Carbonic anhydrase in the amphibia

Pulmonary carbonic anhydrase in vertebrate gas exchange organs

Carbonic anhydrase (CA) catalyzes the interconversion of CO(2) and HCO(3)(-). Intracellular (extravascular) and intravascular (extracellular) CA has been identified and localized in the lungs of reptiles and mammals. Less information is known, however, on the presence of intravascular CA in the lungs of amphibians and avians. In the present study, perfusion studies were used to compare the catalytic activity of pulmonary intravascular CA in reptiles and mammals. In addition, SDS-resistant CA activity was examined in microsomal fractions prepared from gill/lung tissue from representative animals in each vertebrate class. Finally, the CNO(-) sensitivity of the microsomal CA activity was compared. No SDS-resistant CA activity was found in gill microsomal fractions of several fish species. In contrast, the data suggest that SDS-resistant, intravascular pulmonary CA activity is present in air-breathing vertebrates with vastly differing lung morphologies and that the kinetics of inhibition is remarkably comparable between the vertebrate classes.

Comparative physiology and molecular evolution of carbonic anhydrase in the erythrocytes of early vertebrates

The different isozymes of carbonic anhydrase (CA) have been the subject of intensive study in mammals, but there is still much to be learned about the early evolution of this enzyme in vertebrates. Erythrocyte CA plays an essential role in the respiratory processes of most vertebrates and is probably the most well studied CA isozyme. The available evidence indicates that there has been a progressive increase in the efficiency of erythrocyte CA during the early evolution of vertebrates. There also appears to be a substantial increase in erythrocyte CA activity during development in some species. At the present time, however, the selective pressures that may be influencing the properties of erythrocyte CA during vertebrate evolution and development have not been clearly determined. When the available molecular sequence information is examined, it is evident that the erythrocyte CAs of early vertebrates have active sites that are more similar to those of mammalian CA VII and II, rather than CA I. We can now also begin to examine the phylogenetic relationships between the different rbc CAs in vertebrates, but more CA sequence information is clearly required from different groups of vertebrates before we have a complete picture of the molecular evolution of erythrocyte CA.

Carbonic anhydrase and red blood cell anion exchange in the neotenic aquatic salamander, Necturus maculosus

Carbonic anhydrase (CA) activity in blood and other tissues and red blood cell (rbc) anion exchange were measured in the mud puppy, Necturus maculosus, in order to gain insight into the strategy for CO2 transport used by these neotenic salamanders and to further explore evolutionary relationships between rbc CA activity and anion exchange in nonmammalian vertebrates. CA activity was detectable in all of the tissues examined, but CA activity in blood was much lower than that in most vertebrates. There was no indication, however, that additional CA had been incorporated into the membrane fraction of other tissues to compensate for this low blood CA activity. In further contrast to most other animals, low levels of CA activity were also detectable in mud puppy plasma. Preliminary characterization of the rbc CA indicated that the Type II, fast-turnover enzyme was indeed present, but that there are a very low number of active sites in mud puppy rbc's. Further experiments showed that the rbc's were highly permeable to anions and that the relative rate of anion flux could be inhibited by 4, 4-diisothiocyanostilbene-2,2-disulphonic acid. Thus, the process of CO2 transport in the blood of mud puppies probably involves components of the Jacobs-Stewart cycle, as in most other vertebrates.

A developmentally regulated wound epithelial antigen of the newt limb regenerate is also present in a variety of secretory/transport cell types

The role of the wound epithelium in amphibian limb regeneration is not understood. We showed previously that monoclonal antibody (mAb) WE3 stains the wound epithelium but not skin epidermis, suggesting that the WE3 antigen may be a marker for, or be important in, the function of the wound epithelium. In the present study, we conducted an extensive immunohistochemical survey of adult newt tissues to define the distribution of the WE3 antigen. The results show that the antigen is most commonly found in tissues specialized in macromolecular secretion and/or ion transport. Since the enzyme, carbonic anhydrase, serves as a useful marker for a variety of specialized transporting cell types, we examined whether this enzyme was present in WE3-reactive cells. Of the tissues examined, a striking degree of colocalization of carbonic anhydrase and the WE3 antigen was observed, further strengthening the view that the WE3 antigen is an important constituent of specialized transporting cells. A preliminary biochemical characterization suggests that the antigen is probably a glycoprotein, which elutes during gel filtration as a species of over 660 kDa. Possible implications for the function of the wound epithelium are discussed.

Acetazolamide does not disrupt limb regenerate morphogenesis in the salamander, Plethodon cinereus

Acetazolamide, a potent and highly specific inhibitor of carbonic anhydrase, is teratogenic in mammalian embryos and when administered during early limb development causes unique limb defects in a time- and dose-dependent manner. The regenerating urodele limb is often considered to be a good experimental analog of limb development and, if it employs the same mechanisms of tissue interactions during pattern formation, should be susceptible to teratogens which selectively disrupt developmental limb patterning. This study demonstrates that while carbonic anhydrase inhibition is toxic to the red-backed salamander, Plethodon cinereus, it does not have the same teratogenic effect on limb regeneration as seen in mammalian limb development. Several points are considered as to why the regenerating limb, at least in this salamander species, may not be suitable for studying this class of teratogen.

Acid-base status in the toad Bufo viridis in vivo

The acid-base status in the toad Bufo viridis was assessed on blood samples taken through a chronical cannula from unrestrained animals. Blood gases and the pH were very constant in animals which were kept either under control conditions (free access to tap water) or immersed in shallow water. pH was 7.646 +/- 0.032 (mean +/- SEM) in 14 control toads at 26 degrees C. PCO2 was 13.1 +/- 1.2 mm Hg and [HCO3-] was 17.0 +/- 0.7 mmol/l at the same temperature. pH and PCO2 were independent of the hematocrit and the haemoglobin in the blood. Diamox induced a characteristic metabolic acidosis which is apprently the result of the inhibition of carbonic anhydrase both in the kidney and in the skin. The results are discussed in relation to the role of the kidney and the skin in regulating the acid-base balance in the toad.