β-Adrenergic receptor antagonists (β-blockers) are used in the treatment of chronic cardiovascular conditions, as well as in acute control of tachycardia 1,2. There is considerable interindividual variability in the response to β-blockers 3. Two nonsynonymous single nucleotide polymorphisms in the β1 adrenergic receptor (β1AR) gene (ADRB1) are common, the substitution of serine by glycine at position 49 (Ser49Gly, rs1801252) and arginine by glycine at position 389 (Arg389Gly, rs1801253) 4, with minor allele frequencies of 11.0 and 29.3%, respectively, among Caucasians 5. In vitro, the Arg389Gly polymorphism was shown to exert a primary effect on isoproterenol-stimulated adenylate cyclase activity, with the Arg389 variant showing a three-fold higher activity than the Gly3896. In healthy individuals, the Arg389 allele was associated with greater resting systolic blood pressure (SBP) response to β-blocker 7, and β-blockade 5,8, defined as inhibition of exercise-induced tachycardia 9. These studies, however, used orally administered β-blockers, and the effect was measured approximately 2 h after administration 5,7,8. Thus, compensatory hemodynamic responses may have masked the full contribution of genotype toward the variability in β-blockade, hindering its evaluation. Intravenous administration of β-blocker could allow the detection of early changes in heart rate (HR) that occur after drug administration.
Thus, the aim of this study was to determine the effect of the ADRB1 Arg389Gly polymorphism on early HR responses to esmolol, an intravenously administered ultra-short-acting β1AR selective antagonist.
The study was approved by the ethics committee of the Hadassah Medical Center, Jerusalem, Israel. Participants signed a consent form before any study procedure.
The study included nonsmoking healthy individuals. The exclusion criteria were as follows: age above 45 years, pregnancy or lactation, consumption of any medication (3 weeks before the study), smoking, and identification of a clinically significant condition on medical history, physical examination, or ECG. Following genotyping, participants were enrolled in the study according to their ADRB1 49 and 389 genotype.
Genomic DNA was isolated at screening by a commercial kit (Purgene; Gentra Systems, Minneapolis, Minnesota, USA). Genotypes at codons 49 and 389 were determined as described previously 10.
The study was single-blind controlled. Participants were instructed to refrain from consuming alcohol, caffeinated, or salty foods (a list provided at screening), and strenuous exercise for 72 h before the day of the study. Studies began after an overnight fast, in a quiet, temperature-controlled room.
Participants remained in a supine position throughout the study. An indwelling venous catheter was inserted into the participant’s forearm for infusions using two syringe pumps (Perfusor Space; B. Braun, Melsungen AG, Melsungen, Germany). The study protocol consisted of two constitutive 10-min phases of esmolol hydrochloride (10 mg/1 ml, Bedford Laboratories, Ohio, USA)/placebo (normal saline) infusion, separated by 30 min of washout. At the last minute of each infusion phase, participants performed a standardized exercise. The infusion order was placebo first, followed by esmolol. The participants were blinded to the infusion order.
Esmolol was administered at a rate of 500 µg/kg/min for 1 min, followed by 9 min of 300 µg/kg/min. This protocol was found to result in the attainment of 90% steady state of β-blockade within 5 min of infusion initiation 11.
HR and blood pressure (BP) were measured using a semiautomated BP monitor: before initiation of infusion (Baseline), 5 min following initiation of infusion (Resting), and during the last 30 s of exercise (Exercise).
The handgrip dynamic exercise was performed using a handgrip dynamometer (Lafayette Instruments Company, Indiana, USA; Model 78010), according to a previously described protocol 12.
The differences between ‘Exercise’ and ‘Resting’ measurements (Exercise–Resting) were calculated, and defined as exercise-induced tachycardia. The difference between exercise-induced tachycardia (Exercise–Resting) during placebo and esmolol infusions (inhibition of exercise-induced tachycardia, β-blockade) was defined as the primary outcome. The proportion of participants with greater than 10 bpm in primary outcome was calculated.
Continuous variables were expressed as mean±SD. Comparison of continuous hemodynamic measurements among the three genotypes was carried out using the one-way analysis of variance (ANOVA) (polynomial, linear) and the Bonferoni correction for comparisons of any two genotypic groups. Multivariate analyses were carried out using univariate ANOVA. The Shapiro–Wilk test was carried out to ensure normal distribution of data. A P value less than 0.05 was considered significant. Statistical analysis was carried out using SPSS version 17.0 (SPSS Inc., Chicago, Illinois, USA).
We assumed that the difference between Arg389 and Gly389 homozygotes in esmolol inhibition of exercise-induced tachycardia is 10±7 bpm (pooled variance 49). To observe this difference with a power of 80% and an α of 5%, nine participants in each group are required.
Healthy White individuals were genotyped for the ADRB1 Ser49Gly and Arg389Gly polymorphisms. Minor allele frequencies for the 49 and the 389 polymorphisms among the participants genotyped (n=84) were 12.5 and 36.3%, respectively, and were in Hardy–Weinberg equilibrium. Among the 63 Ser49Ser homozygotes, 23 (36.5%) were carriers of the Arg389Arg, 28 (44.4%) Gly389Arg, and 12 (19.0%) Gly389Gly genotypes. Nine participants of each of these three genotypes (18 men) were studied. There was no difference in sex, mean BMI, aerobic exercise (minutes/week), and HR, SBP, and diastolic blood pressure between genotypes. The mean age of the participants in the Arg389Arg group was significantly higher than that of the Gly389Gly group (28.00±1.58 and 24.89±2.67 years; P=0.028).
There was no difference between placebo and esmolol infusions in baseline HR and SBP or diastolic blood pressure.
Exercise tachycardia during esmolol (but not during placebo) varied significantly among carriers of the three 389 genotypes (PANOVA=0.030, post-hoc test P≥0.089) (Table 1). Inhibition of exercise-induced tachycardia (difference in exercise-induced tachycardia between esmolol and placebo) was greatest among Arg389Arg and smallest among Gly389Gly (PANOVA=0.014, post-hoc test P=0.042) (Table 1). The proportion of participants with greater than 10 bpm esmolol inhibition of exercise-induced tachycardia was greater among Arg389Arg (6/9 participants), as compared with Gly389Arg (1/9 participants) and Gly389Gly (0/9 participants) (P=0.003) (Fig. 1).
Among women, there was a significantly greater exercise-induced tachycardia during esmolol, and smaller esmolol inhibition of exercise-induced tachycardia as compared with men (22.5±10.07 vs. 15.05±6.03 bpm, P=0.025; 0.13±8.59 vs. 7.58±7.24 bpm, P=0.029, respectively).
In multivariate analysis, both the 389 genotype and sex contributed significantly toward the variability in exercise-induced tachycardia during esmolol, and in esmolol inhibition of exercise-induced tachycardia.
This study is the first to examine the effect of the ADRB1 Arg389Gly polymorphism on HR response to an intravenously administered β1AR antagonist. We found that esmolol induced more than a 10-fold greater inhibition of exercise-induced tachycardia among carriers of 2 as compared with 0 Arg389 alleles. These findings are in agreement with most previous studies reporting enhanced responsiveness of the Arg389 receptor, both in vitro4,6 and in vivo4,5,7,8,10.
The use of esmolol in our model has several advantages: First, the ultra-short half-life esmolol enables the simulation of near steady-state conditions 2. The infusion protocol we have used has been reported to result in attainment of 90% of steady-state β-blockade within 5 min of infusion 11. In addition, the onset of the effect of esmolol on HR precedes its effect on BP 13, potentially minimizing the confounding effects on HR that are expected with slow-onset β-blockade following oral administration.
Our findings are likely to be clinically relevant: The Arg389Gly polymorphism is relatively common. We studied the early effect of esmolol on exercise-induced tachycardia, which is relevant for the clinical uses of esmolol, the immediate control of tachycardia 2. In addition, the dosing protocol we used is identical to the protocol that is commonly used in clinical practice 2.
We found that female sex was associated with a smaller esmolol effect on HR. Previous studies have shown that metoprolol had a greater effect on HR among women, because of pharmacokinetic differences 14, whereas esmolol had a smaller effect on exercise-induced BP increases in women as compared with men 15. Further study is required to determine the compound effect of genotype and sex as well as factors such as ethnicity, physical activity, and BMI on the variability in β-blockade.
We did not measure esmolol blood levels and it is possible that chance differences in esmolol plasma concentration contributed toward the differences observed between genotypes. However, esmolol was administered intravenously; thus, variability in drug bioavailability was avoided. Esmolol is metabolized by blood esterases 2, avoiding the effect of common genetic variability in cytochrome P450 enzymes on drug metabolism.
Our findings may indicate over a 10-fold difference in the effect of esmolol on exercise-induced tachycardia among ADRB1 389 genotypes. Further study is required to determine whether these differences will translate into different dose requirements in a clinical setting.
This work was partially supported by a research grant from the Hadassah Medical Organization to Mordechai Muszkat, MD.
Conflicts of interest
There are no conflicts of interest.
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