Fig. 31.5 ). This transport process is predominantly localized to the distal part of the colon and is driven by the epithelial Na+ channel (ENaC), which is also involved in reabsorption of Na+ in the kidney. When the channel is opened in response to activation by neurotransmitters or hormones, or both, Na+ flows into the colonocyte cytosol and is then transported across the basolateral membrane by Na+,K+-ATPase. Cl− ions follow passively via the intercellular tight junctions to maintain electrical neutrality. Water is absorbed across the tight junctions as a result of the transepithelial osmotic gradient due to solute absorption. This mode of Na+ absorption is the •Fig. 31.5 ElectrogenicNa+ absorptioninthecolon.ENaC,epithelialsodiumchannel.
last line of defense to prevent excessive loss of water in stool, given its strategic location in the distal part of the colon. Indeed, patients suffering from bowel inflammation often show markedly diminished expression of ENaC, perhaps accounting for their diarrheal symptoms. We also know that expression of ENaC can be acutely regulated in response to whole-body Na+ balance. Thus in situations of reduced Na+ intake, the hormone aldosterone increases ENaC expression in both the colon and kidney, thereby promoting retention of Na+ .
Adequate hydration of the colonic contents is determined by the balance between water absorption and secretion. Fluid secretion in the colon is driven by Cl− ion secretion, by the same mechanism driving fluid secretion in the small intestine, and is subject to the same regulation (see
Fig. 30.18).
Indeed, some cases of constipation may reflect abnormalities in epithelial transport, and constipation that results from abnormally slow motility can be treated by agents that stimulate Cl− secretion. Conversely, excessive Cl− secretion can be one mechanism underlying diarrhea.
The remnants of the meal entering the colon interact with a vast assortment of bacteria and other microorganisms. This enteric microbial ecosystem is established shortly after birth, matures as the child grows, and fluctuates in predictable ways in healthy individuals depending on factors such as diet or circadian rhythms. More drastic perturbations in the microbiota can be provoked by antibiotics or introduction of an aggressive pathogen and may then take considerable time to resolve. The enteric bacterial ecosystem contributes to GI physiology in a surprising number of ways. Indeed, the large intestine (and to a lesser extent the distal portion of the small intestine) is an unusual organ in that it maintains a symbiotic relationship with such an extensive microbiota, whereas other body compartments are largely sterile.