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PubMed1984

Purification and characterization of carbonic anhydrase from the saliva of the rat.

Carbonic anhydrase purified from the saliva of the rat had kinetic properties identical with those of carbonic anhydrase II from rat red cells, but its molecular properties were distinctly different from the type II isozyme. Kinetic parameters were measured under steady state conditions by stopped-flow spectrophotometry and under equilibrium conditions by an 18O exchange method. The turnover number kcat for hydration of CO2 was 6.5 X 10(4) s-1 and the Michaelis constant was 4.2 mM at pH 7.5 and 25 degrees C, values which are equal to the steady state constants for red cell carbonic anhydrase II from the rat. Inhibition of the salivary isozyme by sulfanilamide (Ki = 3.7 microM) was nearly as efficient as inhibition of the erythrocyte isozyme II (Ki = 1.1 microM). The molecular weight for the salivary isozyme was 46,000 and the isoelectric point was 5.5. Salivary carbonic anhydrase had high mannose oligosaccharide components as measured by concanavalin A binding. The amino acid composition for the salivary isozyme was not similar to rat type II, but it was similar to that reported for membrane-bound carbonic anhydrase from bovine lung (Whitney, P.L., and Briggle, T.V. (1982) J. Biol. Chem. 257, 12056-12059). These observations suggest to us that salivary carbonic anhydrase is a secretory product.

Animals↗
PubMed1993

Carotid body chemoreception: the importance of CO2-HCO3- and carbonic anhydrase. (review).

The current hypotheses of carotid body (CB) chemoreception regard the glomus cells as the initial site of stimulus transduction. The consensus is that the transduction of chemical stimulus is coupled with the release of transmitter(s) from the glomus cells, which in turn generates action potentials in the afferent nerve terminals. Carbonic anhydrase (CA) is present in the glomus cells of the CB. Inhibition of CA activity in the CB in situ reduces the carotid chemosensory responses to CO2 and to O2, suggesting a common mechanism of chemosensing for both stimuli. However, CA inhibitors also block the red blood cell enzyme. Thus, the CO2 hydration reaction does not come to completion within the transit time of the blood from the lung to the CB. A steady-state reaction is not reached until later and so the PCO2 and pH levels in arterial blood samples are not the same as those sensed by the CB. Experiments in vitro using cat CB perfused and superfused with cell-free solutions, which had been pre-equilibrated with respiratory gases, strongly support the proposition that the CA activity in CB cells is essential for the speed and amplitude of the initial response to CO2 and for its subsequent adaptation. The immediate response to hypoxia also is delayed, but the late steady-state was less dependent on CA activity. In the nominal absence of CO2-HCO3- from the perfusate, hypoxic chemoreception persisted and its magnitude is not affected by CA inhibition, except for a delay which may be due to the initial alkaline pH of the glomus cells. Recent experiments performed in isolated glomus cells and in the whole CB show that hypoxia does not modify significantly the intracellular pH. By its simple catalytic function, CA can speed up the approach of the CO2 hydration reaction to equilibrium. However, CA may also contribute in the steady-state to the regulation of pHi by providing a continuous supply of H+ and HCO3-. Furthermore, CA performs a facilitatory role in the physiological chemosensory responses to CO2 and O2 in the presence of extracellular CO2-HCO3-. This role is likely to be related to the ion exchanger function and then to pHi regulation in the chemoreceptor cells.

Animals↗
PubMed1994

Basolateral Cl-/HCO3- exchange in epithelial cells from guinea pig distal colon.

In guinea pig distal colon, H+ secretion into the lumen due to the apical ouabain-sensitive H+, K(+)-ATPase activity is associated with HCO3- exit into the serosa. This study aimed at elucidating (1) the transport mechanism of HCO3- exit across the basolateral membrane and (2) the relation of lactate release to the H+, K(+)-ATPase activity, in isolated mucosae mounted in Ussing chamber. The HCO3- exist across the basolateral membrane depended on the serosal Cl-. 4,4'-Diisothiocyanostilbene- anostilbene-2,2'-disulfonic acid (DIDS) applied to the serosal side reduced the rate of HCO3- exit across the basolateral membrane. The lactate was released to the serosal side, but not to the luminal side. The serosal lactate appearance was abolished when the apical H+, K(+)-ATPase was inhibited by luminal ouabain. These results suggest that HCO3- exit across the basolateral membrane is mediated by a DIDS-sensitive Cl-/HCO3- exchange. In addition, the lactic acid production appears to be coupled to the apical H+, K(+)-ATPase activity.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1975

The rates of ion movement from plasma to aqueous humor in the dogfish, Squalus acanthias.

Aqueous humor dynamics were studied in the dogfish, Squalus acanthias, using isotopically labeled inulin, Na+, Cl-, and HCO-3. Fluid production was 100 mul per hour, with a turnover rate constant of 0.4 hr.-1, about half that of mammals. The ciliary process contains carbonic anhydrase and Na-K-ATPase. Diffusion of Na+ and Cl- from plasma to aqueous was four to five times greater than flow, and from aqueous to vitreous, about 15 times greater. Sodium and chloride secretion is masked by the diffusion process; neither ouabain nor acetazolamide yield measurable effects on accumulation of these isotopes. HCO-3 accumulation in aqueous was very rapid and was reduced by inhibition of carbonic anhydrase. Analyses of the data suggest that newly formed aqueous has similar Na+ concentration to plasma, but is high in HCO-3 and low in Cl-. This anion pattern resembles mammalian aqueous humor, and cerebrospinal fluid and endolymph of other vertebrates. These and other data suggest that constant features of formation of these fluids in all phyla are sodium transport and formation of HCO-3 from CO2.

Acetazolamide↗
PubMed1993

Chloride dependent intracellular pH increase induced by bepridil in human red blood cells: a possible involvement in correction of ischemic acidosis.

Inhibitors of Na+/H+ exchanger are reported to exert an anti-ischemic effect. Some calcium antagonists and particularly bepridil are commonly used as anti-ischemic agents. Therefore, in this study, we test the hypothesis that protective effect against ischemia may occur at least in part through an action on Na+/H+ exchanger. The effect of some calcium antagonists on Na+/H+ exchanger from acid-loaded and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid disodium salt (DIDS)-treated human red cells (RBC) has been studied with a pHstat technique. Doses above those required to affect calcium channel (10(-5) and 10(-4) M) of nifedipine, nicardipine, verapamil and diltiazem had no effect on Na+/H+ exchanger activity. Ethyl isoproply amiloride (10(-4) M) completely inhibited the exchanger. Among the calcium antagonists tested, bepridil exhibited a particular effect, dissipating the pH gradient independently from the Na+/H+ exchanger activity. Bepridil's effect on DIDS-treated RBC was compared with that of a well known protonophore, carbamyl cyanide p-(trifluoromethoxy)phenyl hydrazone, and with that of tributyltin, which mediates a Cl-/OH- exchange across the cell membrane. Bepridil (> 2 x 10(-6) M) acts like tributyltin by dissipating the pH gradient whatever the external cation (Na+ or K+) or the membrane potential, and its action depends on the ratio intracellular [Cl-]/extracellular [Cl-]. The dissipation seems to occur through an OH-/Cl- exchange but other mechanisms may intervene. Moreover, intraerythrocytic pH measurement by 31P nuclear magnetic resonance clearly showed that bepridil permits the cell to recover normal pH faster than control.(ABSTRACT TRUNCATED AT 250 WORDS)

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗