Age-related changes in enteric neuromuscular transmission.

Proximal and distal rat small intestine from 4-, 5- and 6-month-old rats were cut into strips measuring 6.0 X 10.0 mm. Longitudinal strips were cut along the oral-caudal axis of the intestine while circular strips were cut 90 degrees to that axis. The strips were stretched to their optimum lengths and subjected to electrical field stimulation (0.1-1.0 msec pulse duration, 30-270 mA, 1-26 Hz) in the presence of Krebs' solution and Krebs' solution plus 10(-6) M atropine. Field stimulation produced atropine-sensitive and atropine-resistant contractions in these tissues. Significant differences among the three groups were found in the amplitudes of atropine-sensitive contractions in strips from proximal longitudinal muscle. The 6-month-old animals showed the highest amplitude contractions and the 4-month-old the lowest, whereas the 5-month-old animals showed contractions that were intermediate in amplitude. No significant differences were noted among the atropine-resistant contractions. Field stimulation delivered at pulse durations of 5.0 and 50.0 msec in the presence of neural blockade with tetrodotoxin (5 X 10(-6) M) produced similar contraction amplitudes among the three groups. These results show that tension development produced by field stimulation of the proximal intestine increases significantly with age. The increases appear to be due to age-related differences in cholinergic neuromuscular transmission.