Effects of Magnesium and Nifedipine Infusions on Insulin Action, Substrate Oxidation, and Blood Pressure in Aged Hypertensive Patients

Eight aged (70.1 ± 2.1 years), nonobese (body mass index [BMI] = 26.3 ± 0.4), hypertensive (systolic blood pressure [SBP] = 184 ± 4.1; diastolic blood pressure [DBP] = 105 ± 2.4 mm Hg) subjects underwent a euglycemic hyperinsulinemic (7.1 pmol/ kg x min during 240 min) glucose clamp with four different infusions: a) 0.9% NaCl; b) 1.0 μg/kg x min nifedipine; c) 100 μιηοΐ/min magnesium; d) 1.0 μg/kg X min nifedipine plus 100 μιηοΐ/min magnesium. All tests were performed in random order. Simultaneous D-3-H glucose infusion and indirect calorimetry allowed us to determine glucose turnover parameters and substrate oxidation. Insulin infusion per se stimulated erythrocyte magnesium (1.83 ± 0.04 ν 1.98 ± 0.03 mmol/L, Ρ < .03) and calcium (4.7 ± 0.3 ν 6.2 ± 0.4 μπιοΙ/L, Ρ < .02) accumulation, and enhanced total body glucose disposal oxidative and nonoxidative glucose metabolisms. Infusion of insulin and nifedipine ν insulin alone reduced insulin-mediated increase in intracellular calcium (5.4 ± 0.3 ν 6.2 ± 0.4 μπιοΙ/L, Ρ < .02), but potentiated the insulin effect upon nonoxidative glucose (15.4 ± 0.4 ν 11.1 ± 0.3 μιηοΐ/kg lean body mass [LBM] x min, Ρ < .03) metabolism. By contrast, insulin plus magnesium infusion ν insulin alone increased the insulin-mediated rise in intracellular magnesium (2.15 ± 0.04 ν 1.98 ± 0.03 mmol/L, Ρ < .003) and potentiated the effect of insulin upon oxidative (26.2 ± 0.5 ν 20.1 ± 0.3 μιηοΐ/kg LBM x min, Ρ < .04) glucose metabolism. Infusion of insulin, magnesium, and nifedipine significantly potentiated oxidative (27.1 ± 0.3 ν 20.1 ± 0.3 μπιοΐ/kg LBM X min, Ρ < .01) and nonoxidative (20.2 ± 0.4 ν 11.1 ± 0.3 μπιοΐ/kg LBM x min, Ρ < .02) glucose metabolisms and lowered arterial blood pressure. At the end of insulin infusion, the changes in intracellular magnesium/ calcium ratio content correlated with the changes in substrate oxidation and arterial blood pressure. In conclusion, total intracellular magnesium/ calcium ratio content seems to play a key role in the modulation of insulin action and arterial blood pressure in aged hypertensive patients. Am J Hypertens 1993;6:920-926