Abstract
The aim of this study was to determine if differences in the force of contraction in different regions of the rabbit colon are associated with variations in the histology of the corresponding muscle tissues. Circular and longitudinal muscles were isolated from strips of proximal and distal colonic muscle. Muscle strips stretched to L0 were either stimulated to contract or were processed for electron microscopy. Cross-sections of the smooth muscle cells of the taenia coli had a larger perimeter (P < 0.001) and were surrounded by increased extracellular matrix (28% of the standardized box) compared with the muscle cells from the other sites in the colon (7%–13%) (P < 0.001). Cross-sections of the proximal circular muscle cells had a smaller perimeter and were present in a greater number than the cells from other areas of the colon. The distal circular muscle generated a larger force than the other muscles after stimulation with bethanechol or K+ (P < 0.05). The taenia developed less force than the other muscles (P < 0.05). Bethanechol was a less potent stimulant for the longitudinal muscles than for the circular muscles (P < 0.05). This study suggests that (1) the decreased efficacy of bethanechol and K+ stimulation of the taenia coli is caused in part by the smaller number of cells that are available to contract and (2) the increased efficacy for the stimulation of the distal circular muscle compared with the proximal circular muscle is unrelated to the mass of muscle and seems to be related to an inherent property of the muscle.
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