Tauro α-muricholate has a biliary transport maximum (Tm) value equivalent to that for tauroursodeoxycholate and tauro β-muricholate in the rat

Abstract

Our previous studies have shown that Tm values for tauroursodeoxycholate (TUDC) and tauro β-muricholate (Tβ-MC) are more than two-fold higher than that for taurocholate (TC) in the rat. The present study attempted to clarify whether tauro α-muricholate (Tα-MC) also has such an unusually large TM value in the rat. Under nembutal anesthesia, male Wistar derived rats (body weight 280–300 g, 13 wks in age) were continuously infused with Tα-MC solution. The infusion rate was raised stepwise every 20 min, until the bile flow begun to decline. Bile was collected every 10 min and bile salt excretion rate was determined. The average of the highest three exretion values was assumed to be the Tm in each animal. The Tm value of Tα-MC was found to be 2.86 ± 0.36 μmol/min/100 g (mean ± SD, n=4), which was even greater than Tm values for TUDC (2.59 ± 0.39 μmol/min/100 g, n=4) and Tβ-MC (1.93 ± 0.31 μmol/min/100 g, n=4) as we reported previously. The relationship between the bile flow rate (μ1/min/100 g, Y axis) and bile salt excretion rate (μmol/min/100 g, X axis) was highly linear [Y=(6.00 ± 0.29) X + (6.60 ± 1.88), P < 0.001, r=0.95, n=54]. The slope value for Tα-MC (6.00 ± 0.29 μl/ μmol) was significantly higher than that for TUDC (4.76 ± 0.71 μl/ μmol) and was comparable to that for Tβ-MC as we previously found for these bile salts in this rat strain. The results suggest that Tα-MC has a very efficient transport system in this species as was observed for the other two bile salts that have a 7β-hydroxy group (TUDC and Tβ-MC). This efficient transport system thus appears to be shared not only by bile salts specifically having a 7β-hydroxy group, but also by other bile salts such as Tα-MC that have a 6β-hydroxy group but not a 7β-hydroxy group.