Cholera toxin: radio-iodination and uptake by the intestine of sucking rats.
1. The chloramine-T procedure was employed to radio-iodinate cholera toxin using Na125I. The procedure was found to be efficient and reproducible. 2. Intragastric injections of both the labelled and the unlabelled toxin produced (a) significant increases in intestinal fluid accumulation as measured by the fluid accumulation ratio; (b) significant increases in cAMP levels; and (c) significant decreases in cAMP-phosphodiesterase activities when compared with the controls suggesting that radio-iodination did not impair the biological activity of the toxin. 3. In vivo uptake studies of the labelled toxin by different parts of the intestine indicated that the uptake by the duodenum and jejunum was high and rapid when compared with the ileum implying that there are more binding sites (or receptor proteins) for cholera toxin in the duodenum and jejunum than in the ileum.