Uptake of Taurocholate by Hepatocytes Isolated from Developing Rats

Abstract

Summaryon Purina rat chow and water ad libitum. Male rats were utilizedTo further define developmental changes in bile acid metabolism, we determined the kinetics of taurocholate uptake by hepatocytes isolated from Sprague-Dawley rats at 7, 14,21,28, and 56 days of age. There was a progressive increase in taurocholate uptake with age. The uptake process exhibited saturable kinetics in every age group with a maximum uptake velocity attained above a taurocholate concentration of 200 pM. There were no differences in Km values but Vmax increased progressively between 7 and 56 days of age. These data suggest that the deficit in hepatic excretory function observed in immature mammals of several species may, in part, be related to decreased transport of bile acids. SpeculationImpaired transport of bile acids by the liver may limit bile flow during development and lead to inefficient fat digestion, altered hepatic excretion of drugs, and an increased susceptibility to cholestasis. Comparable Km values for the uptake process indicate that hepatocyte affinity for taurocholate remains constant during development; whereas the rise in Vmax with postnatal age may reflect an increase in the number of binding sites. Specific changes in the liver cell plasma membrane are likely to be important determinants of the ontogeny of bile acid transport and bile flow.Bile acids are major synthetic and excretory products of the liver and are of primary importance in the generation of bile flow (1 1, 15, 20). There is increasing evidence in humans and in experimental animals that the enterohepatic circulation of bile acids is not fully developed at birth and that the perinatal liver is normally subject to a period of "physiological cholestasis" (13,14,41). In the normal human infant and in the suckling rat, bile acid pool size is decreased (42, 44) and the concentration of bile acids in serum is elevated (5, 6, 41). As a consequence of decreased bile secretion, biliary elimination of many drugs and organic anions such as bilirubin may be impaired (30,43,46) and the concentrations of bile acids reaching the intestine may be inadequate for optimal fat digestion (45). This immaturity of liver function may also place the infant at increased risk for clinical cholestasis as is commonly observed during gram negative infection or during parenteral nutrition (2,8,18). It is our hypothesis that during development, efficient enterohepatic cycling of bile acids may, in part, be limited by immaturity of bile acid transport by the liver. We, therefore, sought maturational changes in the uptake of taurocholate by hepatocytes isolated from suckling and weanling rats.for hepatocyte isolation and all studies were conducted at midday.The chemicals used in this study were: tauro-[~arbonyl-'~C] cholic acid, (52.0 mCi/mmoles; greater than 98% pure by thin layer chromatography) and [3H]-inulin, (1 mCi/6.9 mg) both from New England Nuclear Corp. (Boston, MA); collagenase (type II), Sigma Chemical Corp. (St. Louis, MO); sodium taurocholate (greater than 98% pure ...

Bile Acid Efflux from Suckling Rat Hepatocytes

“…The net effect is that taurocholate will not accumulate within the hepatocyte above the capacity of binding proteins native to suckling rat liver in the absence of a large, inwardly directed concentration gradient. This could explain our observation that suckling rat hepatocytes were able to attain a bile acid concentration similar to mature cells du~ing preloading despite a lower initial uptake velocity as documented earlier (5). Earlier and greater "unloading" seemed to occur once this large inward gradient was removed.…”

Correlation between Fetal and Maternal Serum Bile Acid Concentrations

“…During fetal and neonatal life, synthesis of BA has been postulated to occur via an alternate pathway, which has lithocholic acid and 3 8-hydroxy-5 cholenoic acid as intermediates (16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28). There appears to be synthesis via the usually accepted pathway as well, however, the alternate pathway seems to be of major importance in fetal life (1 3).…”

“…This state of hypercholanemia, reflecting an immaturity of the enterohepatic circulation, has since been corroborated by other investigators using similar radioimmunoassays (4, 25, 27), and "physiologic cholestasis" is now recognized as a normal developmental condition. The factors responsible for physiologic cholestasis have not yet been completely defined in the human infant; data obtained in animal models have documented the immaturity of several of the processes involved in the enterohepatic circulation of BA (hepatic uptake, conjugation, excretion, and intestinal absorption) (5, 10,23,24,26). However, the presence of marked species differences, as has been well demonstrated for other fetal hepatic excretory mechanisms, makes it difficult to extrapolate these data to the human infant.…”

Absence of an Acinar Gradient for Bile Acid Uptake in Developing Rat Liver

“…Recent studies utilizing hepatocytes and basolateral liver plasma membrane vesicles prepared from developing rats have shown that bile acid uptake is markedly decreased probably as a result of fewer functional bile acid carriers on the sinusoidal membrane of the hepatocyte (9,10). These findings led us to the hypothesis that the low transport capacity of the developing liver for bile acids might well result in an acinar distribution for bile acid uptake which differed from the adult.…”