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

Origin of ATP for Ca2+-induced contraction in the guinea-pig femoral artery

Previously, we have described differences between the rat proximal colon and femoral artery with respect to the role of ATP newly synthesized by creatine kinase. In the present study the role of newly synthesized ATP was studied in the guinea-pig femoral artery to examine species differences. In the alpha-toxin-permeabilized preparation of the guinea-pig femoral artery, the rapid Ca(2+)-induced contraction was suppressed when creatine kinase activity was inhibited. The contraction was restored completely by treatment with NaN(3), an inhibitor of ecto-ATPase, the enzyme that breaks down exogenous ATP. Thus, ATP newly synthesized by creatine kinase may have no role in contraction of the guinea-pig femoral artery. This is in marked contrast to the rat femoral artery, in which Ca(2+)-induced contractions are almost completely inhibited by inhibition of creatine kinase activity but only partly restored by NaN(3). To characterize the difference between the guinea-pig and rat tissue, the origin of ATP required for contraction was determined in intact preparations. Monoiodoacetic acid, an inhibitor of glycolysis, inhibited the high K(+)-induced contraction in the guinea-pig femoral artery more potently than in the rat tissue. In contrast, an inhibitor of mitochondrial respiration, carbonylcyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), inhibited contraction in femoral arteries from rats, but not from guinea-pigs. These results suggest that contraction in the rat femoral artery is dependent largely on oxidative phosphorylation, while contraction in the guinea-pig tissue is dependent only on glycolysis. Because oxidative phosphorylation generates ATP and phosphocreatine, while glycolysis generates only ATP, the strong dependence of the contraction of the rat femoral artery on the oxidative phosphorylation is consistent with its dependence on ATP newly synthesized by creatine kinase from ADP and phosphocreatine, as previously shown. Thus, it is proposed that ATP, newly synthesized by creatine kinase, in addition to ATP generated by oxidative phosphorylation, is utilized for contraction in the rat femoral artery, while glycolysis produces sufficient ATP for contraction in the guinea-pig femoral artery.

Ecto-ATPases: identities and functions

Ecto-ATPases are ubiquitous in eukaryotic cells. They hydrolyze extracellular nucleoside tri- and/or diphosphates, and, when isolated, they exhibit E-type ATPase activity, (that is, the activity is dependent on Ca2+ or Mg2+, and it is insensitive to specific inhibitors of P-type, F-type, and V-type ATPases; in addition, several nucleotide tri- and/or diphosphates are hydrolysed, but nucleoside monophosphates and nonnucleoside phosphates are not substrates). Ecto-ATPases are glycoproteins; they do not form a phosphorylated intermediate during the catalytic cycle; they seem to have an extremely high turnover number; and they present specific experimental problems during solubilization and purification. The T-tubule Mg2+-ATPase belongs to this group of enzymes, which may serve at least two major roles: they terminate ATP/ADP-induced signal transduction and participate in adenosine recycling. Several other functions have been discussed and identity to certain cell adhesion molecules and the bile acid transport protein was suggested on the basis of cDNA clone isolation and immunological work.

Ca2+ or Mg2+ nucleotide phosphohydrolases in myometrium: two ecto-enzymes

A high level of Ca2+ or Mg2+ nucleotide phosphohydrolase activity is present on the outside surface of intact myometrial cells and is also observed in the isolated plasma membranes. About half of this activity is labile while the remainder is stable. The characteristics of the activities suggest the presence of at least two different ecto-enzymes. The stable component (Km for Ca2+ about 0.1 mM) accepts XTP or XDP as substrate, is not inhibited by p-chloromercuriphenylsulfonate or inorganic phosphate, but is inhibited by 20 mM NaN3. The labile component (Km for Ca2+ nearly 1 mM) cleaves XTP but not XDP, and is inhibited by p-chloromercuriphenyl-sulfonate and inorganic phosphate, but not by NaN3. The activity of the labile component can be restored by removing the cells from the incubation medium and resuspending them in fresh medium. This suggests that the 'lability' is due to product inhibition, probably by inorganic orthophosphate. While the Ca2+ pump of myometrial plasma membranes was inhibited by 0.1 microM oxytocin, these ecto-enzymes were unaffected by oxytocin concentrations up to 10 microM. Because of its high activity and rapid inactivation by product inhibition, the labile enzyme may be involved in the regulation of purinergic receptors.

Changes in the mechanism of Ca2(+) mobilization during the differentiation of BC3H1 muscle cells

Ca2+ sequestration and release in BC3H1 muscle cells is strongly dependent on the stage of differentiation. In proliferating cells, more than 90% of the sequestered Ca2+ was Ins(1,4,5)P3-sensitive and 25% was caffeine-sensitive. In differentiated cells, the Ca2+ accumulation was 5-fold higher and was InsP3-insensitive, but about 60% of the sequestered Ca2+ was caffeine-sensitive. These changes were reversible upon addition of growth stimuli. Similarly, by measuring the intracellular Ca2+ concentration in single intact BC3H1 cells, it was found that the number of histamine-responsive cells decreased and the number of caffeine-responsive cells increased during muscle cell differentiation. These data indicate that the development of the muscle phenotype in BC3H1 myoblasts induces a major rearrangement of the mechanisms for Ca2+ mobilization.

Heat production in chemically skinned smooth muscle of guinea-pig taenia coli

1. The rate of heat production of chemically skinned guinea-pig taenia coli smooth muscle at 25 degrees C was measured using microcalorimetric techniques. 2. Muscle strips were mounted isometrically and incubated in solutions containing MgATP (3.2 mM) and phosphocreatine (PCr, 12 mM), pH 6.9. Activation was obtained by the injection of Ca2+ into the sample compartment of the calorimeter. 3. The heat production rate of the resting preparation (pCa 9) was 0.40 +/- 0.03 mW g-1 wet weight (n = 23). During maximal activation (pCa 4.8) the heat rate increased to 1.12 +/- 0.07 mW g-1 (mean +/- S.E.M., n = 15). With stepwise increase in [Ca2+] from pCa 9 to 4.8 the energetic cost of force maintenance tended to increase at higher [Ca2+]. 4. After activation by Ca2+, the heat production rate reached its maximum while force was still increasing. 5. Changing ionic strength from 90 to 150 mM had no effect on either basal or activated heat rate. Oligomycin, amphotericin B and the adenylate kinase inhibitor Ap5A had no effect on the basal heat rate. 6. Exchanging ATP in the incubation medium for inosine triphosphate (ITP) reduced the force and heat production after injection of Ca2+. The basal heat production was not lowered when ATP was exchanged for ITP. 7. The observed enthalpy change for PCr splitting at 25 degrees C (pH 6.9, ionic strength 90 mM) was -28 +/- 3 kJ mol-1 (mean +/- S.E.M., n = 9). After correction for the phosphate equilibrium, buffer reactions, and Mg2+ binding to PCr and HPO42-, the net enthalpy change is calculated to be -39 +/- 3 kJ mol-1. 8. Heat production in the skinned smooth muscle consists of one basal component present in relaxed muscle, and one component associated with contraction. The nature of the basal heat production is unclear but does not seem to involve turnover of phosphate on the myosin light chains. The increase in the energetic tension cost with increasing activation by Ca2+ has implications for the understanding of the contractile mechanism in smooth muscle.

Measurement of microsomal ATPase activities: a comparison between the inorganic phosphate-release assay and the NADH-coupled enzyme assay

The specific activity of the Mg2+-ATPase and the (Ca2+ + Mg2+)-ATPase has been measured in a microsomal fraction from pig antral smooth muscle with the phosphate-release assay and the NADH-coupled enzyme assay, and the release of inorganic phosphate as a function of time is compared with the concomitant production of ADP. Both assays are found to overestimate the true Mg2+-ATPase activity. The adenylate kinase inhibitor P1,P5-di(adenosine-5'-)pentaphosphate (Ap5A) reduces the specific activity of the Mg2+-ATPase measured in the NADH-coupled enzyme assay to about half of its original value; however, it does not affect the specific activity of the Mg2+-ATPase in the Pi-release assay. The considerable overestimation of the Mg2+-ATPase activity in the NADH-coupled enzyme assay results from a combined action of an ATP pyrophosphatase (ATP in equilibrium AMP + PPi) and adenylate kinase activity contaminating the microsomes. The adenylate kinase activity in the microsomes catalyses the conversion of AMP formed by the ATP pyrophosphatase together with ATP into two ADP's. Also the phosphate-release assay is prone to an overestimation artefact because an inorganic pyrophosphatase will degrade the pyrophosphate and thus lead to additional Pi-production. Measurements of AMP and NAD+ production by HPLC confirmed our proposed reaction scheme. The same (Ca2+ + Mg2+)-ATPase activity is found in both assays, because the (Ca2+ + Mg2+)-ATPase activity is calculated from the difference in ATPase activity in the presence and absence of Ca2+, so that as a consequence the interfering activities are automatically subtracted.

Effect of ovarian steroids on membrane ATPase activities in microsomes (microsomal fractions) from rat myometrium. Inhibition of a component of the Mg2+-activated ATPase by Ca2+-calmodulin and by oxytocin

The activities of Mg2+-ATPase (Mg2+-activated ATPase), (Ca2+ + Mg2+)-activated ATPase and (Na+ + K+)-activated ATPase have been determined in microsomes (microsomal fractions) obtained from rat myometrium under different hormonal conditions. Animals were either ovariectomized and treated for a prolonged period of time with 17 beta-oestradiol or progesterone, or myometria were obtained at day 21 of pregnancy. In each case the endometrium was carefully removed. The Mg2+-ATPase consists of two components: an inactivating labile component and a second constant component. The rate of ATP hydrolysis by the labile component of the Mg2+-ATPase declines exponentially as a function of time after adding the membranes to the assay medium; this inactivation is caused by the presence of ATP in the medium. This ATPase activity inhibited by ATP is catalysed by a labile enzyme and hence it gradually diminishes within a few hours, even when the microsomes are kept on ice. This labile component has the highest activity in microsomes from pregnant rats, a lower activity in progesterone-treated rats, and the lowest in 17 beta-oestradiol-treated rats. This component of the Mg2+-ATPase is not affected by 90 nM-oxytocin. The constant component of the Mg2+-ATPase must be ascribed to a different enzyme, which, in contrast with the labile component, is very stable and not affected by the hormonal status of the animal. This constant component of the Mg2+-ATPase is inhibited both by Ca2+-calmodulin, and by oxytocin in microsomes from pregnant and from progesterone-treated animals, whereas such inhibition does not occur in microsomes from 17 beta-oestradiol-treated animals. The activity of the (Na+ + K+)-activated ATPase is not dependent on the hormonal status of the animal. Myometrial microsomes present an ATP-dependent Ca2+ transport, irrespective of the hormonal condition, but only in microsomes obtained from rats treated with 17 beta-oestradiol, can a (Ca2+ + Mg2+)-activated ATPase activity be demonstrated. This activity can be stimulated by calmodulin.