Background. The purpose of this study was to elucidate the mechanism of reduced intestinal transit rate in the ileum as compared with the jejunum. Methods. Twenty-one dogs were each instrumented with 12 strain gauge transducer, 2 collection cannulas, and an infusion catheter defining a 100 cm study in the midjejunum (n = 11) and midileum (n = 10). Postprandial motor activity and intestinal transit were measured 1 hour after ingestion of a 650 kcal solid meal. Contractile activity was analyzed by means of computer programs that determine frequency, amplitude, and propagation behavior of circular smooth muscle contractions. Results. Postprandial ileal contractions occurred with frequency (13.7 ± 2.5 versus 11.5 ± 0.4; p = 0.04) and displayed a higher incidence of propagation (61% ± 2% versus 44% ± 3%; p = 0.0001) than jejunal contractions, but traveled at significantly slower rates (1.0 ± 0.7 cm/sec vs 3.7 ± 0.9 cm/sec; p = 0.0001). The net result was significantly slower transit in the ileum compared with the jejunum (4.7 ± 0.7 cm/min versus 13.1 ± 1.5 cm/min; p = 0.0006). Within each region, transit correlated with parameters of propagating contractions. Stepwise regression of the combined data revealed that contraction velocity was the mst important variable determining intestinal transit rate (r = 0.64; p < 0.001). Conclusions. Contrary to previous thinking, postprandial ileal contractions display a high degree of temporal and spatial organization. Slow ileal transit is mainly due to reduced propagation velocity, which is intrinsic to the circular smooth muscle.
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