Blood volume expansion among blacks: an hypothesis

Biohistory of slavery and blood pressure differences in blacks today. A hypothesis

Genetic factors are known to play an important role in the variations in blood pressure levels. However, genetic factors that explain the higher average blood pressure levels of western hemisphere blacks when compared with African blacks have not been seriously considered. Because the genetic makeup of a population is largely determined by biological and ecological forces in the past, an examination of the biohistory of blacks, specifically the slavery era, was conducted. An overview of the salient findings of that investigation is included in this article. The published historical evidence on the transatlantic slave trade and New World slavery (from the 16th century to the 19th century) reveals that conditions existed for "natural selection," and therefore, genetic changes were virtually inevitable in the slave populations. During this period of history, mortality was extremely high, and fertility (or reproductive success) was so low among the survivors that most plantation societies in the western hemisphere depended on a constant importation of captives (over 12 million) from Africa for the viability of the plantation communities. Because the major causes of death were salt-depletive diseases such as diarrhea, fevers, and vomiting, it is argued that individuals with an enhanced genetic-based ability to conserve salt had a distinct survival advantage over others and were, therefore, more likely to bequeath their genotype to subsequent generations of Western hemisphere blacks. Thus, it is predicted that blacks in the Americas have a greater frequency of individuals with an enhanced genetic-based ability to conserve salt than African blacks.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethnic differences in physiological responses associated with the Valsalva maneuver

Ethnic differences in cardiovascular indices at rest and across the Valsalva maneuver (VM) were studied in 30 Asian, 86 Caucasian, and 12 Black healthy men and women aged 30 to 55 years. Subjects simulated the VM by blowing into an aneroid pressure-gauge meter to 40 mmHg for 10 seconds. Resting systolic blood pressure (SBP) was significantly higher in Blacks than in Asians or Caucasians. During the strain phase of the VM, the fall in SBP from baseline was significantly greater in Asians and Blacks than in Caucasians. Further, Asians had the greatest overshoot of SBP above baseline. Although resting heart rate was significantly higher in Asians compared to Caucasians, there were no differences in heart rate responses to the VM. Reduction in carotid artery blood flow velocity during strain was significantly less in Blacks. We conclude that there are ethnic differences in cardiovascular indices at rest and across the VM.

The epidemiology of resting heart rate in a national sample of men and women: associations with hypertension, coronary heart disease, blood pressure, and other cardiovascular risk factors

Few reports have been published on the epidemiology of heart rate in general populations including women and blacks. In the first cycle of the National Health Examination Survey, resting heart rate was significantly higher in women than in men at each age in both races. Age and race were inconsistently related to heart rate, black men having slightly lower rates than white men at ages 18 to 34. The product of heart rate and mean blood pressure was consistently highest in black women. Heart rate was associated with obesity, waist girth, and waist-to-hip girth ratio in men under 55 years and with blood pressure and post-load serum glucose in all groups. Sex differences at each age were larger (6 to 14 beats/min) when adjusted for multiple variables. Multivariate linear regression analysis confirmed the independent associations of heart rate with blood pressure and serum glucose. After controlling for other risk factors, prevalent hypertensive subjects had higher heart rates than normotensive individuals in all groups. Further research is needed to elucidate mechanisms of the associations of resting heart rate with sex, risk factors, coronary heart disease, and mortality.

[3H]ouabain binding of red blood cells in whites and blacks

In a previous study, we demonstrated that the red blood cell Na+ concentration and Na+,K+-ATPase activity are sex-dependent and race-dependent: a higher intracellular Na+ concentration in blacks and men was associated with a lower Na+,K+-ATPase activity. To examine whether the low Na+,K+-ATPase activity is due to a decreased number of enzyme units, altered structure of the enzyme, or the presence of an endogenous digoxinlike substance, ouabain binding studies were performed on the same subject group. The measurements included displacement of [3H]ouabain from its specific binding sites by unlabeled ouabain or potassium. The results demonstrate that groups with lower enzyme activity manifest lower numbers of total specific ouabain binding sites on the surface of the red blood cell (mean +/- SD: blacks, 654 +/- 24.4; whites, 806 +/- 18.3; women, 806 +/- 26.9; men, 728 +/- 21.2). Other kinetic parameters of [3H]ouabain displacement appear to be the same among the groups. The respective red blood cell Na+ and K+ concentrations were negatively and positively correlated with the number of ouabain binding sites. Our findings suggest that the lower activity of red blood cell Na+,K+-ATPase in blacks and men is a function of a lower number of Na+-K+ pump units. The results also indicate that sex and race should be considered when red blood cell ouabain binding is examined.

Race and sex differences in erythrocyte Na+, K+, and Na+-K+-adenosine triphosphatase

Several reports indicate that erythrocytes (RBCs) from blacks and men have higher sodium concentrations than those from whites and women. One possible mechanism to explain this finding is a difference in the activity of Na+-K+-ATPase. To explore this possibility, we have studied the Na+ and K+ kinetics of RBC Na+-K+-ATPase and RBC Na+ and K+ concentrations in 37 normotensive blacks and whites, both males and females. The maximal initial reaction velocity (Vmax) values for RBC Na+-K+-ATPase were lower in blacks and men as compared with whites and women. Higher RBC Na+ levels were observed in blacks and males vs. whites and females. Significant inverse correlations were noted between the Na+-K+-ATPase activity and RBC Na+ concentrations. These findings indicate that cellular Na+ homeostasis is different in blacks and men as compared with whites and women. Since higher RBC Na+ concentrations have also been observed in patients with essential hypertension as compared with normotensive subjects, the higher intracellular Na+ concentrations in blacks and men may contribute to the greater predisposition of these groups to essential hypertension.