Bile acids are becoming increasingly appreciated as a family of hormones that regulate many aspects of intestinal function, including fluid and electrolyte transport. Secondary bile acids, such as deoxycholate and lithocholate (LCA), are formed by bacterial metabolism of primary bile acids in the colon. Acute exposure to LCA does not stimulate colonic epithelial secretion however, LCA may have a more chronic role in to play in regulation of epithelial secretory function, but this is not yet known. The current study aimed to address this gap in our knowledge.Cl- secretion was measured as changes in short circuit current (Isc) across voltage-clamped T84 cells. Transepithelial electrical resistance (Rte) was measured using an EVOM2. Acid phosphatase activity was used as a measure of toxicity. Intracellular Ca2+ levels were measured by Fura2 fluorescence microscopy and protein expression was measured by western blotting. Results are expressed as mean ± SEM for a series of n experiments. Statistical analyses were performed by t-tests or by one way ANOVA, with the Tukey post-test, as appropriate. p values ≤0.05 were considered to be significant.Bilateral treatment of T84 cell monolayers with LCA for 24 hrs resulted in a concentration-dependent reduction in Rte. High concentrations of LCA (500μM) abolished Rte over 24 hrs, whereas more physiologically-relevant levels (10μM) had no effect (n=3-7). Acid phosphatase activity was also abolished by LCA, but only at concentrations ≥ 100μM (p≤0.01, n=4). However, low concentrations of LCA (10μM, 24hrs) attenuated Cl- secretory responses to the Ca2+-dependent agonist, carbachol (CCh) (100μM) to 50.4 ± 9.5% of control responses (p≤0.001, n=5-7). cAMP-dependent responses, stimulated by forskolin (FSK, 10μM), were also inhibited by LCA pre-treatment to 29.2 ± 4.1% of controls (p≤0.001, n=5-7). Measurements of Fura-2 fluorescence revealed that CCh-stimulated Ca2+ mobilisation was significantly attenuated by LCA (10μM, 24hrs) to 36.5 ± 12.6% of control values (n=5, p≤0.01). Furthermore, electrophysiological studies revealed that LCA (10μM, 24hrs) also reduced FSK-stimulated CFTR Cl- currents to 72.6 ± 6.9% of controls (n=5, p≤0.05). Finally, western blot analysis revealed that CFTR expression was dramatically reduced by LCA treatment (10μM, 24hrs) to 9.0 ± 1.7% of control (n=3, p≤0.001).Our studies suggest a novel role for LCA as a physiological regulator of intestinal fluid and electrolyte transport. At physiologically-relevant concentrations, LCA downregulates epithelial secretory capacity, thereby promoting normal colonic absorptive function. However, when LCA levels increase it becomes cytotoxic and is detrimental to epithelial barrier function. Increased understanding of the mechanisms by which bile acids regulate epithelial transport function may lead to the development of new targets for treatment of intestinal disease.