M. Muszkat

A common β1-adrenergic receptor polymorphism (Arg389Gly) affects blood pressure response to β-blockade

A common polymorphism of the β1‐adrenergic receptor Arg389Gly markedly affects function in vitro, but little is known about its in vivo significance.

Differential in Vivo Sensitivity to Inhibition of P-glycoprotein Located in Lymphocytes, Testes, and the Blood-Brain Barrier

A major functional component of the blood-brain barrier is P-glycoprotein. In principle, inhibition of this efflux transporter would permit greater distribution of its substrates into the brain and increased central effects. Tariquidar and elacridar, potent and selective P-glycoprotein inhibitors, were investigated in this regard using the opioid loperamide as an in vivo probe in mice. Pretreatment with both inhibitors converted intravenous loperamide from a drug without central effects to one producing antinociception. Radiolabeled loperamide tissue distribution studies indicated that inhibition was associated with increased uptake into brain and testes in the absence of changes in plasma levels, along with enhanced efflux of rhodamine 123 from CD3e+ T-lymphocytes. However, with tariquidar, the loperamide dose-response curves for testes/plasma and brain/plasma concentration ratios were shifted 6- (p = 0.07) and 25-fold (p < 0.01) to the right, respectively (ED50 = 1.48 and 5.65 mg/kg), compared with the rhodamine 123 efflux curve (ED50 0.25 mg/kg). Less pronounced shifts were noted with elacridar where the brain/plasma ratio was shifted only 2-fold relative to the rhodamine 123 efflux data (ED50 = 2.36 versus 1.34 mg/kg, respectively; p 0.01). These results indicate that the P-glycoprotein localized in the blood-brain barrier and, to a lesser extent, the testes-blood barrier is more resistant to inhibition than at other tissue sites such as the lymphocyte; moreover, the extent of this effect depends on the inhibitor. Such resistance can be overcome by a sufficiently high dose of an inhibitor; however, whether this is safely attainable in the clinical situation remains to be determined.

Beta-1-adrenoceptor genetic variants and ethnicity independently affect response to beta-blockade.

Objectives Black patients may be less responsive to &bgr;-blockers than whites. Genetic variants in the &bgr;1-adrenergic receptor (&bgr;1-AR) associated with lesser response to &bgr;-blockers are more common in blacks than in whites. The purpose of this study was to determine whether ethnic differences in response to &bgr;-blockade can be explained by differing distributions of functional genetic variants in the &bgr;1-AR. Methods We measured sensitivity to &bgr;-blockade by the attenuation of exercise-induced tachycardia in 165 patients (92 whites), who performed a graded bicycle exercise test before and 2.5u2009h after oral atenolol (25u2009mg). We determined heart rate at rest and at three exercise levels from continuous ECG recordings and calculated the area under the curve. We also measured plasma atenolol concentrations and determined genotypes for variants of the &bgr;1-AR (Ser49Gly, Arg389Gly) and &agr;2C-AR (del322-325). The effects of ethnicity, genotype, and other covariates on the heart rate reduction after atenolol were estimated in multiple regression analyses. Results Atenolol resulted in a greater reduction in exercise heart rate in whites than in blacks (P=0.006). &bgr;1-AR Arg389 (P=0.003), but not the &agr;2C-AR 322-325 insertion allele (P=0.31), was independently associated with a greater reduction in heart rate area under the curve. Ethnic differences in sensitivity to atenolol remained significant (P=0.006) after adjustment for &bgr;1-AR and &agr;2C-AR genotypes. Conclusion Ethnic differences in sensitivity to the &bgr;1-blocker atenolol persist even after accounting for different distributions of functional genetic &bgr;1-AR variants, suggesting that additional, as yet unidentified factors contribute to such ethnic differences.

Variations in the α2A-adrenergic receptor gene and their functional effects

The α2A‐adrenergic receptor (ADRA2A) plays a central role in the regulation of systemic sympathetic activity and hence cardiovascular responses such as heart rate and blood pressure. The objectives of this study were to systematically search for variants in the ADRA2A gene, to define the genes haplotype structure, and to examine potential functional effects of these variants.

Variation in the alpha2B-adrenergic receptor gene (ADRA2B) and its relationship to vascular response in vivo.

The &agr;2B-adrenergic receptor (ADRA2B) plays an important role in vasoconstriction and blood pressure regulation. One common variant in the ADRA2B gene (del301–303) has been identified, and results in markedly decreased receptor desensitization in vitro but does not alter vascular sensitivity in vivo. Therefore, we fully characterized genetic variations in ADRA2B and related them to phenotype in vivo. We examined 5812 bp of contiguous sequence of ADRA2B (promoter, exonic, and 3′-untranslated region; 3′-UTR) using the polymerase chain reaction to amplify the genomic target followed by bidirectional sequencing (n=68). Haplotypes were inferred using an expectation maximization algorithm. Vasoconstriction in response to increasing doses of the highly selective &agr;2-adrenergic receptor agonist, dexmedetomidine (0.01–1000 ng/min) was measured in the dorsal hand vein using a linear variable differential transformer. The dose that produced 50% (ED50) of maximum venoconstriction (Emax) was determined for each subject from the individual dose–response curves. ED50 and Emax were compared in subjects with and without variant alleles and haplotypes of interest. We identified 24 variable sites, 12 in the promoter region, five in the coding region (including two previously described as non-synonymous variants) and seven in the 3′-UTR region. Four haplotypes were inferred, representing approximately 95% of the cohort. One haplotype, characterized by two single nucleotide polymorphisms in the promoter region, and one in the 3′-UTR, occurred in seven of 38 African-Americans, and was associated with a lower Emax, 61.3% [95% confidence interval (CI) 39.5–83.0, n=7] compared to 78.1% (CI 73.8–82.5) in wild-types (n=61) (P=0.02). There was no association between the nine common variants and dexmedetomidine ED50. We have described novel variants and haplotypes of the ADRA2B gene. These do not alter sensitivity to a selective &agr;2-adrenergic receptor agonist but some may decrease maximal venoconstriction in vivo.

Alpha2-adrenergic receptor-induced vascular constriction in blacks and whites.

Abstract—Black Americans have a reduced hypotensive response to the &agr;2-adrenergic receptor agonist clonidine compared with whites, despite similar central sympathoinhibition. This reduced hypotensive response might be explained by greater postsynaptic vascular &agr;2-adrenergic receptor vasoconstrictive response. However, clonidine has a low &agr;2/&agr;1 selectivity ratio. Therefore, to determine the role of altered &agr;2-adrenergic receptor vascular sensitivity in ethnic differences in vascular response, we compared local vascular responses with the highly selective &agr;2-adrenergic receptor agonist dexmedetomidine in healthy black (n=18) and white (n=19) subjects. Increasing doses of dexmedetomidine (0.001 to 1000 ng/min) were infused into a dorsal hand vein, and the local response was measured with a linear variable differential transformer. Dexmedetomidine caused pronounced venoconstriction, with an average (±SD) maximum response of 74.5±17.72% but with no difference between blacks and whites. There was substantial intersubject variability in the sensitivity to dexmedetomidine; the dose resulting in 50% (ED50) of maximum vasoconstriction ranged from 0.08 ng/min to 256 ng/min. The geometric mean ED50 was 2.28 ng/min (95% CI, 0.02 to 271.6 ng/min) in blacks and 1.58 ng/min (95% CI, 0.11 to 24.55 ng/min) in whites (P =0.59). Our data indicate that &agr;2-adrenergic receptor–induced venoconstriction is similar in blacks and whites. These findings do not support the hypothesis that altered &agr;2-adrenergic receptor sensitivity is the explanation for the decreased blood pressure response to systemic administration of clonidine in blacks. The response to dexmedetomidine provides a model that will allow further study of the regulation of &agr;2-adrenergic receptor–mediated vascular responses

Desensitization of vascular response in vivo : contribution of genetic variation in the α2b-adrenergic receptor subtype

Objectives Vascular α2B-adrenergic receptors (α2B-ARs) mediate vasoconstriction and contribute to peripheral regulation of vascular tone. In vitro, a common 301–303 deletion in the α2B-AR gene, ADRA2B, results in loss of α2B-AR desensitization. We examined the hypothesis that ADRA2B del301–303 or other common ADRA2B variants alter vascular desensitization in vivo. Methods We measured sensitivity to a highly selective α2-AR agonist, dexmedetomidine, (0.01–1000 ng/min) in the dorsal hand vein in 41 healthy individuals. To induce desensitization a dose of dexmedetomidine that resulted in submaximal constriction was infused for 180 min and dorsal hand vein responses measured. Desensitization was defined as the ratio between the area-under-the-effect curve for each individuals response and the hypothetical area-under-the-effect curve assuming that the initial response had been maintained for 180 min (ratio below 1 reflecting desensitization). The relationship between six ADRA2B variants (one promoter, three coding, and two in the 3 untranslated region ) with an allele frequency of more than 5% and desensitization was determined. Results Forty-one individuals (22 men, 21 whites, age 18–45 years) were studied. The ADRA2B 301–303 deletion allele (ins/del and del/del, n = 18) was associated with resistance to desensitization [1.01 (interquartile range 0.90–1.06)] as compared with ins/ins homozygous individuals (n = 23) [0.91 (interquartile range 0.73–0.99)], P = 0.026. In addition, the −98 GG, 1182 CC, and 1776 CC genotypes were associated with significantly less desensitization than GC or CC, and CA or AA genotypes, respectively. Conclusion Common ADRA2B variants contribute to the interindividual variability in vascular desensitization to an α2-AR agonist in vivo.

Effects of variation in the human α2A- and α2C-adrenoceptor genes on cognitive tasks and pain perception

Background: The mechanisms underlying interindividual variability in pain perception and cognitive responses are undefined but highly heritable. α2C‐ and α2A‐adrenergic receptors regulate noradrenergic activity and are important mediators of pain perception and analgesia. We hypothesized that common genetic variants in these genes, particularly the ADRA2C 322–325 deletion variant, affect pain perception or cognitive responses.

CYP2A6 genetic variation and dexmedetomidine disposition

PurposeThere is a large interindividual variability in dexmedetomidine dose requirements for sedation of patients in intensive care units (ICU). Cytochrome P450 2A6 (CYP2A6) mediates an important route of dexmedetomidine metabolism, and genetic variation in CYP2A6 affects the clearance of other substrate drugs. We examined whether CYP2A6 genotypes affect dexmedetomidine disposition.MethodsIn 43 critically ill ICU patients receiving dexmedetomidine infusions adjusted to achieve the desired level of sedation, we determined a median of five plasma dexmedetomidine concentrations each. Forty subjects were genotyped for five common CYP2A6 alleles and grouped into normal (nu2009=u200933), intermediate (nu2009=u20095), and slow metabolizers (nu2009=u20092).ResultsUsing a Bayesian hierarchical nonlinear mixture model, estimated dexmedetomidine clearance was 49.1xa0L/h (posterior mean; 95% credible interval 41.4–57.6xa0L/h). There were no significant differences in dexmedetomidine clearance among normal, intermediate, and slow CYP2A6 metabolizer groups.ConclusionGenetic variation in CYP2A6 does not appear to be an important determinant of dexmedetomidine clearance in ICU patients.

Genetic variation in the presynaptic norepinephrine transporter is associated with blood pressure responses to exercise in healthy humans

Background The presynaptic norepinephrine transporter (NET) mediates synaptic clearance and recycling of norepinephrine. NET-deficient transgenic mice have elevated blood pressure (BP), heart rate, and catecholamine concentrations. However, the in-vivo effects of common NET variants on cardiovascular regulation at rest and during exercise are unknown. Methods We studied cardiovascular responses and plasma catecholamine concentrations at rest and during bicycle exercise at increasing workloads (25, 50, and 75 W) in 145 healthy participants. We used multiple linear regressions to analyze the effect of common, purportedly functional polymorphisms in NET (rs2242446 and rs28386840) on cardiovascular measures. Results 44 and 58.9% of participants carried at least one variant allele for NET T-182C and A-3081T, respectively. Systolic BP during exercise and systolic BP-area under the curve were higher in carriers of variant NET alleles (P=0.003 and 0.009 for T-182C and A-3081T, respectively) and NET haplotype -182C/-3081T compared with -182T/-3081A (all P<0.01). Diastolic BP during exercise was also higher at lower, but not at higher exercise stages in carriers of NET -182C (P<0.01) and -3081T variants (P<0.05). NET genotypes were not associated with catecholamine concentrations or heart rate. Conclusion Common genetic NET variants (-182C and -3081T) are associated with greater BP response to exercise in humans.