Action of thiazide on renal interstitial calcium.

BACKGROUND: Although thiazides increase urinary sodium excretion, they also decrease urinary calcium excretion. Recent studies in our laboratory have shown that increased dietary salt significantly reduces interstitial fluid calcium in Dahl salt-sensitive (DS) rats, and this was associated with a rise in blood pressure and increased urinary calcium excretion. Owing to the vasorelaxant actions of increased extracellular fluid calcium, we reasoned that the antihypertensive action of hydrochlorothiazide (HCTZ), a commonly used thiazide, may be the result of increased interstitial fluid calcium as a consequence of decreased urinary calcium excretion. METHODS: To test this hypothesis, DS and Dahl salt-resistant (DR) rats were given high salt alone or in combination with HCTZ for 1 week. Renal cortical interstitial fluid calcium was determined by the zero net flux method. RESULTS: High salt decreased cortical interstitial fluid calcium (1.69 +/- 0.25 vs. 1.13 +/- 0.05 mmol/l; P < 0.05) in DS rats as previously reported; thiazide treatment had no effect on the high salt interstitial fluid calcium response in salt-sensitive animals. However, thiazide decreased interstitial fluid calcium in DS on a normal salt diet. Cortical interstitial fluid calcium was unchanged by dietary salt in DR rats, and thiazide did not alter this interstitial fluid calcium response. CONCLUSION: We interpret these data to mean that (i) short-term thiazide treatment does not reduce blood pressure by restoring renal cortical interstitial fluid calcium concentration and (ii) a decrease in renal cortical interstitial fluid calcium may not contribute to the increased renal vasoconstriction seen in salt-sensitivity.

Psychosocial factors contribute to resting blood pressure in African Americans.

OBJECTIVES: African Americans as a group have higher blood pressure than individuals of northern European ancestry (non-Hispanic Whites). We investigate whether psychosocial factors explain the resting blood pressure of healthy, community-dwelling African Americans in our study. PARTICIPANTS: A convenience sample of self-reported normotensive African Americans aged 18-65 years who live in the North Carolina Triangle region. DESIGN: The study protocol consisted of three resting blood pressure sessions with assessment of the following psychosocial factors: anger expression, interpersonal support, anxiety, depression, hostility, active coping, and perceived racism. Additional clinical assessments were height, weight, waist girth, fasting glucose, insulin, triglycerides, and cholesterol. RESULTS: Resting systolic blood pressure was positively associated with male sex (P<.001) and positively correlated with age (P<.0001), waist girth (P<.0001), body mass index (P=.023), and a Cook Medley Hostility subscale identified as aggressive responding (P=.031). Mean arterial pressure was positively correlated with age (P<.0001), waist girth (P=.0041), Spielberger Anger Expression subscale anger control (P=.023), and aggressive responding (P=.020). CONCLUSIONS: Anger and hostility are significantly associated with resting blood pressure and may modulate behavioral and traditional (biologic) risk factors that determine cardiovascular physiology.

Different effects of angiotensin receptor blockade on end-organ damage in salt-dependent and salt-independent hypertension.

BACKGROUND: Although angiotensin II type 1 receptor blockers have emerged as effective antihypertensive agents, it is not known how efficacious these agents are in treating hypertension-associated target organ damage. METHODS AND RESULTS: The present study was undertaken to compare the effect of angiotensin type 1 receptor inhibition on the progression of the organ damage observed in 2 models of hypertension, namely, salt-sensitive and nitric oxide synthase inhibition-mediated hypertension. Effective (16.4 micromol/kg) and ineffective (0.8 to 4.9 micromol/kg) antihypertensive doses of candesartan cilexetil were initiated after hypertension was established. Both low- and high-dose candesartan cilexetil significantly reduced cardiac and renal damage in the nitric oxide synthase inhibitor model of hypertension (P < 0.05 versus untreated); however, high-dose candesartan caused a significant increase in renal damage in the Dahl salt-sensitive model of hypertension (P < 0.05 versus untreated). Interestingly, the beneficial end-organ effects of candesartan in the nitric oxide synthase inhibition model were independent of sustained antihypertensive actions of candesartan, whereas the exacerbation of renal injury with candesartan in the Dahl salt-sensitive model was inversely related to its blood pressure-lowering effect. CONCLUSIONS: These data show that angiotensin type 1 blockade reduces injury in the l-nitroarginine methyl ester model but increases tissue injury in the salt-sensitive model. These data suggest that angiotensin II via angiotensin type 1 receptor activation contributes to organ damage in nitric oxide-deficient salt-independent hypertension but is protective in salt-induced hypertension. These data further suggest that (1) renal injury may evolve independently of blood pressure and (2) the effectiveness of an antihypertensive agent in ameliorating renal injury may depend on the etiology of the hypertension.

Differential Effect of Renal Cortical and Medullary Interstitial Fluid Calcium on Blood Pressure Regulation in Salt-Sensitive Hypertension.

BACKGROUND: Hypercalciuria is a frequent characteristic of hypertension. In this report we extend our earlier studies investigating the role of renal interstitial fluid calcium (ISF(Ca))(2+) as a link between urinary calcium excretion and blood pressure in the Dahl salt-sensitive (DS) hypertensive model. METHODS: Dahl salt-sensitive and salt-resistant (DR) rats were placed on control (0.45%) and high (8%) salt diets to determine if changes in renal cortical and medullary ISF(Ca)(2+)correlated with changes in urinary calcium excretion and blood pressure. RESULTS: We observed that renal ISFCa(2+) was predicted by urinary calcium excretion (P < 0.05) in DS rats but not DR rats. Renal cortical ISF(Ca)(2+) was negatively associated with blood pressure (P < 0.03) while renal medullary ISF(Ca)(2+) was positively associated with blood pressure in DS rats (P < 0.04). In contrast, neither urinary calcium excretion nor renal ISF(Ca)(2+) was associated with blood pressure in the DR rats under the conditions of this study. CONCLUSION: We interpret these findings to suggest that decreased renal cortical ISF(Ca)(2+) plays a role in the increase in blood pressure following a high salt diet in salt hypertension perhaps by mediating renal vasoconstriction; the role of medullary calcium remains to be fully understood. Further studies are needed to determine the mechanism of the altered renal ISF(Ca)(2+) and its role in blood pressure regulation.

Gender differences in preclinical markers of kidney injury in a rural north Carolina african-american cohort.

INTRODUCTION: The incidence rate of end-stage renal disease (ESRD) is highest among African-American (AA) males. The reason for this disparity in ESRD for AA males remains unclear, but it is well established that diabetes is the leading risk factor. Prediabetes may also be a risk for kidney disease since prediabetics have increased risk for cardiovascular disease and often do not receive drug interventions unless their hemoglobin A1c (A1c) level is above 6%. Perhaps, AA males are at greater risk because they often are untreated prediabetics and this predisposes them to renal injury. Therefore, we hypothesize that prediabetic AA males have higher albumin:creatinine ratio (ACr), a biomarker of renal injury, than their female counterparts. METHODS: Male and female AAs were recruited (53 females and 47 males; 45 ± 2 years old) from a rural northeastern region of NC. Blood and urine samples were collected for A1c and albumin measurements, respectively. Participants were stratified based on their A1c levels: non-diabetic: <5.7%, prediabetic: ≥5.7% but <6.5%, and diabetic: ≥6.5%. RESULTS: The proportion of males that are normal, prediabetic, and diabetic differed from that of females (p = 0.002). Interestingly, prediabetic men tended to be younger (41 ± 4 vs. 51 ± 3, respectively; p = 0.027) than prediabetic females (p = 0.027). A1c and ACr were not associated with blood pressure in males or females. AA males had a relative risk of 0.9, 2.5, and 1.4 for microalbuminuria for non-diabetic, prediabetic, and diabetic, respectively, compared to AA females. CONCLUSION: These results support our hypothesis that AA males may be predisposed to prediabetes kidney injury compared to their female counterpart. Thus, young AA males should be screened for biomarkers of kidney injury even if they have normal glucose and blood pressure levels.

Effects of leucine supplementation and serum withdrawal on branched-chain amino acid pathway gene and protein expression in mouse adipocytes.

The essential branched-chain amino acids (BCAA), leucine, valine and isoleucine, are traditionally associated with skeletal muscle growth and maintenance, energy production, and generation of neurotransmitter and gluconeogenic precursors. Recent evidence from human and animal model studies has established an additional link between BCAA levels and obesity. However, details of the mechanism of regulation of BCAA metabolism during adipogenesis are largely unknown. We interrogated whether the expression of genes and proteins involved in BCAA metabolism are sensitive to the adipocyte differentiation process, and responsive to nutrient stress from starvation or BCAA excess. Murine 3T3-L1 preadipocytes were differentiated to adipocytes under control conditions and under conditions of L-leucine supplementation or serum withdrawal. RNA and proteins were isolated at days 0, 4 and 10 of differentiation to represent pre-differentiation, early differentiation and late differentiation stages. Expression of 16 BCAA metabolism genes was quantified by quantitative real-time PCR. Expression of the protein levels of branched-chain amino acid transaminase 2 (Bcat2) and branched-chain alpha keto acid dehydrogenase (Bckdha) was quantified by immunoblotting. Under control conditions, all genes displayed induction of gene expression during early adipogenesis (Day 4) compared to Day 0. Leucine supplementation resulted in an induction of Bcat2 and Bckdha genes during early and late differentiation. Western blot analysis demonstrated condition-specific concordance between gene and protein expression. Serum withdrawal resulted in undetectable Bcat2 and Bckdha protein levels at all timepoints. These results demonstrate that the expression of genes related to BCAA metabolism are regulated during adipocyte differentiation and influenced by nutrient levels. These results provide additional insights on how BCAA metabolism is associated with adipose tissue function and extends our understanding of the transcriptomic response of this pathway to variations in nutrient availability.

State anxiety is associated with cardiovascular reactivity in young, healthy african americans.

Although several studies have shown that enhanced cardiovascular reactivity can predict hypertension development in African Americans, these findings have not been consistent among all studies examining reactivity and hypertension susceptibility. This inconsistency may be explained by the influence of anxiety (state and trait) on the blood pressure response to stress. Therefore, this study sought to determine whether anxiety is associated with blood pressure response to cold pressor (CP) and anger recall (AR) stress tests in young healthy African Americans. Modeling using state and trait anxiety revealed that state anxiety predicts systolic (SBP) and diastolic blood pressure DBP response to CP and AR (P ≤ 0.02). Interestingly, state anxiety predicted heart rate changes only to CP (P < 0.01; P = 0.3 for AR). Although trait anxiety was associated with SBP response to AR and not CP, it was not a significant predictor of reactivity in our models. We conclude that anxiety levels may contribute to the variable blood pressure response to acute stressors and, therefore, should be assessed when performing cardiovascular reactivity measures.

Physiological stress increases renal injury in eNOS-knockout mice.

African Americans have a fourfold greater likelihood of developing end-stage renal disease (ESRD) compared with Caucasians. It has been proposed that the increased prevalence may be explained by non-traditional factors such as environmental stress and psychosocial factors. In this study, we used infrequent running to exhaustion as a physiological stressor to mimic real life experiences, such walking up stairs when an elevator is malfunctioning or running to catch a bus, to study its effect on renal injury in a hypertensive mouse model (endothelial nitric oxide synthase-deficient mice; eNOS(-/-)). This model has previously been shown to have renal injury comparable to that observed in African Americans. The effect of physiological stress on renal injury was examined in the setting of low (0.12%), control (0.45%) and high (8%) dietary salt. Following bouts of physiological stress, eNOS(-/-) mice had significantly greater interstitial inflammation compared with unstressed eNOS(-/-) mice (two-way analysis of variance (2-ANOVA), Holm-Sidak; P<0.01). Interestingly, eNOS(-/-) mice on a high-salt diet had greater interstitial inflammation compared with similarly stressed eNOS(-/-) mice on a low- or control-salt diet (2-ANOVA, Holm-Sidak; P<0.03). These effects of stress were independent of systolic blood pressure (141±7, 143±4, and 158±8 vs. 141±4, 138±5, 150±4 mm Hg; end of study vs. baseline, respectively). There was no significant effect of stress or dietary salt on renal injury in control wild-type mice (eNOS(+)/(+)). These data demonstrate that physiological stress exacerbates the renal injury associated with hypertension and that high-salt compounds this effect of stress. These results provide support for the idea that psychosocial and environmental factors contribute to the increased prevalence of ESRD in hypertensive African Americans.

Mesenteric artery contraction and relaxation studies using automated wire myography.

Proximal resistance vessels, such as the mesenteric arteries, contribute substantially to the peripheral resistance. These small vessels of between 100-400 µm in diameter function primarily in directing blood flow to various organs according to the overall requirements of the body. The rat mesenteric artery has a diameter greater than 100 µm. The myography technique, first described by Mulvay and Halpern(1), was based on the method proposed by Bevan and Osher(2). The technique provides information about small vessels under isometric conditions, where substantial shortening of the muscle preparation is prevented. Since force production and sensitivity of vessels to different agonists is dependent on the extent of stretch, according to active tension-length relation, it is essential to conduct contraction studies under isometric conditions to prevent compliance of the mounting wires. Stainless steel wires are preferred to tungsten wires because of oxidation of the latter, which affects recorded responses(3).The technique allows for the comparison of agonist-induced contractions of mounted vessels to obtain evidence for normal function of vascular smooth muscle cell receptors. We have shown in several studies that isolated mesenteric arteries that are contracted with phenylyephrine relax upon addition of cumulative concentrations of extracellular calcium (Ca(2+)(e;)). The findings led us to conclude that perivascular sensory nerves, which express the G protein-coupled Ca(2+)-sensing receptor (CaR), mediate this vasorelaxation response. Using an automated wire myography method, we show here that mesenteric arteries from Wistar, Dahl salt-sensitive(DS) and Dahl salt-resistant (DR) rats respond differently to Ca(2+)(e;). Tissues from Wistar rats showed higher Ca(2+)-sensitivity compared to those from DR and DS. Reduced CaR expression in mesenteric arteries from DS rats correlates with reduced Ca(2+)(e;)-induced relaxation of isolated, pre-contracted arteries. The data suggest that the CaR is required for relaxation of mesenteric arteries under increased adrenergic tone, as occurs in hypertension, and indicate an inherent defect in the CaR signaling pathway in Dahl animals, which is much more severe in DS. The method is useful in determining vascular reactivity ex vivo in mesenteric resistance arteries and similar small blood vessels and comparisons between different agonists and/or antagonists can be easily and consistently assessed side-by-side(6,7,8).

The effect of salt on renal damage in eNOS-deficient mice.

African Americans have an increased incidence of end-stage renal disease and are characterized as having reduced bioavailability of nitric oxide and salt-sensitivity. We propose that endothelial nitric oxide synthase (eNOS) knockout mice (eNOS(-/-)) are a suitable model of hypertension-associated renal injury as seen in African Americans. Therefore, the purpose of this study was to determine whether older eNOS(-/-) mice have hypertension-associated renal injury and if dietary salt modulates this injury. Six-month-old eNOS(-/-) mice were placed on 0.12%, 0.45% or 8% NaCl diet for 8 weeks and blood pressure measured weekly; kidneys were collected for pathology evaluation and scoring at the end of the 8-week period. Mice deficient of eNOS were hypertensive at baseline compared with control mice in all three groups (128+/-3 vs. 112+/-3, P<0.05). Blood pressure was significantly elevated from baseline in eNOS(-/-) on 0.45 and 8% salt diets (P<0.02). The composite renal pathology scores for eNOS(-/-) mice were significantly greater than wild-type mice, indicating high salt intake exacerbates the injury (P<0.001 vs. normal salt diet). eNOS(-/-) mice may be used as a model of salt-induced and hypertension-associated renal injury as seen in African Americans.