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Direct effect of insulin on 45calcium uptake in human erythrocytes
L Pettersson1, G Frithz, G Ronquist
1Department of Clinical Chemistry, Central Hospital, Eskistuna, Sweden.
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 mU L-1 significantly decreased the 45calcium uptake in a time-dependent 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 (+/- 109) and 767 (+/- 127), respectively (P = 0.02). Inconsistent results were obtained on 45calcium uptake at 60 min by varying insulin concentrations from 40-640 mU L-1 and a dual effect of insulin on 45calcium uptake could not be excluded, one at a fairly low concentration of insulin (40-120 mU L-1) and another at a high concentration (160-640 mU L-1).
Conclusion:
The data indicate a direct role of insulin in the transport process of calcium into normal human erythrocytes.