Bile acids in bile after monensin treatment

Impairment of vitamin A uptake by rat hepatocytes and fat storing cells determined by Monensin--morphological observations

The action of the Na+/H+ antiport monensin on vitamin A uptake by rat liver has been studied in vivo. The quickfading autofluorescence of vitamin A has been used for the determination of vitamin A uptake by the liver. Pretreatment of rats intraperitoneally with monensin decreases the uptake of vitamin A by hepatocytes and its transfer for storage to fat storing cells. Pretreatment of rats intraperitoneally with vitamin A for a short time, then with monensin, shows that the hepatocytes no longer transfer vitamin A to fat storing cells for storage. These results might indicate that monensin impairs the uptake of vitamin A by the hepatocytes and might also impair the transport of vitamin A from parenchymal to perisinusoidal cells.

Subcellular and molecular mechanisms of bile secretion

One of the liver's principal functions is the formation of bile, which is requisite for digestion of fat and elimination of detoxified drugs and metabolites. Bile is a complex fluid made up of water, electrolytes, bile acids, pigments, proteins, lipids, and a multitude of chemical breakdown products. In this review, we have summarized the source of various biliary components, the route by which they end up in bile, including the underlying subcellular and molecular mechanisms, and their contribution to bile formation. One of the reasons why bile formation is so complex is that there are many mechanisms with overlapping substrate specificities, i.e., many biochemically unrelated biliary constituents share common transport mechanisms. Additionally, biliary constituents may reach bile by more than one pathway. Some biliary components are critical for bile formation; others are of minor significance for bile formation but play a major physiological role. The major driving force for bile formation is the uptake and transcellular transport of bile salts by hepatocytes. The energy for bile formation comes from the sodium gradient created by the basolateral Na+/K(+)-ATPase, to which bile salt transport is coupled. The secretory pathway for bile salts involves uptake at the basolateral surface of the hepatocyte, vectorial transcellular movement, and transport across the canalicular membrane into the canalicular lumen. Hydrophilic bile salts are taken up via a sodium-dependent, saturable, carrier-mediated process coupled to the Na+/K(+)-ATPase. This uptake mechanism is also shared by other substrates, such as electroneutral lipids, cyclic oligopeptides, and a wide variety of drugs. Hydrophobic bile acids are taken up by a sodium-independent facilitated carrier-mediated mechanism in common with other organic ions, including sulfated bile acids, sulfobromophthalein, bilirubin, glutathione, and glucuronides, or by nonsaturable passive diffusion. Two major carrier proteins have been identified on the hepatocyte basolateral membrane: a 48-kDa protein that appears to be involved with Na(+)-dependent bile salt uptake, and a 54-kDa protein, thought to be associated with Na(+)-independent bile salt uptake. The intracellular transport of bile salts may involve cytosolic carrier proteins, of which several have been identified. Some evidence suggests a vesicular transport mechanism for bile salts. Since bile acids clearly do not enter the cell by endocytosis, formation of transport vesicles must be a more distal event in the transcellular translocation process. Some bile salts appear to be transported within the same unilamellar vesicles that are involved in the secretion of cholesterol and phospholipid.(ABSTRACT TRUNCATED AT 400 WORDS)

Transcellular transport of organic anions in the isolated perfused rat liver: the differential effects of monensin and colchicine

Nonbile salt cholephiles and bile salts are two classes of organic anions that are efficiently taken up and excreted by the liver. Recent evidence suggests that a microtubular system-dependent, colchicine-sensitive transcellular pathway may transport both classes of these ligands. The relationship of this pathway to flux rates, however, remains unclear. Some structural evidence suggests an important role for a Golgi-associated vesicular system. Monensin, like colchicine, is a perturbing agent that is believed to target primarily Golgi and related organelles. The effects of a minimal effective dose of both colchicine (0.06 mg to 0.12 mg/100 gm body wt) and monensin (0.6 mg/100 gm body wt) were examined in the isolated perfused rat liver in a single-pass mode. The nonbile salt cholephile, phenol red, was studied at two doses: 1 nmol and 5 mumol. Sodium taurocholate was studied at three doses: 2 nmol, 1 mumol and 5 mumol. Colchicine affected the transcellular transport for both classes of organic anions equally. Partially inhibitory effects on both anions occurred only at high ligand flux rates. In contrast, monensin greatly impaired the transport of nonbile salt cholephiles but had no influence on transcellular bile salt flux. We conclude that the monensin effect appears to define a distinct transcellular transport pathway for each of the two classes of organic anions.

Glycerophospholipids and cholesterol composition of bile in bile-fistula rats treated with monensin

Data regarding the action of monensin on the concentrations of glycerophospholipids and cholesterol in bile of rats subjected to total biliary diversion for 3 h are reported. After monensin their concentration in bile drops significantly in the first 60 min collections with respect to the control. Differences seem to be produced between the rates of transport to the bile of glycerophospholipids and cholesterol, not sufficiently explained by the inhibition of bile salt uptake determined by monensin at the sinusoidal pole of the hepatocyte.