Insulin binding and degradation studies on erythrocytes at different temperatures

Red blood cell insulin receptors in health and disease

CONTENTS

Structure and characteristics of erythrocyte insulin receptor. Red blood cell age and insulin receptors. Insulin receptors in human disease states. Obesity. Chronic renal failure. Acanthosis nigricans. Miscellaneous disease states. Insulin receptors in children. Insulin receptors in women during pregnancy. Insulin binding and other hormones. Comparison of biosynthetic insulin, pancreatic human insulin and porcine insulin binding to erythrocytes. Effect of exercise on insulin binding to red blood cells of normal human volunteers. Miscellaneous insulin binding studies. Insulin internalization and degradation. Insulin and erythrocyte metabolism. Summary and conclusion.

Insulin effects on human red blood cells

The effect of insulin on human red blood cells was investigated, both on intact cells and on isolated plasma membranes, testing the responsiveness of membrane-bound enzymes--such as (Na+-K+)-ATPase and 5'-nucleotidase--as well as the ouabain binding and ionic fluxes. It appears that insulin stimulates Na-pumping mechanisms increasing (Na+-K+)-ATPase activity through an enhanced availability of pumping sites, as can be inferred from the increased ouabain binding. The apparent unresponsiveness of fluorescence polarization parameters, following insulin treatment of isolated plasma membranes and intact cells, rules out--at present--an involvement of membrane lipid fluidity in the mechanism of action of insulin on human erythrocytes.

Regulation of insulin receptor activity of human erythrocyte membrane by prostaglandin E1

Incubation of human erythrocyte membrane with low concentration of prostaglandin E1 or prostacyclin increased the binding of 125I-labeled insulin to the membrane. The binding of the radioiodinated hormone was maximally stimulated at 3 nM prostaglandin E1 and the use of higher concentrations (above 8 nM) of the autacoid tended to reverse its own effect at lower concentrations. While prostaglandins A1, A2, B1, B2, D2, F1 alpha, F2 alpha or 6-keto-prostaglandin F1 alpha had no effect on the binding of insulin to the erythrocyte membrane, prostaglandin E2 at similar concentrations decreased the binding of the hormone. The effect of prostaglandin E1 on the increased binding of the insulin was found to be reversible and depended on the occupancy of the autacoid molecules on the membrane and showed positive cooperativity. Scatchard analysis of the binding of 125I-labeled insulin to the erythrocyte ghosts indicated that in the presence of the autacoid, the binding capacity of the insulin receptor increased 2-fold (from 207 to 424 fmol/mg protein) without any change in the ghosts affinity for the ligand (Kd 2.4 X 10(-9) versus 2.49 X 10(-9) M). As a consequence of increased binding of insulin to the erythrocyte membrane in the presence of prostaglandin E1 (3.0 nM), the optimal concentration of the peptide hormone for the maximal reduction of the membrane microviscosity decreased from approx. 1.6 to approx. 0.4 nM. Addition of prostaglandin E1 alone at the above concentration to the assay mixture had no effect on the membrane microviscosity.

Control of erythrocyte membrane microviscosity by insulin

The human erythrocyte membrane binds insulin through high-affinity, low-capacity binding sites (dissociation constant Kd1 2.45 X 10(-9)M; capacity n1 207 fmol/mg protein) and low-affinity, high-capacity binding sites (Kd2 0.63 X 10(-6) M; n2 37 pmol/mg protein). Treatment of the erythrocyte membrane or the intact cells with the physiological concentration of insulin, which is within the range of Kd value of the high-affinity sites, results in a significant reduction of the membrane microviscosity and the filtration time of the intact cells. Use of supraphysiological concentrations of the hormone reverses the effect of the lower concentration of insulin on the membrane microviscosity and the filtration time.