Effects of absorption rate, flow rate and fluid load on colonic motor activity in dogs

We investigated the relationship between absorption rate, flow rate, fluid load and colonic motor activity in an in vivo isolated colonic loop model. Motor activity was recorded by implanted strain gauge transducers. Two electrolyte solutions were perfused at 0.4 and 1.6 mL min-1 through the open colonic loop (distal end open) or infused into the closed loop (distal end closed). The first solution resembled ileostomy fluid (ES1) and the second solution was an iso-osmolar mannitol solution (ES2). The absorption rate for H2O measured by 14C PEG concentrations of ES2, 0.2 +/- 0.03 mL min-1, was significantly less than that of ES1, 0.6 +/- 0.06 mL min-1. Infusion of ES1 under open loop conditions served as control. Motor activity was analysed for area under contractions and expressed as motor index ratio in comparison to the control. None of the solutions altered motor activity when perfused at the two rates through the open loop. Under closed loop conditions, the infusion of either solution at 0.4 mL min-1 had no significant effect on colonic motor activity. Infusion of ES1 or ES2 at 1.6 mL min-1 into the closed loop, however, increased the motor index ratio 2.5-fold and 3.6-fold, respectively (P < 0.01). The proximal half of the colon was less affected than the distal half during infusion of ES1 but not during infusion of ES2. We conclude that flow rate and absorption rate do not influence colonic motor activity as long as the fluids can leave the colon. When the distal end is closed, inflow, resulting in fluid accumulation and distension, leads to stimulation of colonic motor activity.