Bile production

The liver secretes about 500-600 ml of bile per day. Bile is iso-osmotic to blood plasma and consists primarily of water, electrolytes, bile salts, phospholipids (mainly lecithin), cholesterol, bilirubin and other endogenous or exogenous components such as proteins regulating the function of the gastrointestinal tract, drugs or their metabolites. Bilirubin is a product of decomposition of heme components upon destruction of hemoglobin. The formation of bile salts, otherwise called bile acids, causes the secretion of other elements of bile, in particular sodium and water. Functions of bile salts include the excretion of potentially toxic substances (for example, bilirubin, metabolites of drugs), solubilization of fats and fat-soluble vitamins in the intestines, facilitating their absorption, and activation of osmotic purification of the intestine.

For the synthesis and secretion of bile, active transport mechanisms are necessary, as well as processes such as endocytosis and passive diffusion. Bile is formed in the tubules between adjacent hepatocytes. Secretion of bile acids in the tubules is the stage of the formation of bile, which limits its rate. Secretion and absorption also occur in the bile ducts.

In the liver, bile from the intrahepatic collecting system enters the proximal, or general, hepatic duct. Approximately 50% of the bile secreted outside food intake from the common hepatic duct enters the gallbladder through the cystic duct; The remaining 50% are sent directly to the common bile duct, formed by the fusion of the common hepatic and cystic ducts. Outside the meal, a small part of the bile comes directly from the liver. The gallbladder absorbs up to 90% of the water from the bile, concentrating it and accumulating it.

Bile comes from the gallbladder into the common bile duct. The common bile duct connects to the duct of the pancreas, forming the pharynx of the papilla, which opens into the duodenum. Before joining the pancreatic duct, the common bile duct narrows in diameter to <0.6 cm. The sphincter of Oddi surrounds both the pancreatic and common bile ducts; in addition, each duct has its own sphincter. Bile, as a rule, does not flow retrograde into the pancreatic duct. These sphincters have a high sensitivity to holicystokinin and other intestinal hormones (for example, a gastrin-activating peptide), as well as changes in the cholinergic tone (for example, when exposed to anticholinergic substances).

With a standard meal, the gallbladder begins to contract, and the bile duct sphincters relax under the action of secreted intestinal hormones and cholinergic stimulation, which promotes approximately 75% of the contents of the gall bladder into the duodenum. And vice versa, when fasting, the tone of sphincters rises, which helps fill the gallbladder. Bile salts are poorly absorbed with passive diffusion in the proximal part of the small intestine; most bile acids reach the distal ileum, in which 90% is actively absorbed into the portal venous pathway. Once in the liver, the bile acids are effectively extracted and quickly modified (for example, binding of free acids) and secreted back into the bile. Bile salts are circulated along the enterohepatic circle 10-12 times a day.

[1], [2], [3], [4], [5]

Anatomy of the biliary tract

Salts of bile acids, conjugated bilirubin, cholesterol, phospholipids, proteins, electrolytes and water are secreted by the hepatocytes into the bile canals. The biliary secretion apparatus includes transport proteins of the tubular membrane, intracellular organelles, and cytoskeleton structures . Dense contacts between the hepatocytes separate the lumen of the tubules from the circulatory system of the liver.

The tubular membrane contains transport proteins for bile acids, bilirubin, cations and anions. Microvilli increase its area. Organelles are represented by the Golgi apparatus and lysosomes. With the help of vesicles, transport of proteins (for example, IgA) from the sinusoidal to the tubular membrane is carried out, the delivery of transport proteins synthesized in the cell for cholesterol, phospholipids and, possibly, bile acids from microsomes to the tubular membrane.

The cytoplasm of the hepatocyte around the tubule in the cytoskeleton structures: microtubules, microfilaments and intermediate filaments.

Microtubules are formed by polymerization of tubulin and form a network inside the cell, especially near the basolateral membrane and the Golgi apparatus, taking part in receptor-mediated vesicular transport, secretion of lipids, and under certain conditions, of bile acids. The formation of microtubules is inhibited by colchicine.

In the construction of microfilaments involved interacting polymerized (F) and free (G) actin. Microfilaments, concentrating around the tubular membrane, determine the contractility and motility of the tubules. Phalloidin, enhancing the polymerization of actin, and cytochalasin B, which weaken it, inhibit the motility of the tubules and cause cholestasis.

Intermediate filaments consist of cytokeratin and form a network between the plasma membranes, the nucleus, intracellular organelles and other structures of the cytoskeleton. The rupture of intermediate filaments leads to disruption of intracellular transport processes and obliteration of the lumen of the tubules.

Water and electrolytes affect the composition of the tubular secretion, penetrating through tight contacts between the hepatocytes due to the osmotic gradient between the lumen of the tubules and the Disse spaces (paracellular current). The integrity of the tight contacts depends on the presence on the inner surface of the plasma membrane of a ZO-1 protein with a molecular weight of 225 kDa. The rupture of tight contacts is accompanied by the ingress of dissolved larger molecules into the tubules, which leads to a loss of the osmotic gradient and the development of cholestasis. In this case, there may be a regurgitation of the tubular bile in the sinusoids.

The bile ducts flow into the ductules, sometimes called the cholangiols or the canals of Goering. Ductules are located mainly in the portal zones and flow into the interlobular bile ducts, which are the first of the bile ducts accompanied by branches of the hepatic artery and portal vein and are found in the portal triads. Interlobular ducts, merging, form septal ducts until two major hepatic ducts are formed, leaving the right and left lobes in the region of the liver gates.

[6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16],

Biliary secretion

The formation of bile occurs with the participation of a number of volatile transport processes. Its secretion is relatively independent of the perfusion pressure. The total bile current in humans is approximately 600 ml / day. Hepatocytes provide secretion of two fractions of bile: dependent on bile acids ("225 ml / day) and not dependent on them (" 225 ml / day). The remaining 150 ml / day are secreted by the cells of the bile ducts.

Secretion of bile salts is the most important factor in the formation of bile (a fraction that depends on bile acids). Water moves after osmotically active salts of bile acids. The change in osmotic activity can regulate the flow of water into the bile. There is a clear correlation between the secretion of bile salts and bile current.

The existence of bile fraction, which does not depend on bile acids, is proved by the possibility of the formation of bile, which does not contain bile salts. Thus, the continuation of the bile current is possible, despite the absence of excretion of bile salts; the secretion of water is due to other osmotically active soluble substances, such as glutathione and bicarbonates.

[17], [18], [19], [20], [21], [22], [23], [24], [25]

Cellular mechanisms of bile secretion

The hepatocyte is a polar secretory epithelial cell having a basolateral (sinusoidal and lateral) and an apical (tubular) membrane.

Bile formation involves the capture of bile acids and other organic and inorganic ions, transporting them through the basolateral (sinusoidal) membrane, the cytoplasm, and the tubular membrane. This process is accompanied by osmotic filtration of water contained in the hepatocyte and paracellular space. The identification and characterization of transport proteins of the sinusoidal and tubular membranes are complex. Especially difficult is the study of the secretory apparatus of the tubules, but by now a technique has been developed and proved reliable in many studies for obtaining double hepatocytes in a short-lived culture. Cloning transport proteins allows us to characterize the function of each of them separately.

The process of biliary formation depends on the presence of certain carrier proteins in the basolateral and tubular membranes. The role of the driving force of secretion performs the Na ⁺, K ⁺ - ATPase of the basolateral membrane, providing a chemical gradient and a potential difference between the hepatocyte and the surrounding space. Na ⁺, K ⁺ - ATPase exchanges three intracellular sodium ions for two extracellular potassium ions, maintaining a sodium concentration gradient (high outside, low inside) and potassium (low outside, high inside). As a result, the cell content has a negative charge (-35 mV) as compared to the extracellular space, which facilitates the capture of positively charged ions and the excretion of negatively charged ions. Na ⁺, K ⁺ -ATPase is not found in the tubular membrane. The fluidity of membranes can affect the activity of the enzyme.

[26], [27], [28], [29], [30], [31], [32], [33]

Capture on the surface of a sinusoidal membrane

Basolateral (sinusoidal) membrane has a variety of transport systems for the capture of organic anions, the substrate specificity of which partially coincides. Characteristics of carrier proteins were previously given on the basis of the study of animal cells. The recent cloning of human transport proteins has made it possible to better characterize their function. The transport protein for organic anions (organic anion transporting protein (OATP)) is sodium-independent, it transports molecules of a number of compounds, including bile acids, bromsulfalein and, probably, bilirubin. It is believed that the transport of bilirubin to the hepatocyte is also carried out by other carriers. The capture of bile acids conjugated with taurine (or glycine) is carried out by the sodium / bile acid cotransporting protein (NTCP) protein.

In the transfer of ions through the basolateral membrane involved protein, exchanging Na ⁺ / H ⁺ and adjusting the pH inside the cell. This function is also performed by the cotransport protein for Na ⁺ / HCO ₃ ^-. On the surface of the basolateral membrane is also the capture of sulfates, unesterified fatty acids, organic cations.

[34], [35], [36], [37], [38], [39], [40]

Intracellular transport

Transport of bile acids in the hepatocyte is carried out with the help of cytosolic proteins, among which the main role belongs to Za-hydroxysteroid dehydrogenase. Glutathione-S-transferase and fatty acid binding proteins are of lesser importance. In the transfer of bile acids, the endoplasmic reticulum and the Golgi apparatus are involved. Vesicular transport is included, apparently, only with a significant entry into the cell of bile acids (at concentrations exceeding physiological).

Transport of liquid phase proteins and ligands such as IgA and low-density lipoproteins is carried out through vesicular transcytosis. The time of transfer from the basolateral to the tubular membrane is about 10 min. This mechanism is responsible only for a small part of the total bile current and depends on the state of the microtubules.

Tubular secretion

The tubular membrane is a specialized region of the plasma membrane of the hepatocyte, which contains transport proteins (mostly ATP-dependent), responsible for the transfer of molecules in bile against the concentration gradient. In the tubular membrane, enzymes such as alkaline phosphatase and GGTP are also localized. The transfer of glucuronides and glutathione-S-conjugates (eg, bilirubin diglucuronide) is carried out using the tubular multispecific transport protein for organic anions (saplicular multispecific organic anion transporter-cMOAT), the transport of bile acids with the canalic transport protein for bile acids (canalicular bile acid transporter - сВАТ), whose function is partially controlled by the negative intracellular potential. The bile current, which does not depend on bile acids, is apparently determined by the transport of glutathione, as well as the tubular secretion of bicarbonate, possibly with the participation of a protein exchanging Cl ^- / HCO ₃ ^-.

An important role in the transport of substances through the tubular membrane belongs to two enzymes of the family of P-glycoproteins; both enzymes are ATP-dependent. The protein of multiple drug resistance 1 (multidrug resistance protein 1 - MDR1) transfers organic cations, and also removes cytostatic preparations from cancer cells, causing their resistance to chemotherapy (hence the name of the protein). The endogenous substrate MDR1 is unknown. MDR3 tolerates phospholipids and acts as a flipase for phosphatidylcholine. The MDR3 function and its importance for the secretion of phospholipids in bile are refined in experiments in mice lacking mdr2-P-glycoprotein (analogous to human MDR3). In the absence of phospholipids in bile, bile acids cause damage to the biliary epithelium, inflammation of the ductula and periductular fibrosis.

Water and inorganic ions (especially sodium) are excreted into the bile capillaries along an osmotic gradient through diffusion through negatively charged semipermeable tight contacts.

The secretion of bile is regulated by many hormones and secondary messengers, including cAMP and protein kinase C. An increase in the intracellular calcium concentration inhibits bile secretion. Passage of bile along the tubules is due to microfilaments, which provide motility and contraction of the tubules.

Ductular secretion

Epithelial cells of the distal ducts produce a bicarbonate enriched secret that modifies the composition of the tubular bile (the so-called ductular current, bile). During the secretion, cAMP, some membrane transport proteins, including the protein exchanging Cl ^- / HCO ₃ ^-, and the transmembrane conductivity regulator for cystic fibrosis - the membrane channel for Cl ^- regulated by cAMP are produced. Ductular secretion is stimulated by secretin.

It is assumed that ursodeoxycholic acid is actively absorbed by the ductular cells, is exchanged for bicarbonates, recycled in the liver and subsequently re-excreted into bile (a "cholegeptic shunt"). Perhaps, this explains the choleretic effect of ursodeoxycholic acid, accompanied by high biliary secretion of bicarbonates in experimental cirrhosis.

The pressure in the bile ducts, at which the secretion of bile occurs, normally amounts to 15-25 cm of water. Art. Increase of pressure up to 35 cm of water. Art. Leads to the suppression of bile secretion, the development of jaundice. The secretion of bilirubin and bile acids can be completely stopped, while the bile becomes colorless (white bile) and resembles a mucous fluid.

[41], [42], [43], [44], [45], [46], [47],