Nighttime blood pressure dipping: the role of the sympathetic nervous system.

There is a marked diurnal variation in blood pressure (BP), with BP dipping to its lowest levels during nighttime sleep. A day-night dip in systolic BP (SBP) of <10% has been used to characterize individuals as nondippers, and is associated with an increased risk for cardiovascular disease. The present study examined the contribution of the sympathetic nervous system (SNS) to BP dipping in a biracial sample of 172 men and women aged 25 to 45 years. Assessments included 24-h ambulatory BP monitoring and both waking and sleeping urinary catecholamines. In addition, cardiovascular alpha- and beta-adrenergic receptor (AR) responsiveness was determined by the doses of isoproterenol and phenylephrine required to attain an increase in heart rate of 25 points (CD25) and BP (PD25), respectively. Compared with dippers (n = 116), nondippers (n = 56) were more likely to be African American and to have a family history of hypertension as well as a higher body mass index (BMI). The nighttime fall in both norepinephrine (NE) and epinephrine (EPI) excretion rates was reduced in nondippers compared with dippers (NE dip 9.3 v 13.1 microg/mg; EPI dip 2.7 v 4.0 microg/mg; both P < .05). Nondippers also were characterized by heightened alpha1-AR responsiveness compared with dippers (PD25 = 252 v 321 microg, P < .05). These data suggest that the SNS may contribute to individual differences in nighttime BP dipping, and appears to account in part for blunted BP dipping in African Americans.