Influence of age on cholinergic and inhibitory nonadrenergic noncholinergic responses in the rat ileum

In view of the previously reported marked reduction in the number of neurons in the rat small intestine, the influence of age on the responses to electrical stimulation of the longitudinal muscle-myenteric plexus preparation of the ileum of young (3-4 months), adult (12-13 months) and old (24-25 months) rats was investigated. The differences in responses between the three age groups were varied. At the basal tone level, electrical train stimulation induced complex responses consisting of a primary contraction, which was partly cholinergic, a secondary contraction, which was completely cholinergic and a rebound contraction, which was partly cholinergic. These responses did not differ between young, adult and old rats. The inhibiting effects of atropine and potentiating effects of physostigmine on these responses did not change with age. The acetylcholine-induced concentration-response curve did not differ between the three age groups. Electrical train stimulation of the precontracted longitudinal muscle-myenteric plexus preparation under nonadrenergic noncholinergic (NANC) conditions induced multiphasic responses consisting of a primary contraction, a primary relaxation, a postrelaxation and a rebound contraction. The primary contraction and the postrelaxation, which were nitrergic in nature, were not influenced by age. The primary relaxation was slightly more pronounced in young rats. In young rats, this relaxation was partly inhibited by NG-nitro-L-arginine methyl ester (L-NAME) and ATP desensitization, while in adult and old rats these primary relaxations were only influenced by ATP desensitization, but not by L-NAME. Nitric oxide-induced relaxations were similar in the three age groups. The cAMP content was not increased by electrical stimulation, while the cGMP content increased with electrical stimulation, but no differences due to age were observed. These results suggest that cholinergic responses in the rat small intestine are well-maintained with age while the nitrergic contribution to NANC relaxation decreases with age.