (all P > 0.05). When individuals with baseline hypertension (ABPM ≥ 130/80 mm Hg) or nondippers (with nighttime-to-daytime systolic BP ratio > 0.9) were excluded, PRAL was not a predictor of incident cases (P > 0.30). Kidney function did not modify these null relations. Similar findings were obtained with the use of NEAP as the exposure.

CONCLUSION

Our analyses linking estimated dietary acid load with BP outcome measurements both cross-sectionally and after 7 y in community-based older Swedish men of similar age did not reveal an association between dietary acid load and BP.

Plasma bicarbonate and odds of incident hypertension

BACKGROUND

Several biomarkers of metabolic acidosis, including lower plasma bicarbonate, have been associated with prevalent hypertension in cross-sectional studies. We sought to examine prospectively whether lower plasma bicarbonate is associated with incident hypertension.

METHODS

We conducted a prospective case-control study nested within the Nurses' Health Study II. Plasma bicarbonate was measured in 695 nonobese women without hypertension at time of blood draw who subsequently developed hypertension during 6 years of follow-up. Control subjects were matched to case subjects according to age, race, time and day of blood draw, and day of menstrual cycle. We used unconditional logistic regression to generate odds ratios (ORs) for development of hypertension by quintile of baseline plasma bicarbonate.

RESULTS

After adjusting for matching factors, body mass index, family history of hypertension, plasma creatinine, and dietary and lifestyle factors, higher plasma bicarbonate was associated with lower odds of developing hypertension across quintiles (P for linear trend = 0.04). Those in the highest compared with the lowest quintile of plasma bicarbonate had 31% lower odds of developing hypertension (OR = 0.69; 95% confidence interval = 0.48-0.99). Further adjustment for diet-estimated net endogenous acid production, plasma insulin, 25-hydroxyvitamin D, and uric acid did not alter these findings.

CONCLUSIONS

Our case-control study is consistent with a modest association between higher plasma bicarbonate and reduced odds of developing hypertension among nonobese women, although our findings are of borderline statistical significance. Further research is required to confirm this finding as part of a larger prospective cohort study and to elucidate the mechanism for this relation.

Sensory renal innervation: a kidney-specific firing activity due to a unique expression pattern of voltage-gated sodium channels?

Sensory neurons with afferent axons from the kidney are extraordinary in their response to electrical stimulation. More than 50% exhibit a tonic firing pattern, i.e., sustained action potential firing throughout depolarizing, pointing to an increased excitability, whereas nonrenal neurons show mainly a phasic response, i.e., less than five action potentials. Here we investigated whether these peculiar firing characteristics of renal afferent neurons are due to differences in the expression of voltage-gated sodium channels (Navs). Dorsal root ganglion (DRG) neurons from rats (Th11-L2) were recorded by the current-clamp technique and distinguished as "tonic" or "phasic." In voltage-clamp recordings, Navs were characterized by their tetrodotoxoxin (TTX) sensitivity, and their molecular identity was revealed by RT-PCR. The firing pattern of 66 DRG neurons (41 renal and 25 nonrenal) was investigated. Renal neurons exhibited more often a tonic firing pattern (56.1 vs. 12%). Tonic neurons showed a more positive threshold (-21.75 ± 1.43 vs.-29.33 ± 1.63 mV; P < 0.05), a higher overshoot (56.74 [53.6-60.96] vs. 46.79 mV [38.63-54.75]; P < 0.05) and longer action potential duration (4.61 [4.15-5.85] vs. 3.35 ms [2.12-5.67]; P < 0.05). These findings point to an increased presence of the TTX-resistant Navs 1.8 and 1.9. Furthermore, tonic neurons exhibited a relatively higher portion of TTX-resistant sodium currents. Interestingly, mRNA expression of TTX-resistant sodium channels was significantly increased in renal, predominantly tonic, DRG neurons. Hence, under physiological conditions, renal sensory neurons exhibit predominantly a firing pattern associated with higher excitability. Our findings support that this is due to an increased expression and activation of TTX-resistant Navs.

Lower serum bicarbonate and a higher anion gap are associated with lower cardiorespiratory fitness in young adults

Lower levels of serum bicarbonate and a higher anion gap have been associated with insulin resistance and hypertension in the general population. Whether these associations extend to other cardiovascular disease risk factors is unknown. To clarify this, we examined the association of serum bicarbonate and anion gap with cardiorespiratory fitness in 2714 adults aged 20–49 years in the 1999–2004 National Health and Nutrition Examination Survey. The mean serum bicarbonate was 24.6 mEq/L and the mean anion gap was 10.26 mEq/L, with fitness determined by submaximal exercise testing. After multivariable adjustment, gender, length of fasting, soft drink consumption, systolic blood pressure, serum phosphate, and hemoglobin were independently associated with both the serum bicarbonate and the anion gap. Low fitness was most prevalent among those in the lowest quartile of serum bicarbonate or highest quartile of anion gap. After multivariable adjustment, a one standard deviation higher serum bicarbonate or anion gap was associated with an odds ratio for low fitness of 0.80 (95% CI 0.70–0.91) and 1.30 (95% CI 1.15–1.48), respectively. The association of bicarbonate with fitness may be mediated by differences in lean body mass. Thus, lower levels of serum bicarbonate and higher levels of anion gap are associated with lower cardiorespiratory fitness in adults aged 20–49 years in the general population.

High sodium intake increases HCO(3)- absorption in medullary thick ascending limb through adaptations in basolateral and apical Na+/H+ exchangers

A high sodium intake increases the capacity of the medullary thick ascending limb (MTAL) to absorb HCO(3)(-). Here, we examined the role of the apical NHE3 and basolateral NHE1 Na(+)/H(+) exchangers in this adaptation. MTALs from rats drinking H(2)O or 0.28 M NaCl for 5-7 days were perfused in vitro. High sodium intake increased HCO(3)(-) absorption rate by 60%. The increased HCO(3)(-) absorptive capacity was mediated by an increase in apical NHE3 activity. Inhibiting basolateral NHE1 with bath amiloride eliminated 60% of the adaptive increase in HCO(3)(-) absorption. Thus the majority of the increase in NHE3 activity was dependent on NHE1. A high sodium intake increased basolateral Na(+)/H(+) exchange activity by 89% in association with an increase in NHE1 expression. High sodium intake increased apical Na(+)/H(+) exchange activity by 30% under conditions in which basolateral Na(+)/H(+) exchange was inhibited but did not change NHE3 abundance. These results suggest that high sodium intake increases HCO(3)(-) absorptive capacity in the MTAL through 1) an adaptive increase in basolateral NHE1 activity that results secondarily in an increase in apical NHE3 activity; and 2) an adaptive increase in NHE3 activity, independent of NHE1 activity. These studies support a role for NHE1 in the long-term regulation of renal tubule function and suggest that the regulatory interaction whereby NHE1 enhances the activity of NHE3 in the MTAL plays a role in the chronic regulation of HCO(3)(-) absorption. The adaptive increases in Na(+)/H(+) exchange activity and HCO(3)(-) absorption in the MTAL may play a role in enabling the kidneys to regulate acid-base balance during changes in sodium and volume balance.

Diet-dependent net acid load and risk of incident hypertension in United States women

Animal and human studies suggest a potential link between acid-base status and blood pressure. Contemporary Western diets yield a daily systemic acid load of varying amounts, yet the association with hypertension has never been explored. We prospectively examined the association between the diet-dependent net acid load (also known as the estimated net endogenous acid production) and the risk of incident hypertension among 87 293 women without a history of hypertension in the Nurses' Health Study II. We also used the ratio of animal protein intake to potassium intake as an alternative evaluation of diet-dependent net acid load. We identified 15 385 incident cases of hypertension during 995 239 person-years of follow-up. After adjusting for potential confounders, women in the top decile of estimated diet-dependent net acid load had an increased risk of hypertension (relative risk: 1.14; 95% CI: 1.05 to 1.24; P for trend=0.01) compared with women in the bottom decile. To test whether the association between estimated diet-dependent net acid load and hypertension is independent of its individual components, an additional adjustment for intakes of protein and potassium was made and resulted in a relative risk of 1.23 (95% CI: 1.08 to 1.41; P for trend=0.003) for the top decile of estimated diet-dependent net acid load. Results of the ratio of animal protein intake to potassium intake were similar with those of estimated diet-depend net acid load. In conclusion, a high diet-dependent net acid load is independently associated with a higher risk of incident hypertension.

Increasing sodium intake from a previous low or high intake affects water, electrolyte and acid-base balance differently

Contrasting data are published on the effects of high salt intake (between 300 and 660 mmol/d) on Na balance and fluid retention. In some studies high levels of NaCl intake (400, 440, 550 and 660 mmol/d) led to positive Na balances without fluid retention. To test the relevance of different baseline NaCl intake levels on changes in metabolic water, Na, K, chloride and acid-base balance, a 28 d clinical trial ('Salty Life 6') was carried out in a metabolic ward. Nine healthy male volunteers (aged 25.7 (SD 3.1) years; body mass (BM) 71.4 (SD 4.0) kg) participated in the present study. Four consecutive levels of NaCl intake: low (6 d, 0.7 mmol NaCl/kg BM per d), average normal (6 d, 2.8 mmol NaCl/kg BM per d), high (10 d, 7.7 mmol NaCl/kg BM per d), and low again (6 d, 0.7 mmol NaCl/kg BM per d) were tested. Urine osmolality, extracellular volume (ECV) and plasma volume (PV), cumulative metabolic Na, K, chloride and fluid balances, mRNA expression of two glycosaminoglycan (GAG) polymerisation genes, capillary blood pH, bicarbonate and base excess were measured. During average normal NaCl intake, 193 (SEM 19) mmol Na were retained and ECV (+2.02 (SEM 0.31) litres; P<0.001) and PV (+0.57 (SEM 0.13) litres; P<0.001) increased. During high NaCl intake, 244 (SEM 77) mmol Na were retained but ECV did not increase (ECV -0.54 (SEM 0.30) litres, P=0.+89; PV +0.27 (SEM 0.25) litres, P=0.283). mRNA expression of GAG polymerisation genes increased with rise in NaCl intake, while pH (P<0.01) and bicarbonate (P<0.001) levels decreased. We conclude that a high NaCl intake may increase GAG synthesis; this might play a role in osmotically inactive Na retention in humans.

Nephrolithiasis as a systemic disorder

PURPOSE OF REVIEW

Nephrolithiasis is a prominent public health issue. It imposes a substantial burden on human health and is a considerable financial expenditure for the nation. Numerous epidemiologic studies have shown a significant association between nephrolithiasis, obesity, hypertension and chronic kidney disease. The review highlights many of those emerging studies and sheds light on the importance of our recognition of kidney stones as a systemic illness.

RECENT FINDINGS

Several cross-sectional retrospective studies have investigated the relationship between kidney stones and the metabolic syndrome. The various silent features of the metabolic syndrome, including type 2 diabetes, increased BMI, hypertension and dyslipidemia, are becoming progressively more recognized and independently associated with an increased risk of kidney stone formation.

SUMMARY

Our further understanding of the underlying mechanisms in the connection between nephrolithiasis and the metabolic syndrome will stimulate the development of more effective preventive and therapeutic measures.

Salt-sensitive men show reduced heart rate variability, lower norepinephrine and enhanced cortisol during mental stress

Salt sensitivity (SS) represents a risk factor for essential hypertension, which has been related to enhanced cardiovascular stress reactivity possibly mediated by increased noradrenergic susceptibility. We investigated biophysiological responses to mental stress in salt-sensitive (ss) and salt-resistant (sr) subjects, hypothesizing lower heart rate variability (HRV) and higher cortisol in the ss. A total of 48 healthy normotensive Caucasian men (age 25.6+/-2.6, body mass index 22.9+/-2.3) were phenotyped for SS (defined as significant drop in mean arterial pressure>3 mm Hg under the low-salt diet) by a 2-week high- versus low-salt diet. Subjects underwent a standardized mental stress task with continuous cardiovascular monitoring before, during and after the test (Finapres; Ohmeda, Louisville, CO, USA). Blood samples were drawn to examine cortisol and catecholamines before, after and 20 min after stress. The task elicited significant increases of systolic blood pressure (SBP), diastolic BP (DBP) and heart rate (HR) and a significant decrease of HRV (all time effects P<0.0001). The ss subjects showed lower norepinephrine (NE) and higher cortisol, indicated by significant group effects (P=0.009 and 0.025, respectively). HR increased and HRV decreased more in the ss under the stress, shown by significant time by group interactions (P=0.045 and 0.003, respectively). The observation of a more pronounced HR rise coupled with a greater decrease of HRV in healthy ss men under the influence of brief mental stress confirms their enhanced physiological stress reactivity. The lower peripheral NE may represent an effort to compensate for increased noradrenergic receptor sensitivity. The enhanced cortisol levels are backed by recent genetic findings on HSD11B2 polymorphisms and may promote hypertension.

Association between dietary acid-base load and cardiometabolic risk factors in young Japanese women

Mild metabolic acidosis, which can be caused by diet, may adversely affect cardiometabolic risk factors, possibly by increasing cortisol production. Methodologies for estimating diet-induced acid-base load using dietary-intake information have been established. To our knowledge, however, the possible association between dietary acid-base load and cardiometabolic risk factors has not been investigated. We cross-sectionally examined associations between dietary acid-base load and cardiometabolic risk factors in a free-living population. The subjects were 1136 female Japanese dietetic students aged 18-22 years. Dietary acid-base load was characterized as the potential renal acid load (PRAL), which was determined using an algorithm including dietary protein, P, K, Ca and Mg, as well as the ratio of dietary protein to K (Pro:K). Estimates of each nutrient were obtained from a validated comprehensive self-administered diet history questionnaire. Body height and weight, waist circumference and blood pressure were measured. Fasting blood samples were collected. After adjustment for potential confounding factors, higher PRAL and Pro:K (more acidic dietary acid-base loads) were associated with higher systolic and diastolic blood pressure (P for trend = 0.028 and 0.035 for PRAL and 0.012 and 0.009 for Pro:K, respectively). PRAL was also independently positively associated with total and LDL-cholesterol (n 1121; P for trend = 0.042 and 0.021, respectively). Additionally, Pro:K showed an independent positive association with BMI and waist circumference (P for trend = 0.024 and 0.012, respectively). In conclusion, more acidic dietary acid-base load was independently associated with adverse profile of several cardiometabolic risk factors in free-living young Japanese women.

Association between the serum anion gap and blood pressure among patients at Harvard Vanguard Medical Associates

Animal and human data suggest a link between endogenous acid production with elevations in blood pressure and the development of hypertension; increases in endogenous organic acid production can lead to a higher anion gap. We studied the cross-sectional association between the serum anion gap and blood pressure among 1057 non-diabetic patients who were not taking antihypertensive drugs, and who received their care at a multisite, multispecialty group practice in eastern Massachusetts. Using linear regression controlling for age, sex, race, BMI, estimated GFR and presence of impaired fasting glucose, every 1 mEq l(-1) higher serum anion gap was associated with a 0.27 mm Hg (P=0.08) higher systolic, 0.20 mm Hg (P=0.05) higher diastolic and 0.22 mm Hg (P=0.04) higher mean arterial pressure; these results suggest that endogenous acid production may raise the risk of hypertension.

Serum Anion Gap and Blood Pressure in the National Health and Nutrition Examination Survey

Increased production of organic acid can result in an elevated serum anion gap and may play a role in the development of hypertension. We studied the cross-sectional associations between anion gap and blood pressure and between serum bicarbonate and blood pressure in the 1999-2000 and 2001-2002 National Health and Nutrition Examination Surveys. We included 5043 adult participants who were not taking antihypertensive medications or diuretics and who denied hypertension, cardiovascular disease, diabetes, and other diseases. Linear regression was used to adjust for age, race, body mass index, creatinine, albumin, and other factors. Sample weights were used to produce weighted regression parameters. In the lowest quintile of anion gap, mean values of sodium, chloride, and bicarbonate were 139 mEq/L, 105 mEq/L, and 25 mEq/L, respectively. In the highest quintile, mean values of sodium, chloride, and bicarbonate were 140 mEq/L, 101 mEq/L, and 22 mEq/L, respectively. Mean blood pressure was 118/72 mm Hg. After multivariable adjustment, participants in the highest quintile of anion gap had systolic blood pressure 3.73 mm Hg higher (95% CI: 1.83 to 5.63 mm Hg; P for trend: <0.01) than participants in the lowest quintile. Participants in the highest quintile of bicarbonate had systolic blood pressure 2.73 mm Hg lower (95% CI: 1.26 to 4.20 mm Hg; P for trend: <0.01) than participants in the lowest quintile. No associations were observed between anion gap or bicarbonate and diastolic blood pressure. The results were unchanged after excluding participants with estimated glomerular filtration rate <60 cc/min per 1.73 m(2). The anion gap is independently associated with higher blood pressure. Further research is needed to elucidate the relation between organic acid and hypertension.

Association of prevalent hypertension with 24-hour urinary excretion of calcium, citrate, and other factors

BACKGROUND

The relation between hypertension and the urinary excretion of calcium, citrate, and other factors is unclear. It has been proposed that increased urinary calcium excretion is a central feature of essential hypertension. Metabolic acidosis also may be associated with hypertension and decreases urinary citrate levels.

METHODS

To compare the urine composition of individuals with and without hypertension, we studied 24-hour urinary excretion of calcium, citrate, oxalate, uric acid, sodium, magnesium, potassium, phosphorus, and creatinine and pH in a subset of participants with and without nephrolithiasis in the Nurses' Health Study I (older women; N = 1,284), Nurses' Health Study II (younger women; N = 952), and the Health Professionals Follow-up Study (men; N = 788). Logistic regression models adjusted for age, weight, dietary intake, and urinary factors.

RESULTS

In participants with and without nephrolithiasis, citrate was the only urinary factor consistently related to hypertension. Compared with those in the lowest quartile of urinary citrate excretion, multivariate odds ratios of prevalent hypertension in the highest quartile were 0.37 (95% confidence interval [CI], 0.24 to 0.55; P trend < 0.001) for older women, 0.54 (95% CI, 0.32 to 0.92; P trend = 0.03) for younger women, and 0.27 (95% CI, 0.16 to 0.45; P trend < 0.001) for men. Urinary calcium levels were not related consistently to hypertension. Excluding participants with single 24-hour urine collections and those administered thiazide diuretics or angiotensin-converting enzyme inhibitors did not change the results.

CONCLUSION

Lower urinary citrate excretion is associated independently with prevalent hypertension. Factors that regulate urinary citrate excretion may play a role in hypertension.

Effects of a low sodium diet on bone metabolism

Osteoporosis is a serious public health problem, and dietary interventions may potentially be helpful in preventing this disorder. The purpose of this study was to determine the effects of a low sodium diet on bone metabolism in postmenopausal women. This was a longitudinal study to determine the effects of a low sodium (2-g/day) diet on bone. Forty postmenopausal African-American and Caucasian women were enrolled in a 2-g/day sodium diet for 6 months. Sodium and calcium excretion, bone turnover, and calcitropic hormones (intact parathyroid hormone (PTH) and 1,25 dihydroxyvitamin D) were measured before and 6 months after the intervention. In women who had baseline sodium excretions equal to or greater than the average sodium intake in the United States (> or =3.4 g/day), the low sodium diet resulted in significant decreases in sodium excretion (P = 0.01), in calcium excretion (P = 0.01), and in a biomarker of bone turnover, aminoterminal propeptide of type I collagen (P = 0.04). However, there were no significant changes in calcitropic hormones, including intact PTH (P = 0.97) or 1,25 dihydroxyvitamin D (P = 0.49) with the low sodium diet. These findings suggest that in postmenopausal women with sodium intakes > or =3.4 g/day, a low sodium diet may have benefits for skeletal health.

Enhanced affective startle modulation in salt-sensitive subjects

Salt-sensitive normotensive men exhibit an enhanced pressor response to mental stress. Although an enhanced pressor response is associated with higher affective startle modulation in men, an association between salt sensitivity of blood pressure and affective startle modulation has not been studied so far. We studied reactivity to mental stress and startle modulation in 14 salt-sensitive healthy white male students and 14 salt-resistant control subjects, who were well matched for age, body mass index, physical fitness, and family history of hypertension. Subjects performed a computerized information-processing task under time pressure (manometer test), while heart rate and blood pressure were continuously registered. In a separate session, subjects viewed a series of 42 pictures of the International Affective Picture System (IAPS), varying in pleasure and arousal, while acoustic startle probes were administered randomly, and electromyogram activity of the orbicular eye muscle was continuously recorded. Startle modulation was calculated as the difference between startle responses under negative and positive affective stimuli. In contrast to salt-resistant subjects, salt-sensitive subjects showed significantly enhanced startle amplitudes under negative stimuli and diminished amplitudes under positive stimuli. Thus, salt-sensitive subjects displayed a significantly higher startle modulation than did salt-resistant subjects (P<0.05). Subjective ratings of the presented IAPS pictures did not differ between the groups. The increased startle modulation of salt-sensitive subjects suggests an enhanced activity of the central nucleus of the amygdala. This enhanced central nervous responsiveness may contribute to higher sympathetic pressor reactivity and, thus, to the later development of hypertension in salt-sensitive individuals.

G-protein beta3 subunit 825T allele and response to dietary salt in normotensive men

BACKGROUND AND AIMS

A functional single-nucleotide variant of the gene encoding the beta3 subunit of heterotrimeric G proteins (Gbeta3 C825T), associated with enhanced G-protein activation and increased activity of the sodium-proton exchanger (NHE1), has been implicated in the development of hypertension. Given the possible involvement of NHE1 in sodium homeostasis, we tested the hypothesis that the Gbeta3 825T allele determines the response of the renin-angiotensin system and blood pressure to dietary salt restriction.

METHODS

Young normotensive men (20-30 years old, n = 193) were recruited within the framework of the Berlin Salt-Sensitivity Trial and studied on low- (20 mmol/day) and high-salt (220 mmol/day) dietary protocols. Subjects were characterized for parameters of the renin-angiotensin system and blood pressure response and genotyped for the Gbeta3 C825T polymorphism.

RESULTS

The genotype distribution was in Hardy-Weinberg equilibrium (CC = 90, CT = 81 and TT = 22). The responses of the renin-angiotensin system and blood pressure to the dietary protocol were virtually identical between the genotypic groups. Furthermore, when subjects were classified as salt-resistant (n = 145) or salt-sensitive (n = 48), genotype distribution was comparable between the two groups (salt-resistant: TT = 17, CT = 60, CC = 68, qT = 0.32; salt-sensitive: TT = 5, CT = 21, CC = 22, qT = 0.32).

CONCLUSION

These findings do not support the hypothesis that the Gbeta3 C825T polymorphism determines the response of the renin-angiotensin system to salt depletion or can serve as an early genetic marker of salt sensitivity in young normotensive men.

Aldosterone synthase gene (CYP11B2) C-344T polymorphism in Caucasians from the Berlin Salt-Sensitivity Trial (BeSST)

OBJECTIVE

Aldosterone synthase (CYP11B2) is a key enzyme in the biosynthesis of aldosterone. Recently, the T allele of a polymorphism in the 5'-flanking region of the CYP11B2 gene (C-344T) has been reported to be more frequent in hypertensives than in normotensives, and has also been associated with increased plasma aldosterone levels. We therefore hypothesized that this variant may be related to increased blood-pressure response to dietary salt intake.

SUBJECTS AND METHODS

We genotyped 1 63 young normotensive men recruited within the framework of the Berlin Salt-Sensitivity Trial (BeSST) for the CYP11B2 C-344T polymorphism. Subjects were characterized for family history of hypertension, plasma parameters of the renin-angiotensin-aldosterone system and blood-pressure response to a high (220 mmol/day) and low (20 mmol/day) salt diet

RESULTS

The frequency of the -344T allele (0.45) was similar to that reported previously and genotype distribution was in Hardy-Weinberg equilibrium (CC, n = 55; CT, n = 71; TT, n = 37). There was a trend towards a higher frequency of the T allele in subjects with a positive family history of hypertension (0.48 versus 0.42), but the C-344T genotype was not related to blood pressure under either diet Furthermore, when subjects were classified into salt-sensitive and salt-resistant groups, allelic distribution did not differ between the two groups (qT = 0.43 versus qT = 0.45). While renin activity and plasma aldosterone levels were not related to genotype, plasma angiotensinogen was significantly higher in T-allele carriers under both the high (P = 0.02) and low (P = 0.008) salt diet.

CONCLUSION

Our findings do not support the hypothesis that the C-344T polymorphism of the CYP11B2 gene is associated with salt sensitivity or increased activity of the renin-angiotensin system in young normotensive subjects. It is, therefore, unlikely that the C-344T polymorphism is a genetic marker for salt sensitivity in young normotensive Caucasian men.

Angiotensinogen M235T variant and salt sensitivity in young normotensive Caucasians

BACKGROUND AND AIMS

A single-nucleotide variant of the angiotensinogen gene (AGT 235T) has been associated with essential hypertension and increased plasma levels of angiotensinogen. This variant may also serve as a genetic marker for the increased blood pressure response to dietary salt intake, but the relationship between AGT genotype and salt sensitivity has not been studied until now. We therefore examined the relationship between the AGT 235T genotype and the blood pressure response to short-term dietary salt restriction in young normotensive men.

SUBJECTS AND METHODS

A total of 187 young normotensive men were characterized for family history of hypertension, salt sensitivity, plasma parameters of the renin-angiotensin system under high- and low-salt diets, and the AGT 235T genotype.

RESULTS

While the T allele was significantly associated with a positive family history of hypertension (chi2 = 7.0; P< 0.03) and higher plasma angiotensinogen levels (P< 0.015) and renin activity (P < 0.037), blood pressure under both diets was not significantly affected by the AGT genotype. When the subjects were classified into salt-resistant and salt-sensitive groups, genotypic distribution was nearly identical between both groups (frequency of T allele: 0.45 versus 0.46).

CONCLUSION

Our findings demonstrate that the AGT 235T allele is significantly associated with a positive family history of hypertension, but is not an important determinant of the blood pressure response to dietary salt intake in young normotensive subjects. It is therefore unlikely that the AGT 235T genotype can serve as an early genetic marker of salt sensitivity.

Sodium sensitivity in young adults with high resting end-tidal CO2

BACKGROUND

Previous research with normotensive adults aged over 40 years ('older') found that sensitivity of blood pressure of subjects with high resting end-tidal partial pressures of CO2 to high sodium intake was greater than normal.

OBJECTIVE

To test the hypothesis that the lesser sensitivity of blood pressure of young normotensive adults to high sodium intake is also a function of resting end-tidal partial pressure of CO2.

DESIGN

Forty-eight Caucasian men and women (age 28.5 +/- 1.4 years) had a lower than normal dietary intake of sodium chloride for 4 days, and then ingested sodium chloride capsules for 7 days (an additional 190 mmol/day sodium chloride). Resting end-tidal partial pressure of CO2 and blood pressure, and 24 h ambulatory blood pressure, were measured before and after the high-sodium diet. Overnight urine samples were collected before and after the high-sodium diet to determine dietary compliance, and to assess changes in urinary excretion of endogenous digitalis-like factors (a ouabain-like factor, and a marinobufagenin-like factor) that covary with plasma volume.

RESULTS

Subjects with high end-tidal partial pressures of CO2 had lower resting heart rates and lower urinary excretion of ouabain-like factor before sodium loading. Sodium loading decreased mean partial pressure of CO2 (by 0.8 +/- 0.2 mmHg) and increased only ambulatory systolic blood pressure (by 2.1 +/- 0.8 mmHg) for the whole group. However, the changes in resting systolic (r = 0.32, P < 0.025) and diastolic (r = 0.36, P < 0.01) blood pressures and in 24 h systolic (r = 0.28, P < 0.05) blood pressure after sodium loading were all positive functions of individual resting end-tidal partial pressures of CO2. Sodium loading increased urinary excretion of marinobufagenin-like factor (by 1.78 +/- 0.88 nmol) and the magnitude of the individual increase was a function of end-tidal partial pressure of CO2.

CONCLUSIONS

The results indicate that a high resting partial pressure of CO2 augments the effects of high sodium intake on plasma volume, levels of endogenous digitalis-like factors, and blood pressure in young normotensive humans.

Psychophysiological reactivity of salt-sensitive normotensive subjects

OBJECTIVE

To evaluate the psychophysiological response to mental stress of young healthy salt-sensitive normotensive subjects.

METHODS

Thirty-two healthy volunteers who had previously been phenotyped for salt sensitivity were selected for the study. The 16 salt-sensitive and 16 salt-resistant subjects, who were matched for age, body mass index and family history of hypertension, underwent a mental stress test consisting of an information-processing task performed under time pressure (the Manometer test). During the experimental session the blood pressure, heart rate and pulse-wave velocity were registered continuously. Before and after the mental task subjects were instructed to complete several standardized psychological state and trait questionnaires.

RESULTS

Mental stress resulted in a greater rise in blood pressure (P < 0.05) and in pulse-wave velocity (P < 0.01) in salt-sensitive than in salt-resistant individuals. Salt-sensitive subjects also displayed significantly higher levels of anxiety (P < 0.01) and a lower level of control of anger (P < 0.01) than did salt-resistant subjects. Furthermore, the level of irritation of the salt-sensitive subjects was higher both before (P < 0.01) and after (P < 0.05) the stress test

CONCLUSIONS

An increased responsiveness of the blood pressure to mental stress and an increased level of irritation are associated with salt sensitivity in normotensive subjects. These findings are in line with the hypothesis that psychophysiological traits play a role in the development of salt-sensitive hypertension.

Salt sensitivity of blood pressure in humans

A variety of different techniques have been used for the assessment of the blood pressure response to changes in salt and water balance in humans. These have generally been found to be reproducible and to yield congruent results. This review surveys the characteristics of subjects identified as salt sensitive and salt resistant by different investigators from demographic and physiological perspectives.