Crypt cells

The physiological role of the ICOR is not clear and may be heterogeneous in the various tissues. In the thick ascending limb of the loop of Henle this channel appears to serve as the exit for CP at the basal cell pole [16,65,66], This conductive mechanism, therefore, is required for the reabsorption of Na and CP by this segment of the nephron [16]. In the rectal gland of Squalus acanthias a very similar channel is utilized for Na" and CP secretion. In these latter cells the CP-channel is present in the luminal membrane and is controlled by cytosolic cAMP [15,56,71]. It has been claimed that this kind of channel is also responsible for the secretion of CP in the colonic crypt cell, in colonic carcinoma cells and in respiratory epithelial cells [17,19,20,22]. Recent data have cast some doubt on this concept ... 

I have noted that NPPB is structurally related to loop diuretics of the furosemide (Fig. 2) type. These latter compounds bind to the Na 2CNK -cotransporter [16] and inhibit NaCl reabsorption in the TAL segment and NaCl secretion in epithelia such as the colonic crypt cell and rectal gland of Squalus acanthias [15]. We were able to show that only minor modification of the NPPB molecule on one side and of furosemide on the other led to compounds with altered selectivities [70,91-93]. One prototype of an intermediate blocker, i.e., a substance blocking both Na 2Cl K -cotransport and CP-channels, is torasemide (Fig. 2). Hence we have performed a systematic study in order to define the constraints defining the effectiveness of this class of substances [91]. 

Cahill, R.J., O Sullivan, K.R, Mathias, P.M., Beattie, S., Hamilton, H. and O Morain, C. (1992). The effect of nutrient antioxidants on colonic crypt cell proliferation in patients with adenomatous polyps. Gastroenterology 102, A919. 

Additional epithelial aqueous pathways of significantly smaller radius (<3 A) have also been documented utilizing both equivalent pore and circuit theory [25], These pathways may correspond to specific channels through lipid membranes as opposed to paracellular pathways. Osmotically activated ion channels [35] and even specific water channels [36] have been characterized in renal epithelia. In intestinal epithelia, mucosal chloride channels have been studied in secreting crypt cells, and basolateral potassium channels in colonic epithelia serve cellular ion and volume homeostatic functions. 

Figure 14 Ion transport pathways responsible for water flux across intestinal epithelia. Sodium absorption in villus tip cells (left) stimulates water absorption, while chloride channel exit in crypt cells (right) stimulates water secretion.

Figure 8.2 Rat duodenal cells divide in the crypts of Lieberktihn and differentiate while migrating to the villus tips within approximately 48 h. The crypt cells take up iron from the blood, and are thereby able to sense the body s state of iron repletion. They migrate to the villus tips where this information determines their iron absorption capacity from the intestinal lumen. Adapted from Schumann et al., 1999, by permission of Blackwell Science.

The situation prevailing in the crypt cell at the beginning of its differentiation into an enterocyte and before it has begun to climb towards the villus is shown in the lower panel. The cell s iron requirements are supplied by receptor-mediated diferric transferrin uptake from the basolateral membrane. The TfR in turn is involved in an interaction with the HFE protein, which decreases the affinity of TfR for diferric transferrin. The level of transferrin saturation, or some other factor, determines the amount of iron taken up, and presets the IRP system at a level that corresponds to the iron requirements of the organism. 

Another human colonic cancer cell line is T84 this develops monolayers of high TER ( 1000 Q cm2) when grown on permeable supports, but cells are not well differentiated and have been described as resembling a colonic crypt cell phenotype. Hence, these cells have been used mainly in studies of epithelial ion transport and are generally not considered to be adequate for drug transport studies, particularly with respect to carrier-mediated processes [10, 79, 82-84]. 

Additionally, tegaserod at low nanomolar concentrations increases intracellular cAMP concentrations in crypt cells isolated from rat distal colon and stimulates chloride and water secretion by activation of 5-HT4 receptors [34,35], These findings suggest a modulatory effect on intestinal electrolyte and water secretion in vivo. 

Other cell lines used in permeability studies include the T84 human colonic adenocarcinoma colonic crypt cell model. This line has a reduced carrier expression, secrets mucus, and has very high resistance [31, 32], The IEC cell line is a rat fetal intestinal epithelium cell with higher permeabilities than Caco-2 cells [33], LLC PKi is a pig kidney epithelial cell line with low expression of efflux systems, but expression systems for transport proteins [32], 2/4/A1 cells are a conditionally immortalized rat fetal intestinal epithelium line with crypt cell-like morphology and temperature-sensitive differentiation [34], They form differentiated monolayers with tight junctions, increased brush border enzymes when grown on extracellular matrices with laminin. Transport of drugs with LP in 2/4/A1 monolayers was comparable to that in the human jejunum and up to 300 times faster than that in Caco-2 monolayers. In contrast, the permeability of HP drugs was comparable in both cell lines [34],... 

M. S. Kulkarni and K. L. Yielding, DNA damage and repair in epithelial (mucous) cells and crypt cells from isolated colon, Chem. Biol. Interact., 1985, 52(3), 311. 

Glickman, L.T., Suissa, S. Fleiszer, D.M. (1987) Proliferative characteristics of colonic crypt cells in C57BL/6J and A/J mice as predictors of subsequent tumor formation. Cancer Res., 47, 4766-4770... 

Epithelial cells of small intestine were prepared in a fractional way (4), the older, less adherent villus tip cells being washed out by EDTA-containing phosphate buffer first, while mitotic crypt cells appeared in the final fractions. The enzyme characteristics of the series of fractions obtained (Fig. 13) followed conventional criteria for differentiated (villus) and less differentiated (crypt) cells (3, 4). The thymidine kinase activity decreased from crypt to villus while the activity of alkaline phosphatase increased (Fig. 13). 

Crypt cells actively secrete electrolytes, leading to water secretion (Figure 4.8). The apical membranes of crypt epithelial cells contain an ion channel of immense medical significance, a cyclic adenosine monophosphat (cAMP)-dependent chloride channel, known as the cystic... 

Figure 4.8 Crypt cells actively secrete electrolytes.

Elevated intracellular concentrations of cAMP in crypt cells activate the CFTR, resulting in secretion of chloride ions into the lumen. 

A defective CFTR channel will disturb electrochemical gradients in both intestinal crypt cells and pancreatic epithelial cells, with a common end resutt, namely the inability to secrete water into the lumen. In crypt cells, chloride is normally transported to the lumen, drawing sodium ions across the tight junctions and creating an osmotic potential that in mrn draws water into the lumen (Figure 4.11). A defective CFTR wiU prevent the gradient forming and hence halt water movement to the lumen. 

---