SUMMARY To examine the relationship between body mass index, blood pressure, and the Na + ,K + -adenosine triphosphatase (ATPase) system, we measured the erythrocyte ghost Na + ,K + -ATPase and the erythrocyte Na + concentration in 120 blacks and 127 whites (136 males and 111 females). Blacks showed a 13.9% higher erythrocyte Na + (7.63 ± 0.19 vs 6.70 ± 0.11 [SEM] mEq/L; p = 0.0001) and a 16.1% lower erythrocyte ghost Na + ,K + -ATPase activity (140.3 ± 4.2 vs 167.3 ± 4.7 nmol inorganic phosphate/mg protein/hr; p = 0.0002) than whites. Male subjects demonstrated a 6.4% higher erythrocyte Na + (7.35 ± 0.17 vs 6.91 ± 0.14 mEq/L; p= 0.043) and an 11.5% lower Na + ,K + -ATPase activity (145.7 ±3.7 vs 164.7 ± 5.5 nmol inorganic phosphate/mg protein/hr; p = 0.0015) than female subjects. Significant (p<0.001) negative correlations were identified for the systolic, diastolic, and mean blood pressure levels and the erythrocyte ghost Na + ,K + -ATPase. These findings were complemented by positive correlations for the blood pressure levels and erythrocyte Na + concentrations. The body mass index was negatively correlated with erythrocyte ghost Na + ,K + -ATPase and it accounted for 6.7%, 5.6%, and 6.1% of the variabilities in the systolic, diastolic, and mean blood pressure levels, respectively. Variabilities of 1.4% systolic, 12.3% diastolic, and 11.1% in mean arterial pressure were attributable to the erythrocyte ghost Na + ,K + -ATPase activity. Provided that findings in erythrocytes also reflect the relative status of the vascular smooth muscle cell Na + ,K + -ATPase, the predisposition of black, male, and obese persons to hypertension may relate, among other factors, to a lower activity of this enzyme system, which results in an increased vascular tone. Supported in part by grants-in-aid from the American Heart Association (84-765) and the American Heart Association New Jersey Affiliate (83-23) and a grant from the National Heart, Lung, and Blood Institute (R23HL29221). Dr. Kuriyama is a recipient of a research fellowship from the National Kidney Foundation of New York-New Jersey.Address for reprints: Abraham Aviv, M.D., Hypertension Research Unit, Room F-578 Medical Science Building, University of Medicine and Dentistry of New Jersey, 185 South Orange Avenue, Newark, NJ 07103.Received October 6, 1986; accepted April 22, 1987.(for a review see References 1-3) and obesity 4 " 6 of abnormalities in erythrocyte Na + and the Na + ,K + -ATPase system. Such studies have usually examined a relatively small number of subjects. Furthermore, by comparing hypertensive with normotensive or obese with nonobese subjects, these studies have treated the BMI and blood pressure level as discontinuous entities rather than as variables with continuous distributions. In the present report we describe the relationships among the erythrocyte ghost (EG) Na + ,K + -ATPase, RBC Na + , BMI, blood pressure, as well as other related parameters in a large group of men and women who participated in this investigation without being categorized a pr...