Liver glucagon receptors (GluR): effect of exercise and fasting on binding characteristics, GluR-mRNA, and GluR protein content in rats.

The purpose of the study was to investigate the effects of acute exercise and fasting on glucagon receptor (GluR) binding characteristics, GluR-mRNA, and protein content in rat liver. Liver homogenates were prepared and plasma membranes were purified by aqueous 2-phase affinity partitioning in rats fed at rest (control) and after 180 min of swimming exercise and 24 h of fasting (7 rats/group). Saturation curve of plasma membranes incubated with [125I]-glucagon showed significant higher GluR density following exercise and fasting than in the control group (8.19±0.29 and 8.01±0.65 vs. 3.09±0.12 pmol/mg of proteins, respectively). When compared to control rats, GluR Kd was also higher following exercise and fasting (0.46±0.05 and 0.56±0.13 vs. 0.33±0.05 nM, respectively; significantly different for fasting only). Expression of GluR-mRNA and protein content were both significantly higher (~100% and ~90%, respectively) following the 24-h fast than in the control rats, but not following exercise. These results, in line with the literature showing an increased sensitivity of the liver to glucagon following exercise and fasting, indicate that an increased density of GluR on plasma membranes can be obtained by 2 complementary mechanisms: externalization of pre-existing GluR from intracellular pools operative in response to the prolonged exercise, and de novo synthesis of GluR operative only in response to fasting. The reduction in plasma insulin concentration and/or depletion of liver glycogen stores, which results from both prolonged exercise and fasting, could be involved in the control of these mechanisms.

SU-E-T-527: Assessment of Field-Junction Dosimetry: Pinnacle V9.0 versus Film (EBT2).

PURPOSE: To investigate dose agreement of Pinnacle V9.0 with measurements across the junction between abutting photon and electron fields. METHODS: Photon-photon, photon-electron, and electron-electron field junctions of energies 6 MV and 9 MeV were evaluated with 0, 5, and 10 mm gaps between the field edges at depths of 2, 15, and 30 mm with Gafchromic film (EBT2). Photon and electron fields (10×10 cm) were setup to deliver 300 cGy in their field centers to 30 mm depth. Film optical density was measured every millimeter across the field-junction with a densitometer (light beam 2 mm). Optical Density to dose conversion was accomplished using a calibration curve specific to the batch of this film. For comparison, all field combinations were generated in Pinnacle, and doses were calculated with collapsed-cone convolution algorithm employing calculation-grid sizes 2, 3, and 4 mm. RESULTS: Measured doses at junction are as much as 20% different than Pinnacle calculation. 3 profiles with zero gap, 1 profile with 0.5 cm gap, and 0 profiles with 1.0 cm gap showed a disagreement greater than 12%. Measurement showed a bias towards higher dose in the junction when compared to Pinnacle calculation. With dose-calculation grid reduced below 4 mm, Pinnacle calculations showed improved agreements, but only a few percent at most. Pinnacle versus measured dose disagreements do not show clear trends with field separation or depth. CONCLUSIONS: Doses calculated by Pinnacle V9.0 in the field-junction region may not be accurate for abutting photon and electron fields, particularly when a coarse dose-calculation grid is utilized. Lack of clear trends in dose-agreement with field separation and depth suggests that uncertainties in other factors such as jaw positions and phantom set-ups may be additional contributors. Significant inaccuracies in doses reported by the treatment-planning system in field-junction region should be considered while making clinical decisions.