Determination of Bile Acids in Canine Biological Samples: Diagnostic Significance

Abstract

The comprehensive examination of bile acids is of paramount importance across various fields of health sciences, influencing physiology, microbiology, internal medicine, and pharmacology. While enzymatic reaction-based photometric methods remain fundamental for total BA measurements, there is a burgeoning demand for more sophisticated techniques such as liquid chromatography-tandem mass spectrometry (LC-MS/MS) for comprehensive BA profiling. This evolution reflects a need for nuanced diagnostic assessments in clinical practice. In canines, a BA assessment involves considering factors, such as food composition, transit times, and breed-specific variations. Multiple matrices, including blood, feces, urine, liver tissue, and gallbladder bile, offer insights into BA profiles, yet interpretations remain complex, particularly in fecal analysis due to sampling challenges and breed-specific differences. Despite ongoing efforts, a consensus regarding optimal matrices and diagnostic thresholds remains elusive, highlighting the need for further research. Emphasizing the scarcity of systematic animal studies and underscoring the importance of ap-propriate sampling methodologies, our review advocates for targeted investigations into BA alterations in canine pathology, promising insights into pathomechanisms, early disease detection, and therapeutic avenues.

Keywords: LC-MS/MS; bile acids; canine; enterohepatic circulation.

Figure 1

The process of cholic acid 7α-dehydroxylation, also known as the Hylemon–Björkhem pathway, occurs in intestinal anerobic microbes. Intermediate compounds formed in the synthesis of secondary bile acid deoxycholic acid are indicated [15,18,33,34].

Figure 2

Enterohepatic circulation of bile acids (BAs). BAs are synthesized in the hepatocytes and then, after tauro- or glycoconjugation, secreted into the lumen of the small intestine via bile following their production by the liver. In the distal segment of the small intestine, enterocytes recover the majority of conjugated BAs through an active mechanism involving sodium-dependent BA transporter (ASBT) embedded in the apical membrane. Bile acids that are not absorbed (5% of the total amount) then travel to the colon, where they are modified further by the gut microbes, while 95% of the BAs stay in the cycle. In ileal enterocytes, ileal BA-binding protein (IBABP) serves as the primary intracellular transporter for reabsorbed conjugated BAs. The heterodimeric basolateral membrane organic solute transporter OSTα/OSTβ facilitates the elimination of BAs from ileal enterocytes, allowing them to be transported back to the liver through the portal circulation. Bile acids are reabsorbed into the hepatocytes, assisted by sodium taurocholate co-transporting polypeptide (NTCP) and organic anion-transporting polypeptides (OATPs), which are located in the basolateral (sinusoidal) membrane of the liver in direct contact with the portal blood. The occurrence of this uptake mechanism has negative feedback on the process of endogenous synthesis from cholesterol. Since the cycle is continuous, the liver also excretes these reabsorbed BAs into the bile [20,36,37,49].

Figure 3

Number of matrices tested for BAs within a trial (n = 94) (number of relevant papers).

Figure 4

Matrices studied the most frequently for BA composition by species (in number of papers; n = 94).

Figure 5

Ratio and number of measurement methods in the bibliographic sources processed (n = 94).