Background The colon undergoes distension-induced changes in motor activity as luminal contents or feces increase wall pressure. Input from enteric motor neurons regulates this motility. Here we examined stretch-dependent responses in circular muscle strips of murine colon.
Methods Length ramps (6–31μm s−1) were applied in the axis of the circular muscle layer in a controlled manner until 5 mN isometric force was reached.
Key Results Length ramps produced transient membrane potential hyperpolarizations and attenuation of action potential (AP) complexes. Responses were reproducible when ramps were applied every 30 s. Stretch-dependent hyperpolarization was blocked by TTX, suggesting AP-dependent release of inhibitory neurotransmitter(s). Atropine did not potentiate stretch-induced hyperpolarizations, but increased compliance of the circular layer. Nω-nitro-l-arginine (l-NNA) inhibited stretch-dependent hyperpolarization and decreased muscle compliance, suggesting release of NO mediates stretch-dependent inhibition. Control membrane potential was restored by the NO donor sodium nitorprusside. Stretch-dependent hyperpolarizations were blocked by l-methionine, an inhibitor of stretch-dependent K+ (SDK) channels in colonic muscles. Loss of interstitial cells of Cajal, elicited by Kit neutralizing antibody, also inhibited responses to stretch. In presence of l-NNA and apamin, stretch responses became excitatory and were characterized by membrane depolarization and increased AP firing. A neurokinin-1 receptor antagonist inhibited this stretch-dependent increase in excitability.
Conclusions and Inferences Our data show that stretch-dependent responses in colonic muscles require tonic firing of enteric inhibitory neurons, but reflex activation of neurons does not appear to be necessary. NO causes activation of SDK channels, and stretch of muscles further activates these channels, explaining the inhibitory response to stretch in colonic muscle strips.