Selective and reversible inhibition of heart sarcolemmal (Na+ + K+)-ATPase by p-bromophenyl isothiocyanate. Evidence for a sulfhydryl group in the ATP-binding site of the enzyme

Isothiocyanates are potent modifiers of thiol groups, and they have been successfully applied in studying the active site structure of renal (Na+ + K+)-ATPase. However, very little has been known on interactions of isothiocyanates with myocardial sarcolemmal ATPases. In the present study the mode of interaction and inhibitory effect of p-bromophenyl isothiocyanate (BPITC) on isolated rat heart sarcolemmal preparation ATPase activities not exhibiting (Mg-Ca)-ATPase activity was investigated. BPITC in concentrations of 10(-7)-10(-4) mol . l-1 inhibited selectively and non-competitively the (Na+ + K+)-ATPase activity in the sarcolemma with an ID50 around 2.10(-7) mol . l-1. The non-specific interaction of BPITC with bivalent cations, namely with Mg2+ and Ca2+, in the reaction system was eliminated by preincubation of membranes with BPITC keeping the ratio of inhibitor to membrane protein concentration constant. Under these conditions no considerable inhibitory effects were observed on Mg2+-ATPase or the low-affinity Ca2+-ATPase of sarcolemma. Preincubation of membranes with 2 mmol . l-1 ATP protected (Na+ + K+)-ATPase activity against inhibition by BPITC. The interaction of BIPTC with the sarcolemma proved to be reversible in the presence of beta-mercaptoethanol or dithiothreitol.     

Inhibition of red cell Ca2+-ATPase by vanadate

1. The Mg2+- plus Ca2+-dependent ATPase (Ca2+-ATPase) in human red cell membranes is susceptible to inhibition by low concentrations of vanadate. 2. Several natural activators of Ca2+-ATPase (Mg2+, K+, Na+ and calmodulin) modify inhibition by increasing the apparent affinity of the enzyme for vanadate. 3. Among the ligands tests, K+, in combination with Mg2+, had the most pronounced effect on inhibition by vanadate. 4. Under conditions optimal for inhibition of Ca2+-ATPase, the K 1/2 for vanadate was 1.5 microM and inhibition was nearly complete at saturating vanadate concentrations. 5. There are similarities between the kinetics of inhibition of red cell Ca2+-ATPase and (Na+ + K+)-ATPase prepared from a variety of sources; however, (Na+ + K+)-ATPase is approx. 3 times more sensitive to inhibition by vanadate.     

Effects of lanthanum on the heart sarcolemmal ATPase and calcium binding activities.

Effects of lanthanum on Ca2+-ATPase, Mg2+-ATPase, Na+-K+-ATPase, and calcium binding activities were studied in rat heart sarcolemma. Ten to 100 micrometers lanthanum depressed significantly the Ca2+-ATPase activity and 50--200 micrometers lanthanum inhibited the calcium binding activity. Lineweaver-Burk plots of the Ca2+-ATPase activity showed that the inhibition by lanthanum was competitive with calcium concentration. Neither Mg2+-ATPase nor Na+-K+-ATPase activities were affected by lanthanum when the assay medium contained 1 mM EDTA; however, in the absence of EDTA, these enzyme activities were significantly decreased by 10--100 micrometers lanthanum. Rat hearts perfused with HEPES buffer containing 0.5 mM lanthanum showed electron-dense deposits restricted to the outer cell surface and the sarcolemma obtained from these hearts also had the deposits, indicating that the membrane fraction isolated by the hypotonic shock--LiBr treatment method is of sarcolemmal origin. The Ca2+-ATPase activity of the sarcolemma isolated from lanthanum-perfused hearts, unlike the Mg2+-ATPase, Na+-K+-ATPase, and calcium binding activities, was significantly less than the control value. From these observations it is suggested that lanthanum may influence calcium movement across the sarcolemma by affecting sarcolemmal ATPase and calcium binding activities.     

Ca2+-stimulated, Mg2+-dependent ATPase in bovine thyroid plasma membranes

An isolated plasma membrane fraction from bovine thyroid glands contained a Ca2+-stimulated, Mg2+-dependent adenosine triphosphatase ((Ca2+ + Mg2+)-ATPase) activity which was purified in parallel to (Na+ + K+)-ATPase and adenylate cyclase. The (Ca2+ + Mg2+)-ATPase activity was maximally stimulated by approx. 200 microM added calcium in the presence of approx. 200 microM EGTA (69.7 +/- 5.2 nmol/mg protein per min). In EGTA-washed membranes, the enzyme was stimulated by calmodulin and inhibited by trifluoperazine.     

Calmodulin-dependent Ca2+-pump ATPase of human smooth muscle sarcolemma

An enzymatically active Ca2+-stimulated ATPase has been isolated from the sarcolemmal sheets of human smooth muscle (myometrium). Ca2+-ATPase activity was quantitated in an assay medium which simulated the characteristic free ionic concentrations of the cytosol. New computer programs for calculating the composition of solutions containing metals (Ca, Mg, Na, K) and ligands (EGTA, ATP), based on the updated stability constants, were used. In detergent-soluble form the enzyme has a high Ca2+-affinity expressed by an apparent Km (Ca2+) of 0.25 +/- 0.04 microM. The maximum specific activity (about 20 nmol of Pi/mg protein/min) was found in the micromolar domain of free-Ca2+ concentrations, the same levels required for normal maximal contractions in smooth muscle. The variation of free-Ca2+ concentration in the assay medium over 4 orders of magnitude (pCa 9 to pCa 5) resulted in a sigmoidal dependence of enzymatic activity, with a Hill coefficient of 1.4, which suggested the regulation of Ca2+-ATPase by allosteric effectors. The presence and the activator role of endogenous calmodulin in smooth muscle sarcolemma was proved by calmodulin-depletion experiments and by using suitable anticalmodulinic concentrations of trifluoperazine. The addition of exogenous calmodulin restored the enzyme activity. Apparently, the concentration of calmodulin in isolated smooth muscle sarcolemma is about 0.1% of sarcolemmal proteins, as deduced from the comparison of calmodulin-depletion and calmodulin-readdition experiments. Calmodulin increased significantly the enzyme Ca2+-affinity and Vmax (by a factor of about 10). At variance with the sarcoplasmic reticulum Ca2+-ATPase, the sarcolemmal Ca2+-ATPase is extremely sensitive to orthovanadate, half-maximal inhibition being observed at 0.8 microM vanadate. In conclusion, the Ca2+-ATPase isolated from smooth muscle sarcolemma appears very similar to the well-known Ca2+-pump ATPases of erythrocyte membrane, heart sarcolemma or axolemma. We suggest that this high-affinity Ca2+-ATPase represents the calmodulin-regulated Ca2+-extrusion pump of the smooth muscle sarcolemma.     

Exaprolol as a modulator of heart sarcolemmal (Na+ + K+)-ATPase. Evidence for interaction with an essential sulfhydryl group in the catalytic centre of the enzyme

Beta-adrenoceptor blocking agents may have, in addition to their primary action, also ancillary effects on the cell membrane. In the present paper the non-specific interaction of exaprolol with the ATPase systems in isolated rat heart sarcolemmal membranes was investigated. When preincubated with sarcolemmal membranes in vitro, exaprolol in concentrations below 10(-4) mol.l-1 had no significant effect on sarcolemmal Mg2+-, Ca2+- and (Na+ + K+)-ATPase activities. At exaprolol concentration of 10(-4) mol.l-1 the Mg2+- and Ca2+-ATPase activities became inhibited whereas the (Na+ + K+)-ATPase activity was markedly stimulated. A kinetic analysis of these interactions revealed a non-competitive inhibition of Mg2+- and Ca2+-ATPase. In the case of (Na+ + K+)-ATPase a synergistic type of stimulation characterized by an exaprolol-induced conversion of an essential sulfhydryl group in the active site of the enzyme to the more reactive [S-] form has been observed thus increasing the affinity of the enzyme to ATP. Exaprolol concentrations exceeding 5 X 10(-4) mol.l-1 induced an overall depression of the investigated enzyme activities.     

Cholesterol effect on enzyme activity of the sarcolemmal (Ca2+ + Mg2+)-ATPase from cardiac muscle

The effect of cholesterol incorporation and depletion of the cardiac sarcolemmal sacs on (Ca2+ + Mg2+)-ATPase activity was examined. Cholesterol incorporation to the sarcolemmal sacs was achieved utilizing an in vivo and an in vitro procedure. Cholesterol depleted membranes were obtained in vitro after incubation of the sarcolemmal sacs with inactivated plasma. Arrhenius plots of the (Ca2+ + Mg2+)-ATPase activity showed a triphasic curve when the assays were carried out using a temperature range between 0 and 40 degrees C. The sarcolemmal (Ca2+ + Mg2+)-ATPase activity was shown to be inversely proportional to the cholesterol concentration of the membranes, showing a low ATPase activity with a high cholesterol content and a high ATPase activity when the cholesterol concentration was low. Although the (Ca2+ + Mg2+)-ATPase activity was found to be inhibited in the cholesterol incorporated sarcolemmal sacs, the withdrawal of small amounts of cholesterol from the membranes produced an important stimulatory effect. Changes in (Ca2+ + Mg2+)-ATPase activity due to variation in the membrane cholesterol concentration were shown to be reversible. Our results indicate the possibility of a slow exchange of cholesterol between the tightly bound lipid surrounding the (Ca2+ + Mg2+)-ATPase and the bulk lipid of the sarcolemma.     

Effect of calcium on the structure-function relationship of (Na+ + K+)-ATPase in cardiac sarcolemma

Calcium-induced changes in (Na+ + K+)-ATPase activity and structural changes of membrane bound proteins in rat heart sarcolemma were investigated. Increasing concentrations of Ca2+ (0.1-8.0 mmol.l-1) gradually inhibited the (Na+ + K+)-ATPase activity and decreased the alpha-helix content of sarcolemmal proteins. Mathematical and graphical analysis of observed data yielded a quantitative relationship between Ca2+-induced changes in (Na+ + K+)-ATPase activity and the secondary structure of membrane proteins in cardiac sarcolemma.     

The effect of lipid intermediates on Ca2+ and Na+ permeability and (Na+ + K+)-ATPase of cardiac sarcolemma. A possible role in myocardial ischemia

The effect of fatty acid and acylcarnitine on Ca2+ and Na+ transporting enzymes and carriers was studied in sealed cardiac sarcolemma vesicles of mixed polarity. Palmitoylcarnitine markedly reduced the Na+ gradient-induced Ca2+ uptake. Half-maximal reduction was obtained at 15 microM of the carnitine derivative. In a same concentration range palmitoylcarnitine caused a rapid release of accumulated Ca2+ when added to Ca2+-filled vesicles, which suggests that palmitoylcarnitine increases the permeability of the sarcolemma vesicles to Ca2+. A rapid release of Ca2+ was also observed if Ca2+ was taken up by action of the Ca2+ pump. The (Ca2+ + Mg2+)-ATPase, which most likely drives this active Ca2+ uptake, was 90% increased by 50 microM palmitoylcarnitine and evidence was presented that the acylcarnitine effect again was linked to an alteration of Ca2+ permeability of the vesicles. At the same concentration acylcarnitine was not able to unmask the latent protein kinase, so that probably the sarcolemma ATP permeability was not affected. Palmitoylcarnitine at 25 microM did not affect the ouabain-sensitive (Na+ + K+) -ATPase in native sarcolemma vesicles, however, it inhibited markedly if the enzyme was measured in SDS-treated vesicles. The effect of increased free fatty acid concentration on some of the sarcolemma transporting properties was tested by adding oleate-albumin complexes with different molar ratios to the sarcolemma vesicles. In contrast to molar ratios 1 and 5, the ratio of 7 was able to induce a rapid Ca2+ release and to inhibit (Na+ + K+)-ATPase in either native or SDS-treated vesicles markedly. 22Na release from 22Na-preloaded sarcolemma vesicles was shown to be stimulated by either palmitoylcarnitine (50 microM) or oleate-albumin complex (with a molar ratio of 7). The possible significance of the observed effects of lipid intermediates on ion permeability and (Na+ + K+)-ATPase activity in isolated sarcolemma vesicles for the derangement of cardiac cell function in ischemia is discussed.     

Localization of (Ca2+ + Mg2+)-ATPase,Ca2+ pump and other ATPase activities in cardiac sarcolemma

N-Ethylmaleimide was employed as a surface label for sarcolemmal proteins after demonstrating that it does not penetrate to the intracellular space at concentrations below 1·10^?4 M. The sarcolemmal markers, ouabain-sensitive (Na⁺ + K⁺)-ATPase and Na⁺/Ca²⁺-exchange activities, were inhibited in N-ethylmaleimide perfused hearts. Intracellular activities such as creatine phosphokinase, glutamate-oxaloacetate transaminase and the internal phosphatase site of the Na⁺ pump (K⁺-p-nitrophosphatase) were not affected. Almost 20% of the (Ca²⁺ + Mg²⁺)-ATPase and Ca²⁺ pump were inhibited indicating the localization of a portion of this activity in the sarcolemma. Sarcolemma purified by a recent method (Morcos, N.C. and Drummond, G.I. (1980) Biochim. Biophys. Acta 598, 27–39) from N-ethylmaleimide-perfused hearts showed loss of approx. 85% of its (Ca²⁺ + Mg²⁺-ATPase and Ca²⁺ pump compared to control hearts. (Ca²⁺ + Mg²⁺)-ATPase and Ca²⁺ pump activities showed two classes of sensitivity to vanadate ion inhibition. The high vanadate affinity class ($\text{K}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ for inhibition approx. 1.5 μM) may be localized in the sarcolemma and represented approx. 20% of the total inhibitable activity in agreement with estimates from N-ethylmaleimide studies. Sucrose density fractionation indicated that only a small portion of Mg²⁺-ATPase and Ca²⁺-ATPase may be associated with the sarcolemma. The major portion of these activities seems to be associated with high density particles.     

Mg2,Ca2+-ATPase activity of sarcolemmas of intestinal smooth muscle cells in the rabbit

Preparations of rabbit small intestine smooth muscle cell sarcolemma are capable of hydrolyzing ATP in the presence of millimolar concentrations of Mg2+ and Ca2+ and possess the activity of Mg2+,Ca2+-ATPase having a high affinity for Ca2+ (Km = 5.8 X 10(-6) M). The optimal conditions for the Mg2+,Ca2+-ATPase reaction were established. It was demonstrated that sarcolemmal preparations hydrolyze ATP, GTP, ITP and UTP almost at the same rates. The enzyme contains SH-groups that are unequally exposed to the water phase and are inhibited by 50% by p-chloromercurybenzoate and by 90% by dithionitrobenzoate. The Mg2+,Ca2+-ATPase activity is highly sensitive to oxytocin: at the concentration of 10(-7) MU/ml, the hormone completely inhibits the enzyme without affecting its Mg2+-, Ca2+- and Na+,K+-ATPase activities.     

Trifluoperazine inhibition of calmodulin-sensitive Ca2+ -ATPase and calmodulin insensitive (Na+ +K+)- and Mg2+ -ATPase activities of human and rat red blood cells

Trifluoperazine dihydrochloride-induced inhibition of calmodulin-activated Ca2+ -ATPase and calmodulin-insensitive (Na+ +K+)- and Mg2+ -ATPase activities of rat and human red cell lysates and their isolated membranes was studied. Trifluoperazine inhibited both calmodulin-sensitive and calmodulin-insensitive ATPase activities in these systems. The concentration of trifluoperazine required to produce 50% inhibition of calmodulin-sensitive Ca2+ -ATPase was found to be slightly lower than that required to produce the same level of inhibition of other ATPase activities. Drug concentrations which inhibited calmodulin-sensitive ATPase completely, produced significant reduction in calmodulin-insensitive ATPases as well. The data presented in this report suggest that trifluoperazine is slightly selective towards calmodulin-sensitive Ca2+ -ATPase but that it is also capable of inhibiting calmodulin-insensitive (Na+ +K+)-ATPase and Mg2+ -ATPase activities of red cells at relatively low concentrations. Thus the action of the drug is not due entirely to its interaction with calmodulin-mediated processes, and trifluoperazine cannot be assumed to be a selective inhibitor of calmodulin interactions under all circumstances.     

(Ca2+ + Mg2+)-ATPase in enriched sarcolemma from dog heart

An enriched fraction of plasma membranes was prepared from canine ventricle by a process which involved thorough disruption of membranes by vigorous homogenization in dilute suspension, sedimentation of contractile proteins and mitochondria at 3000 X g followed by sedimentation of a microsomal fraction at 200 000 X g. The microsomal suspension was then fractionated on a discontinuous sucrose gradient. Particles migrating in the density range 1.0591--1.1083 were characterized by (Na+ + K+)-ATPase activity and [3H]ouabain binding as being enriched in sarcolemma and were comprised of nonaggregated vesicles of diameter approx. 0.1 micron. These fractions contained (Ca2+ + Mg2+)-ATPase which appreared endogenous to the sarcolemma. The enzyme was solubilized using Triton X-100 and 1 M KCl and partially purified. Optimal Ca2+ concentration for enzyme activity was 5--10 microM. Both Na+ and K+ stimulated enzyme activity. It is suggested that the enzyme may be involved in the outward pumping of Ca2+ from the cardiac cell.     

The presence of two (Na+ + K+)-ATPase inhibitors in equine muscle ATP: vanadate nad a dithioerythritol-dependent inhibitor.

A potent inhibitor of (Na+ + K+)-ATPase activity was purified from Sigma equine muscle ATP by cation- and anion-exchange chromatography. The isolated inhibitor was identified by atomic absorption spectroscopy and proton resonance spectroscopy to be an inorganic vanadate. The isolated vanadate and a solution of V2O5 inhibit sarcolemma (Na+ + K+)-ATPase with an I50 of 1 micrometer in the presence of 1 mM ethyleneglycol-bis-(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA), 145 mM NaCl, 6mM MgCl2, 15 mM KCl and 2 mM synthetic ATP. The potency of the isolated vanadate is increased by free Mg2+. The inhibition is half maximally reversed by 250 micrometer epinephrine. Equine muscle ATP was also found to contain a second (Na+ + K+)-ATPase inhibitor which depends on the sulfhydryl-reducing agent dithioerythritol for inhibition. This unknown inhibitor does not depend on free Mg2+ and is half maximally reversed by 2 micrometer epinephrine. Prolonged storage or freeze-thawing of enzyme preparations decreases the susceptibility of the (Na+ + K+)-ATPase to this inhibitor. The adrenergic blocking agents, propranolol and phentolamine, do not block the catecholamine reactivation. The inhibitors in equine muscle ATP also inhibit highly purified (Na+ + K+)-ATPase from shark rectal gland and eel electroplax. The inhibitors in equine muscle ATP have no effect on the other sarcolemmal ATPases, Mg2+-ATPase, Ca2+-ATPase and (Ca2+ + Mg2+)-ATPase.     

Regulation of the (Ca2+ + Mg2+)-ATPase activity and calcium transport in reconstituted cardiac sarcolemmal vesicles. Effect of sodium and potassium

A reconstitution procedure for a cardiac sarcolemmal enriched fraction is described. In the reconstituted cardiac sarcolemmal inside-out vesicles, a difference in calcium transport and (Ca2+ + Mg2+)-ATPase activity was found depending on the side of the membrane at which sodium and potassium were placed. Having inhibited the (Na+ + K+)- ATPase activity with ouabain, the active transport of calcium was increased when potassium was located outside and sodium inside the reconstituted vesicles. Nevertheless, this activity was maximal having potassium present on both sides. During calcium transport it was also shown that 86Rb moves opposite to calcium. When the experiment was carried out having 22Na located at the inside, there was no movement of this cation despite the low calcium transport observed. The present study supports the possibility of potassium having a stimulatory effect upon the sarcolemmal (Ca2+ + Mg2+)-ATPase activity and suggests the existence of a Ca2+, K+ co-transport carried out by this enzyme.     

Calcium inhibition of the ATPase and phosphatase activities of (Na+ + K+)-ATPase

In experiments performed at 37 degrees C, Ca2+ reversibly inhibits the Na+-and (Na+ + K+)-ATPase activities and the K+-dependent phosphatase activity of (Na+ + K+)-ATPase. With 3 mM ATP, the Na+-ATPase was less sensitive to CaCl2 than the (Na+ + K+)-ATPase activity. With 0.02 mM ATP, the Na+-ATPase and the (Na+ + K+)-ATPase activities were similarly inhibited by CaCl2. The K0.5 for Ca2+ as (Na+ + K+)-ATPase inhibitor depended on the total MgCl2 and ATP concentrations. This Ca2+ inhibition could be a consequence of Ca2+-Mg2+ competition, Ca . ATP-Mg . ATP competition or a combination of both mechanisms. In the presence of Na+ and Mg2+, Ca2+ inhibited the K+-dependent dephosphorylation of the phosphoenzyme formed from ATP, had no effect on the dephosphorylation in the absence of K+ and inhibited the rephosphorylation of the enzyme. In addition, the steady-state levels of phosphoenzyme were reduced in the presence both of NaCl and of NaCl plus KCl. With 3 mM ATP, Ca2+ alone sustained no more than 2% of the (Na+ + K+)-ATPase activity and about 23% of the Na+-ATPase activity observed with Mg2+ and no Ca2+. With 0.003 mM ATP, Ca2+ was able to maintain about 40% of the (Na+ + K+)-ATPase activity and 27% of the Na+-ATPase activity seen in the presence of Mg2+ alone. However, the E2(K)-E1K conformational change did not seem to be affected. Ca2+ inhibition of the K+-dependent rho-nitrophenylphosphatase activity of the (Na+ + K+)-ATPase followed competition kinetics between Ca2+ and Mg2+. In the presence of 10 mM NaCl and 0.75 mM KCl, the fractional inhibition of the K+-dependent rho-nitrophenylphosphatase activity as a function of Ca2+ concentration was the same with and without ATP, suggesting that Ca2+ indeed plays the important role in this process. In the absence of Mg2+, Ca2+ was unable to sustain any detectable ouabain-sensitive phosphatase activity, either with rho-nitrophenylphosphate or with acetyl phosphate as substrate.     

ATP-dependent Na+ transport in cardiac sarcolemmal vesicles

Although the enzyme (Na+ + K+)-ATPase has been extensively characterized, few studies of its major role, ATP-dependent Na+ pumping, have been reported in vesicular preparations. This is because it is extremely difficult to determine fluxes of isotopic Na+ accurately in most isolated membrane systems. Using highly purified cardiac sarcolemmal vesicles, we have developed a new technique to detect relative rates of ATP-dependent Na+ transport sensitively. This technique relies on the presence of Na+-Ca2+ exchange and ATP-driven Na+ pump activities on the same inside-out sarcolemmal vesicles. ATP-dependent Na+ uptake is monitored by a subsequent Nai+-dependent Ca2+ uptake reaction (Na+-Ca2+ exchange) using 45Ca2+. We present evidence that the Na+-Ca2+ exchange will be linearly related to the prior active Na+ uptake. Although this method is indirect, it is much more sensitive than a direct approach using Na+ isotopes. Applying this method, we measure cardiac ATP-dependent Na+ transport and (Na+ + K+)-ATPase activities in identical ionic media. We find that the (Na+ + K+)-ATPase and the Na+ pump have identical dependencies on both Na+ and ATP. The dependence on [Na+] is sigmoidal, with a Hill coefficient of 2.8. Na+ pumping is half-maximal at [Na+] = 9 mM. The Km for ATP is 0.21 mM. ADP competitively inhibits ATP-dependent Na+ pumping. This approach should allow other new investigations on ATP-dependent Na+ transport across cardiac sarcolemma.     

Characterisation of a high affinity Ca2+-stimulated, Mg2+-dependent ATPase in the rat parotid plasma membrane

Two Ca2+-stimulated ATPase activities have been identified in the plasma membrane of rat parotid: (a) a (Ca2+ + Mg2+)-ATPase with high affinity for free Ca2+ (apparent Km = 208 nM, Vmax = 188 nmol/min per mg) and requiring micromolar concentration of Mg2+ and (b) a (Ca2+ or Mg2+)-ATPase with relatively low affinity for free Ca2+ (K0.5 = 23 microM) or free Mg2+ (K0.5 = 26 microM). The low-affinity (Ca2+ or Mg2+)-ATPase can be maximally stimulated by Ca2+ alone or Mg2+ alone. The high-affinity (Ca2+ + Mg2+)-ATPase exhibits sigmoidal kinetics with respect to ATP concentration with K0.5 = 0.4 mM and a Hill coefficient of 1.91. It displays low substrate specificity with respect to nucleotide triphosphates. Although trifluoperazine inhibits the activity of the high affinity (Ca2+ + Mg2+)-ATPase only slightly, it inhibits the activity of the low-affinity (Ca2+ or Mg2+)-ATPase quite potently with 22 microM trifluoperazine inhibiting the enzymic activity by 50%. Vanadate, inositol 1,4,5-trisphosphate, phosphatidylinositol 4,5-bisphosphate, Na+,K+ and ouabain had no effect on the activities of both ATPases. Calmodulin added to the plasma membranes does not stimulate the activities of both ATPases. The properties of the high-affinity (Ca2+ + Mg2+)-ATPase are distinctly different from those of the previously reported Ca2+-pump activity of the rat parotid plasma membrane.     

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

Ouabain-sensitive (Na+ + K+)-ATPase activity in the rat myometrial microsome fraction could only be determined following detergent treatment. The (Na+ + K+)-ATPase activity manifested by detergent treatment proved very stable even to high concentrations of NaN3, in contrast Mg+-ATPase activity was reduced to about 30 percent of the control. The major part of the Mg2+-ATPase in the myometrial membrane preparation was found to be identical with the NaN3-sensitive ATP diphosphohydrolase capable of ATP and ADP hydrolysis. This monovalent-cation-insensitive ATP hydrolysis could be extensively reduced by DMSO. Furthermore DMSO prevented the inactivation of the (Na+ + K+)-ATPase activity. 10-100 microM Ca2+ inhibited the (Na+ + K+)-ATPase activity obtained in the presence of SDS by 15-50 percent. The Ca2+ sensitivity of the enzyme was considerably decreased if the proteins solubilized by the detergent had been separated from the membrane fragments by ultracentrifugation. The inhibitory effect could be regained by combining the supernatant with the pellet. Ca2+ sensitivity of the (Na+ + K+)-ATPase activity was preserved even after removal of the solubilized proteins provided that DMSO had been applied. It appears that a factor in the plasma membrane solubilized by SDS may be responsible for the loss of Ca2+ sensitivity of the (Na+ + K+)-ATPase activity, the solubilization of which can be prevented by DMSO.     

Influence of gramicidin S on cardiac membrane Ca2+/Mg2+ ATPase activities and contractile force development

The effects of gramicidin S (GS), an antibiotic, on the rat heart membrane ATPases and contractile activity of the right ventricle strips were investigated. GS inhibited sarcolemmal Ca2+-stimulated ATPase (IC50 = 3 microM), Ca2+/Mg2+ ATPase which is activated by millimolar Ca2+ or Mg2+ (IC50 = 3.4 microM), and sarcoplasmic reticulum Ca2+-stimulated ATPase (IC50 = 6 microM). The type of inhibition for the sarcolemmal Ca2+/Mg2+ ATPase by GS was apparently uncompetitive, while that for Ca2+-stimulated ATPases in sarcolemma or sarcoplasmic reticulum was of mixed type. Other ATPases, including mitochondrial ATPase, sarcolemmal Na+-K+ ATPase, and myofibrillar ATPase, were not inhibited by this agent. GS also decreased the rat right ventricle maximum force development (half-maximal inhibitory concentration was 2-4 microM), maximum velocity of contraction, and maximum velocity of relaxation. The resting tension was increased by GS to over 200%. The contractile actions of GS were mostly irreversible upon washing the muscle 3 times over a 10-min period. Decreased Ca2+, Mg2+, Na+, K+ concentrations in the perfusate increased the effects of GS. These findings showed that GS was a potent inhibitor of divalent cation ATPases of heart sarcolemma and sarcoplasmic reticulum and it is suggested that these membrane effects may explain the cardiodepressant action of this agent.