Histochemical demonstration of carbonic anhydrase activity in the alimentary canal

Distribution of the carbonic anhydrase isoenzymes I, II, and VI in the human alimentary tract

The distribution of carbonic anhydrase isoenzymes I, II, and VI was studied in the human alimentary tract using specific antibodies to human isoenzymes in conjunction with the immunoperoxidase technique to elucidate the physiological role and possible functional interplay of carbonic anhydrases (CAs) in alimentary canal functions. From the isoenzymes studied, CA II was found to be the most widely distributed in the various epithelia throughout the alimentary canal. In addition to the acinar cells of the parotid and submandibular glands and the duodenal Brunner's glands, it was present in the mucosal epithelium of the oesophagus, stomach, duodenum, and colon. The epithelial cells of the hepatic bile ducts, gall bladder, and pancreatic ducts also contained CA II in abundance. In contrast, CA VI was present only in the serous acinar and ductal cells of the parotid and submandibular glands, and CA I in the mucosal epithelium of the colon and the A cells of the pancreatic Langerhans's islets. These results suggest that CA II as a widely distributed isoenzyme in the epithelia of the alimentary canal and CA VI as secreted into saliva, may form a mutually complementary system protecting oesophageal, gastric, and intestinal mucosa from acidity.

Carbonic anhydrase isoenzymes in the caecum and colon of normal and germ-free rats

Histochemical and immunocytochemical methods were used to study the presence of carbonic anhydrase (CA) isoenzymes in the caecum and colon of normal and germ-free rats. Very high enzyme activity was demonstrated by histochemistry in the caecum and proximal colon of normal rats, while the activity decreased in the distal colon. Very strong immunostaining for the isoenzyme CA I was found in the cytoplasm of surface cells and upper gland cells in the caecum and colon of normal rats. In the distal colon the staining was less intense with a marked cell-to-cell variation. Ca II was found in the apical (luminal) cell region of the surface epithelium in all regions. Ca III was possibly present in small amounts, but this could not be judged with certainty. There was no difference in carbonic anhydrase between normal and germ-free rats (except for less staining of the mucosal capillaries in germ-free animals). Therefore, our data give no support to the hypothesis that CA I participates in the absorption of microbial fermentation products. The location of CA II in the apical cell region suggests a role for this isoenzyme in regulation of the microclimate close to the epithelial cells.

Occurrence, absorption and metabolism of short chain fatty acids in the digestive tract of mammals

Short chain fatty acids (SCFA) also named volatile fatty acids, mainly acetate, propionate and butyrate, are the major end-products of the microbial digestion of carbohydrates in the alimentary canal. The highest concentrations are observed in the forestomach of the ruminants and in the large intestine (caecum and colon) of all the mammals. Butyrate and caproate released by action of gastric lipase on bovine milk triacylglycerols ingested by preruminants or infants are of nutritional importance too. Both squamous stratified mucosa of rumen and columnar simple epithelium of intestine absorb readily SCFA. The mechanisms of SCFA absorption are incompletely known. Passive diffusion of the unionized form across the cell membrane is currently admitted. In the lumen, the necessary protonation of SCFA anions could come first from the hydration of CO2. The ubiquitous cell membrane process of Na+-H+ exchange can also supply luminal protons. Evidence for an acid microclimate (pH = 5.8-6.8) suitable for SCFA-protonation on the surface of the intestinal lining has been provided recently. This microclimate would be generated by an epithelial secretion of H+ ions and would be protected by the mucus coating from the variable pH of luminal contents. Part of the absorbed SCFA does not reach plasma because it is metabolized in the gastrointestinal wall. Acetate incorporation in mucosal higher lipids is well-known. However, the preponderant metabolic pathway for all the SCFA is catabolism to CO2 except in the rumen wall where about 80% of butyrate is converted to ketone bodies which afterwards flow into bloodstream. Thus, SCFA are an important energy source for the gut mucosa itself.

Amount and distribution of carbonic anhydrases CA I and CA II in the gastrointestinal tract

The levels of carbonic anhydrase (CA) activity and the amounts of carbonic anhydrase isoenzyme CA I and CA II proteins in the human gastrointestinal tract were determined by kinetic assays and radioimmunoassays. Cellular distribution of carbonic anhydrase activity in the various segments of the gastrointestinal tract were studied by the histochemical method of Hansson and the distribution of CA I and CA II by an immunohistochemical method. The stomach and the colon showed high carbonic anhydrase activity, the jejunum had intermediate activity, and the ileum had low activity. In the stomach CA II was the dominating isoenzyme, whereas the jejunum and the colon contained considerable amounts of both forms. Small amounts of both isoenzymes were found in the ileum. Carbonic anhydrase II immunofluorescence was demonstrated in the surface epithelium and in the parietal cells of the gastric mucosa, and in the epithelium of the jejunal villi. The surface epithelium of the colon contained both CA I and CA II. Carbonic anhydrase I was found in many superficial capillaries in all regions studied. The histochemical method demonstrated enzyme activity also at the cell membranes of gastric chief cells, intestinal crypt cells, and a subpopulation of ileal surface cells. This probably indicates presence of the membrane-bound isoenzyme CA IV.

Carbonic anhydrase in the monkey stomach and intestine

The distribution of carbonic anhydrase in the stomach and intestine of the cynomolgus monkey (Macaca fascicularis) was studied by the histochemical method of Hansson. In the gastric surface epithelium high enzyme activity was found in the cytoplasm and at the lateral cell borders. The parietal cells in the gastric glands also showed high enzyme activity, while the chief cells were less active. The mucous cells in the pyloric glands and in the Brunner's glands demonstrated a staining pattern similar to that of gastric surface cells. The mucosa of the duodenum and the jejunum was less intensely stained than the gastric mucosa. The enzyme activity was located at the lateral cell borders of the enterocytes, with weak or no cytoplasmic activity. Goblet cells and Paneth cells were unstained. In the ileum a small number of epithelial cells displayed high enzyme activity; their identity is not clear at present. In the cecum and colon large amounts of cytoplasmic carbonic anhydrase were found in the surface epithelium and in the upper part of the glands. Capillaries showing clear enzyme staining were found in the mucosa of all tissues; they were often located close to the surface epithelium and the glands. In the stomach, cecum and colon the distribution of the enzyme in the monkey appears very similar to that reported for other mammalian species, but in the small intestine clear differences exist. The functional role of carbonic anhydrase at the various sites in the gastrointestinal tract is only partly understood.

Ultrastructural localization of carbonic anhydrase activity in the rat choroid plexus epithelial cell

Localization of carbonic anhydrase activity was studied electron microscopically on cells of the rat choroid plexus epithelium. For the ultracytochemical detection of these activities, Yokota's technique (1969), which is the modification of Hansson's method (1967) was employed. Numerous electron dense reaction products were observed in the microvilli of the choroidal epithelial cell. The reaction deposits were also remarkably present in the infoldings of the basal plasmalemma but to a lesser extent than in the microvilli. The localization sites were mainly on the plasma membrane, but some reaction products were also observed in the cytoplasm near the plasma membrane. Hardly any reaction product was found in the intracellular organelles except for the mitochondria in which reaction products were occasionally observed on the cristae. These activities were completely inhibited by acetazolamide. As the carbonic anhydrase activity was histochemically seen in the microvilli and the basal infoldings, it is likely that carbonic anhydrase is related to an active transport process in the secretion of cerebrospinal fluid as is Na+, K+ -ATPase (Masuzawa et al. 1980).

Immunohistochemical localization of human carbonic anhydrase isoenzyme C

Methods for immunohistochemical localization of human carbonic anhydrase isoenzyme C (HCA C) with indirect fluorescent antibody and immunoperoxidase techniques are described. Both methods revealed large amounts of this "high activity" isoenzyme in the mucosae of human stomach and appendix. With the indirect immunofluorescent method the presence of the enzyme in human erythrocyte cytoplasm was also demonstrated. Correlations of present findings with those obtained with the traditional histochemical methods for demonstration of carbonic anhydrase activity are discussed.

Carbonic anhydrase in the intestinal tract of the guinea-pig

The distribution of carbonic anhydrase activity in the intestinal tract of the guinea-pig was studied by the histochemical method of Hansson. Enzyme activity was demonstrated in epithelial cells, erythrocytes and capillary walls. In the gastric mucosa parietal cells, surface mucous cells and neck mucous cells were highly active. In the small intestine only a few epithelial cells on the villi and in the upper part of the crypts showed enzyme activity. They seemed to be randomly scattered among inactive ones. It is not clear at present if they represent a distinct cell type or specialized absorptive cells. In the proximal colon most surface epithelial cells were highly active (goblet cells were inactive), whereas the surface cells in the distal colon showed less activity with a more varying degree of staining. In the cecum enzyme activity was found in the surface epithelium and in the upper part of the crypts, the staining being most marked at the luminal border of the surface cells. The staining reaction was completely inhibited in all tissues by 10 muM acetazolamide, except for the luminal staining of the cecum, which was inhibited only by 100 muM acetazolamide. This indicates the presence of high concentrations of carbonic anhydrase, probably of the "low activity" form, at this locus. Mucosal scrapings were taken from the intestinal tissues, homogenized and assayed for carbonic anhydrase activity by a changing-pH indicator method. The results confirm those of previous studies and correlate well with the histochemical findings.

The isoenzymes of carbonic anhydrase: tissue, subcellular distribution and functional significance, with particular reference to the intestinal tract

1. The total carbonic anhydrase activity in some guinea-pig tissues has been measured using a pH-stat procedure. Stomach, gall bladder, proximal colon and caecum all possess more carbonic anhydrase activity per unit amount of protein than does whole blood.2. The carbonic anhydrase activity of the small intestine is low. Reasons are given for supposing that activity found there is not entirely due to contamination by whole blood, and it is suggested that in this tissue the enzyme may be localized in some cell type other than the columnar absorbing cells.3. Evidence is presented which indicates that heavy metals interfere with the activity of the enzyme as measured in tissue homogenates.4. The distribution and concentration of the two major isoenzymes of carbonic anhydrase have been measured in different tissues. Blood and proximal colon contain both isoenzymes in comparable concentrations, the ratio of the concentration of the ;low activity' isoenzyme to that of the ;high activity' being about 2. The gastric mucosa contains much ;high activity' carbonic anhydrase, but only a negligible amount of the ;low activity' isoenzyme. In the caecal mucosa, the ;low activity' isoenzyme is predominant, the ratio of its concentration to that of the ;high activity' isoenzyme being about 9. It is also found that more than 1.5% of the protein in the caecal mucosa is accounted for as carbonic anhydrase enzymes.5. It is found that some 45% of the total carbonic anhydrase activity of sucrose homogenates of the guinea-pig colon is bound to particles. The activity is located mainly in the nuclear and microvillous fraction and in the ;high-speed supernatant' fraction. The form of enzyme bound is largely of the ;high activity' variety. When the tissue is homogenized in potassium chloride solutions less than 4% of the total activity is recovered in particulate fractions. The amount of activity which is bound to particulate fractions increases as the ionic strength or pH of the homogenate is lowered.6. The findings are discussed in relation to the possible physiological roles of the isoenzymes in tissues other than blood. Possible relationships between the presence of the enzymes and the metabolism and transport of ammonium and fatty acids are considered.