SUMMARY1. Bromsulphthalein (BSP) was administered throughout the experiments at a constant rate well in excess of its excretory rate, to anaesthetized dogs in which the common bile duct had been cannulated. The maximal excretory rate of BSP into bile (BSP Tm) obtained in this manner was greatly elevated by choleresis arising from the administration of bile salt (usually taurocholate) at constant rate.2. When bile flow rate was increased in stages by raising the taurocholate administration rate, successive increments in BSP excretion rate were obtained up to a limiting value of about 3 times the original Tm. Beyond this point further increases in taurocholate administration rate caused either no further enhancement of BSP Tm or a decline in the extent of enhancement produced at a previous lower rate of infusion.3. When taurocholate maximal secretion was established first, the subsequent administration of BSP at progressively increasing rates led to reduction in the taurocholate secretion rate.4. Portal infusion of secretin at constant rate (usually 0x2 units/kg body wt. min) which caused substantial increases in bile flow rate, had no effect on BSP Tm. Increases of bile flow rate of the same order following constant taurocholate infusion produced marked elevation of the BSP Tm.5. These findings are discussed and the following conclusions reached: (a) The limiting factor in BSP maximal transfer is the concentration of BSP in bile; increased bile flow rate at the site of BSP excretion (canaliculi) produced by bile salt administration permits an increase in the original Tm to occur without the limiting biliary concentration being exceeded.(b) There is excretory competition between BSP and bile salt but over a certain range of bile salt administration the facilitatory effects of increased bile flow rate outweigh the inhibitory effects due to competition. BILIARY TRANSPORT MAXIMA (c) Since secretin administration had no effect on BSP Tm, it is likely that the hydrocholeresis it produces originates downstream from the canaliculi, i.e. in the bile ductules or ducts; this supports previous evidence obtained in a different manner.

SUMMARY1. Sodium taurocholate or cholate was administered systemically at a constant rate of about 2-9 #mole/min.kg body wt. to anaesthetized dogs in which the common bile duct had been cannulated. In steady-state conditions blood was sampled from systemic and hepatic veins and the fraction of bile salt removed in a single passage through the liver was determined. Total hepatic blood flow was estimated by application of the Fick principle.2. The hepatic extraction fraction for synthetic taurocholate in ten experiments was 92 % ± 5 % (S.D.) over the blood flow range encountered (1.1-2.8 ml./min.g liver). The extraction of cholate extensively conjugated in the liver before excretion into bile was 79 % ± 8 % (S.D.)(twenty-one observations, thirteen experiments). In circumstances of similar hepatic blood flow the extraction of cholate transferred to bile in the free form (after acute taurine depletion) was significantly less than that of either synthetic taurocholate or cholate which could be actively conjugated before excretion. These results, which are discussed and criticized, support previous work on the advantage of conjugation in the transfer of cholic acid from blood to bile. 3. The hepatic clearance ofbile salt decreases with increasing administration rate, but the values obtained may be influenced by changes in hepatic blood flow. With regard to taurocholate an increase in total hepatic flow was observed when its administration rate exceeded about 5 ,umole/min . kg body wt.4. The secretory maximum for glycocholate, a bile salt not normally found in dog bile, was of the same order as that for taurocholate.

The importance of side-chain charge on hepatic uptake and biliary secretion of bile acids and analogues was studied using the isolated, perfused rat liver and the anesthetized rat with a bile fistula. Derivatives of cholic acid with negative, neutral, zwitterionic, or positive charges on the side chain were synthesized and studied. Hepatic uptake by the isolated perfused liver, determined by measuring the rate of disappearance of a single 20-mumol bolus added to the perfusate, was strongly influenced by side-chain charge. A fully positively charged bile acid derivative (cholylcholamine) and two fully zwitterionic bile acid derivatives (CHAPS and cholyllysine) showed no appreciable uptake (less than 1% of the uptake rate of cholyltaurine). Bile acid derivatives existing mostly in cationic form (cholylamine) at pH 7.4, in neutral form (cholylglycylhistamine), or in divalent anion form (cholylaspartate and cholylcysteate) had an uptake rate that was greater but only 7-19% that of cholyltaurine. Side-chain charge also appeared to influence the rate of secretion into bile. Bile acids existing in mono- or dianionic form were well secreted (greater than 95% of dose in 2 h) into the bile, but all other derivatives had much lower secretion rates (less than 20% of dose in 2 h). When the biliary secretion of each bile acid derivative was expressed in relation to the amount that had entered the liver, relative secretion rates (presumably from liver cell) into bile decreased in the following order: cholyltaurine greater than cholylaspartate and cholylcysteate greater than CHAPS greater than cholyllysine greater than cholylglycylhistamine approximately equal to cholylamine. In bile fistula rats, cholylaspartate was quantitatively secreted into bile when infused at rates below its secretory maximum, whereas only very low biliary secretion rates of CHAPS were observed even during relatively high infusion rates; cholylamine was cholestatic. The above data show that, although uncharged and anionic derivatives of cholic acid may be taken up by the liver at a moderate rate, only anionic derivatives (both monovalent and divalent) are well secreted from within the liver cell into bile. A single negative charge on the side chain appears to be required for optimal transport of a bile acid from sinusoidal blood to bile.

1. The influence of micelle formation on bile salt secretion was assessed by analysing the secretory characteristics of, and interaction between, the natural micelle-forming bile salts, taurocholate and cholate, and the artificial non-micelle-forming bile salts, taurodehydrocholate and dehydrocholate (both of which are subjected to reductive metabolism), in anaesthetized dogs. 2. Competitive secretory interaction between these two classes of bile salt was demonstrated thereby indicating that they share the same biliary transport system. Taurodehydrocholate had a lower affinity for the transport system than that of taurocholate and the metabolic derivatives of dehydrocholate. 3. The (initially determined) biliary secretory maxima for taurodehydrocholate (4 . 9 +/- 1 . 9 (S.D.) mumole/min. kg, n = 6) and total 'dehydrocholate' in taurine replete dogs (4 . 2 +/- 1 . 0 mumole/min. kg, n = 16) were both significantly less than those for taurocholate (8 . 0 +/- 1 . 8 mumole/min. kg, n = 16) and total cholate in taurine replete dogs (6 . 9 +/- 1 . 2 mumole/min. kg, n = 12). 4. The initially determined secretory maxima of taurodehydrocholate and 'dehydrocholate' were elevated by about 30 and 36%, respectively, by an earlier period of taurocholate administration; the most likely explanation (which is supported by independent morphological studies) for this effect is that taurocholate increases the number of functional 'carriers' in the canalicular membrane. When calculated for optimal conditions, the secretory maxima of the non-micelle-forming bile salts closely approached those of the micelle formers. 5. The above results would seem to indicate that micelle formation (in the hepatocyte, canalicular membrane or bile) is not essential for the effective translocation of bile salt by the specific canalicular membrane receptors. The results also suggest that the effective concentration of bile salt in bile (possibly 60--70 times greater in the case of the non-micelle-formers) is not an important determinant of the net secretory performance of conjugated bile salt. 6. At the same bile salt secretion rate (3 . 06 mumole/min. kg), the bile flow rate associated with taurodehydrocholate (44 . 3 +/- 2 . 7 (S.D. along regression line) microliter/min. kg, n = 38) was significantly greater than that associated with taurocholate (29 . 5 +/- 7 . 7 microliter/min. kg, n = 80) but significantly less than that associated with 'dehydrocholate' in taurine replete dogs (51 . 7 +/- 4 . 8 microliter./min. kg, n = 33), 'dehydrocholate' after acute taurine depletion (61 . 2 microliter./min. kg, n = 1) and free cholate after taurine depletion (49 . 8 +/- 9 . 8 microliter/min. kg, n = 92). The extra flow associated with the free bile salts is derived by means that are largely or entirely independent of their osmotic activity in bile.

The effects of norcholate (a C23 bile acid that differs from cholate in having a side chain containing four rather than five carbon atoms) on bile flow and biliary lipid secretion were compared with those of cholate, using the anesthetized rat with a bile fistula. Norcholate and cholate were infused intravenously over the range of 0.6-6.0 mumol X min-1 X kg-1. Both bile acids were quantitatively secreted into bile; norcholate was secreted predominantly in unconjugated form in contrast to cholate, which was secreted predominantly as its taurine or glycine conjugates. The increase in bile flow per unit increase in bile acid secretion induced by norcholate infusion [17 +/- 3.2 (SD) microliters/mumol, n = 8] was much greater than that induced by cholate infusion (8.6 +/- 0.9 microliters/mumol, n = 9) (P less than 0.001). Both bile acids induced phospholipid and cholesterol secretion. For an increase in bile acid secretion (above control values) of 1 mumol X min-1 X kg-1, the increases in phospholipid secretion [0.052 +/- 0.024 (SD) mumol X min-1 X kg-1, n = 9] and cholesterol secretion (0.0071 +/- 0.0033 mumol X min-1 X kg-1, n = 9) induced by norcholate infusion were much less than those induced by cholate infusion (0.197 +/- 0.05 mumol X min-1 X kg-1, n = 9, and 0.024 +/- 0.011 mumol X min-1 X kg-1, n = 9, respectively; P less than 0.001 for both phospholipid and cholesterol). The strikingly different effects of norcholate on bile flow and biliary lipid secretion were attributed mainly to its possessing a considerably higher critical micellar concentration than cholate.