Eitan A. Friedman

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.

The Effects of Tadalafil on Cold‐induced Vasoconstriction in Patients with Raynaud's Phenomenon

Raynauds phenomenon (RP) is a disorder characterized by episodic periods of vasoconstriction typically provoked by exposure to cold. Phosphodiesterase 5 (PDE5) inhibitors may improve digital blood flow and clinical symptoms in patients with RP, but the mechanisms are unknown. We examined the hypothesis that a PDE5 inhibitor, tadalafil, attenuates cold‐induced vasoconstriction. Additionally, we examined whether tadalafil reduced vascular dysfunction following ischemia, thus altering the response to repeated cooling. We conducted a double‐blind, placebo‐controlled crossover study in 20 subjects with RP on two separate study days, when subjects received either placebo or tadalafil (10 mg). Digital blood flow (flux) was measured by laser Doppler flowmetry at rest and during two graduated local heat and cold exposure cycles. Temperature–response curves were evaluated by Emax (maximal flux during heating), Emin (minimal flux during cooling), and ET50 and ET90 (the local temperature at which flux decreased by 50% and 90% of Emax–Emin, respectively). Tadalafil did not increase baseline flux (81.0±73.0 vs 91.3±114.0 arbitrary unit (AU), P=0.57), Emax (280.0±107.6 vs 279.5±119.8 AU, P=0.94), ET50 (25.4±4.4 vs 26.6±5.7°C, P=0.62), or ET90 (21.2±3.9 vs 21.8±5.0°C, P=0.78), (cycle 1 values presented). There were no differences between cycles on either study day. In conclusion, in patients with RP, single‐dose tadalafil does not increase digital blood flow at baseline or in response to heating, nor does it attenuate cold‐induced vasoconstriction. Furthermore, it does not precondition the endothelium to resist a second cooling challenge. The clinical benefit in patients with RP treated with PDE5 inhibitors probably involves mechanisms other than acute inhibition of cold‐induced vasoconstriction.

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.

Genetic variants in the α2C-adrenoceptor and G-protein contribute to ethnic differences in cardiovascular stress responses

Objectives Cardiovascular responses to stressors are regulated by sympathetic activity, increased in black Americans, and associated with future cardiovascular morbidity. Our aim was to determine whether two functional variants in genes regulating sympathetic activity, a deletion in the &agr;2C-adrenergic receptor (ADRA2C del322–325) and a G-protein &bgr;3-subunit variant (GNB3 G825T), affect cardiovascular responses to physiologic stressors and contribute to their ethnic differences. Methods We measured heart rate and blood pressure responses to a cold pressor test (CPT) in 79 healthy participants (40 blacks, 39 whites), aged 25.7±5.3 years, and determined genotypes for the ADRA2C and GNB3 variants. We examined the response variables (increase in heart rate and blood pressure) in multiple linear regression analyses adjusting first for baseline measures, ethnicity, and other covariates, and then additionally for genotypes. Results Black participants had a greater heart rate response to CPT than whites [mean difference, 9.9 bpm; 95% confidence interval (CI), 4.1 to 15.6; P=0.001]. Both the ADRA2C del/del (15.8 bpm; 95% CI, 8.0–23.7; P<0.001) and GNB3 T/T genotypes (6.8 bpm; 95% CI, 0.9–12.7; P=0.026) were associated with greater heart rate response. After adjusting for genotypes, the ethnic difference was abrogated (1.3 bpm; 95% CI, −5.4–8.0; P=0.70), suggesting that the genetic variants contributed substantially to ethnic differences. Conclusion Variation in genes that regulate sympathetic activity affects hemodynamic stress responses and contributes to their ethnic differences. This study elucidates how genetic factors may in part explain ethnic differences in cardiovascular regulation.

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.

Effect of the α2C-adrenoreceptor deletion322–325 variant on sympathetic activity and cardiovascular measures in healthy subjects

Background The α2C-adrenergic receptor plays an important role in the regulation of the sympathetic nervous system and, therefore, blood pressure and heart rate. A deletion polymorphism in its gene (ADRA2C del322–325), ten times more common in black than white Americans, has been associated with a loss of function in vitro and, under controlled study conditions, raised blood pressure and catecholamine secretion. We therefore examined the hypothesis that the ADRA2C deletion variant would alter sympathetic activity and contribute to ethnic differences in blood pressure. Methods We measured resting plasma norepinephrine and epinephrine concentrations, blood pressure and heart rate in 224 healthy subjects (127 whites), and determined their ADRA2C del322–325 genotype. Additionally, we analyzed heart rate variability (HRV) in a subgroup of 50 black subjects. Results Systolic (SBP) and diastolic blood pressure (DBP) were higher in blacks than whites [difference (95% confidence interval), 4.4 (1.5–7.4) mmHg, P = 0.003; and 2.7 (0.7–4.6) mmHg, P = 0.01, respectively]. Norepinephrine concentrations did not differ among subjects with 0, 1 and 2 copies of the deletion variant [median (interquartile range), 185.0 (147.5–269.8), 200.0 (154.9–257.0) and 173.8 (158.5–235.8) pg/ml, respectively; P = 0.54]. Similarly, none of the HRV parameters differed among the genotype groups. In multiple linear regression analyses adjusting for multiple covariates, the deletion genotype was not associated with SBP or DBP. In contrast, black ethnicity was associated with higher SBP (P = 0.001) and DBP (P = 0.005). Conclusion The ADRA2C deletion polymorphism had no effect on markers of resting sympathetic activity and cardiovascular measures, and did not account for ethnic differences in blood pressure.

Catecholamine pathway gene variation is associated with norepinephrine and epinephrine concentrations at rest and after exercise.

Objective To examine the hypothesis that genetic variation in enzymes and transporters associated with synthesis, storage, release, and metabolism of catecholamines contributes to the interindividual variability in plasma catecholamine concentrations at rest and after exercise. Methods We measured plasma norepinephrine (NE) and epinephrine concentrations at rest and after a standardized exercise protocol in 165 healthy individuals (60% White, 40% African-American) and examined 29 functional or common variants in 14 genes involved in synthesis, transport, or metabolism of catecholamines. We examined the relationship between genotypes and NE concentrations at rest and the increase after exercise (&Dgr;NE) by multiple linear regression with adjustment for covariates [age, race, sex, BMI, fitness, and resting NE (for &Dgr;NE)]. As a secondary outcome, we carried out similar analyses for epinephrine concentrations. Results There was large interindividual variability in resting NE (mean, 204±102 pg/ml; range, 39–616 pg/ml) and &Dgr;NE (mean, 256±206 pg/ml; range, −97 to 953 pg/ml). Resting NE was significantly associated with variants of four genes: CYB561 (P<0.001), VMAT2 (P=0.016), CHGA (P=0.039), and PNMT (P=0.038). &Dgr;NE after exercise was associated with three variants of PNMT (P=0.041) and COMT (P=0.033 and 0.035), and resting and exercise epinephrine concentrations were associated with two variants each. Conclusion The findings of this exploratory study suggest that variation in catecholamine pathway genes contributes to the interindividual variability in plasma NE and epinephrine concentrations at rest and after exercise.

Understanding the role of prostaglandin E2 in regulating human platelet activity in health and disease.

The platelet thrombus is the major pathologic entity in acute coronary syndromes, and antiplatelet agents are a mainstay of therapy. However, individual patient responsiveness to current antiplatelet drugs is variable, and all drugs carry a risk of bleeding. An understanding of the complex role of Prostaglandin E2 (PGE2) in regulating thrombosis offers opportunities for the development of novel individualized antiplatelet treatment. However, deciphering the platelet regulatory function of PGE2 has long been confounded by non-standardized experimental conditions, extrapolation of murine data to humans, and phenotypic differences in PGE2 response. This review synthesizes past and current knowledge about PGE2 effects on platelet biology, presents a rationale for standardization of experimental protocols, and provides insight into a molecular mechanism by which PGE2-activated pathways could be targeted for new personalized antiplatelet therapy to inhibit pathologic thrombosis without affecting hemostasis.

Genetic variations in the α2A-adrenoreceptor are associated with blood pressure response to the agonist dexmedetomidine

Background—&agr;2A-Adrenoceptors (&agr;2A-ARs) have important roles in sympathetic cardiovascular regulation. Variants of ADRA2A affect gene transcription and expression and are associated with insulin release and risk for type 2 diabetes. We examined whether ADRA2A variants are also associated with cardiovascular responses to the selective &agr;2-AR-agonist dexmedetomidine. Methods and Results—Seventy-three healthy subjects participated in a placebo-controlled, single-blind study. After 3 infusions of placebo, subjects received 3 incremental infusions of dexmedetomidine (cumulative dose, 0.4 &mgr;g/kg). Primary outcomes were changes in systolic blood pressure (SBP) and plasma norepinephrine concentrations, measured as difference of the area-under-the-curve during placebo and dexmedetomidine infusions (&Dgr;AUC). We used multiple linear regression analysis to examine the associations between 9 ADRA2A tagging variants and 5 inferred haplotypes and &Dgr;AUC after adjustment for covariates. Homozygous carriers of rs553668 and the corresponding haplotype 4, previously associated with increased &agr;2A-AR expression, had a 2.2-fold greater decrease in AUCSBP after dexmedetomidine (adjusted P=0.006); similarly, the maximum decrease in SBP was 24.7±8.1 mm Hg compared with 13.6±5.9 mm Hg in carriers of the wild-type allele (P=0.007). Carriers of haplotype 3, previously associated with reduced &agr;2A-AR expression, had a 44% smaller decrease in AUCSBP (P=0.013). Haplotype information significantly improved the model predicting the decrease in SBP (P<0.001). There were similar but nonsignificant trends for diastolic blood pressure and heart rate. Genotypes were not significantly associated with norepinephrine responses. Conclusions—Common ADRA2A variants are associated with the hypotensive response to dexmedetomidine. Effects of specific variants/haplotypes in vivo are compatible with their known effects on gene expression in vitro.

Genetic variations in the α(2A)-adrenoreceptor are associated with blood pressure response to the agonist dexmedetomidine.

Background—&agr;2A-Adrenoceptors (&agr;2A-ARs) have important roles in sympathetic cardiovascular regulation. Variants of ADRA2A affect gene transcription and expression and are associated with insulin release and risk for type 2 diabetes. We examined whether ADRA2A variants are also associated with cardiovascular responses to the selective &agr;2-AR-agonist dexmedetomidine. Methods and Results—Seventy-three healthy subjects participated in a placebo-controlled, single-blind study. After 3 infusions of placebo, subjects received 3 incremental infusions of dexmedetomidine (cumulative dose, 0.4 &mgr;g/kg). Primary outcomes were changes in systolic blood pressure (SBP) and plasma norepinephrine concentrations, measured as difference of the area-under-the-curve during placebo and dexmedetomidine infusions (&Dgr;AUC). We used multiple linear regression analysis to examine the associations between 9 ADRA2A tagging variants and 5 inferred haplotypes and &Dgr;AUC after adjustment for covariates. Homozygous carriers of rs553668 and the corresponding haplotype 4, previously associated with increased &agr;2A-AR expression, had a 2.2-fold greater decrease in AUCSBP after dexmedetomidine (adjusted P=0.006); similarly, the maximum decrease in SBP was 24.7±8.1 mm Hg compared with 13.6±5.9 mm Hg in carriers of the wild-type allele (P=0.007). Carriers of haplotype 3, previously associated with reduced &agr;2A-AR expression, had a 44% smaller decrease in AUCSBP (P=0.013). Haplotype information significantly improved the model predicting the decrease in SBP (P<0.001). There were similar but nonsignificant trends for diastolic blood pressure and heart rate. Genotypes were not significantly associated with norepinephrine responses. Conclusions—Common ADRA2A variants are associated with the hypotensive response to dexmedetomidine. Effects of specific variants/haplotypes in vivo are compatible with their known effects on gene expression in vitro.