[Effect of hyperthyroidization on (pro)insulin biosynthesis and secretion]

Insulin production and secretion in simulated rat hyperthyrosis induced by L-thyroxin, injected intraperitoneally during 8 to 30 days, were studied on isolated Langerhans' islets, using collagenase fermentation. Insulin secretion in vitro was determined by radioimmunoassay, its biosynthesis being evaluated according to 3H-leucine incorporation into de novo formed islet proteins with insulin immunoreactivity. The dissociated effect of thyroxin on the secretory response of beta-cells and their hormone production was revealed. In hyperthyroidized animals a decrease in the islet insulin secretion was seen in the presence of a low glucose content in the incubating medium (5 mM), (pro-)insulin biosynthesis remaining unchanged. (Pro-)insulin concentration increased comparatively to the control following 8-day thyroxin injection under condition of the islet incubation with 15 mM of glucose. Insulin secretion returns to normal after augmentation of hexose content in the incubating medium up to 15 mM (hormone production being not inhibited), indicating the functional character of a decrease in beta-cell secretory response and a significant role in its genesis of the changed beta-cell sensitivity to glucose action, inducing insulin secretion.