Cellular ionic effects of insulin in normal human erythrocytes: a nuclear magnetic resonance study

Summary Elevated erythrocyte cytosolic free calcium, and suppressed free magnesium and pH values are associated with the hyperinsulinaemia and insulin resistance of hypertension, obesity, and Type 2 (non-insulin-dependent) diabetes mellitus. To determine the role of insulin in this process, we utilized ¹⁹F- and ³¹P-nuclear magnetic resonance spectroscopy to study the cellular ionic effects of insulin in vitro on normal human erythrocytes. Insulin elevated cytosolic free calcium levels in a dose- and time-dependent manner. The effect began at 10 U/ml, peaked at 200 U/ml, and continued at both the 500 U/ml and 1000 U/ml doses. At 200 U/ml, free calcium levels rose from 24.6±2.5 nmol/l to a peak value at 120 min of 66.4±11 nmol/l (p<0.05 vs basal), levels remaining elevated throughout the incubation (45.7±5.6 nmol/l at 60 min, and 47.9±9.1 nmol/l at 180 min, p<0.05 vs basal, respectively). Similarly, insulin also increased intracellular free magnesium at all time points (basal: 177± 11 mol/l; 60 min: 209±19 mol/l; 120 min: 206±22 mol/l; and 180 min: 202±12 mol/l; p<0.05 vs basal at all times). No insulin-induced changes in pH were observed. We conclude (i) that insulin in physiological concentrations may participate in regulating divalent cations in the mature human erythrocyte, (ii) that insulin per se cannot account for the previously described cellular ionic lesions of hypertension and diabetes, and (iii) that future clinical studies of cell ion metabolism should be conducted in the fasting state, be controlled for ambient circulating insulin levels, or both.