Regional variation in canine intestinal muscle mass and function

Our aim was to investigate the contribution of variations in intestinal muscle morphology or function to regional differences in motor properties in vivo. We quantitated intestinal muscle thickness and surface area along the canine gut and compared the in vitro contractile properties of the jejunum and ileum. The thickness and cross-sectional surface area of both circular and longitudinal muscle demonstrated a parabolic distribution along the intestine, with the greatest values occurring in the proximal and distal regions. The terminal ileum had the greatest circular (885 +/- 194 microns) and longitudinal muscle (367 +/- 135 microns) thickness. Circular muscle was 2.5-3 times thicker than longitudinal muscle at all points. Passive tension was similar in muscle strips from the mid-jejunum, mid-ileum, and terminal ileum (2.8 +/- 0.8, 2.5 +/- 0.4, and 2.3 +/- 0.8 vs 2.5 +/- 0.5, 1.9 +/- 0.5, and 2.8 +/- 1.0, longitudinal and circular, respectively). Active and total tension, however, were significantly greater in longitudinal than circular muscle in mid-jejunum (active; 8.5 +/- 1.4 vs 5.6 +/- 1.2, P < 0.05 and total 11.3 +/- 1.7 vs 8.1 +/- 1.2) and in mid-ileum (active 9.5 +/- 1.6 vs 5.8 +/- 1.2 and total 12.0 +/- 1.6 vs 7.7 +/- 1.2). Values for each layer were similar in both sites. In contrast, in the terminal ileum, longitudinal and circular muscle strips demonstrated similar active (10.1 +/- 1.7 vs 9.0 +/- 2.7, NS) and total tension (12.4 +/- 2.0 vs 11.9 +/- 3.4, NS). Dose-response curves to carbachol (10(-8)-10(-2) M) were similar in all these regions. We conclude (1) there are regional variations in muscle mass but contractile properties are similar in jejunum and ileum; and (2) the unique motor properties of the terminal ileum may be related more to differences in muscle morphology and neural input than intrinsic function.