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Investigation of the effect of in vivo inhibition of dipeptidyl peptidase IV (DP IV) on glucose tolerance in Zucker rats and of DP IV expression in pancreatic beta cells

University of British Columbia

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are two hormones released from the gut post-prandially which act at the pancreatic β cell to potentiate glucose-induced insulin secretion. DP IV is the primary enzyme responsible for the cleavage and inactivation of the incretins and it is known that inhibition of the enzyme by the specific inhibitor Isoleucine-thiazolidide (Ile-thiazolidide), prevents degradation of GIP and GLP-1. It was hypothesised that inhibition of the enzyme would prolong the biological half-lives of the incretins, thereby enhancing insulin release post-prandially and improving glucose tolerance. In addition to the circulation, a site where a portion of this proteolysis occurs may be the β-cell, the incretin target tissue. DP IV has been previously localised to the pancreatic islets and specifically to the α-cells, and its presence has yet to be demonstrated in the β-cell. Using conscious, unrestrained Zucker rats (n=6) Ilethiazolidide was administered (20 μmol) simultaneously with an oral glucose challenge (OGTT) and resulted in rapid and sustained inhibition of DP IV activity in obese and lean animals. The integrated insulin response in both phenotypes was significantly different between the inhibitor-treated and control groups (obese, 1696.6 ± 80.1 vs. 725.5 ± 88.3 μU, respectively; lean, 2324.1 ± 177.4 vs. 1890.8 ± 67.9 μU, respectively). The integrated glucose response to an OGTT was reduced by 39% in inhibitor-treated obese animals versus 22% in treated lean rats. Pre-treatment of animals with Ile-thiazolidide, Pro-Ile-thiazolidide or Gly-Pro-Ile-thiazolidide prior to OGTT significantly inhibited DP IV activity as before and resulted in a more rapid insulin response compared to the previous protocol. Gly-Pro-Ile thiazolidide demonstrated the greatest effect on insulin secretion with a peak of 218.0 ± 22.5 μU/ml. Pro-Ile-thiazolidide and Gly-Pro-Ile-thiazolidide improved glucose tolerance in obese rats while Ile-thiazolidide did not. Due to a possible "priming" effect on the enteroinsular axis by pre-treatment with inhibitor, this protocol was of greater value in improving glucose handling in the obese Zucker rat, a model of NIDDM. In an attempt to localise DP IV to the β cell, pancreata from lean and obese Zucker rats, and DP IV-negative rats were fixed in paraformaldehyde and cryosectioned (20 μm). Sections were double stained for insulin and DP IV and visualised using fluorescence microscopy. Double labelling was observed in both phenotypes with no difference between the two. These results indicate that DP IV is expressed in the pancreatic β-cell and may be co-secreted with insulin where it could cleave and inactivate the incretin hormones at their target cell.

Thesis/Dissertation

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2009-05-28

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http://hdl.handle.net/2429/8349

Physiology

University of British Columbia

UBCV

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