Evidence for the presence of carbonic anhydrase in the plasma membrane of rat hepatocytes

Guanylin and functional coupling proteins in the hepatobiliary system of rat and guinea pig

Guanylin, a bioactive intestinal peptide, is involved in the cystic fibrosis transmembrane conductance (CFTR)-regulated electrolyte/water secretion in various epithelia. In the present work we report on the expression and cellular localization of guanylin and its affiliated signaling and effector proteins, including guanylate cyclase C (Gucy2c), Proteinkinase GII (Pkrg2), CFTR and the solute carrier family 4, anion exchanger, member 2 (Slc4a2) in the hepatobiliary system of rat and guinea pig. Localization studies in the liver and the gallbladder revealed that guanylin is located in the secretory epithelial cells of bile ducts of the liver and of the gallbladder, while Gucy2c, Pkrg2, CFTR, and Slc4a2 are confined exclusively to the apical membrane of the same epithelial cells. Based on these findings, we assume that guanylin is synthesized as an intrinsic peptide in epithelial cells of the hepatobiliary system and released luminally into the hepatic and cystic bile to regulate electrolyte secretion by a paracrine/luminocrine signaling pathway.

The action of zymosan on octanoate transport and metabolism in the isolated perfused rat liver

The effects of zymosan on transport, distribution, and metabolism of octanoate in the perfused rat liver were investigated using the multiple-indicator dilution technique. Livers were perfused with 300 microM octanoate in the absence or in the presence of 100 microg/mL zymosan. Tracer amounts of [1-14C]octanoate, [3H] water, and [131I]albumin were injected into the portal vein, and the effluent perfusate was fractionated. The normalized dilution curves were analyzed by means of a space-distributed variable transit time model. Zymosan decreased the space into which octanoate undergoes flow-limited distribution, possibly the first cellular exchanging pool represented by plasma membranes and their adjacencies. However, the rate of transfer of octanoate from the plasma membrane into the rest of the cell was not modified as indicated by the similar values of the influx rates and also the net uptake of octanoate per unit of accessible cellular volume. However, when referred to the wet weight of the liver, the net uptake of octanoate was 37.5% reduced, a value corresponding to the diminution of the cellular accessible space. It can be concluded that an exclusion of a fraction of the liver parenchyma from the microcirculation is the main mechanism by which zymosan reduces the metabolism of exogenous octanoate.

The hemodynamic effects of zymosan in the perfused rat liver

The actions of zymosan on hepatic microcirculation and on the cell membrane permeability were investigated using the multiple-indicator dilution technique. The experimental system was the perfused rat liver. [(3)H]Water, [(3)H]sucrose and [(14)C]urea or [(14)C]bicarbonate were simultaneously injected into the portal vein. Mean transit times, distribution spaces, variances, linear superpositions and transfer coefficients across the plasma membrane were calculated. Zymosan had no net effect on the great vessels space but increased the extracellular sucrose space and decreased the aqueous cell space. Zymosan impaired the flow-limited distribution and increased the normalized variances of all tracers. The increase in the portal pressure caused by zymosan results most probably from a constriction just after or at the exit of the sinusoids. Impairment of the flow-limited distribution of tracers in the sinusoidal bed indicates that zymosan induces the formation of permeability barriers, which could make the access of the solutes to transporters or enzymes located on the outer surface of the plasma membrane difficult.

A histochemical study of carbonic anhydrase in the plasma membranes of human oral epithelial cells

Carbonic anhydrase (EC 4.2.1.1) was detected histochemically from the following regions in patients of various ages (14-84 yr): buccal mucosa, buccal flap, hard palate and tongue. The enzyme was principally located in the cell membranes but was also present in nuclei. There was a gradation in activity from basal (strong) to superficial cells (weak/negative). The carbonic anhydrase inhibitors ethoxyzolamide and acetazolamide abolished activity at 0.001 mM, but were ineffective, even at 1.2 mM, against a reaction associated with the granules of the stratum granulosum. No activity was detected in the absence of bicarbonate from the substrate.

Solvent isotope effect on bile formation in the rat

2H2O affects many membrane transport processes by solvent and kinetic isotope effects. Since bile formation is a process of osmotic filtration where such effects could be important, we investigated the effects of 2H2O on bile formation in the in situ perfused rat liver. Dose finding experiments showed that at high concentrations, 2H2O increased vascular resistance and induced cholestasis; at 60% 2H2O however, a clear dissociation between the vascular and biliary effects was observed. Therefore, further experiments were carried out at this concentration. The main finding was a reduction in bile salt-independent bile flow from 0.99 +/- 0.04 to 0.66 +/- 0.04 microliters.min-1.g-1 (P < 0.001). This was associated with a 40% reduction in biliary bicarbonate concentration (P < 0.001). Choleretic response to neither taurocholate nor ursodeoxycholate was altered by 2H2O; in particular, there was a similar stimulation of bicarbonate secretion by ursodeoxycholate in the presence of 60% 2H2O. To further elucidate this phenomenon, the effect of 2H2O on three proteins potentially involved in biliary bicarbonate secretion was studied in vitro. 2H2O slightly inhibited cytosolic carboanhydrase and leukocyte Na+/H(+)-exchange, these effects reached statistical significance at 100% 2H2O only, however. In contrast, Cl-/HCO(3-)-exchange in canalicular membrane vesicles was already inhibited by 50% (P < 0.001) at 60% 2H2O. Finally, there was a slight reduction in biliary glutathione secretion while that of the disulphide was not affected. Our results are compatible with an inhibition of canalicular Cl-/HCO(3-)-exchange by 2H2O. Whether this is due to altered hydration of the exchanger and/or of the transported bicarbonate remains to be determined.

Carbonic anhydrase in the membrane of the endoplasmic reticulum of male rat liver

We have prepared subcellular fractions of male rat liver homogenate by the method of Lewis and Tata [Lewis, J. A. & Tata, J. R. (1973) J. Cell Sci. 23, 447-459], further purifying the membranes of the microsomal fraction by exposure to 0.01% Triton X-100 and centrifugation. We determined the purity of the fractions with marker enzymes and measured carbonic anhydrase (CA; EC 4.2.1.1) activity in intact and solubilized particulates with 18O exchange between CO2/HCO3- and water. We measured the concentration of CA by titration with a sulfonamide inhibitor, ethoxzolamide, obtaining an average value of 3.8 mumol/mg of microsomal membrane protein. The equilibrium constant for binding ethoxzolamide was 0.49 x 10(-9) M. The Km for CO2 was 1.7 mM and the turnover number was 560,000 sec-1, characterizing this as a membrane-bound, high-activity isozyme of type IV. By electron microscopy of tissue sections after staining with a cobalt precipitation technique, CA was seen in small cytoplasmic vesicles in hepatocytes and in microsomal particles and membranes. There was a sulfonamide-resistant (isozyme type III) and a sulfonamide-sensitive (isozyme type II) CA in the cytosol but none in the rapidly sedimenting endoplasmic reticulum. We conclude that there is no CA normally within the matrix of the cell endoplasmic reticulum but that the CA type III found in the microsome may have been captured from the cytosol during resealing. Thus the adult male rat hepatocyte contains CA type IV in the membrane of the endoplasmic reticulum and CA type II and CA type III in the cytoplasm.

Plasma membrane-bound carbonic anhydrase activity in the regenerating rat liver

Part of the carbonic anhydrase activity of hepatocytes has been reported to be located in the plasma membrane. This strategic location suggests a physiological role other than that located within the cell and probably related to the specific secretory function of these cells. Furthermore, after two-thirds hepatectomy an enzymatic retrodifferentiation has been reported. We reasoned that liver regeneration probably affects the carbonic anhydrase activity in different ways depending upon its location and hence presumably physiological role. We measured, therefore, carbonic anhydrase activity in a soluble fraction or in a plasma membrane-enriched fraction obtained from liver homogenate from rats undergoing hepatectomy (two-thirds) one, three or seven days before liver resection and homogenation. No changes in carbonic anhydrase activity were found as far as soluble fraction was concerned. However, the carbonic anhydrase activity in plasma membrane was reduced (by 55%) soon after hepatectomy, there after it increased, returning to near control value at seven days. Lactate dehydrogenase activities in soluble and plasma membrane fractions were not modified by the regenerative process. Neither was 5'-nucleotidase activity determined in plasma membrane affected by liver regeneration. In summary, these results indicate a higher sensitivity of plasma membrane carbonic anhydrase activity to the regenerative process than soluble carbonic anhydrase activity. This suggests a different control of the turnover of these isoenzymes during rat liver regeneration. The phenomenon is consistent with a different physiological role for these activities; i.e., one (plasma membrane-bound carbonic anhydrase activity) may be involved in specific functions of differentiated hepatocytes, and another (soluble carbonic anhydrase activity) may be involved in general functions shared by both differentiated and undifferentiated cells.

Influence of backward perfusion on ursodeoxycholate-induced choleresis in isolated in situ rat liver

Ursodeoxycholate-induced bicarbonate-rich hypercholeresis was studied in isolated in situ forward- or backward-perfused rat livers. Both spontaneous bile flow and bile acid secretion were similar regardless of the direction of the perfusion. The choleretic effect of tauroursodeoxycholate infusion (400 nmol.min-1.100 g-1 body weight) was not significantly different in forward- or backward-perfused livers either. Ursodeoxycholate infusions at low rate (800 nmol.min-1.100 g-1 body weight) induced similar bile flow, bile acid output and bicarbonate output in both forward- and backward-perfused livers. Net ursodeoxycholate uptake, measured as [14C]ursodeoxycholate uptake over the bile acid infusion period (30 min), was not significantly different during forward- or backward-perfusion (4.8 and 5.1 mumol/g liver, respectively); i.e., approx. 67% of infused dose (approximately 7.5 mumol/g liver per 30 min). A 2-fold increase in the dose of ursodeoxycholate infusion (1600 nmol.min-1.100 g-1 b.wt.) induced additional enhancement in both bile flow and bicarbonate biliary secretion, but not in bile acid uptake or output, in forward-perfused livers. Moreover, infusion of the same dose of ursodeoxycholate to backward-perfused livers had a significantly lower choleretic effect (-29%, p less than 0.001) even though ursodeoxycholate uptake and biliary output were similar regardless of perfusion direction. Net ursodeoxycholate uptake, was only 2.4 mumol/g liver; i.e., approx. 16% of infused dose (approximately 15 mumol/g liver per 30 min). These findings indicate that a process related with the hepatic microanatomy may be involved in the hypercholeretic response to ursodeoxycholate.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane specific carbonic anhydrase (CAIV) expression in human tissues

Membrane-bound carbonic anhydrase IV (CAIV) expression has been evaluated in a range of fetal and adult human tissues and in cell culture. All tissues tested showed expression of CAIV, assessed by Western blotting, with a single immunodetected band at 55 kDa. The levels varied in fetal lung and liver during development and in various zones of the fetal brain. CAIV was clearly expressed in lung, pancreatic tumour and skin cell cultures.

Differential regulation of hepatic carbonic anhydrase isozymes in the streptozotocin-diabetic rat

Most work with the male rat liver carbonic anhydrase isozymes in the past decade has centered on the cytosolic CA III and the mitochondrial CA V. This paper reports that the relative activity of both isozymes is altered in streptozotocin-diabetes. Carbonic anhydrase activity of perfused liver homogenates and disrupted, isolated mitochondria was measured by the mass spectrometric 18O decay technique at 37 degrees C. The contributions of the different isozymes were determined based on intracellular location and sensitivity to acetazolamide inhibition. Diabetes resulted in a twofold increase in the activity of CA V but a halving in the activity of CA III. This is the first time that liver CA V has been shown to be altered by physiological stress. The total carbonic anhydrase activity in the diabetic rat liver was unaltered compared with control rats; however, CA III never accounted for more than 50% of this activity. Since CA isozymes I, II, and IV together account for 30% of the CA activity in control rats and 70% in diabetic rats it is concluded that one or more of these isozymes is subject to regulation in the diabetic male rat. The increase in CA V during diabetes is in accord with this isozyme having an important function in provision of substrate for hepatic gluconeogenesis and ureagenesis.