Diurnal variation in insulin binding to human monocytes

We studied insulin binding to monocytes isolated from dieting and fasting young healthy persons during a 24-h period. In the persons who ate their usual food, we found a characteristic diurnal variation in the cellular insulin binding. Daytime insulin binding was low with a minimum in the afternoon; binding increased during the evening, reaching a maximum in the early morning. This variation seemed to be diet related, as total fasting abolished the circadian rhythm. Changes in cellular insulin binding were due to fluctuations in the binding affinity rather than alterations in receptor concentration. The circadian variation in insulin binding was grossly parallel to the well known variations in insulin sensitivity and glucose tolerance during the day.

Insulin receptors on monocytes of young healthy persons correlated with glucose tolerance and insulin sensitivity

We have studied in normal man the interrelationships between insulin binding to monocytes, glucose tolerance and insulin sensitivity. In 25 young healthy persons we found a significant positive correlation between insulin binding and glucose disappearance rate both after glucose (R = 0.68, p less than 0.001) and insulin (R = 0.49, p less than 0.02) given intravenously.

The monocyte as a model for the study of insulin receptors in man

We have characterized the cellular composition of preparations isolated from peripheral blood by Ficoll-Isopaque gradient centrifugation. 125I-insulin binding to every cell type was measured. A highly significantly positive correlation between specific cell binding fraction and the monocyte concentration of the heterogeneous cell suspension was demonstrated. Depletion of monocytes reduced the insulin binding approximately 80%, which confirms previous findings by other investigators. The granulocytes possessed the second highest binding ability, but only one fourteenth of that of monocytes. Compared to the lymphocyte the monocyte had about 25 times greater insulin binding. Also thrombocytes bound insulin and contamination with these meant that their contribution to the total specific cell binding was not negligible. A reduction in these contaminants is essential. We found that insulin binding to erythrocytes was insignificant. A method of calculating the specific insulin binding to monocytes alone is introduced. The monocyte-insulin-receptor possesses specificity. Only an insignificant degradation of receptor bound insulin could be shown. Evidence of negative cooperativity between receptors was found. Consequently monocytes are considered a useful model for insulin receptor studies in man.

Decreased binding of insulin to its receptor in patients with congenital generalized lipodystrophy

Patients with congenital generalized lipodystrophy are extremely insulin resistant. To ascertain whether this resistance is due to an insulin receptor defect, we tested four young patients with congenital generalized lipodystrophy and seven healthy persons of comparable age for the binding of 125I insulin to mononuclear leukocytes isolated from peripheral blood. Mononuclear leukocytes from patients with congenital generalized lipodystrophy bound significantly less insulin than cells from normal subjects (P less than 0.01). When patients with lipodystrophy fasted for 60 hours, the insulin binding increased. Altered insulin receptors may be responsible for the pronounced insulin resistance and the decreased synthesis of triglycerides in congenital generalized lipodystrophy.

A study of insulin receptors in human mononuclear leucocytes

Insulin binding sites were demonstrated in human mononuclear leucocytes by use of a technique which includes isolation of mononuclear leucocytes from defibrinated blood and separation of cell bound and free [125I]insulin with silicone oil. The binding was time and temperature dependent. At 15 degrees C equilibrium was reached after 90 min and a plateau maintained for at least 50 min. Incubations were carried out at 4 degrees C, 15 degrees C and 37 degrees C. Maximal binding was obtained at 15 degrees C. The optimum pH for insulin receptor interaction occurred at about 8. [125I]insulin binding to mononuclear leucocytes was demonstrated to be a linear function of cell number concentration over a range of 17-70 X 10(6) X ml-1. The binding was a displaceable function of native insulin concentration. In a group of 21 young healthy persons with normal body weight we found a mean specific cell binding fraction of 1.92 +/- 0.58 (s) X 10(-2). Analysis of the equilibrium between insulin and its receptor revealed an apparent heterogeneity of insulin receptors.

The insulin receptor in normal and obese persons

Using [125I]insulin at 172 pmol/1 (1 ng/ml) the binding of insulin to mononuclear leucocytes isolated from peripheral blood was studied. Our present study comprised 21 healthy subjects (22-33 years old, 90-110% of ideal weight) and a comparable group of 22 obese subjects (20-37 years old, minimum 150% of ideal weight). A significant difference in insulin binding was found between the two groups, the mean specific insulin binding fraction in normals being 1.92 +/- 0.58 (s) X 10(-2) and that for the obese 1.19 +/- 0.41 (s) X 10(-2) (P less than 0.01). No correlation was found between body weight and the number of insulin receptors in the obese subjects. However, the number of insulin receptors was negatively correlated to fat cell size (P less than 0.05). Insulin receptors in subjects were also negatively correlated to fasting plasma insulin (P less than 0.05). Insulin receptors were studied in 11 obese subjects before and after 10 days of fasting. A significant increase in the number of insulin receptors was observed with a simultaneous decrease in plasma insulin to normal values. The results indicate that obesity complicated by hyperinsulinism is associated with a decrease in the number of insulin receptors compared with the normal. This finding may in part explain the decreased insulin sensitivity of the hyperinsulinaemic obese.