Insulin-producing cells in the gut of freshwater bivalve molluscs Anodonta cygnea and Unio pictorum and the role of insulin in the regulation of their carbohydrate metabolism.

Abstract In the epithelium of the small intestine of freshwater bivalve molluscs Anodonta cygnea and Unio pictorum cells were found which are reactive with antibodies to mammalian insulin. The apical parts of the cells are extended to the intestinal lumen. The nuclei are located in the basal parts resting on the basement membrane. The cytoplasm contains fine aldehyde fuchsin (AF)-positive and periodic acid-Schiff (PAS)-positive granules. The height of the immunostained cells is 10 to 15 times their width. Glucose, insulin, and anti-insulin serum (AIS) were injected into the molluscs and the changes in glycaemia, muscle glycogen content, and muscle glycogen synthetase activity induced by these substances were observed. Immunoreactive insulin (IRI) content in haemolymph and in gut extracts was assayed in experiments on the same animals; the quantity of AF-positive cells was calculated in the region of the gut where immunostained cells were present. The action of insulin on glucose uptake by slices of mollusc muscles was also studied. The results of the experiments demonstrate that insulin or some substance closely related to insulin is actually produced in the small intestine of the molluscs studied and makes a direct contribution to the regulation of carbohydrate metabolism in these animals.