(Na + -K + )-activated ATPase in isolated mucosal cells of toad bladder

Identification of Na+/K+-ATPase α/β isoforms in Rhinella marina tissues by RNAseq and a molecular docking approach at the protein level to evaluate α isoform affinities for bufadienolides

Na⁺/K⁺-ATPase (NKA) function is inhibited by Bufadienolides (BD), a group of cardiotonic steroids (CTS) primarily produced by anurans of the Bufonidae family, such as Rhinella marina. This study characterized the presence of α and β NKA subunit isoforms in R. marina via RNAseq in four tissues: oocytes, skin, heart, and skeletal muscle. Transcripts encoding three α-like isoforms (α₁, α₂, α₃) and three β-like isoforms (β₁, β₂, β₄) were identified. The amino acid sequence of α₁-like isoform shared 99.4% identity with the α₁ isoform previously published for R. marina. Sequences for α₂, α₃, and β₄ from R. marina were previously unavailable. The first extracellular loop in the α₂-like isoform in R. marina showed similar substitutions to those found in their susceptible homologues in other taxa (L/Q111T and S119T); in contrast, this same loop in α₃-like isoform showed similar substitutions (Q111L and G120R) to those reported for toad-eating animals such as snakes, which suggests relatively lower affinity for CTS. Docking results showed that all three α-like isoforms identified in R. marina transcriptomes have low affinity to CTS compared to the susceptible α₁ isoform of Sus scrofa (pig), with α₁-like isoform being the most resistant. The tissue-specific RNAseq results showed the following expression of NKA α-like and β-like subunit isoforms: Oocytes expressed α₁ and β₁; skin α₁, β₁, and low levels of β₂; heart α₁, α₃, and β₁; skeletal muscle α₁, β₄, with low levels of α₂, α₃, and β₁. R. marina could be used as an important model for future structural, functional and pharmacological studies of NKA and its isoforms.

Oral exposure to the organophosphorus insecticide, Monocrotophos induces intestinal dysfunction in rats

There is limited experimental evidence to imply the role of organophosphorus insecticides on intestinal dysfunctions. Residues of Monocrotophos (MCP), above maximum residue limits (MRL), have been reported in fruits and vegetables from various parts of India. Hence, in this study, we investigated the potential of MCP to induce intestinal dysfunction in rats. MCP was administered orally to rats at sublethal doses (0.45, 0.9 and 1.8 mg/kgb.w/d) for 30 days. MCP at the highest dose significantly increased the unit weight of the small intestine. MCP increased the activities of intestinal brush border disaccharidases, intestinal alkaline phosphatase, glycyl-glycine dipeptidase, and Na(+)/K(+)-ATPase while it decreased cholesterol: phospholipid ratio. Histology and scanning electron microscopy of small intestine of MCP treated rats revealed disruption in terms of congestion, increased length of villi, goblet cell hyperplasia, infiltration of inflammatory cells and necrotic villi tip. Further, the intestinal transit rate was found to be increased in MCP treated rats. Collectively, our findings provide evidence that repeated oral intake of MCP has the propensity to alter small intestinal structure and functions, which might lead to intestinal dysfunctions and abnormal nutrient uptake and thereby affect the human health. Although we have employed doses, which are higher than those likely to be encountered as residues, we speculate that further studies should be performed to determine whether MCP residues in foods in the long-term will interfere with the digestive capacity of the small intestine and thus exert adverse effects on the health of human.

Ouabain-sensitive Na+,K(+)-ATPase activity in toad brain

Toads of the genus Bufo are highly resistant to the toxic effects of digitalis glycosides, and the Na+,K(+)-ATPase of all toad tissues studied to date has been relatively insensitive to inhibition by digitalis and related compounds. In studies of brain microsomal preparations from two toad species, Bufo marinus and Bufo viridis, inhibition of ATPase activity and displacement of [3H]ouabain from Na+,K(+)-ATPase occurred over broad ranges of ouabain or bufalin concentrations, consistent with the possibility that more than one Na+,K(+)-ATPase isoform may be present in toad brain. The data could be fitted to one- or two-site models, both of which were consistent with the presence of Na+,K(+)-ATPase activity with high sensitivity to ouabain and bufalin. Ki (concentration capable of producing 50% inhibition of activity) values for ouabain in the one-site model were in the 0.2 to 3.7 microM range, whereas Ki1 values in the two-site model ranged from 0.085 to 0.85 microM, indicating that brain ATPase was at least three orders of magnitude more sensitive to ouabain than B. marinus bladder ATPase (Ki = 5940 microM). Ouabain was also an effective inhibitor of 86Rb+ uptake in B. marinus brain tissue slices (Ki = 3.1 microM in the one-site model; Ki1 = 0.03 microM in the two-site model). However, the relative contribution of the high ouabain-sensitivity site to the total activity was 17% in the transport assay as compared with 63% in the Na+,K(+)-ATPase enzymatic assay. We conclude that a highly ouabain-sensitive Na+,K(+)-ATPase activity is present and functional in toad brain but that its function may be partially inhibited in vivo.

The rat distal colon P-ATPase alpha subunit encodes a ouabain-sensitive H+, K+-ATPase

The functional properties and the pharmacological profile of the recently cloned cDNA colonic P-ATPase alpha subunit (Crowson, M.S., and Shull, G.E. (1992) J. Biol. Chem. 267, 13740-13748) were investigated by using the Xenopus oocyte expression system. Xenopus oocytes were injected with alpha subunit cRNAs from Bufo marinus bladder or rat distal colon and/or with beta subunit cRNA from B. marinus bladder. Two days after injection, K+ uptake was measured by using 86 Rb+ as a K+ surrogate, and pH measurements were performed by means of ion-selective microelectrodes. Co-injection of alpha and beta subunit cRNAs led to a large increase in 86Rb+ uptake, an intracellular alkalinization, and an extracellular medium acidification, as compared to alpha or beta injection alone. These results indicate that the colonic P-ATPase alpha subunit, like the bladder alpha subunit, acts as a functional H+,K+-ATPase, and that co-expression of alpha and beta subunits is required for the function. External K+ activation of the 86Rb+ uptake had a K1/2 of approximately 440 microM for the bladder isoform (consistent with the previously reported value (Jaisser, F., Horisberger, J.D., Geering, K., and Rossier, B.C. (1993) J. Cell. Biol. 123, 1421-1431) and a K1/2 of approximately 730 microM for the colonic isoform. Sch28080 was ineffective to reduce 86Rb+ uptake whereas ouabain inhibited the activity expressed from rat colon alpha subunit with a Ki of 970 microM when measured at the Vmax of the enzyme. We conclude that, when expressed in Xenopus oocytes, the rat colon P-ATPase alpha subunit encodes a ouabain-sensitive H+,K+-ATPase.

Myocardial beta-adrenergic adenylate cyclase complex in a canine model of chagasic cardiomyopathy

Infection of beagles with an opossum-derived strain of Trypanosoma cruzi (Tc-O) results in features of early and chronic chagasic cardiomyopathy, that is, increases in PR interval, atrioventricular block, and frequent ventricular premature contractions, ventricular tachycardia, and decreased left ventricular ejection fraction. These signs are not observed in animals infected with a canine strain of T. cruzi (Tc-D). To understand the biochemical basis for these early cardiac effects, we examined the beta-adrenergic adenylate cyclase complex in myocardial membranes prepared from animals infected with either of the two strains. In animals infected with Tc-O (symptomatic), the maximum velocity (Vmax) decreased and concentration of agonist resulting in 50% of Vmax (Kact) increased for isoproterenol-dependent adenylate cyclase activity; in animals infected with Tc-D (asymptomatic), Vmax and Kact for isoproterenol were unchanged from control, uninfected animals. beta-Receptor density decreased by 20% in symptomatic animals with no change in affinity, whereas no differences were observed between uninfected and infected asymptomatic animals. A complex pattern of changes was apparent in the guanine nucleotide binding protein, Gs, in the setting of infection. Alterations in cholera toxin-dependent ADP-ribosylation patterns as well as immunochemical detection with anti-G alpha s antisera suggested a change in the biochemical nature of the Gs species and not necessarily a physical loss of this protein. Reconstitution of adenylate cyclase activity in cyc- membranes demonstrated a decrease in hormone-sensitive Gs activity in membranes prepared from symptomatic animals without a change in activity demonstrable in the presence of Gpp(NH)p. Collectively, the results suggest that the depression in beta-adrenergic adenylate cyclase activity associated with symptomatic infection of beagles with T. cruzi occurs primarily as a result of changes in the Gs protein complex, most likely resulting in an uncoupling of the beta-adrenergic receptor from the Gs protein.

Isoforms of Na,K-ATPase in Artemia salina: II. Tissue distribution and kinetic characterization

To characterize the molecular properties conveyed by the isoforms of the alpha subunit of Na,K-ATPase, the two major transepithelial transporting organs in the brine shrimp (Artemia salina), the salt glands and intestines, were isolated in pure form. The alpha isoforms were quantified by ATP-sensitive fluorescein isothiocyanate (FITC) labeling. The salt gland enzyme exhibits only the alpha 1 isoform, whereas the intestinal enzyme exhibits both the alpha 1 and the alpha 2 isoforms. After 32 hours of development, Na,K-ATPase activity [in mumol Pi/mg protein/hr (1 mu)] in whole homogenates was 32 +/- 6 in the salt glands and 12 +/- 3 in the intestinal preparations (mean +/- SEM). The apparent half-maximal activation constants (K1/2) of the salt gland enzyme as compared to the intestinal enzyme were 3.7 +/- 0.6 mM vs. 23.5 +/- 4 mM (P less than 0.01) for Na+, 16.6 +/- 2.2 mM vs. 8.29 +/- 1.5 mM for K+ (P less than 0.01), and 0.87 +/- 0.8 mM vs. 0.79 +/- 1.1 mM for ATP (NS). The apparent Ki's for ouabain inhibition were 1.1 x 10(-4) M vs. 2 x 10(-5) M, respectively. Treatment of whole homogenates with deoxycholic acid (DOC) produced a maximal Na,K-ATPase activation of 46% in the salt gland as compared to 23% in the intestinal enzyme. Similar differences were found with sodium dodecyl sulfate (SDS). The two distinct forms of Na,K-ATPase isolated from the brine shrimp differed markedly in three kinetic parameters as well as in detergent sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative effects of ouabain, natriuretic factor and ammonium chloride in the toad urinary bladder

Electrical changes and direct effects on Na-K ATPase activity induced by an endogenous digitalis-like natriuretic factor (NF), NH4Cl and ouabain were studied in toad bladders. NF inhibited the SCC and the Na-K ATPase activity in a similar manner to ouabain, but induced a greater increase in calculated direct current resistance (R) (p less than 0.05). NH4Cl was a weak inhibitor of Na-K ATPase activity, although it produced steeper SCC inhibition slopes than those observed with ouabain or NF (p less than 0.01). The data suggested the same mechanism of action of NF and ouabain on the sodium pump, with an additional effect of the former on apical sodium permeability of the cells and/or closure of paracellular routes leading to an increased tissue resistance. In contrast, the effects of NH4Cl were mostly compatible with intracellular inhibition of apical sodium entry into the cell.

Isolation and characterization of granules of the toad bladder

The electron-dense granules that lie just below the apical plasma membrane of granular epithelial cells of toad urinary bladder contribute glycoproteins to that apical membrane. Also, exocytosis of granules (and tubules) elicited by antidiuretic hormone potentially doubles that apical surface, during the same period the transport changes characteristic of the hormonal response occur. Granules separated from other membrane systems of the cells provide the material to assess the importance of the granules as glycocalyx precursors and in hormone action. We used isosmotic media to effect preliminary separations by differential centrifugation. Then granules were isolated by centrifugation on self-forming gradients of Percoll of decreasing hypertonicity. We find qualitative and quantitative changes in protein composition and enzymic activities in the isolated fractions. The primary criterion for granule purification was electron microscopic morphology. In addition, polypeptide species found in the granule fraction are limited in number and quantity. The granules are enzymically and morphologically not lysosomal in nature. Granules may provide the glycoproteins of the apical glycocalyx but they differ from the isolated plasma membrane fraction enzymically, in protein composition and in proportion of esterified cholesterol. We conclude that the granules are not "average" plasma membrane precursors. Their role in the membrane properties of the toad urinary bladder may now be evaluated by characterizing permeability and other properties of the isolated organelles.

Energetics of sodium transport in the urinary bladder of the toad. Effect of aldosterone and sodium cyanide

Experiments were designed to determine whether the stimulatory effect of aldosterone on sodium transport involves an increase in tissue ATP. Urinary bladders that were removed from toads presoaked in 0.6% saline for 48-72 h, mounted as sacs, and maintained in open circuit except for brief observation of short circuit current every 30 min responded to 100 nM aldosterone added to the serosal bath with an increase in short circuit current to 170% of control hemibladders, which plateaus at 2-3 h. Tissue (ATP)/(ADP) X (Pi) measured in perchloric acid extracts increased to a maximum of 208% of controls (P less than 0.001) and ATP increased to 116% of controls (P less than 0.01) at 180 min. The short circuit current response to aldosterone paralleled the increase in ATP and (ATP)/(ADP) X (Pi) measured at 75, 120, 180, and 240 min. In bladders clamped at -150 mV, the short circuit current response to aldosterone was greater: 280% of controls (P less than 0.001) and tissue (ATP)/(ADP) X (Pi) increased to 191% of controls (P less than 0.001). In continuously short circuited bladders and bladders clamped at +75 mV, the short circuit current response to aldosterone and the change in ATP, ADP, or Pi were markedly diminished. 100 microM amiloride added to mucosal bath decreased the short circuit current to zero and inhibited the short circuit current response to aldosterone, whereas tissue ATP increased to 141% (P less than 0.05). 100, 250, and 500 microM NaCN dropped the short circuit current to 59, 35, and 24% of control values, respectively. Concurrently, tissue ATP measured at 60 min after the addition of NaCN dropped to 79, 66, and 56% of control values, respectively, and tissue ATP/ADP dropped to 68, 50, and 40%, respectively. The data revealed significant correlation between the change in the rate of sodium transport produced by aldosterone or NaCN as measured by the short circuit current and the concentration of ATP (r = 0.96, P less than 0.001), as well as ATP/ADP (r = 0.95, P less than 0.001). In conclusion, these results support the view that the stimulatory effects of aldosterone on sodium transport involve an increase in ATP or (ATP)/(ADP) X (Pi).

Purification and characterization of (Na+ + K+)-ATPase from toad kidney

This report describes the partial purification and the characteristics of (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) from an amphibian source. Toad kidney microsomes were solubilized with sodium deoxycholate and further purified by sodium dodecyl sulphate treatment and sucrose gradient centrifugation, according to the methods described by Lane et al. [(1973) J. Biol. Chem. 248, 7197--7200], Jørgensen [(1974) Biochim. Biophys. Acta 356, 36--52] and Hayashi et al. [(1977) Biochim. Biophys. Acta 482, 185--196]. (Na+ + K+)-ATPase preparations with specific activities up to 1000 mumol Pi/mg protein per h were obtained. Mg2+-ATPase only accounted for about 2% of the total ATPase activity. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis revealed three major protein bands with molecular weights of 116 000, 62 000 and 26 000. The 116 000 dalton protein was phosphorylated by [gamma-32P]ATP in the presence of sodium but not in the presence of potassium. The 62 000 dalton component stained for glycoproteins. The Km for ATP was 0.40 mM, for Na+ 12.29 mM and for K+ 1.14 mM. The Ki for ouabain was 35 micron. Temperature activation curves showed two activity peaks at 37 degrees C and at 50 degrees C. The break in the Arrhenius plot of activity versus temperature appeared at 15 degrees C.

Aldosterone action and sodium- and potassium-activated adenosine triphosphatase in toad bladder

Urinary hemibladders obtained from toads soaked in water or saline were treated with aldosterone, 10(-6) M, either 1(1/2) or 16 h after mounting. After 2(1/2) h exposure to the hormone, short-circuit current was increased by 110-192% and open-circuit potential by 20-44% as compared with untreated paired hemibladders. Mucosal cells were then assayed for sodium-potassium-stimulated adenosine triphosphatase (ATPase). No increase occurred in activity per milligram protein or in the portion of total activity dependent on sodium. Activity at low sodium concentrations was also measured and analyzed by means of the Hill equation in terms of K, the apparent dissociation constant of the enzyme-sodium complex, and n, a number that expresses the degree of interaction between binding sites. Neither K nor n was significantly altered by aldosterone. A few experiments were also carried out at low ATP concentrations (0.3 mM); again no change in sodium-dependent activity was noted. The results indicate that aldosterone does not stimulate sodium transport by increasing the quantity of sodium-potassium adenosine triphosphatase in mucosal cells or the dependence of this activity on sodium or ATP concentrations.