IN-VIVO DYNAMICAL DISTRIBUTION OF I-131 VIP IN THE RAT STUDIED BY GAMMA-CAMERA

The in vivo distribution of vasoactive intestinal peptide (VIP) was st udied for the first time using a rat model in combination with labeled VIP (I-131-VIP) and a gamma-camera. A dynamic scan showed that I-131- VIP was cleared rapidly from the blood circulation. The radioactivity was taken up and accumulated in the lungs during the first minute. Dur ing the next 15 min, the radioactivity was slowly removed from the lun gs and redistributed into the kidneys, gastric mucosa, liver and small intestine. However, the radioactivity extracted by the lungs was abou t 6-fold lower during the first minute when a large amount of the non iodinated VIP was coinjected with the I-131-VIP. I-131-VIP was elimina ted rapidly from the blood with a half-life of 0.44 +/- 0.05 (min +/- SD) while in lung the elimination half-life was determined to 2.3 +/- 0.8 (min +/- SD). Of the radioactivity in the lungs, 2% was found to b e intact I-131-VIP after 20 min. In all other organs the radioactivity found was assumed to be low molecular weight fragments of I-131-VIP. We suggest that lungs play an important role to extract VIP from the c irculation after an i.v. administration. I-131-VIP degradation product s are redistributed mostly to the kidneys and to the gastric mucosa to be excreted through urine and stomach contents, respectively.