Since little is known about the in vivo disposition of circulating somatostatin-14 (SRIF-14), we examined hepatic processing of SRIF-14 in the rat. Three minutes after the intraportal injection of iodine 125 (125I)-labeled SRIF-14, 16.0 ± 2.0% of the injected dose is localized to the liver. In the presence of unlabeled SRIF-14, hepatic uptake can be decreased by 68%. Five minutes after the intraportal injection of 125I-SRIF-14, 9.5 ± 1.4% of the tracer is localized to the liver, more than any other organ tested. Serial collections of bile reveal peak radioactivity at between 10 and 20 minutes. Simultaneous administration of unlabeled SRIF-14 decreases biliary radioactivity by 40%. HPLC analysis of radioactive bile reveals a chromatographic profile similar to that of intact SRIF and is 73% immunoprecipitable by an anti-SRIF antibody. Pretreatment with chloroquine, a lysosomal enzyme inhibitor, does not significantly decrease biliary radioactivity. We conclude that the data are consistent with saturable hepatic uptake and predominantly nonlysosomal transcellular transport.
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