Differences in longitudinal smooth muscle response along the length of the rabbit small intestine

To determine whether there is a gradient in the ability of longitudinal smooth muscle along the length of the small intestine to develop tension we prepared longitudinal muscle strips from proximal (P), medial (M) and distal (D) segments of small intestine removed from 900–1200 g New Zealand white rabbits (n = 9). Isometric tension was recorded in standard tissue baths perfused with oxygenated Krebs solution and 10−6m tetrodotoxin. Basal and active (the response to 10−5m carbachol) length-tension curves were generated, and the length at which tissues developed maximal active tension (the optimum length or Lo) was determined for P, M and D tissues. Then the dose-response to carbachol (over the range 10−2-10−4m) was determined for each of the P, M and D tissues at Lo. Non-linear regression was used to determine the dose at which the contractile response was half maximal (the ED so), and the maximum tension generated (Tm) by the tissue at each location. The basal tension generated in response to graded stretch increased exponentially between 100 and 140% of initial length and was not significantly different for P, M and D tissues. The active tension generated in response to 10−5m carbachol peaked at 125% of initial length; the Lo for P, M and D tissues was identical. With tissues equilibrated at Lo, dose-response curves for carbachol and graded concentrations of KCl showed a significant increment in maximal tension generated, but no significant difference in ED50 in the more distal compared to proximal segments. Thus, the passive elastic properties and Lo were identical for the three regions of small intestine examined. However, there was a significant increase in active tension generated in distal compared to more proximal regions of the small intestine, which was not associated with a change in the ED50 value for carbachol. As the response occurs in the presence of tetrodotoxin and can be reproduced with KCl depolarization, it probably reflects post-receptor changes in smooth muscle function.