Tauro alpha-muricholate has a biliary transport maximum (Tm) value equivalent to that for tauroursodeoxycholate and...

Our previous studies have shown that Tm values for tauroursodeoxycholate (TUDC) and tauro beta-muricholate (T beta-MC) are more than two-fold higher than that for taurocholate (TC) in the rat. The present study attempted to clarify whether tauro alpha-muricholate (T alpha-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 T alpha-MC solution. The infusion rate was raised stepwise every 20 min, until the bile flow began to decline. Bile was collected every 10 min and bile salt excretion rate was determined. The average of the highest three excretion values was assumed to be the Tm in each animal. The Tm value of T alpha-MC was found to be 2.86 +/- 0.36 mumol/min/100 g (mean +/- SD, n = 4), which was even greater than Tm values for TUDC (2.59 +/- 0.39 mumol/min/100 g, n = 4) and T beta-MC (1.93 +/- 0.31 mumol/min/100 g, n = 4) as we reported previously. The relationship between the bile flow rate (microliter/min/100 g, Y axis) and bile salt excretion rate (mumol/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 T alpha-MC (6.00 +/- 0.29 microliters/mumol) was significantly higher than that for TUDC (4.76 +/- 0.71 microliters/mumol) and was comparable to that for T beta-MC as we previously found for these bile salts in this rat strain. The results suggest that T alpha-MC has a very efficient transport system in this species as was observed for the other two bile salts that have a 7 beta-hydroxy group (TUDC and T beta-MC). This efficient transport system thus appears to be shared not only by bile salts specifically having a 7 beta-hydroxy group, but also by other bile salts such as T alpha-MC that have a 6 beta-hydroxy group but not a 7 beta-hydroxy group.