Transcellular compartments

Figure 41-14. The transcellular movement of glucose in an intestinal cell. Glucose follows Na+ across the luminal epithelial membrane. The Na+ gradient that drives this symport is established by Na+ -K+ exchange, which occurs at the basal membrane facing the extra-ceiiuiarfiuid compartment. Glucose at high concentration within the ceii moves "downhill" into the extracel-iuiarfiuid by fadiitated diffusion (a uniport mechanism).

FIG. 2 Mechanisms of drug transfer in the cellular layers that line different compartments in the body. These mechanisms regulate drug absorption, distribution, and elimination. The figure illustrates these mechanisms in the intestinal wall. (1) Passive transcellular diffusion across the lipid bilayers, (2) paracellular passive diffusion, (3) efflux by P-glycoprotein, (4) metabolism during drug absorption, (5) active transport, and (6) transcytosis [251]. 

The transcellular fluid includes the viscous components of the peritoneum, pleural space, and pericardium, as well as the cerebrospinal fluid, joint space fluid, and the gastrointestinal (GI) digestive juices. Although the transcellular fluid normally accounts for about 1% of TBW, this amount can increase significantly during various illnesses favoring fluid collection in one of these spaces (e.g., pleural effusions or ascites in the peritoneum). The accumulation of fluid in the transcellular space is often referred to as third spacing. To review the calculations of the body fluid compartments in a representative patient, see Patient Encounter 1. 

Figure 3. Transcellular transport of HRP-S-PLL in filter-grown MDCK cell monolayers. Confluent MDCK monolayers in Transwells were treated at the basal compartment (closedsquares)or the apical compartment (open squares) with 3 pg/mL HRP-S-PLL conjugate.

In whole tissue or cell monolayer experiments, transcellular membrane resistance (Rm = Pm1) lumps mucosal to serosal compartment elements in series with aqueous resistance (R = P ). The operational definition of Lm depends on the experimental procedure for solute transport measurement (see Section VII), but its magnitude can be considered relatively constant within any given experimental system. Since the Kp range dwarfs the range of Dm, solute differences in partition coefficient dominate solute differences in transcellular membrane transport. The lumped precellular resistance and lumped membrane resistance add in series to define an effective resistance to solute transport ... 

Secretory epithelia control transport of water and solutes from the subluminal compartment (blood) into the lumen or body exterior. At present, there is no single unifying model for transepithelial fluid or water transport. In some epithelia, transcellular routes of fluid transport via water channels may predominate [88a], However, in other types of epithelia, such as the cervical-vaginal epithelia, transport of fluids usually occurs via the paracellular route [1, 14], In the latter, movement of fluid can be driven by three main mechanisms (Figure 15.1C) ... 

The transcellular leakage is determined by incubating cells with 50 pM 14C-inulin carboxylic acid in the basolateral compartments for 30 min and measuring the radioactivity in the apical compartments. The transcellular leakage should be less than 1 %. 

Passively absorbed compounds diffuse either through the cell itself (transcellular pathway) or in between cells (paracellular pathway). The lipid bilayers of which the mucosal and basolateral epithelial cell membranes are composed of, define the primary transcellular diffusion resistance to solute transport across the intestinal barrier. Transcellular permeabihty, particularly of lipophilic solutes, depends on their partitioning between intestinal membrane and aqueous compartments (Fig. 1). 

In conclusion, it may be clear that RMT and CMT are important processes for the influx and efflux of substances to and from the BBB endothelial compartment. Changed activity of these processes due to disease, for example, can have serious consequences for functionality and integrity of the BBB. On one hand, this can result in increased para- and transcellular BBB permeability, and therefore, in changed CNS homeostasis. Ultimately, this can lead to CNS diseases, e.g., Alzheimer s or other neurodegenerative diseases (25-27). On the other hand, this offers opportunities for site-specific or targeted drug delivery to the brain when transport processes are selectively upregulated under disease conditions. 

This buffer is used in both the unidirectional (permeability) and the bidirectional (Efflux, Section 4.3.2). In the permeability assay, NCEs are placed into the apical compartment of the Transwell. Duplicate samples are taken immediately after compound addition from the apical compartment (zero time) and then after 2h from both the apical and basolateral compartments for LC-MS/MS analysis. The integrity of the monolayer is confirmed by the measurement of the transepithelial electrical resistance (TEER), which must be above a certain limit to be used for transport experiments. In addition, with each experiment a transcellular and a paracellular marker are included for quality control. 

Figure 4 Transcellular transport in the proximal and distal intestine of the suckling rat. (A) In the proximal intestine, macromolecules may be selectively absorbed in coated pits along luminal membranes (M) and transported via coated vesicles to the basolateral surface (arrows). Alternatively, selection may occur in tubular compartments (T) or from endosomal vesicles (V). (B) In the distal intestine selective absorption may occur in coated pits at the luminal membrane (M) or from the numerous tubules (T) of the elaborate endocytic complex. (From Ref. 18.)...

Extracellular fluid (ECF) is divided into smaller compartments. These spaces between the cells are called the interstitial space. The space is occupied by plasma and lymph, transcellular fluid, and fluid in the bone and connective tissues. This makes up 20% of body weight. About a third is plasma and two thirds of extracellular fluid is in the space between the cells. Transcellular fluid is also ECF but is found in the gastrointestinal (GI) tract, cerebrospinal space, aqueous humor, pleural space, synovial space, and the peritoneal space. Although fluid in the transcellular space is a small volume when compared with intracellular and extracellular compartments, the increase or decrease in volumes in transcellular spaces can have a dramatic effect on the fluid-electrolyte balance. 

Fig. 7. Transport pathways of a polymer in the liver 1 — diffusion through theintercellular junctions (molecular-size limited process) 2 — transcellular route of polymer transport into the bile including pinocytosis into hqtatocyte and exocytosis of the vesicles or residual bodies at the lateral side of the cell 3 — pinocytosis into the Kupffer cells occurs regularly, from either the central or the interstitial compartment. H.C. — hepatocyte E.C. — endothelial cell of capillary wall K.C. — Kupffer cell I.S. — interstitial space B.C. — bile canaliculi P — pinocytic vesicle L — lysosomes primary) S.L. — secondary lysosome R.B. — residual body N — nudeus...

The electrometric intracellular [Cl ] of 18.7 .1.3 mM, while it accounts for only 2/3 of the total Cl content of proximal tubule cells, is still significantly greater than that expected from a simple passive distribution of this ion between the intracellular fluid and the two extracellular fluid compartments (luminal and peritubular). Therefore, chloride must be actively transported across the luminal membrane by an anionic pump or a neutral NaCl pump. This constitutes the first or luminal step in transcellular chloride reabsorptive transport. In the second or, peritubular step. Cl could passively accompany the actively and electrogenically extruded Na" as well as be a component of a peritubular electroneutral NaCl active transport process. 

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