The Diurnal Rhythm in Natriuretic Peptide Levels

Obese and African-American individuals are at greater risk for cardiovascular morbidity and mortality than lean and white individuals. One of the key reasons for this health disparity is a higher risk of hypertension among obese and African-American individuals. The reasons for why these disparities develop are not well understood.

Natriuretic peptides are hormones produced by the heart and have a wide range of favorable cardiovascular effects such as natriuresis (sodium excretion), vasodilation, and direct inhibition of the renin-angiotensin-aldosterone system. Human studies showed the existence of 24-hour (diurnal) variations in the circulating natriuretic peptide levels.

Prior work from the investigators and others demonstrated that individuals with genetically-determined lower circulating natriuretic peptides levels have higher blood pressure and greater risk of hypertension. Further, the investigators have shown that obesity and African-American race are associated with lower natriuretic peptide levels, suggesting that relatively low natriuretic peptide levels may be a biologic determinant contributing to health disparities.

Obese and African-American individuals have a greater prevalence of nocturnal hypertension [nighttime blood pressure >120/80 mmHg], which is an independent risk factor for cardiovascular events. The underlying reasons for 24-hour variations in blood pressure are unknown.

The investigators hypothesize that loss of the natural 24-hour rhythm of natriuretic peptide levels plays a role in the development of nocturnal hypertension in obese and African-American individuals. The aims of this study are:

• to examine whether there is a presence of a 24-hour rhythm in natriuretic peptide levels among normotensive obese and African-American individuals and whether there is a difference in the rhythmicity of natriuretic peptide levels between obese and lean as well as in African-Americans and whites;
• to examine whether there is an existence of a relationship between 24-hour variability of natriuretic peptide levels and 24-hour patterns of blood pressure and whether this relationship of rhythmicity of natriuretic peptide levels and nocturnal blood pressure differed in obese and lean individuals and by race.

Inclusion criteria

• Age 18 to 40 years
• Blood pressure less than 140/90
• Body Mass Index between 18.5 to 25 kg/m2 (lean) or 30 to 45 kg/m2 (obese)
• Self identified African-American or white individuals
• Willingness to comply with the study diet
• Provide informed consent

Exclusion criteria

• History of hypertension
• History of cardiovascular, renal, or liver disease
• Diabetes or use of glucose-lowering medications
• Use of vasoactive or diuretic medications
• Atrial fibrillation
• Anemia (Hematocrit <41%)
• Abnormal serum sodium or potassium
• Abnormal liver function tests (>3x upper limit of normal)
• Current smokers
• Regular users of non-steroid anti-inflammatory medications