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2-adrenergic receptor polymorphism alters the cardiac output response to isometric exercise
1 Department of Anesthesiology, Mayo Clinic College of Medicine, Rochester, MN, USA
2 Department of Biostatistics, Mayo Clinic College of Medicine, Rochester, MN, USA
3 Department of Internal Medicine, Mayo Clinic College of Medicine, Rochester, MN, USA
* To whom correspondence should be addressed. E-mail: eisenach.john{at}mayo.edu.
Normotensive adults homozygous for glycine (Gly) of the Arg16/Gly 
2-adrenergic receptor polymorphism have: 1) greater forearm 2-receptor mediated vasodilation, and 2) a higher heart rate (HR) response to isometric handgrip than arginine (Arg) homozygotes. To test the hypothesis that the higher HR response in Gly16 subjects serves to maintain the pressor response (increased cardiac output, CO) in the setting of augmented peripheral vasodilation to endogenous catecholamines, we measured continuous HR (ECG), arterial pressure (AP, Finapres), and CO (transthoracic echocardiography) during isometric, 40% submaximal handgrip to fatigue in healthy subjects homozygous for Gly (n = 30; mean age ± SE: 30 ± 1.2, 13 women) and Arg (n = 17, age 30 ± 1.6, 11 women). Resting data were similar between groups. Handgrip produced similar increases in AP, venous norepinephrine and epinephrine concentrations; however, HR increased more in the Gly group (60.1 ± 4.3 % increase from baseline vs. 45.5 ± 3.9%, P = 0.03) which caused CO to be higher (Gly: 7.6 ± 0.3 l/m vs. Arg: 6.5 ± 0.3, P = 0.03), while the decrease in systemic vascular resistance in the Gly group did not reach significance (P = 0.09). We conclude that Gly16 homozygotes generate a higher CO to maintain the pressor response to handgrip. The influence of polymorphic variants in the 2-adrenergic receptor gene on the cardiovascular response to sympathoexcitation may have important implications in the development of hypertension and heart failure.
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