Serum Ionized Magnesium: Relation to Blood Pressure and Racial Factors

To study potential ionic factors predisposing to vascular disease in hypertension, particularly among black subjects, we used a recently developed combined magnesium and calcium specific, ion selective electrode apparatus to measure extracellular ionized calcium (Ca-ion), ionized magnesium (Mg-ion), and Ca-ion/Mg-ion ratios in the serum of fasting, nonmedicated white and black normotensive (n = 61) and hypertensive (n = 23) subjects, studied consecutively in a tertiary referral center. Both race and blood pressure status had independent effects on the distribution of Mg-ion values. Although Mg-ion levels for the group as a whole were lower in hypertensive versus in normotensive subjects (0.571 ± 0.012 v 0.601 ± 0.005 mmol/L; P< .01), this was only true of white subjects (0.579 ± 0.021 v 0.620 ± 0.006 mmol/L; P = .0095). The lack of a significant difference in Mg-ion levels between black hypertensive versus normotensive subjects (0.553 ± 0.012 v 0.577 ± 0.007 mmol/L, P = NS) was attributable to the significantly lower Mg-ion levels present in normotensive blacks compared to those in normotensive white subjects (0.577 ± 0.007 v 0.620 ± 0.006 mmol/L, P = .0001). Resultant Ca-ion/Mg-ion ratios were elevated in all black subjects and in white hypertensive subjects. These data support the presence among hypertensives and among black subjects (independently of blood pressure) of a consistent depletion of circulating magnesium and of an imbalance of calcium and magnesium that may potentiate vascular disease among these subjects.