Different Effects of Hormonal Peptides and Cyclic Adenosine 3',5'-Monophosphate on Colonic Transport In Vitro

The effect of peptide hormones and other small intestinal secretory stimulants on ion and water transport by ascending and descending rat colon was investigated in vitro using a muscle-stripped everted open sac preparation. Net water flux was measured gravimetrically at 30min intervals for 150 min, each sac serving as its own control. Water flux rate was constant over the entire period and equal in ascending (15.6 ± 1.1) and descending (14.9 ± 1.1 μl hr-1 mg-1 of dry weight) colon. Both segments responded identically to all test substances. Neither glucagon 10-5 m nor pentagastrin 10-5 m, singly or in combination, had a significant effect on net water flux. In contrast, theophylline 10-2 M and dibutyryl cyclic AMP 10-3 M reduced net flux significantly by 23% (P < 0.01) and 38% (P < 0.01), respectively. The greatest reduction was observed with ricinoleic acid 2 x 10-3 M applied to the mucosal side (71%; P < 0.01). Final concentrations of Na, Cl, and HCO3 and osmolalities in the serosal compartment were significantly different on addition of theophylline or cyclic AMP. These changes could be interpreted as a shift from Na to H absorption or an increase in NaHCO3 secretion. The effects of theophylline and cyclic AMP paralleled their effect on ileal transport in other species . The effects of hormonal peptides did not: for the rat, this appeared to be a true species difference, probably unrelated to the insensitivity of the distal gut to secretory stimulants.