Regulation of hepatic insulin receptors by pancreatic polypeptide in fasting and feeding

Pancreatic polypeptide (PP) increases hepatic insulin receptor (IR) binding activity in fasted PP-deficient rats, but not fasted normal animals. PP-induced alteration of hepatic IR levels in normal animals may be detectable in the fed state when IR concentrations are lower than during fasting. In the current study, the effect of exogenous PP on IR concentrations in the fed and fasted states was determined in healthy 300- to 350-g male Sprague-Dawley rats. Ten animals were administered PP 100 μg/kg/day for 3 days by intraperitoneal injection and 10 weight-matched control animals received saline vehicle. Five PP- and five saline administered rats were fasted for 12 hr prior to organ procurement, while 5 PP- and 5 saline-treated rats were given free access to food for this period. Livers were removed and snap-frozen. IRs were isolated from solubilized hepatocyte membranes by affinity chromatography with agarose-bound wheat germ agglutinin. Western blots were performed using a specific antibody to the β subunit of the IR, which was detected by a chemiluminescence technique after 45-min exposure to X-ray film. Exposed films were examined by scanning densitometry and IR concentration was expressed as absorbance units per milligram of hepatic protein (mean ± SE). Statistical comparisons were by Student's t test with significance taken at P < 0.05. Feeding was associated with a significantly lower IR concentration in saline-administered animals compared with the fasted state (24.2 ± 4.0 vs 53.3 ±11.1). PP administration in fed rats resulted in significantly increased IR concentration as compared with that seen in saline-administered fed animals (43.8 ± 8.9 vs 24.2 ± 4.0). This difference may be due to increased IR synthesis with long-term PP administration, and supports the role of PP as a regulatory factor in hepatic carbohydrate metabolism.