Volume shifts, unfolding and rolling of haustra in the isolated guinea pig caecum.

The large intestine extracts water from chyme and compacts chyme into faecal conglomerates; it is unclear what role the special pockets known as colonic haustra have in these events. Here we monitored the movements of haustra in isolated preparations of guinea pig caecum using videocamera and ultrasound and related them to contractions of muscle flaps and movements of glass beads in haustral pockets. We found that in partially filled caecal loops localized contractions of taeniae shift volume back and forth between adjacent haustra; volume unfolds haustral walls in a characteristic sweep with sequential intrahaustral folds popping out; cyclic contractions and relaxations of the fold then produce the caterpillar-like movement known as haustral rolling; ultrasound showed that haustral rolling made the haustral flow channel narrower and longer as haustral folds increase their height from 7.5 +/- 1.5 mm to 16 +/- 4 mm and their distance from 4.1 +/- 0.2 mm to 7.9 +/- 0.3 mm; luminal contents were alternatively shaken off the haustral wall, whirled around the lumen or left to settle. We also suspended the row of haustra between two taeniae inside a frame and attached flaps of taeniae and haustral folds to strain gauges to record their mechanical activity; both taeniae and haustral folds produced an undulating baseline tension; during rolling, folds produced phasic contractions at 17 +/- 2 cycles min-1 which propagated distally across haustral septa; rolling constantly shuffled around glass beads placed inside the haustra. When we stimulated the intramural nerves to the caecum through bipolar electrodes, all contractile activity was temporarily inhibited and haustral septa flattened; a rebound contraction then propagated aborally from the caecal pole and swept the glass beads ahead of it. Thus, tonic contractions of taeniae shift caecal contents back and forth across haustral septa; expansion of haustra triggers haustral rolling which shuffles contents; both these movements produce local flow within and between haustra which might enhance the separation of solid and liquid colonic contents.