Persistence of altered 5‐hydroxytryptamine turnover following hemibody X‐irradiation in the rat distal colon

Purpose: Acute gastrointestinal responses to ionizing radiation exposure include a role for 5‐hydroxytryptamine (5‐HT), but it is not known whether involvement of 5‐HT persists and contributes to late effects. The aim was to investigate the acute and later effects of lower hemibody irradiation on 5‐HT turnover and the biological effect in the rat distal colon. Materials and methods: Rats were exposed to 10 Gy lower hemibody X‐radiation. 5‐HT and 5‐hydroxyindole acetic acid tissue levels were measured in the distal colon along with the serotonin re‐uptake transporter and tryptophan hydroxylase mRNA. 5‐HT‐containing cells and crypt cell numbers were estimated in addition to 5‐HT‐stimulated short‐circuit current responses in isolated mucosa. Studies were performed from 3 days to 3 months post‐exposure. Results: During the acute phase, at 3 days post‐irradiation, reductions in cell number, tissue resistance, serotonin re‐uptake transporter expression and secretory responses to 5‐HT were observed. However, at later times when secretory responses were normal, 5‐HT tissue levels and enterochromaffin cell numbers were increased. Conclusions: The results provide evidence that after 10 Gy hemibody irradiation, modifications persist past the acute phase. In particular, 5‐HT turnover in the distal colon is altered during a longer period.