Myometrial (Na+ + K+)-activated ATPase and its Ca2+ sensitivity

ATP and ADP hydrolysis in cell membranes from rat myometrium

Extracellular nucleotides affect female reproductive functions, fertilization, and pregnancy. The aim of this study was to investigate biochemical characteristics of ATP and ADP hydrolysis and identify E-NTPDases in myometrial cell membranes from Wistar albino rats. The apparent K (m) values were 506.4 ± 62.1 and 638.8 ± 31.3 μM, with a calculated V (max) (app) of 3,973.0 ± 279.5 and 2,853.9 ± 79.8 nmol/min/mg for ATP and ADP, respectively. The enzyme activity described here has common properties characteristic for NTPDases: divalent cation dependence; alkaline pH optimum for both substrates, insensitivity to some of classical ATPase inhibitors (ouabain, oligomycine, theophylline, levamisole) and significant inhibition by suramine and high concentration of sodium azides (5 mM). According to similar apparent K(m) values for both substrates, the ATP/ADP hydrolysis ratio, and Chevillard competition plot, NTPDase1 is dominant ATP/ADP hydrolyzing enzyme in myometrial cell membranes. RT-PCR analysis revealed expression of three members of ectonucleoside triphosphate diphosphohydrolase family (NTPDase 1, 2, and 8) in rat uterus. These findings may further elucidate the role of NTPDases and ATP in reproductive physiology.

Identification of a phospholemman-like protein from shark rectal glands. Evidence for indirect regulation of Na,K-ATPase by protein kinase c via a novel member of the FXYDY family

The Na,K-ATPase provides the driving force for many ion transport processes through control of Na(+) and K(+) concentration gradients across the plasma membranes of animal cells. It is composed of two subunits, alpha and beta. In many tissues, predominantly in kidney, it is associated with a small ancillary component, the gamma-subunit that plays a modulatory role. A novel 15-kDa protein, sharing considerable homology to the gamma-subunit and to phospholemman (PLM) was identified in purified Na,K-ATPase preparations from rectal glands of the shark Squalus acanthias, but was absent in pig kidney preparations. This PLM-like protein from shark (PLMS) was found to be a substrate for both PKA and PKC. Antibodies to the Na, K-ATPase alpha-subunit coimmunoprecipitated PLMS. Purified PLMS also coimmunoprecipitated with the alpha-subunit of pig kidney Na, K-ATPase, indicating specific association with different alpha-isoforms. Finally, PLMS and the alpha-subunit were expressed in stoichiometric amounts in rectal gland membrane preparations. Incubation of membrane bound Na,K-ATPase with non-solubilizing concentrations of C(12)E(8) resulted in functional dissociation of PLMS from Na,K-ATPase and increased the hydrolytic activity. The same effects were observed after PKC phosphorylation of Na,K-ATPase membrane preparations. Thus, PLMS may function as a modulator of shark Na,K-ATPase in a way resembling the phospholamban regulation of the Ca-ATPase.

Na+/K+ ATPase in lower airway epithelium from cystic fibrosis and non-cystic-fibrosis lung

The basolateral Na+/K+ ATPase plays a critical role in sodium reabsorption across airway epithelium. Nasal epithelium shows increased Na+/K+ ATPase activity in cystic fibrosis (CF) but Na+/K+ ATPase has not been characterized in human lung epithelium or compared in CF and normal lung. We measured 3[H] ouabain binding and Na+/K+ ATPase activity in human tracheal epithelium and compared Na/K ATPase activity in bronchial epithelium in CF and control subjects. In tracheal epithelium Na+/K+ ATPase pumps were abundant and of high 3[H] ouabain affinity (Kd 4.7 nM, Bmax 38 pmol/mg) and Na+/K+ ATPase activity was 55 +/- 8 nmol/mg protein/min. Bronchial epithelial Na+/K+ ATPase activity was twofold higher in CF patients than in controls. The increased Na+/K+ ATPase activity may contribute to the increased sodium reabsorption seen in cystic fibrosis.

Influence of global ischemia on the sarcolemmal ATPases in the rat heart

To elucidate the effect of global ischemia on the energy utilizing processes, regarding the molecular principles, the kinetic and thermodynamic properties of the sarcolemmal ATPases were investigated in the rat heart. The activation energy for hydrolysis of ATP during ischemia was higher when the reaction was catalyzed by Ca-ATPase or Mg-ATPase. For the Na,K-ATPase reaction, no changes in the activation energy were observed. With respect to the enzyme kinetics, ischemia in a time-dependent manner induced important alterations in KM and Vmax values of Na,K-ATPase, Ca-ATPase and Mg-ATPase. The Vmax value decreased significantly already after 15 min of ischemia, and it also remained low after 30, 45 and 60 min for all 3 enzymes. The significant diminution of KM values occurred later in the 30th min for Ca-ATPase, in the 45th min for Na,K-ATPase. The observed drop in KM indicates the increase in the affinity of the enzymes to substrate, suggesting thus the adaptation to ischemic conditions on the molecular level. This effect could be attributed to some conformational changes of the protein molecule in the vicinity of the ATP-binding site developing after longer duration of ischemia.

Anaerobic glycolysis and postanoxic recovery of respiration of rat cortical synaptosomes are reduced by synaptosomal sodium load

Synaptosomes of rat cerebral cortex were used to study the effect of veratridine-induced Na+ load on postanoxic recovery of respiration and on aerobic and anaerobic ATP turnover, calculated from rates of oxygen consumption and lactate production. Non-stimulated synaptosomes: after onset of anoxia lactate synthesis of synaptosomes rose immediately from 0.8 to 17.7 nmol lactate/min/mg protein indicating an anaerobic ATP turnover of 17.7 nmol ATP/min/mg protein. This value accounts for 80% of ATP synthesized during oxygenated conditions and seems to cover the energetic demand of anoxic synaptosomes. This assumption was supported by linearity of lactate production throughout anoxia (90 min), by unaffected synaptosomal integrity and by complete recovery of postanoxic respiration after 90 min of anoxia. Stimulated synaptosomes: stimulation of oxygenated synaptosomes with 10(-5) mol/l veratridine enhanced ATP turnover 5-fold, due to activation of Na+/K+ ATPase, as a result of veratridine-induced Na+ influx. Consequently, if not limited in capacity, anaerobic ATP synthesis should be enhanced after addition of veratridine during anoxia. However, the opposite effect was observed. Veratridine reduced anaerobic glycolysis in a concentration-dependent manner. This inhibitory effect could be prevented by tetrodotoxin applied 5 min prior to veratridine. Inhibition of anaerobic glycolysis was independent of extrasynaptosomal glucose (1-30 mmol/l) and Ca2+ concentration (Ca(2+)-free and 1.2 mmol/l Ca2+). Veratridine stimulation of anoxic synaptosomes reduced also the recovery of postanoxic respiration. The data indicate that Na+ load inhibits anaerobic ATP synthesis, the only energy source during anaerobic conditions. To our knowledge, inhibition of anaerobic glycolysis due to increased Na+ influx has not been shown so far.

The effect of micromolar Ca2+ on the activities of the different Na+/K(+)-ATPase isozymes in the rat myometrium

In the present work we show the existence of two Na+/K(+)-ATPase isozymes in rat myometrial microsomes and suggest that they have different Ca2+ sensitivities. The catalytic subunits (alpha 1, alpha 2) of Na+/K(+)-ATPase were labelled by fluorescein-isothiocyanate and separated by SDS gel electrophoresis. The two isozyme Ca2(+)-sensitivities were studied by comparing the kinetics of Ca2+, strophantidin, ouabain and N-ethylmaleimide inhibitions. Our results indicate that the activity of the high ouabain-sensitive part (alpha 2 type) of Na+/K(+)-ATPase enzyme could only be inhibited by micromolar Ca2+. Furthermore, treatment of the microsomal preparation with 1mM N-ethylmaleimide selectively inactivated the high Ca2+ sensitive isoform of myometrial Na+/K(+)-ATPase.

Possible regulation of the myometrial Na+/K+-ATPase activity by Ca2+ and cAMP-dependent protein kinase

We have previously reported an inhibition of the rat myometrial Na+/K+-ATPase by micromolar Ca2+ concentrations which was abolished by SDS treatment of the microsomal preparation. Application of dimethyl sulphoxide (DMSO) prevented this effect of SDS. In this report, we present our investigation into the mechanism of the inhibitory effect of Ca2+ on the myometrial Na+/K+-ATPase. We observed that, in parallel with inhibition by Ca2+, phosphorylation of a number of membrane components was abolished by SDS treatment of the microsome fraction. Exogenously added calmodulin had no effect. However, the catalytic subunit of cAMP-dependent protein kinase restored Ca2+ sensitivity of the Na+/K+-ATPase and phosphorylation of the other components. Furthermore, addition of the heat-stable protein kinase inhibitor reduced drastically the Ca2+ sensitivity of the Na+/K+-ATPase, as well as the phosphorylation of a number of proteins in the myometrial microsome fraction. It is concluded that the cAMP-dependent protein kinase may be involved in the modulation of Na+/K+-ATPase activity by Ca2+ in the myometrial plasma membrane.

Characterization of the Mg2+-activated ATPase activity in smooth-muscle membranes. NADH oxidase and adenylate kinase interfere with the NADH-coupled enzyme assay

The apparent Mg2+-activated ATPase activity measured by the continuous NADH-coupled enzyme assay was studied in a number of microsomal preparations obtained from smooth muscle of the myometrium from pregnant or 17 beta-oestradiol-pretreated rats, the bovine aorta, the guinea-pig taenia coli, the rabbit ear artery and pig antrum. It was shown that this ATPase assay is prone to the effects of a number of artefacts that are tissue-dependent. The apparent Mg2+-ATPase activity in microsomes (microsomal fractions) from myometrium, aorta and taenia coli declines non-linearly during the assay. Its initial high rate gradually diminishes over 15-60 min, depending on the type of smooth muscle, to a constant value. This decline depends on the presence of ATP and can be partially prevented by concanavalin A. The non-linearity is limited in microsomes from rabbit ear artery. In microsomes from antrum the apparent Mg2+-ATPase activity actually increases with time, albeit gradually. Storage on ice of the microsomes of the aorta, and especially of myometrium of pregnant rats and of taenia coli, is accompanied over a few hours after their preparation by a gradual suppression of the component of the Mg2+-ATPase activity that is inhibited by ATP. The Mg2+-ATPase activity in microsomes from antrum remains constant. NADH oxidase activity accounts for 10% of the Mg2+-ATPase activity in microsomes from stomach smooth muscle. The apparent initial non-linearity of the Mg2+-ATPase activity in that tissue is due to a time-dependent decrease of a rotenone-sensitive NADH oxidase activity. The adenylate kinase activity, as deduced from the effect of the adenylate kinase inhibitor P1,P5-di(adenosine-5') pentaphosphate, could account for 45.0, 35.0 and 31.0% respectively of the Mg2+-ATPase activity in microsomes from stomach, myometrium and aorta. No adenylate kinase activity could be detected in microsomes from ear artery and taenia coli. When microsomes from stomach smooth muscle were separated on a sucrose gradient, the contribution of adenylate kinase and NADH oxidase to the Mg2+-ATPase activity was most pronounced in the higher-density fractions. Part of the NADH oxidase activity and of the Mg2+-ATPase activity, and most of the adenylate kinase activity, are not sedimented at 224000 gmax. for 30 min and may therefore be present as soluble enzymes.(ABSTRACT TRUNCATED AT 400 WORDS)

Sarcolemmal ATPase activities of the rat heart ventricle--dependence on age and sodium ion

Na-K-ATPase activity of the rat heart was similar throughout the postnatal growth when measured from crude unpurified fraction. Instead in the cardiac sarcolemmal fraction, isolated by hypotonic shock LiBr-treatment method, the activity was over two times higher in 10-day old neonates than in adult rats. The conflicting results are partly explained by different effects of the isolation procedure on neonatal and adult tissues. Na concentration for half-maximal activity of the Na-K-ATPase was similar in neonates (7.0 mM) and adults (6.4 mM). Ca-ATPase activity was not affected by Na concentration (2-100 mM) in the two age-groups studied.

(Na+ + K+)-ATPase in artificial lipid vesicles: influence of the concentration of mono- and divalent cations on the pumping rate

(Na+ + K+)-ATPase from kidney outer medulla was incorporated into artificial dioleoylphosphatidylcholine vesicles. Transport activity was induced by adding ATP to the external medium. A voltage-sensitive dye was used to detect the ATP-driven potassium extrusion in the presence of valinomycin. The observed substrate-protein interactions of the reconstituted (Na+ + K+)-ATPase largely agree with that from native tissues. An agreement between ATP hydrolysis and transport activity is given for concentration dependences of sodium, potassium, magnesium and calcium ions. The only significant deviations were observed in the influence of pH. Protons were found to have different influence on transport, enzymatic activity and phosphorylation of the enzyme. The transport studies showed a twofold interaction of protons with the protein: competition with sodium at the cytoplasmic ion binding sites, a non competitive inhibition of transport which is not correlated with protein phosphorylation.