DIRECT EFFECT OF INSULIN ON CA-45 UPTAKE IN HUMAN ERYTHROCYTES

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 intr a-erythrocytic levels of calcium ions. We tested the hypothesis of whe ther insulin itself might be responsible for the abnormal red cell cyt osolic free calcium. Design. The ionic effects of insulin were studied on the kinetics of (45)calcium uptake in vitro in normal human erythr ocytes. 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 outcom e measures. Blood from eight individuals was used for time-dependent s tudies of (45)calcium uptake in erythrocytes and blood from another ei ght individuals was used for dose-dependent studies of insulin effect. Results. The rate of (45)calcium influx in red blood cells has two co mponents, a fast component (0-10 min), which measures the initial rate of (45)calcium influx, and a slow component (10-60 min) probably refl ecting a relatively large backflux of calcium (calcium efflux), which accordingly determines an apparent low rate of (45)calcium influx betw een 10-60 min. The uptake was linear with time between 10-120 min rega rdless of insulin being present or not. Insulin at a concentration of 120 mU L(-1) significantly decreased the (45)calcium uptake in a time- dependent fashion between 10-120 min. The uptake was 508 (+/- 59) at 6 0 min in the presence of insulin vs. a control value of 529 (+/- 59) p mol mt red blood cells(-1) (P < 0.001). The corresponding figures at 1 20 min were 742 (+/-109) and 767 (+/-127), respectively (P = 0.02). in consistent results were obtained on (45)calcium uptake at 60 min by va rying insulin concentrations from 40-640 mU L(-1) and a dual effect of insulin on (45)calcium uptake could not be excluded, one at a fairly low concentration of insulin (40-120 mU L(-1)) and another at a high c oncentration (160-640 mU L(-1)). Conclusion, The data indicate a direc t role of insulin in the transport process of calcium into normal huma n erythrocytes.