Opioid peptides in rat islets of Langerhans. Immunoreactive met- and leu-enkephalins and BAM-22P.

Previous studies have shown that met- and leu-enkephalins are present in extracts of whole pancreas obtained from guinea pigs and human cadavers. The present studies demonstrate that immunoreactive methionine (met)- and leucine (leu)-enkephalins present in rat pancreas are localized in islets of Langerhans. Immunohistochemical staining of fixed, whole pancreas indicated that only islet endocrine cells were heavily stained when any of four different met- and leu-enkephalin-directed antisera or an anti-BAM-22P (bovine adrenal medulla docosapeptide) antiserum was used. The peptides were characterized by a combination of gel-filtration chromatography, high-performance liquid chromatography (HPLC), and specific radioimmunoassay. Free met-enkephalin content in extracts of rat islets was 90-fold enriched over content in extracts of whole pancreas (1.72 +/- 0.35 versus 0.019 +/- 0.007 pmol/mg protein). Treatment with trypsin and carboxy-peptidase-B of high-molecular-weight peptides extracted from pancreas or islets resulted in release of additional met-enkephalin immunoreactivity, which was 39-fold enriched in islets compared with pancreas (5.90 +/- 0.58 and 0.153 +/- 0.032 pmol/mg protein, respectively). Total islet content (per milligram protein) of met-enkephalin-containing peptides was similar to that reported elsewhere for bovine hypothalamus. The immunohistochemical data as well as the enrichment of extractable enkephalins in islets compared with whole pancreas indicate that essentially all the met-enkephalin present in pancreas is localized in islets, while the presence of BAM-22P immunoreactivity in islets is consistent with biosynthesis of enkephalins in islet cells via a preprohormone, such as that described in the bovine adrenal medulla and rat brain.

Role of the hypophysis in regulation of pancreatic and gastric somatostatin.

Since hypophysectomy and GH deficiency are associated with decreases in hypothalamic content and release of SRIF, it was of interest to determine whether these hormonal alterations also affect peripheral tissue levels of SRIF. Hypophysectomized (hypox) rats were studied at various times after surgery and compared with age-matched controls. Pancreatic, gastric, and hypothalamic SRIF levels were measured by RIA and expressed as nanograms per mg protein or nanograms per organ. Decreased levels of hypothalamic SRIF were observed in hypox animals at all time periods after surgery. In contrast, pancreatic SRIF concentrations increased within 1 week of hypophysectomy, and the tissue content increased as much as 3-fold after 20 weeks. Measurement of the SRIF content of isolated rat islets of Langerhans revealed a 67% increased content/islet in hypox rats compared with controls. The gastric SRIF concentration was not changed early, but subsequently, the total organ content was significantly decreased compared with that in controls. The changes in stomach and pancreas SRIF contents became more marked with duration of pituitary deficiency. Studies in genetically dwarfed Snell mice, lacking primarily GH but also other anterior pituitary hormones, were similar to the findings noted in hypox rats; the SRIF concentration was significantly increased in the pancreas and decreased in the stomach and hypothalamus. It is probable that deficiencies in other hormones as well as GH are involved in producing the changes in pancreatic SRIF in hypox and dwarfed animals. This contention was supported in that replacement of T3 (5 micrograms/kg . day) reduced pancreatic SRIF concentration by 30%, while GH plus T3 produced a significantly greater (60%) decrease in pancreatic SRIF in hypox rats.