Gbenga G. Sofowora

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.

Arg389Gly β1-adrenoceptor polymorphism varies in frequency among different ethnic groups but does not alter response in vivo

There are marked interethnic differences in beta 1-adrenoceptor-mediated responsiveness, with sensitivity decreased in African-Americans and increased in Chinese compared with Caucasians. Therefore, the frequency of a common naturally occurring polymorphism of the human beta 1-adrenoceptor gene (Arg389Gly), which has functional importance in vitro, was determined in 194 African-Americans, 316 Caucasian-Americans, 221 Hispanic-Americans and 142 Chinese. African-Americans were found to have a significantly lower frequency of the Arg389 allele than the other three ethnic groups (all P < 0.01). In the populations studied, the order of the distribution of the Arg389 allele was: Chinese (74%) > Caucasians (72%) > Hispanics (67%) > African-Americans (58%). To determine the functional significance of the Arg389Gly beta 1-adrenoceptor polymorphism, in-vivo heart rate responses to exercise were compared in healthy subjects homozygous for the Arg (n = 9) and Gly (n = 8) alleles. Heart rate response to exercise was not affected by genotype (P = 0.4). Although ethnic differences in the frequency of the beta 1-adrenoceptor Arg389Gly polymorphism exist, the polymorphism does not appear to have functional significance in healthy subjects and therefore may not contribute to ethnic differences in response to drugs acting through the beta 1-adrenoceptor.

Human β2‐adrenergic receptor polymorphisms: No association with essential hypertension in black or white Americans

The most common polymorphisms of the human β2‐adrenergic receptor‐Arg16→Gly and Gln27→Glu‐are associated with alterations in β2‐adrenergic receptor responses, both in vitro and in vivo. β2‐Adrenergic receptor–mediated vascular responses are affected by ethnicity, blood pressure, and genotype. We tested the hypothesis that these two common β2‐adrenergic receptor genetic variants are associated with essential hypertension in black or white Americans.

The effect of sildenafil on nitric oxide–mediated vasodilation in healthy men

Sildenafil, a treatment for erectile dysfunction, is a specific phosphodiesterase type 5 (PDE 5) inhibitor that enhances nitric oxide (NO)–mediated vasodilation in the corpus cavernosum by inhibiting cyclic guanosine monophosphate breakdown. Since PDE 5 is widely expressed in the vasculature, we examined the hypothesis that sildenafil could enhance NO‐mediated vasodilation in other vascular beds and improve endothelial function.

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 2.5 h after oral atenolol (25 mg). 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.

In-vivo effects of Glu298Asp endothelial nitric oxide synthase polymorphism.

Endothelial nitric oxide synthase catalyses the formation of the vasodilator nitric oxide, a major regulator of vascular tone. The Asp298 polymorphism of the nitric oxide synthase gene is associated with altered function and expression of the enzyme in vitro and myocardial infarction and coronary artery spasm in vivo. We examined the effect of the Glu298Asp polymorphism on: (1) local vascular responses to phenylephrine, acetylcholine, glyceryl trinitrate and prostaglandin E1 in the dorsal hand vein; (2) changes in forearm blood flow during mental stress, a measure of nitric oxide-mediated effect on resistance vessels; (3) excretion of urinary nitrite/nitrate as a measure of total body nitric oxide production; and (4) F2-isoprostane metabolite, a measure of oxidative stress, in healthy Glu298 (n = 12) and Asp298 (n = 13) homozygotes. There were no significant differences in acetylcholine dose responses (P = 0.29) in Glu298 and Asp298 homozygotes. Responses to glyceryl trinitrate, prostaglandin E1 and the alpha-adrenergic agonist phenylephrine did not differ by genotype. Forearm blood flow was similar at rest and increased significantly (from 7.5 ml/min/100 ml to 12.2 ml/min/100 ml; P = 0.003), but similarly (P = 0.2), during mental stress in both genotypes. Asp298 homozygotes excreted significantly less nitrate/nitrite than Glu298 homozygotes (nitrate + nitrite/creatinine ratio 0.05 +/- 0.01 vs. 0.09 +/- 0.01, respectively; P < 0.005). Urinary F2-isoprostane metabolite excretion did not differ (Glu298, 2.04 +/- 0.25 ng/mg creatinine; Asp298, 1.85 +/- 0.37 ng/mg creatinine; P = 0.7). We conclude that in healthy volunteers the Glu298Asp polymorphism affects endogenous nitric oxide production without affecting nitric oxide-mediated vascular responses. This polymorphism may only have clinical significance in the presence of endothelial dysfunction.

Ethnic and Genetic Determinants of Cardiovascular Response to the Selective α2-Adrenoceptor Agonist Dexmedetomidine

The &agr;2-adrenoceptor agonist clonidine reduces blood pressure more effectively in White than Black Americans despite similar degrees of sympatholysis. Functional genetic variation in receptor signaling mechanisms, for example in the &bgr;3 G-protein subunit (GNB3 C825T) and in the &agr;2C-adrenoceptor subtype (ADRA2C del322–325), may affect drug responses. We examined the hypothesis that there are ethnic differences in the responses to the highly selective &agr;2-agonist, dexmedetomidine, and that these genetic variants contribute to interindividual variability in drug responses. In a placebo-controlled, single-masked study, 73 healthy subjects (37 whites and 36 blacks) received 3 placebo infusions and then 3 incremental doses of dexmedetomidine (cumulative dose, 0.4 &mgr;g/kg), each separated by 30 minutes. Blood pressure, heart rate, and plasma catecholamine concentrations were determined after each infusion. We measured dexmedetomidine concentrations after the last infusion and determined ADRA2C del322–325 and GNB3 C825T genotypes. Dexmedetomidine lowered blood pressure and plasma catecholamine concentrations significantly (all P<0.001). There was substantial interindividual variability in the reduction of systolic blood pressure (range, 1 to 34 mm Hg) and plasma norepinephrine concentrations (range, 24 to 424 pg/mL). However, there were no differences between black and white subjects in dexmedetomidine responses (P>0.16 for all outcomes) before or after adjustment for covariates. Neither ADRA2C del322–325 nor GNB3 C825T genotypes affected the responses to dexmedetomidine (all P>0.66). There is large interindividual variability in response to the selective &agr;2-AR agonist dexmedetomidine, and neither ethnicity nor ADRA2C and GNB3 genotypes contribute to it. Further studies to identify determinants of &agr;2-AR–mediated responses will be of interest.

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.

Tariquidar, a selective P-glycoprotein inhibitor, does not potentiate loperamide's opioid brain effects in humans despite full inhibition of lymphocyte P-glycoprotein.

Background:Loperamide, a potent opioid, has been used as an in vivo probe to assess P-glycoprotein activity at the blood–brain barrier, because P-glycoprotein inhibition allows loperamide to cross the blood–brain barrier and exert its central opioid effects. In humans, studies with nonselective and moderately potent inhibitors resulted in mild opioid effects but were confounded by the concurrent inhibition of loperamide’s metabolism. The authors studied the effect of the highly selective, potent P-glycoprotein inhibitor tariquidar on loperamide’s central opioid effects. Methods:In a randomized, double-blind, crossover study, nine healthy subjects received on 2 study days oral loperamide (32 mg) followed by an intravenous infusion of either tariquidar (150 mg) or placebo. Central opioid effects (pupil diameter, sedation) were measured for 12 h, and blood samples were drawn up to 48 h after drug administration to determine plasma loperamide concentrations and ex vivo P-glycoprotein activity in T lymphocytes. Values for pupil diameter and loperamide concentrations were plotted over time, and the areas under the curves on the tariquidar and placebo study day were compared within each subject. Results:Tariquidar did not significantly affect loperamide’s central effects (median reduction in pupil diameter area under the curve, 6.9% [interquartile range, −1.4 to 12.1%]; P = 0.11) or plasma loperamide concentrations (P = 0.12) but profoundly inhibited P-glycoprotein in lymphocytes by 93.7% (95% confidence interval, 92.0–95.3%). Conclusion:These results suggest that despite full inhibition of lymphocyte P-glycoprotein, the selective P-glycoprotein inhibitor tariquidar does not potentiate loperamide’s opioid brain effects in humans.