Release of 5-hydroxytryptamine by pentagastrin and its role in the 'fade' of stimulated gastric secretion in cats.

1. The release of 5-hydroxytryptamine (5-HT) by pentagastrin and the 'fade' phenomenon seen in acid gastric secretion during a continuous intravenous infusion of pentagastrin have been studied in anaesthetized cats. 2. During an intravenous infusion of pentagastrin, acid secretion rose steeply to a maximum at 45 min. It was maintained at this rate for only 15-30 min, after which it decreased gradually. Meanwhile there was a marked increase in the 5-HT content of the plasma which reached a peak at 120 min and remained just below the peak concentration for the rest of the experiment. 3. Acid secretion due to an intravenous infusion of histamine on the other hand, steadily increased over the 3 h collection period. Meanwhile there was only a gradual small increase in the 5-HT content of the plasma. 4. Whereas there was a continuous steady increase in acid secretion in response to an intravenous infusion of pentagastrin in cats pre-treated with reserpine, acid secretion rose to a sharp peak and then decreased gradually when reserpinized cats were given a simultaneous infusion of 5-HT along with the pentagastrin. 5. It is concluded that 5-HT is released by pentagastrin and that it is responsible for the 'fade' phenomenon seen in acid secretion during infusions of pentagastrin, first of all augmenting acid secretion and then inhibiting it.