Direct effect of insulin on 45calcium uptake in human erythrocytes

Abstract. Objective. High blood pressure is prevalent in obesity and non-insulin dependent diabetes mellitus: both conditions, with insulin resistance and essential hypertension, have been associated with increasing intra-erythrocytic levels of calcium ions. We tested the hypothesis of whether insulin itself might be responsible for the abnormal red cell cytosolic free calcium. Design. The ionic effects of insulin were studied on the kinetics of 45calcium uptake in vitro in normal human erythrocytes. Setting. The study was performed in the outpatient clinic of a central hospital. Subjects. Sixteen healthy, normotensive individuals with normal body mass index were recruited for the study. Main outcome measures. Blood from eight individuals was used for time-dependent studies of ⁴⁵calcium uptake in erythrocytes and blood from another eight individuals was used for dose-dependent studies of insulin effect. Results. The rate of ⁴⁵calcium influx in red blood cells has two components, a fast component (0–10 min), which measures the initial rate of ⁴⁵calcium influx, and a slow component (10–60 min) probably reflecting a relatively large backflux of calcium (calcium efflux), which accordingly determines an apparent low rate of ⁴⁵calcium influx between 10–60 min. The uptake was linear with time between 10–120 min regardless of insulin being present or not. Insulin at a concentration of 120 mUL^-1 significantly decreased the ⁴⁵calcium uptake in a timedependent fashion between 10–120 min. The uptake was 508 (±59) at 60 min in the presence of insulin vs. a control value of 529 (±59) pmol mL red blood cells^-1 (P < 0.001). The corresponding figures at 120 min were 742 (±log) and 767 (±127), respectively (P = 0.02). Inconsistent results were obtained on ⁴⁵calcium uptake at 60 min by varying insulin concentrations from 40–640 mUL^-1 and a dual effect of insulin on ⁴⁵calcium uptake could not be excluded, one at a fairly low concentration of insulin (40–120 mUL^-1) and another at a high concentration (160–640 mUL^-1). Conclusion. The data indicate a direct role of insulin in the transport process of calcium into normal human erythrocytes.