Effect of taurocholate on electrical potential difference across rat small intestine

Intestinal bile acid physiology and pathophysiology

Bile acids (BAs) have a long established role in fat digestion in the intestine by acting as tensioactives, due to their amphipathic characteristics. BAs are reabsorbed very efficiently by the intestinal epithelium and recycled back to the liver via transport mechanisms that have been largely elucidated. The transport and synthesis of BAs are tightly regulated in part by specific plasma membrane receptors and nuclear receptors. In addition to their primary effect, BAs have been claimed to play a role in gastrointestinal cancer, intestinal inflammation and intestinal ionic transport. BAs are not equivalent in any of these biological activities, and structural requirements have been generally identified. In particular, some BAs may be useful for cancer chemoprevention and perhaps in inflammatory bowel disease, although further research is necessary in this field. This review covers the most recent developments in these aspects of BA intestinal biology.

Activation of CFTR by ASBT-mediated bile salt absorption

In cholangiocytes, bile salt (BS) uptake via the apical sodium-dependent bile acid transporter (ASBT) may evoke ductular flow by enhancing cAMP-mediated signaling to the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel. We considered that ASBT-mediated BS uptake in the distal ileum might also modulate intestinal fluid secretion. Taurocholate (TC) induced a biphasic rise in the short circuit current across ileal tissue, reflecting transepithelial electrogenic ion transport. This response was sensitive to bumetanide and largely abrogated in Cftr-null mice, indicating that it predominantly reflects CFTR-mediated Cl- secretion. The residual response in Cftr-null mice could be attributed to electrogenic ASBT activity, as it matched the TC-coupled absorptive Na+ flux. TC-evoked Cl- secretion required ASBT-mediated TC uptake, because it was blocked by a selective ASBT inhibitor and was restricted to the distal ileum. Suppression of neurotransmitter or prostaglandin release, blocking of the histamine H1 receptor, or pretreatment with 5-hydroxytryptamine did not abrogate the TC response, suggesting that neurocrine or immune mediators of Cl- secretion are not involved. Responses to TC were retained after carbachol treatment and after permeabilization of the basolateral membrane with nystatin, indicating that BS modulate CFTR channel gating rather than the driving force for Cl- exit. TC-induced Cl- secretion was maintained in cGMP-dependent protein kinase II-deficient mice and only partially inhibited by the cAMP-dependent protein kinase inhibitor H89, suggesting a mechanism of CFTR activation different from cAMP or cGMP signaling. We conclude that active BS absorption in the ileum triggers CFTR activation and, consequently, local salt and water secretion, which may serve to prevent intestinal obstruction in the postprandial state.

Characterization and effect of phospholipid on bile acid absorption by villi isolated from hamster small intestine

The effects of phospholipid on absorption of bile acids by hamster small intestine were studied to determine if this compound inhibits absorption of bile acids. Absorption of taurocholic and cholic acids was studied using a new in vitro technique that relates uptake rates to the weight of the villi present on the intestinal sample rather than to the weight of the entire segment of intestine used for the study. This procedure removes from consideration various components of the intestinal wall that are not directly involved with the absorptive process. Using radioactive techniques absorption of each type of bile acid was determined over a broad range of concentrations both in the presence and absence of phospholipid in the incubation medium. Absorption of taurocholic acid by villi from jejunum was determined to be a passive process, as previously reported by others. Villi from ileum absorbed both bile acids by an apparent active process when initial concentrations of bile acids were below 2.0 mM. Above this concentration bile acid absorption by the ileum appeared to be mainly passive. Phospholipid was found to inhibit bile acid absorption by ileum when initial bile acid concentrations were moderately high. However, at low substrate concentration, phospholipid has no appreciable effect on bile acid transport.

The effect of short chain fatty acids on transmural electrical potential across rat small intestine in vivo

Short chain fatty acids suddenly produce a phasic increase in transmural electrical potential difference (PD) when placed in the lumen of rat small intestine in vivo. With concentrations of propionate ranging from 50 muM to 1000 muM the amplitude of the response in jejunum is about 5.5 mV. The concentration giving half this effect is about 20 muM. With 10 mM propionate the duration of the response is 3-5 min; after this, PD again equals the control value and the gut is refractory to further additions. Removing propionate from the mucosal surface produces no change in PD, but does restore responsiveness to subsequent exposure to short chain fatty acids. This effect is independent of a variety of other alterations in PD such as those caused by sugars, amino acids, bile salts, theophylline, prostaglandins, and ATP. Mechanism and significance of this surprisingly sensitive response remain obscure.