Small Bowel Transplantation Induces Adrenergic Hypersensitivity in Ileal Longitudinal Smooth Muscle in Rats

Our aim was to determine the effects of small bowel transplantation on contractility of longitudinal muscle in the rat ileum. Full-thickness longitudinal muscle strips from four groups of rats (naive controls, sham-operated controls, and 1 week and 8 weeks after syngeneic orthotopic small bowel transplantation) were studied in vitro. Neither baseline contractility nor response to neural blockade (tetrodotoxin) or adrenergic/cholinergic blockade differed among the groups. Although the dose response to the cholinergic agonist bethanechol and to nitric oxide did not differ among groups, the ED₅₀ (negative log of concentration giving half-maximal effect) for the adrenergic agonist norepinephrine was increased 1 week and 8 weeks after transplantation, indicating a hypersensitivity response not blocked by tetrodotoxin. Nonadrenergic, noncholinergic inhibitory responses to electrical field stimulation were of greater amplitude and occurred at lesser frequencies (≤5 Hz) 1 week after small bowel transplantation, but returned to control values 8 weeks postoperatively. These inhibitory responses were blocked by the nitric oxide synthase inhibitor L-NMMA but not by methylene blue, a nonspecific inhibitor of guanylate cyclase. Small bowel transplantation induces a persistent adrenergic denervation hypersensitivity at the muscle and appears to upregulate, at least transiently, other inhibitory mechanisms mediated by neural release of nitric oxide. Small bowel transplantation does not alter muscle response to cholinergic pathways. These alterations in smooth muscle contractility may affect gut function early after clinical small bowel transplantation.