Binding properties of the insulin receptor of human placenta under different conditions of incubation.

The effect of incubation temperature and media on insulin receptor has been investigated employing membranes (4,000 x g pellet; particulate receptor) and solubilized material (solubilized receptor) from normal human placenta. Particulate receptor concentration (Ro) and affinity (Ke) were seen to be affected by buffer employed in binding experiments, both at 24 degrees C as well as at 4 degrees C; solubilized receptor affinity, on the other hand, is affected by buffer at 24 degrees C but not at 4 degrees C and receptor concentration does not vary at either temperature. Solubilization induced an increase in receptor affinity or in receptor concentration depending upon the buffer used in binding experiments. Insulin binding increases at the lowest incubation temperature; this phenomenon is due entirely to the greater receptor affinity on both particulate and solubilized receptor. From these data it appears that buffers induce variations in binding properties of the human placenta insulin receptor, thus, at ths present state of our knowledge, no definite conclusions can be drawn on the effect of procedures, such as solubilization, on human placenta insulin receptor affinity and/or concentration.

[Evaluation of intestinal absorption of calcium by means of double-isotope methods and oral administration of the tracer]. / Valutazione dell'assorbimento intestinale del calcio me diante i metodi del doppio isotopo e la somministrazione orale del tracciante.

The percent intestinal absorption of calcium was measured in normal volunteers and in patients with idiopathic hypercalciuria employing the deconvolution method, the ratio of the two administered isotopes at equilibrium and the percent of dose present in plasma 2 hr after oral administration of the tracer. Comparison of results obtained showed that the technique based on the ratio between the two radioisotopes overestimates intestinal absorption by about 9% with respect to values calculated with the deconvolution method, but gives results comparable to those determined by oral administration of the isotope. The percent dose of the tracer 2 h after i.v. administration is closely correlated with the size of the miscible calcium pool. A less significant correlation exists between the size of the pool and percent of the dose 2 h after oral administration.

Insulin antibodies prevent insulin-receptor interactions.

The study was carried out to investigate whether insulin bound to antibody is able to bind the insulin receptor of target tissues. Three specific rabbit anti-insulin sera as well as sera from eight diabetic patients with insulin antibodies were incubated, free of insulin, with labelled insulin for 48 h at 4 degrees C; following incubation labelled insulin was employed in binding experiments on monocytes, erythrocytes and placenta membranes. Using rabbit sera, receptor binding was absent when insulin was totally combined with antibody, and appeared in increasing amounts as the percentage of free insulin increased to reach a maximum when no insulin was combined with antibody. The same experiment using sera from diabetic patients showed a close negative relationship (r = 0.95) between the amount of insulin bound to the antibody and the amount bound to receptors. The influence of the insulin-antibody complex on the insulin receptor interaction was evaluated by exposing the insulin-antibody complex to the receptor in pH, temperature and competition-inhibition curve experiments. The complex had no effect on receptor affinity or on the pH and temperature relationship influence with insulin-receptor interaction. The findings suggest that insulin resistance in the presence of insulin antibodies is due only to an alteration occurring before the interaction of insulin with its receptor, and demonstrate that the insulin-antibody complex does not influence the insulin receptor interaction.

Response of insulin receptors to oral glucose in normal subjects.

Insulin binding to circulating monocytes was studied in ten male volunteers before and 1, 2, and 5 hours after the oral intake of 100 g of glucose. Results indicate an increase in the specific cell binding fraction with a change in receptor affinity 5 hours after glucose. Since the same changes appear 3 hours after food intake they are probably not directly induced by insulinemia.