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A genetic polymorphism of the ₂-adrenergic receptor increases autonomic responses to stress

¹Department of Otolaryngology, Naval Medical Center, San Diego, California 92134; ²Department of Neurosurgery, Wayne State University School of Medicine, Detroit, Michigan 48201; ³Department of Audiology, Speech Pathology, and Balance Center, University of Pennsylvania, Philadelphia, Pennsylvania 19104; ⁴Department of Physiology, University of Michigan Medical School, Ann Arbor, Michigan 48109; ⁵Department of Medicine, University of California, San Diego 92103; and ⁶Special Projects, Naval Special Warfare Command, San Diego, California 92155

Submitted 16 May 2003 ; accepted in final form 16 January 2004

We hypothesized that individual differences in autonomic responses to psychological, physiological, or environmental stresses are inherited, and exaggerated autonomic responsiveness may represent an intermediate phenotype that can contribute to the development of essential hypertension in humans over time. ₂-Adrenergic receptors (₂-ARs), encoded by a gene on chromosome 10, are found in the central nervous system and also mediate release of norepinephrine from the presynaptic nerve terminals of the peripheral sympathetic nervous system and the exocytosis of epinephrine from the adrenal medulla. We postulated that, because this receptor mediates central and peripheral autonomic responsiveness to stress, genetic mutations in the gene encoding this receptor may explain contrasting activity of the autonomic nervous system among individuals. The restriction enzyme Dra I identifies a polymorphic site in the 3'-transcribed, but not translated, portion of the gene encoding the chromosome 10 ₂-AR. Southern blotting of genomic DNA with a cDNA probe after restriction enzyme digestion results in fragments that are either 6.7 kb or 6.3 kb in size. Transfection studies of these two genotypes resulted in contrasting expression of a reporter gene, and it is suggested from these findings that this is a functional polymorphism. In a study of 194 healthy subjects, we measured autonomic responses to provocative motion, a fall in blood pressure induced by decreasing venous return and cardiac output, or exercise. Specifically, we measured reactions to 1) Coriolis stress, a strong stimulus that induces motion sickness in man; 2) heart rate responses to the fall in blood pressure induced by the application of graded lower body negative pressure; and 3) exercise-induced sweat secretion. In all of these paradigms of stress, subjective and objective evidence of increased autonomic responsiveness was found in those individuals harboring the 6.3-kb allele. Specifically, volunteers with the 6.3-kb allele had greater signs and symptoms of motion sickness mediated by the autonomic nervous system after off-axis rotation at increasing velocity (number of head movements a subject could complete during rotation before emesis ± SE: 295 ± 18 vs. 365 ± 11; P = 0.001). They also had greater increases in heart rate in responses to the lower body negative pressure-induced fall in blood pressure (increase in heart rate ± SE: 3.0 ± 0.4 vs. 1.8 ± 0.3; P = 0.012), and the 6.3-kb group had higher sweat sodium concentrations during exercise (mean sweat sodium concentration in meq/l over 30 min of exercise ± SE: 43.2 ± 7.1 vs. 27.6 ± 3.4; P < 0.05). This single-nucleotide polymorphism may contribute to contrasting individual differences in autonomic responsiveness among healthy individuals.

motion sickness; baroreceptors; lower body negative pressure; hypertension; single-nucleotide polymorphism

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