Impaired propulsive motility in the distal but not proximal colon of BK channel β1-subunit knockout mice

Background Large-conductance Ca2+-activated K+ (BK) channels regulate smooth muscle tone. The BK channel beta 1-subunit increases Ca2+ sensitivity of the a-subunit in smooth muscle. We studied beta 1-subunit knockout (KO) mice to determine if gastrointestinal (GI) motility was altered. Methods Colonic and intestinal longitudinal muscle reactivity to bethanechol and colonic migrating motor complexes (CMMCs) were measured in vitro. Gastric emptying and small intestinal transit were measured in vivo. Colonic motility was assessed in vivo by measuring fecal output and glass bead expulsion time. Myoelectric activity of distal colon smooth muscle was measured in vitro using intracellular microelectrodes. Key Results Bethanechol-induced contractions were larger in the distal colon of beta 1-subunit KO compared to wild type (WT) mice; there were no differences in bethanechol reactivity in the duodenum, ileum, or proximal colon of WT vs beta 1-subunit KO mice. There were more retrogradely propagated CMMCs in the distal colon of beta 1-subunit KO compared to WT mice. Gastrointestinal transit was unaffected by beta 1-subunit KO. Fecal output was decreased and glass bead expulsion times were increased in beta 1-subunit KO mice. Membrane potential of distal colon smooth muscle cells from beta 1-subunit KO mice was depolarized with higher action potential frequency compared to WT mice. Paxilline (BK channel blocker) depolarized smooth muscle cells and increased action potential frequency in WT distal colon. Conclusions & Inferences BK channels play a prominent role in smooth muscle function only in the distal colon of mice. Defects in smooth muscle BK channel function disrupt colonic motility causing constipation.