[Effect of long-term thyroxine administration on the endocrine epithelium of rat pancreas].

The effect of prolonged thyroxine administration (0.001 mg/g BW) on pancreatic islets has been studied on 64 Wistar male rats by means of radioautographic, morphometric and electron microscopic methods. The phase response in the amount of the DNA-synthesising cells of the middle class islets has been revealed: the initial increase (5 days) is followed by a decrease (30 days) and then by a return to the control levels (60 days). The level of metabolism in sulphur-containing proteins has decreased in both A- and B-cells. After 30 days of the experiment, B/A cell volume ratio has been shown to increase. Electron microscopic studies have revealed ultrastructural reorganization of B-cells from "resting" B-cells into "dark" B-cells at increased excretion of secretory material.