Intracellular localization of salivary peptide P-C-like immunoreactivity in the human pancreatic B-cells

Abstract. In order to clarify the intracellular localization of salivary peptide P-C-like immunoreactivity in human pancreatic B-cells, an immunohistochemical study at electron microscopic levels was carried out by the protein A-gold technique using antisera against insulin and salivary peptide P-C. Both salivary peptide P-C-like immunoreactivity and insulin-like immunoreactivity were present only in the insulin secretory granules of the pancreatic B-cells. However, the former immunoreactivity was lacking in many insulin secretory granules of foetal pancreatic B-cells while the latter immunoreactivity was seen in all insulin secretory granules. Salivary peptide P-C-like immunoreactivity was not found in the other kinds of cells in the islets. In a previous immunohistochemical study at light microscopic level, salivary peptide P-C-like immunoreactivity appeared in a few pancreatic B-cells at about the 16th week of gestation, in an increasing number during gestation, and was seen in all pancreatic B-cells a few months after birth. The present finding together with the above results suggest that absence of salivary peptide P-C-like immunoreactivity in some foetal pancreatic B-cells may be due to the underdevelopment of salivary peptide P-C-like immunoreactivity in each insulin secretory granule. From the examination of cross-reactivity of antisera against salivary peptide P-C to other kinds of salivary peptides and salivary Protein C, and from the results of an indirect immunofluorescence technique using three kinds of antisera including antisera against salivary peptide P-C, salivary peptide P-B and salivary Protein C, it was thought that salivary peptide P-C-like immunoreactivity in human pancreatic B-cells belongs neither to salivary Protein C nor to salivary peptide P-B nor to salivary peptide P-E, but either to salivary peptide P-C itself or to an unknown substance which has common antigenic determinants with salivary peptide P-C, salivary peptide P-B and salivary Protein C. Salivary peptide P-C-like immunoreactivity was not found in the pancreatic B-cells of other mammals. Thus, although a new substance other than insulin is present in the insulin secretory granules of the human pancreatic B-cells, its pathophysiological function remains unclear.