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Background: Chromogranin A, coreleased with catecholamines by exocytosis, is cleaved to the catecholamine release-inhibitory fragment catestatin. We identified a natural nonsynonymous variant of catestatin, Gly364Ser, that alters human autonomic function and blood pressure., Methods and Results: Gly364Ser heterozygotes and controls underwent physiological and biochemical phenotyping, including catecholamine production, chromogranin A precursor, and its catestatin product. Case-control studies replicated effects of the gene on blood pressure in the population. Gly364Ser displayed diminished inhibition of catecholamine secretion from cultured neurons. Gly/Ser heterozygotes displayed increased baroreceptor slope during upward deflections (by approximately 47%) and downward deflections (by approximately 44%), increased cardiac parasympathetic index (by approximately 2.4-fold), and decreased cardiac sympathetic index (by approximately 26%). Renal norepinephrine excretion was diminished by approximately 26% and epinephrine excretion by approximately 34% in Gly/Ser heterozygotes. The coalescent dated emergence of the variant to approximately 70,000 years ago. Gly364Ser was in linkage disequilibrium with 1 major Chromogranin A promoter haplotype, although promoter haplotypes did not predict autonomic phenotypes. The 364Ser variant was associated with lower diastolic blood pressure in 2 independent/confirmatory groups of patients with hypertension; genotype groups differed by approximately 5 to 6 mm Hg, and the polymorphism accounted for approximately 1.8% of population diastolic blood pressure variance, although a significant gene-by-sex interaction existed, with an enhanced effect in men., Conclusions: The catestatin Gly364Ser variant causes profound changes in human autonomic activity, both parasympathetic and sympathetic, and seems to reduce risk of developing hypertension, especially in men. A model for catestatin action in the baroreceptor center of the nucleus of the tractus solitarius accounts for these actions.