Morphine impedes renal tubular excretion of 99mTc-MAG3 in rats by binding preferentially to MRP-2 transporter

375 Objectives: Renal tubules play an important role in the excretion of morphine and its metabolites via the MRP-2 transporter. The aim of our study was to investigate the effect of morphine on tubular secretion of 99mTc-MAG3, whose clearance also depends on MRP-2. Methods: 8 rats were divided into 2 groups: Group 1 (n = 4) controls, Group 2 (n = 4) morphine-treated. An Alzet mini-osmotic pump was inserted under anesthesia into a subcutaneous pocket on the dorsal aspect of each animal’s neck. In Group 1, the pump was filled with 200 µL of normal saline; in Group 2 the pump was filled with a morphine solution that was released at the rate of 1µl /hr, equivalent to a 20 mg/kg dose of morphine each day. On day 5 after pump insertion all animals underwent renal imaging. Data were collected with a gamma camera equipped with a low energy high resolution parallel-hole collimator, using a 2.19 zoom factor. Beginning immediately after injection of 100 µCi (3.75 MBq) of 99mTc-MAG3 via the tail vein, 10-sec dorsal images were acquired as 128x128x16 matrixes for 30 minutes. Regions of interest were drawn around each kidney to generate time-activity curves. The time to peak kidney activity (Tpeak) & percent of Tpeak 99mTc-MAG3 activity remaining in the kidneys 30 min post injection (R30) were determined. Tissue immunostaining and western blots of kidney lysate were performed to determine the expression of MRP-2 transporter. Results: Tpeak values in both groups were similar (2.1±0.6 versus 2.0±0.8 min, p = 0.82). In Group 1, the saline treated animals, 99mTc-MAG3 cleared rapidly from the kidneys. In Group 2, the morphine treated animals, 99mTc-MAG3 clearance was significantly delayed. At R30, morphine treated animals had significantly higher 99mTc-MAG3 retention than controls (77.1±8.5% versus 31.5±3.8%, ANOVA F-ratio = 95.6, p