Atrial natriuretic factor and exocrine pancreas: autoradiographic localization of binding sites and ultrastructural evidence for internalization of endogenous ANF.

Atrial natriuretic factor (ANF) binding sites in adult rat exocrine pancreas were studied by autoradiography using slide-mounted frozen tissue sections with mono-iodinated ANF (101-126) as the tracer. Radiolabel was displaced by unlabeled atrial peptide (IC50 = 2 X 10(-11) M). High specific labeling was found in pancreatic acini. The presence of endogenous ANF has also been demonstrated in the exocrine pancreas by immunocytochemistry on ultra-thin sections obtained by cryoultramicrotomy. ANF-like immunoreactivity was found in acinar and centro-acinar cells as well as cells of the intercalated duct. For these cells, immunostaining was observed at the plasma membrane level, in the cytoplasm and nucleus. In the cytoplasm, ANF-like immunoreactivity was observed in the cytoplasmic matrix, mitochondria, and zymogen granules. In the nucleus, ANF-like immunoreactivity was distributed in the vicinity of the heterochromatin region primarily in the euchromatin. It was also detected in the plasma membrane of microvilli of acinar and duct cells, and in the lumen of secretory ducts. In centro-acinar cells, the reaction product was also found sparsely at the nuclear envelope. No immunoreactivity was observed when anti-human ANF serum preincubated with rat ANF was used. No modifications were observed when this antiserum was preincubated with heterologous peptides (NPY, CRF, GRF, TRH, somatostatin). These data provide autoradiographic evidence of ANF binding sites, indicate the presence of this peptide in acinar and centro-acinar cells as well as cells of the intercalated duct, and immunocytochemical evidence for the internalization of endogenous ANF by exocrine pancreas.