The cellular response of saurian pancreatic islets to chronic insulin administration

Abstract

Moderately degranulated pancreatic A cells were observed by light, phase, and electron microscopy after chronic insulin administration in the lizard Anolis carolinensis. The reduced granularity of A cells is presumably due to glucagon release in response to insulin-induced hypoglycemia. Secretion of glucagon is compatible with the observed reduction in glycogen content of the hepatocytes. The ultrastructural observations made during the periods of maximal A cell degranulation suggest a new interpretation of glucagon secretion. The salient feature of the proposed secretory mechanism is intracytoplasmic release of glucagon. Other ultrastructural changes appear to be associated with the secretory process. The two most prominent changes were: pleomorphic A granule profiles often exhibiting knoblike protuberances, and short, basal infoldings of the A cell plasma membrane abutting the capillary space. The role these changes might have in the release of glucagon is discussed. A, B, and D cells were identified by light, phase, and electron microscopy. With the light microscope, A cells were stained by phosphotungstic acid-hematoxylin, and Ponceau-acid fuchsin. B cells were aldehyde fuchsin positive. D cells were argyrophilic. With the phase microscope, A cells contained large, dense secretory granules; B cells had the smallest secretory granules of the three major cell types; and D cells had large granules which were less dense than A granules. With the electron microscope, A granules tended to be oval in profile, were electron opaque with a predominantly homogeneous matrix, and had a long axis of 700 to 800 nm. B granules were round to oval in profile, had a moderately electron opaque matrix with a central dense core, and were approximately 450 nm in diameter. D granules were large, approximately 650 nm in diameter, round in profile and moderately electron opaque. The matrix of D granules was separated from the limiting membrane of the granule by a narrow, electron lucent space. Using the above criteria, the islet cell population contained approximately 50% A cells, about 40% B cells, and 10–15% D cells.