The effect of hypophysectomy on pancreatic islet hormone and insulin-like growth factor I content and mRNA expression in rat

The growth arrest after hypophysectomy in rats is mainly due to growth hormone (GH) deficiency because replacement of GH or insulin-like growth factor (IGF) I, the mediator of GH action, leads to resumption of growth despite the lack of other pituitary hormones. Hypophysectomized (hypox) rats have, therefore, often been used to study metabolic consequences of GH deficiency and its effects on tissues concerned with growth. The present study was undertaken to assess the effects of hypophysectomy on the serum and pancreatic levels of the three major islet hormones insulin, glucagon, and somatostatin, as well as on IGF-I. Immunohistochemistry (IHC), in situ hybridization (ISH), radioimmunoassays (RIA), and Northern blot analysis were used to localize and quantify the hormones in the pancreas at the peptide and mRNA levels. IHC showed slightly decreased insulin levels in the cells of hypox compared with normal, age-matched rats whereas glucagon in cells and somatostatin in cells showed increase. IGF-I, which localized to cells, showed decrease. ISH detected a slightly higher expression of insulin mRNA and markedly stronger signals for glucagon and somatostatin mRNA in the islets of hypox rats. Serum glucose concentrations did not differ between the two groups although serum insulin and C-peptide were lower and serum glucagon was higher in the hypox animals. These changes were accompanied by a more than tenfold drop in serum IGF-I. The pancreatic insulin content per gram of tissue was not significantly different in hypox and normal rats. Pancreatic glucagon and somatostatin per gram of tissue were higher in the hypox animals. The pancreatic IGF-I content of hypox rats was significantly reduced. Northern blot analysis gave a 2.6-, 4.5-, and 2.2-fold increase in pancreatic insulin, glucagon, and somatostatin mRNA levels, respectively, in hypox rats, and a 2.3-fold decrease in IGF-I mRNA levels. Our results show that the fall of serum IGF-I after hypophysectomy is accompanied by a decrease in pancreatic IGF-I peptide and mRNA but by partly discordant changes in the serum concentrations of insulin and glucagon and the islet peptide and/or mRNA content of the three major islet hormones. It appears that GH deficiency resulting in a low IGF-I state affects translational efficiency of these hormones as well as their secretory responses. The maintenance of normoglycemia in the presence of reduced insulin and elevated glucagon serum levels, both of which would be expected to raise blood glucose, may result mainly from the enhanced insulin sensitivity, possibly due to GH deficiency and the subsequent decrease in IGF-I production.     

Cortisone-induced islet cell hyperplasia in hamsters

Pancreatic islet cell hyperplasia was studied in hamsters during one to eight weeks of cortisone treatment. Measurement of serum glucose and insulin; pancreatic insulin, glucagon, somatostatin, pancreatic polypeptide as well as islet tissue morphometry were performed. Serum glucose was highest at week 2, followed by mild to moderate hyperglycemia. Serum insulin was increasingly higher from week 1 to week 8. Pancreatic insulin was maximal at week 5 then declined through week 8 in the presence of beta cell neurosis in markedly hyperplastic islets. Pancreatic concentration of somatostatin and pancreatic polypeptide moderately increased more than the control levels; however, compared with the controls, glucagon was reduced by cortisone treatment. Effect of cortisone in the four types of islet cells is discussed, particularly on beta cell hyperplasia, which appears to be a response to decreased insulin binding to the target organs with no changes in receptor concentration.     

Genetic analysis of early endocrine pancreas formation in zebrafish

Endocrine pancreas of zebrafish consist of at least four different cell types that function similarly to mammalian pancreatic islet. No mutants specifically affecting formation of the endocrine pancreas have been identified during the previous large-scale mutagenesis screens in zebrafish due to invisibility of a pancreatic islet. We combined in situ hybridization method to visualize pancreatic islet with an ethyl-nitroso-urea mutagenesis screen to identify novel genes involved in pancreatic islet formation in zebrafish. We screened 900 genomes and identified 11 mutations belonging to nine different complementation groups. These mutants fall into three major phenotypic classes displaying severely reduced insulin expression, reduced insulin expression with abnormal islet morphology, or abnormal islet morphology with relatively normal number of insulin expressing cells. Seven of these mutants do not have any other visible phenotypes associated. These mutations affect different processes in pancreatic islet development. Additional analysis on glucagon and somatostatin cell specification revealed that somatostatin cells are specified at a separate domain from insulin cells whereas glucagon cells are specified adjacent to insulin cells. Furthermore, glucagon cells and somatostatin cells are always associated with insulin cells in mutants that have scattered insulin expression. These data indicate that there are separate mechanisms regulating endocrine cell migration, proliferation, and differentiation. Further study on these mutants will reveal important information on novel genes involved in pancreatic islet cell specification and morphogenesis.     

Tissue-specific regulation of angiotensinogen mRNA accumulation by dexamethasone

The regulation of angiotensinogen gene expression in response to adrenalectomy and dexamethasone treatment was examined in multiple rat tissues. Angiotensinogen mRNA as quantitated by slot blot hybridization utilizing an angiotensinogen cRNA probe was most abundant in the liver with levels in the brain, kidney, and adrenal of 50, 25, and 10%, respectively. No angiotensinogen mRNA was detected in testes or heart. Although no change in the quantity of angiotensinogen mRNA was found following adrenalectomy and maintenance on 0.9% saline, dexamethasone treatment of both normal and adrenalectomized rats resulted in a time-dependent and tissue-specific accumulation of angiotensinogen mRNA. In normal animals, the hepatic response to treatment was a 4.5-fold increase in angiotensinogen mRNA by 8 h which remained 2.4-fold above basal levels by 24 h. Angiotensinogen mRNA levels in the brains of normal rats treated with dexamethasone increased only 60% by 6 h and returned to basal levels by 24 h. In contrast to the increases seen in brain and liver, angiotensinogen mRNA derived from kidney did not significantly change following dexamethasone treatment. In adrenalectomized animals, the hepatic response to dexamethasone was similar to normal animals with a 3.7-fold increase by 6 h. The accumulation in brain was greater in these animals compared to normals and increased 3-fold by 8 h. Finally, dexamethasone did not significantly increase levels in the kidney. These results clearly demonstrate glucocorticoid regulation of angiotensinogen mRNA levels in liver and brain. In contrast, the kidney, an organ known to contain glucocorticoid receptors, does not respond with increased angiotensinogen mRNA levels following glucocorticoid stimulation. These studies provide the first evidence for tissue-specific differences in the control of angiotensinogen mRNA.     

Phe-met-arg-phe-amide (FMRF-NH2) inhibits insulin and somatostatin secretion and anti-FMRF-NH2 sera detects pancreatic polypeptide cells in the rat islet

FMRF-NH2-like immunoreactivity was localized in the pancreatic polypeptide containing cells of the rat islet. FMRF-NH2 was investigated with regard to its effect on insulin, somatostatin and glucagon secretion from the isolated perfused rat pancreas. FMRF-NH2 (1 microM) significantly inhibited glucose stimulated (300 mg/dl) insulin release (p less than 0.005) and somatostatin release (p less than 0.01) from the isolated perfused pancreas. FMRF-NH2 (1 and 10 microM) was without effect on glucagon secretion, either in low glucose (50 mg/dl), high glucose (300 mg/dl), or during arginine stimulation (5 mM). These findings indicate that these FMRF-NH2 antisera recognize a substance in the pancreatic polypeptide cells of the islet which may be capable of modulating islet beta and D cell activity.