Mala T. Kailasam

Plasma Hydrogen Peroxide Production in Human Essential Hypertension Role of Heredity, Gender, and Ethnicity

Oxygen free radicals, including hydrogen peroxide, may mediate oxidative stress in target organ tissues and contribute to cardiovascular complications in hypertension. To examine heritability of hydrogen peroxide production, we investigated this trait in a family-based cohort consisting of family members (n=236) ascertained through probands (n=57) with essential hypertension. Significant effects on hydrogen peroxide production were found for gender and ethnicity, with men having greater values than women (P <0.001) and white subjects having greater values than black subjects (P =0.025). Hydrogen peroxide production correlated directly with plasma renin activity (P =0.015), suggesting an important interaction between circulating oxygen radicals and the renin-angiotensin system and a potential mechanism for lower hydrogen peroxide values observed in blacks. Heritability estimates from familial correlations revealed that approximately 20% to 35% of the observed variance in hydrogen peroxide production could be attributed to genetic factors, suggesting a substantial heritable component to the overall determination of this trait. Hydrogen peroxide production negatively correlated with cardiac contractility (r =−0.214, P =0.001) and renal function (r =−0.194, P =0.003). In conclusion, these results indicate that hydrogen peroxide production is heritable and is related to target organ function in essential hypertension. Genetic loci influencing hydrogen peroxide production may represent logical candidates to investigate as susceptibility genes for cardiovascular target organ injury.

Early decline in the catecholamine release-inhibitory peptide catestatin in humans at genetic risk of hypertension.

Background Hypertension is a complex trait with an ill-defined genetic predisposition, in which adrenergic mechanisms seem to be involved even at the early stages. Chromogranin A is a pro-hormone stored and released with catecholamines by exocytosis; its fragment catestatin, formed in vivo, inhibits further catecholamine release as an antagonist at the physiologic trigger for secretion, the neuronal nicotinic cholinergic receptor. Methods We measured catestatin by radioimmunoassay in n = 277 subjects stratified by blood pressure (n = 61 hypertensive, n = 216 normotensive), and if normotensive by genetic risk of developing hypertension: family history positive (n = 176) versus negative (n = 40). Maximum likelihood analysis tested for bimodality. Involvement of catestatin in pathophysiology was probed by measurements of catecholamines and leptin, and the hemodynamic responses to environmental (cold) stress. Results The normotensive offspring of patients with hypertension already had diminished catestatin (P = 0.024), and family history was a better predictor of catestatin than age, ethnicity or gender (P = 0.014). Greater catestatin variance among family history-positive individuals (P = 0.021) suggested heterogeneity in this group, and a bimodal distribution (P < 0.001) identified 4.3% of individuals in a lower mode of catestatin values, all with positive family histories (P = 0.05). Catestatin correlated inversely with body mass index (r = −0.215, r2 = 0.046, n = 276, P < 0.001) and plasma leptin (r = −0.203, r2 = 0.041, n = 212, P = 0.003), while body mass index and leptin correlated directly (r = 0.59, r2 = 0.350, n = 212, P < 0.001). Family history-positive individuals had greater epinephrine excretion (P = 0.037) in addition to diminished catestatin, suggesting an inhibitory effect of catestatin on chromaffin cells in vivo. Low plasma catestatin predicted enhanced pressor response to a sympathoadrenal stressor (cold stress;r = −0.184, r2 = 0.034, n = 211, P = 0.007), suggesting an adrenergic mechanism whereby diminished catestatin might predispose to later development of hypertension. In white subjects, diminished catestatin also predicted greater systemic vascular resistance responses to cold stress (r = −0.307, r2 = 0.094, n = 75, P = 0.007), a relationship not found in Blacks (r = 0.122, r2 = 0.015, n = 94, P = 0.243). Conclusions We conclude that catestatin is diminished early in the course of development of hypertension, even in the normotensive offspring of patients with the disease. Low catestatin predicts augmented adrenergic pressor responses, suggesting a mechanism whereby diminished catestatin might increase the risk for later development of hypertension.

Development and Validation of a Noninvasive Method to Determine Arterial Pressure and Vascular Compliance

The ability not only to record automated systolic and diastolic pressure, but also to derive measurements of the rate of pressure change during the cardiac cycle, would have great potential clinical value. A new method has been developed to obtain pressure measurements at 20-ms intervals by oscillometric cuff signal pattern recognition. Derivation of noninvasive pressure measurements is based on a T tube aorta and straight tube brachial artery, and assumes that the systolic phase of the suprasystolic cuff signal and the diastolic phase of the subdiastolic cuff signal most closely approximate systolic and diastolic aortic pressures, respectively. Arterial pressures obtained by this method were compared with simultaneous invasive measurements from the thoracic aorta in 36 patients. Good agreement was observed between noninvasive and invasive methods for systolic (146 +/- 4 vs 145 +/- 5 mm Hg), diastolic (80 +/- 2 vs 77 +/- 2 mm Hg), and mean (100 +/- 3 vs 100 +/- 3 mm Hg) arterial pressures, and correlation coefficients were r = 0.94, 0.91, and 0.95, respectively. To assess the validity of measurements of the rate of pressure change, oscillometric cuff signals from a subgroup of 14 patients were analyzed in detail for the peak positive pressure derivative (dP/dt(Max)), peak negative pressure derivative (dP/dt(Min)), and time interval between peak positive and peak negative pressure derivatives [t(pp)]. Results (mean +/- SEM) were: [table in text]. The incorporation of measurements of the rate of pressure change into a physical model of the brachial artery was used to derive vascular compliance. A significant correlation was observed between vascular compliance derived from the oscillometric signal and determinations by either thermodilution or Fick methods and noninvasive pressures (n = 20, r = 0.83, p <0.001). Day-to-day variability for blood pressure and vascular compliance derived by the noninvasive method did not differ by >4%, representing a reproducible measure of vascular structure and function. We conclude that the measurement of absolute pressure and rate of pressure change show good correlation with catheter data and that vascular compliance can be reliably assessed by this new method. The technology should provide a valuable noninvasive tool for the assessment of both cardiac function and vascular properties.

Chromogranin A in Human Hypertension: Influence of Heredity

Multiple heritable traits are associated with essential (genetic) hypertension in humans. Because chromogranin A is increased in both human and rodent genetic hypertension, we examined the influence of heredity and blood pressure on chromogranin A in humans. In estimates derived from among- and within-pair variance in monozygotic versus dizygotic twins, plasma chromogranin A displayed significant (F15,18 = 2.93, P = .016) genetic variance (sigma 2 g), and its broad-sense heritability was high (h2B = 0.983). Plasma chromogranin A was increased in essential hypertension (99.9 +/- 6.7 versus 62.8 +/- 4.7 ng/mL, P < .001) but was influenced little by genetic risk for (family history of) hypertension (in normotensive or hypertensive subjects), by race, or by several antihypertensive therapies (angiotensin-converting enzyme inhibitor, diuretic, or beta-adrenergic antagonist). In normotensive subjects at genetic risk for essential hypertension, neither basal nor sympathoadrenal stress-evoked chromogranin A differed from values found in subjects not at risk. In established essential hypertension, plasma chromogranin A responses to adrenal medullary (insulin-evoked hypoglycemia) or sympathetic neuronal (dynamic exercise) activation were exaggerated, whereas responses to sympathoadrenal suppression (ganglionic blockade) were diminished, suggesting increased vesicular stores of chromogranin A and an adrenergic origin of the augmented chromogranin A expression in this disorder. We conclude that plasma chromogranin A displays substantial heritability and is increased in established essential hypertension. Its elevation in established hypertension is associated with evidence of increased vesicular stores of the protein and with adrenergic hyperactivity but is influenced little by customary antihypertensive therapies. However, the chromogranin A elevation is not evident early in the course of genetic hypertension.

Divergent Effects of Dihydropyridine and Phenylalkylamine Calcium Channel Antagonist Classes on Autonomic Function in Human Hypertension

Calcium channel antagonists differ by class in reported frequency of side effects that suggest reflex sympathoadrenal activation. Do such differences result from differential effects on autonomic and baroreflex function? The present study compared acute and chronic effects of two classes of calcium channel antagonists, the dihydropyridine type (felodipine) and the phenylalkylamine type (verapamil), on efferent sympathetic outflow and baroreflex slope in 15 essential hypertensive subjects. Blood pressure, heart rate, hemodynamics, and biochemistries were determined at baseline and after acute (first dose) and chronic (4 weeks) administration of the drugs versus placebo. Acutely, felodipine caused a greater decrease in blood pressure associated with a larger decline in systemic vascular resistance than the corresponding effects produced by verapamil. Chronically, there were similar, significant declines in blood pressure (P = .001) and systemic vascular resistance (P = .001) after each drug. Acutely, increased sympathetic activity after felodipine was suggested by reflex tachycardia (from 69 +/- 3 to 74 +/- 2 beats per minute, P = .014) and elevation of plasma norepinephrine (from 264 +/- 25 to 323 +/- 25 pg/mL, P = .037), whereas after verapamil the corresponding changes were closely similar to those after placebo.(ABSTRACT TRUNCATED AT 250 WORDS)

Arterial Compliance by Cuff Sphygmomanometer Application to Hypertension and Early Changes in Subjects at Genetic Risk

Abnormalities of the arterial pulse waveform reflect changes in cardiovascular structure and function. These abnormalities may occur early in the course of essential hypertension, even before the onset of blood pressure elevation. Previous studies of cardiovascular structure and function have relied on invasive intra-arterial cannulation to obtain the arterial pulse wave. We evaluated arterial structure and function using a noninvasive cuff sphygmomanometer in hypertensive (n = 15) and normotensive (n = 36) subjects stratified by genetic risk (family history) for hypertension. Using a simple physical model in which the aorta was assumed to be a T tube and the brachial artery a straight tube, we determined vascular compliance and peripheral resistance by analyzing the brachial artery pulsation signal from a cuff sphygmomanometer. Essential hypertensive subjects tended to have higher peripheral resistance (P = .06) and significantly lower vascular compliance (P = .001) than normotensive subjects. Vascular compliance correlated with simultaneously determined pulse pressure in both groups (n = 51, r = .74, P < .0001). Higher peripheral resistance (P = .07) and lower vascular compliance (P = .04) were already found in still-normotensive offspring of hypertensive parents (ie, normotensive subjects with a positive family history of hypertension) than in normotensive subjects with a negative family history of hypertension. Multivariate analysis demonstrated that both genetic risk for hypertension (P = .030) and blood pressure status (P = .041), although not age (P = .207), were significant predictors of vascular compliance (multiple R = .47, P = .011). However, by two-way ANOVA, genetic risk for hypertension was an even more significant determinant (F = 7.84, P = .007) of compliance than blood pressure status (F = 2.69, P = .089). Antihypertensive therapy with angiotensin-converting enzyme inhibitors (10 days, n = 10) improved vascular compliance (P = .02) and reduced resistance (P = .003) significantly; treatment with calcium channel antagonists (4 weeks, n = 8) tended to improve vascular compliance (P = .07) and significantly reduced peripheral resistance (P = .006). We conclude that arterial vascular compliance abnormalities detected by a noninvasive cuff sphygmomanometer reflect treatment-reversible changes in vascular structure and function. Early changes in vascular compliance in still-normotensive individuals at genetic risk for hypertension may be a heritable pathogenetic feature of this disorder.

β2-Adrenergic receptor polymorphisms at codon 16, cardiovascular phenotypes and essential hypertension in whites and African Americans*

Abstract β2-Adrenergic receptors (β2-AR) contribute to cardiovascular regulation by influencing several functions and previous studies suggest that a decreased function of the β2-AR may be involved in essential hypertension. β2-AR are polymorphic and certain polymorphisms of these receptors are of functional importance. We focus here on the Arg16→Gly16 β2-AR polymorphism, which shows enhanced agonist-promoted downregulation of the receptor and which, in two recent studies, yielded opposite results in terms of association with essential hypertension: an increased frequency of the Gly16 variant in African–Caribbean hypertensives and of the Arg16 variant in offspring of Norwegian white hypertensive parents. In the current study, we genotyped 243 subjects, including both African-American and white individuals, for codon 16 polymorphism and assessed blood pressure and cardiovascular function using impedance cardiography and pressor sensitivity to phenylephrine. We found similar patterns of cardiovascular function and expression of hypertension with the two genotypes of codon 16. There was no statistically significant difference in the overall allelic distribution of the two genotypes: among African-Americans, 51% of the hypertensives and 50% of the normotensives carried the Arg16 allele, whereas among the white subjects 40% of the hypertensives and 47% of the normotensives were carriers of the Arg16 allele. Although we observed a statistically significant increase in the Arg16/Gly16 heterozygotes in the African-American population, the Gly16 allele was not significantly increased in the African-Americans compared to whites. These findings indicate that the codon 16 polymorphisms are not associated with hypertension in a mixed American study population nor do they appear to substantially impact on a variety of hemodynamic variables.

Circulating amylin in human essential hypertension: heritability and early increase in individuals at genetic risk

Background Human essential hypertension is a complex trait with poorly understood genetic determination. Insulin resistance is frequently associated with this trait. Objective To determine whether a potentially pathogenic feature of the insulin-resistant state, circulating amylin (islet amyloid polypeptide, co-released with insulin from pancreatic islet β-cells), is already increased in prehypertensive individuals (normotensive persons at genetic risk of hypertension because of family history), whether such individuals already differ in their amylin response to β-cell stimulation, and whether plasma amylin concentration is heritable. Such features could establish increased circulating amylin as a hereditary ‘intermediate phenotype’ useful in genetic analyses of hypertension. Methods Plasma amylin and insulin were measured in 283 medication-free individuals stratified by blood pressure status (82 hypertensive and 201 normotensive), and genetic risk (family history) of hypertension. Differences in means were tested by ANOVA, variances by F test, and frequency distributions by maximum likelihood analysis. Co-release of amylin and insulin was provoked by intravenous infusion of mixed amino acids. The effect of antihypertensive treatment was evaluated after monotherapy with either angiotensin converting enzyme inhibition or calcium-channel blockade in hypertension. Results Plasma amylin was increased in hypertension (P = 0.027), and body mass index was a strong predictor of increased circulating amylin (P = 0.0001). Plasma amylin and plasma renin activity were not correlated (P = 0.395), and effective antihypertensive monotherapy with either angiotensin converting enzyme inhibition or calcium-channel blockade did not affect either amylin (P = 0.87–0.97) or insulin (P = 0.55–0.59). Among normotensive individuals, those at genetic risk of hypertension (with positive family history) already had increased concentrations of amylin (P < 0.001), despite exhibiting no difference in blood pressure or body mass index compared with the family-history-negative group; however, among normotensive individuals, both family history (P = 0.043) and body mass index (P = 0.0059) were significant predictors of increased concentrations of amylin. By maximum likelihood analysis, plasma amylin was distributed heterogeneously in the normotensive individuals, with two modes best explaining the distribution (χ2 = 77.4, P < 0.001), and family-history-positive individuals completely accounting for the upper mode (χ2 = 4.63, P = 0.031). Family-history-positive normotensive individuals showed greater plasma amylin concentrations both before and during β-cell stimulation by amino acid infusion (P = 0.014). Black (n = 111) and white (n = 172) individuals did not differ in mean (P = 0.946) or variance (P = 0.172) of plasma amylin concentrations. Conclusions These results suggest that plasma amylin concentration is in part determined by heredity. Both basal and stimulated plasma amylin excess may identify a subgroup of individuals bearing an inherited predisposition to hypertension. Measurement of amylin might identify a useful ‘intermediate phenotype’ in the genetic analysis of essential hypertension and its relationship to insulin resistance.

Early alteration in glomerular reserve in humans at genetic risk of essential hypertension: mechanisms and consequences.

Essential hypertension has a familial predisposition, but the phenotype of elevated blood pressure has delayed penetrance. Because the kidney is a crucial determinant of blood pressure homeostasis, we studied early glomerular alterations in still-normotensive young subjects at genetic risk of hypertension. Thirty-nine normotensive adults (mean age 29 to 31 years), stratified by genetic risk (parental family history [FH]) of hypertension (26 with positive FH [FH+], 13 with negative FH [FH−]), underwent intravenous infusion of mixed amino acids. Before and during amino acid administration, we measured glomerular filtration rate (GFR), putative second messengers of amino acids (nitric oxide [NO·] metabolites and cGMP), serum insulin and amino acid concentrations, and the FELi+ as an index of renal proximal tubular reabsorption. The FH+ group had a blunted GFR rise in response to amino acids (2.43±8.16% versus 31.0±13.4% rise, P =0.0126). The amino acid–induced change in GFR correlated (r =0.786, P <0.01) with the change in urinary NO· metabolite excretion; a diminished rise in urinary NO· metabolite excretion in the FH+ group (P =0.0105) suggested a biochemical mechanism for the different GFR responses between FH groups: a relative inability to convert arginine to NO·. The FH+ group had a far lower initial cGMP excretion at baseline (261±21.1 versus 579±84.9 nmol · h−1/1.73 m2, P =0.001), although cGMP did not change during the amino acid infusion (P =0.703). FH status, baseline GFR, and baseline serum insulin jointly predicted GFR response to amino acids (P =0.0013), accounting for ≈45% of the variance in GFR response. Decline in FELi+, an inverse index of proximal tubular reabsorption, paralleled increase in GFR (r =−0.506, P =0.01), suggesting differences in proximal tubular reabsorption during amino acids between the FH groups. GFR response to amino acid infusion was blunted in the FH+ group despite significantly higher serum concentrations of 6 amino acids (arginine, isoleucine, leucine, methionine, phenylalanine, and valine) in the FH+ group, suggesting a novel form of insulin resistance (to the amino acid–translocating action of insulin) in FH+ subjects. We conclude that blunted glomerular filtration reserve in response to amino acids is an early-penetrance phenotype seen even in still-normotensive subjects at genetic risk of hypertension and is linked to impaired formation of NO· in the kidney. Corresponding changes in GFR and fractional excretion of Li+ suggest that altered proximal tubular reabsorption after amino acids is an early pathophysiologic mechanism. Resistance to the amino acid–translocating actions of insulin may play a role in the biological response to amino acids in this setting. This glomerular reserve phenotype may be useful in genetic studies of renal traits preceding or predisposing to hypertension.

Diminished renal kallikrein responses to mineralocorticoid stimulation in African Americans: determinants of an intermediate phenotype for hypertension*

BACKGROUND Hypertension is a complex trait with an ill-defined genetic predisposition, in which renal mechanisms seem to be involved even at the early stages. Renal kallikrein excretion is diminished in patients with hypertension, and perhaps even in the early, prehypertensive phases of the syndrome. African Americans, a group at increased risk of developing hypertension, have especially diminished kallikrein expression, coupled with decreased renal excretion of K(+), a known stimulant of kallikrein expression, suggesting an environmental mechanism for their kallikrein deficit. We, therefore, tested whether short-term indirect (K(+)) or direct (fludrocortisone) stimulation of mineralocorticoid activity might be capable of restoring kallikrein excretion in African Americans. METHODS Nineteen healthy normotensive young men (n = 10 white, n = 9 African Americans) were treated with the following sequence of four oral medications, each for 1 week: placebo, KCl (120 mEq/day), placebo, and the mineralocorticoid fludrocortisone (0.4 mg/day). At each stage, we measured vital signs, excretion of kallikrein, aldosterone and electrolytes, and serum renin. Results were evaluated by two-way, repeated measures ANOVA. RESULTS African Americans had diminished urinary excretion of not only kallikrein (P =.007), but also K(+) (P <.001) and aldosterone (P =.015). Kallikrein responses to mineralocorticoid stimulation were substantially blunted in African Americans, whether achieved indirectly (by supplemental K(+); P =.019) or directly (by the exogenous mineralocorticoid fludrocortisone; P =.027), despite achievement of substantial increments in K(+) excretion after KCl (P =.002), and multiple other mineralocorticoid effects after fludrocortisone (P =.005). The kallikrein increment after KCl was best predicted by renin activity (P =.001) rather than ethnicity. Potassium chloride did not lower blood pressure (BP) in either group (P >.4). CONCLUSIONS Restoration of K(+) and aldosterone secretion to levels found in whites does not normalize kallikrein excretion or lower BP in African Americans, at least in the short term. Nor does exogenous mineralocorticoid stimulation fully restore kallikrein expression in African Americans. Therefore, the diminution of kallikrein biosynthesis in African Americans seems to involve mechanisms at or distal to the aldosterone receptor, and perhaps at the level of the kallikrein gene itself.