Plasma membrane Ca2+ transport: antagonism by several potential inhibitors

Chemical inhibitors of Candida albicans hyphal morphogenesis target endocytosis

Candida albicans is an opportunistic pathogen, typically found as a benign commensal yeast living on skin and mucosa, but poised to invade injured tissue to cause local infections. In debilitated and immunocompromised individuals, C. albicans may spread to cause life-threatening systemic infections. Upon contact with serum and at body temperature, C. albicans performs a regulated switch to filamentous morphology, characterized by emergence of a germ tube from the yeast cell followed by mold-like growth of branching hyphae. The ability to switch between growth morphologies is an important virulence factor of C. albicans. To identify compounds able to inhibit hyphal morphogenesis, we screened libraries of existing drugs for inhibition of the hyphal switch under stringent conditions. Several compounds that specifically inhibited hyphal morphogenesis were identified. Chemogenomic analysis suggested an interaction with the endocytic pathway, which was confirmed by direct measurement of fluid-phase endocytosis in the presence of these compounds. These results suggest that the activity of the endocytic pathway, which is known to be particularly important for hyphal growth, represents an effective target for hyphae-inhibiting drugs.

Calcium-Stimulated Protein Phosphorylation in Synaptic Membranes

Synaptic membranes from rat brain contain several calcium-requiring protein kinase (PK) activities with different substrate specificities: (a) an activity (CaH-PK) effective at high concentrations of Ca2+ ion in the absence of Mg2+ (active on class F substrates); (b) a (Ca + Mg)-PK activity that is mediated by Ca2+ ion in the presence of Mg2+ (active on class B substrates); (c) (Ca-CaM)-PK activities that exhibit simultaneous requirements for both Ca2+ ion and CaM (for class C and D substrates). Also described are three activities (d-f) that do not require Ca2+ ion: (d) a Mg-PK activity in which the presence of Ca2+ causes the inhibition of phosphorylation (active on class A substrates); (e) an activity affecting a diverse group of substrates (class E substrates), the phosphorylation of which occurs in the presence of Mg2+ ion alone (Mg-PK activity) and is unaffected by the addition of Ca2+ ion and CaM, the substrates of which show different responses to several types of inhibitors; and, finally, (f) the previously well characterized cAMP-dependent PK activities. Several of the substrates of these kinases have been identified in a fairly unambiguous manner: among them are P43 (class A), as the alpha subunit of pyruvate dehydrogenase; P54 (class B), as the presynaptic protein B50; and the doublet P75-P80, as proteins IA and IB of Ueda and Greengard. The most interesting activity is that requiring both Ca2+ and CaM. The half-maximal stimulation (K0.5) for Ca2+ in the presence of CaM was found to be 1.0 microM Ca2+F in untreated membranes. There is little change in this value on prior EGTA extraction of the membranes, which removes the bulk of its Ca2+ and reduces its residual CaM by greater than or equal to 50%. The apparent K0.5 for CaM in the presence of excess Ca2+ ion was found to equal 0.4 microgram per reaction mixture (8 micrograms/ml) or 1.35 micrograms per reaction mixture (27 micrograms/ml), for the untreated and EGTA-treated membranes, respectively.

Specific 1,25(OH)2D3-mediated regulation of transcellular calcium transport in Caco-2 cells

Calcium transport in the apical-to-basolateral (A-to-B) or B-to-A direction was examined in cells treated with 10 nM 1, 25-dihydroxyvitamin D3 [1,25(OH)2D3, calcitriol] for up to 72 h. Net A-to-B calcium transport was positive at all time points and increased from 0.14 +/- 0.06 to 0.50 +/- 0.01 nmol. well-1. min-1 after 72 h of calcitriol treatment. Neither phenol red transport nor transepithelial electrical resistance was altered by calcitriol treatment, suggesting that the increase in net A-to-B calcium transport was not due to paracellular movement. Neither 25-hydroxyvitamin D3 nor 24,25-dihydroxyvitamin D3 (100 nM, 48 h) alters basal or calcitriol-stimulated A-to-B calcium transport. Treatment with the calmodulin antagonist trifluoperazine (50 microM) reduced calcitriol-stimulated A-to-B Ca transport by 56%. The transcription inhibitor actinomycin D inhibited calcitriol-regulated A-to-B calcium transport as well as calbindin D9k and 24-hydroxylase mRNA accumulation. These data demonstrate that calcitriol-mediated A-to-B calcium transport in Caco-2 cells is a specific, transcellular process that requires transcriptional events normally mediated through the vitamin D receptor.

Inhibition of Ca2+-pump ATPase and the Na+/K+-pump ATPase by iron-generated free radicals. Protection by 6,7-dimethyl-2,4-DI-1- pyrrolidinyl-7H-pyrrolo[2,3-d] pyrimidine sulfate (U-89843D), a potent, novel, antioxidant/free radical scavenger

Preincubation of red blood cell (RBC) membranes with a model system known to generate reactive oxygen species (ROS) and free radicals (200 microM ferrous sulfate and 200 microM EDTA, Fe2+/EDTA) resulted inhibition of the Na+/K+ -pump ATPases was also associated with membrane protein crosslinking and lipid peroxidation, the latter as monitored by the formation of thiobarbituric acid reactive substances (TBARS). Inhibition of the ion transport ATPases, protein cross-linking and formation of TBARS were prevented by U-89843D in a concentration-dependent manner, with half-maximal protection seen at 0.3 microM. U-89843D was more potent than the classical antioxidant butylated hydroxytoluene. Neither U-89843D nor the solvent DMSO had any effect on the assay of TBARS. U-89843D exerted only minimal inhibitory activity on ATPase activities. Thus, U-89843D was potent in vitro in preventing a variety of membrane-damaging reactions mediated by ROS. It is suggested that protection of membranes from ROS-mediated damage is of potential usefulness in the prevention and treatment of certain disease processes.

Effects of trifluoperazine and amiloride on the extrusion of axoplasmic Ca++ by rapidly adapting and slowly adapting type I cutaneous mechanoreceptors: an electron-microscopic study using the oxalate-antimonate method

Influences of Ca++ transport inhibitors on the extrusion of cytosolic Ca++ from axon terminals of oral mucosal mechanoreceptors were cytochemically studied through the use of trifluoperazine (TFP), a Ca++ pump inhibitor, and amiloride, a Na+/Ca++ exchanger inhibitor. Palates of Mongolian gerbils were isolated after perfusion with normal, TFP-containing, or amiloride-containing HEPES-buffered saline, and then mechanically stimulated in the same buffer. Cytosolic Ca++ in axon terminals at 2 min after the stimulation was visualized by means of an oxalate-antimonate method using microwave fixation, and then evaluated electron-microscopically. TFP was found to inhibit the Ca++ extrusion from axon terminals of Meissner corpuscles, but not from those of Merkel cell-axon complexes. Conversely, amiloride severely reduced the Ca++ extrusion from axon terminals of Merkel cell-axon complexes, but not from those of Meissner corpuscles. These results suggest that cytosolic Ca++ extrusion from axon terminals of rapidly adapting and slowly adapting type I mechanoreceptors is regulated by a Ca++ pump and an Na+/Ca++ exchanger, respectively.

The Ca(2+)-transport ATPases from the plasma membrane

The initial studies on the plasma membrane (PM) Ca(2+)-transport ATPases were made in the erythrocyte, a structure that can not be taken as representing a typical eukaryotic cell. In other cell types however, the study of the PM Ca(2+)-transport ATPase is complicated by the simultaneous expression of related Ca(2+)-pumps in intracellular stores. Whereas there are as yet no known specific inhibitors for the PM Ca(2+)-transport ATPase, a number of selective inhibitors for the endo(sarco)plasmic reticulum Ca2+ pumps have been described: thapsigargin, cyclopiazonic acid and 2,5-di-(tert-butyl)-1,4-benzohydroquinone. With the recent introduction of the molecular biological approach, it became quickly obvious that a family of at least 5 different PM Ca(2+)-transport ATPase genes govern the tissue-dependent expression of PM Ca2+ pumps. Moreover alternative splicing of the primary gene transcripts was found to further enhance the number of pump variants. The PM Ca(2+)-transport ATPase are subject to modulatory control by calmodulin, by acidic phospholipids, and by the known families of protein kinases. Each of the ensuing effects are mutually related and interdependent. The wide variety PM Ca2+ pump isoforms and their regulation by such an intricate modulatory network allows the distinct tissues to adapt most adequately to the prevailing tissue and stimulus specific requirements.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.

Inhibition of erythrocyte Ca2(+)-pump by Ca2+ antagonists

Inside-out vesicularized membrane fragments from human erythrocytes were prepared to study the effects of various Ca2+ channel entry blockers of plasma membrane Ca2+ transport and (Ca2+ + Mg2+)-ATPase activity concomitantly. Verapamil and diltiazem (0.01 to 5 mM) inhibited both (Ca2+ + Mg2+)-ATPase activity and initial rates of 45Ca2+ net uptake analogously. In general, the parameter affected most by these drugs, using either Ca2+ transport or (Ca2+ + Mg2+)-5'-adenosine-triphospho-hydrolase (EC 3.6.1.3) ([Ca2+ + Mg2+]-ATPase) measurements, was the stimulation by calmodulin. However, the specificity and selectivity of inhibition appeared to be highly concentration and membrane preparation dependent. Verapamil and diltiazem inhibited the calmodulin-Ca2+ transport concentration-effect relationship by changing its apparent affinity as well as the maximal velocity of the process. In a "white ghost" membrane preparation, bepridil inhibited calmodulin activation with a high degree of selectivity as opposed to its effects on calmodulin activation in the vesicular preparation. Nifedipine failed to exhibit any specificity and modestly inhibited basal and calmodulin-activated inside-out vesicular Ca2+ transport and (Ca2+ + Mg2+)-ATPase alike. Our results suggest that verapamil, diltiazem and bepridil (0.01 to 0.3 mM), but not nifedipine (1 nM to 0.01 mM), in relatively high concentrations can antagonize the calmodulin-stimulated Ca2(+)-pump, i.e. the ATPase as well as the transport process. The inhibitors differed with regard to potency, selectivity, and the type of inhibition they produced.

Ruthenium red and compound 48/80 inhibit the smooth-muscle plasma-membrane Ca2+ pump via interaction with associated polyphosphoinositides

We will demonstrate the compound 48/80 and ruthenium red inhibit the smooth-muscle plasma-membrane Ca2+ pump by counteracting the stimulant effect of negatively charged phospholipids. Both substances did not affect the purified enzyme re-activated by pure phosphatidylcholine or phosphatidylinositol and measured in the absence of calmodulin, indicating that under these conditions they did not have a direct effect on the ATPase protein. Ruthenium red and compound 48/80 however inhibited the (Ca2(+) + Mg2+)-ATPase in the presence of phosphatidylinositol 4-phosphate and especially phosphatidylinositol 4,5-bisphosphate. The K0.5 for inhibition was 25 microM ruthenium red and 9 micrograms/ml of compound 48/80. The inhibition by ruthenium red developed slowly with half maximal inhibition occurring after about 75 s while that by compound 48/80 developed immediately within the time required for mixing. The efficacy of ruthenium red increased as the concentration of the acidic phospholipid increased, while no such cooperativity was observed for compound 48/80. Ruthenium red reduced the Vmax for Ca2+ without affecting the affinity for Ca2+, while compound 48/80 decreased both parameters. In conclusion, although ruthenium red and compound 48/80 affect the ATPase differently, both substances most likely inhibit the plasma-membrane Ca2+ pumping by counteracting the stimulation by negatively charged phospholipids.

Inhibition of basal and calmodulin-activated Ca2+-pump ATPase by fractionated compound 48/80

Compound 48/80 (48/80), a mixture of polycationic compounds was fractionated using affinity chromatography on calmodulin-Sepharose. Unfractionated 48/80 and various fractions were tested for their potential inhibitory effects on ATPase activities of isolated human red blood cell membranes. ATPase activities tested included: Mg2+-ATPase, the Na+/K+-pump ATPase, and the Ca2+-pump ATPase in both its basal (calmodulin-independent) and calmodulin-activated state. Neither 48/80 nor its various fractions were very potent or efficacious inhibitors of the Mg2+-ATPase or the Na+/K+-pump ATPase. In agreement with previous reports, 48/80 was found to be an inhibitor of the calmodulin-activated Ca2+-pump ATPase. By contrast, we found that unfractionated, as well as some fractionated, material inhibited both the basal (calmodulin-independent) and calmodulin-activated Ca2+-pump ATPase activity. A fraction designated as Fraction III bound to calmodulin-Sepharose in the presence of Ca2+ and low salt and was eluted in the absence of Ca2+ and 0.15 M NaCl. By gel filtration, Fraction III had an apparent average molecular weight of 2064 (1320 for unfractionated material). Fraction III was the most potent inhibitor of the Ca2+-pump ATPase with IC50 values for the basal and calmodulin-activated forms of the enzyme of 0.6 and 1.2 micrograms/ml, respectively. Inhibition by Fraction III was cooperative with n apparent values of 2.4 and 5.7, respectively, for the basal and calmodulin-activated forms of the enzyme. Thus, binding of 48/80 constituents to calmodulin can not fully account for the observed data. Direct interaction of 48/80 constituent(s) with the enzyme and/or the lipid portion of the membrane is suggested.

Calmodulin involvement in TPA and DDT induced inhibition of intercellular communication

The organochlorine pesticide DDT is a liver tumour promoter and a potent inhibitor of intercellular communication. Present knowledge of the mechanism by which DDT inhibits intercellular communication is limited but it has been suggested that increased intracellular free calcium induced by DDT could be of importance. As the effects of calcium are closely associated with the multifunctional protein calmodulin (CaM) in most cells the potential binding of DDT to CaM and subsequent effects on CaM-stimulated Ca2+/Mg2+-ATPase activity were studied. DDT inhibited CaM-stimulated Ca2+/Mg2+-ATPase activity and bound to CaM in a manner similar to established CaM-inhibitors. Subsequently an in vitro assay for measuring inhibition of metabolic cooperation between 6-thioguanine (TG)-sensitive and TG-resistant Chinese hamster (V79) cells was used to investigate the possible involvement of CaM in the regulation of intercellular communication. Calmidazolium (CzM), a potent CaM inhibitor, was tested alone or in combination with the tumour promoters 12-O-tetradecanoyl phorbol-13-acetate (TPA) or DDT known inhibitors of intercellular communication. The results showed that CzM alone was without effect with regard to inhibition of metabolic cooperation but potentiated the response induced by TPA, an effect not noticed with DDT. These results suggest different mechanisms of action of TPA and DDT on metabolic cooperation and support the hypothesis that with calcium CaM may be of importance for drug-induced inhibition of intercellular communication and tumour promotion.

Calcium transport by a calmodulin-regulated Ca-ATPase in the enamel organ

Using aldehyde-fixed rat incisor enamel organ, we localized Ca-ATPase activity ultracytochemically in the plasma membranes, the mitochondrial inner membranes, and the Golgi membranes of secretory ameloblasts and the cells of stratum intermedium at the secretory stage and papillary layer cells at the maturation stage, but not in maturation ameloblasts. This Ca-ATPase activity was totally dependent on substrate ATP, the enzyme activator CaCl2, and also sensitive to the specific calmodulin blocker trifluoperazine (TFP) in the incubation media. Specific antigenic sites of endogenous calmodulin were demonstrated in polyribosomes, the nucleus, mitochondria, and the cytoplasmic matrix along the plasma membranes of secretory ameloblasts, by the protein A-immunogold technique using sheep antiserum against bovine testis calmodulin. All other enamel organ cells-such as stratum intermedium, papillary layer cells, and maturation ameloblasts-were also weakly immunoreactive. In control sections incubated with antiserum pre-absorbed with an excess of calmodulin and protein A-gold complex, only a few gold particles were observed to be randomly associated with the tissues. Daily intraperitoneal injection of TFP (1 and 5 mg per 100 g body weight) for one week resulted in prominent migration of mitochondria from the infranuclear to supranuclear regions of secretory ameloblasts but caused no other morphological alterations in the enamel organ cells. EDX analysis of ultrathin sections revealed significantly lower peaks of Ca and P in the forming enamel of TFP-injected rats than those in controls. However, little reduction in the Ca and P levels in the maturing enamel was observed in TFP-injected rats. When growing enamel surfaces were exposed with NaOCl and examined with SEM, a remarkable defect in the enamel matrix was observed in the forming enamel but not in the maturing enamel. These results suggest that early enamel mineralization is dependent upon an intact calmodulin-regulated Ca-transporting ATPase in secretory ameloblasts and that enamel maturation is controlled by different mechanism(s).

Calmodulin-stimulated plasma membrane (Ca2+ + Mg2+)-ATPase: inhibition by calcium channel entry blockers

Calcium channel entry blockers representing different structural classes were studied for their effects on human erythrocyte basal and calmodulin-stimulated (Ca2+ + Mg2+)-ATPase. Effects on the activity of (Mg2+)-ATPase and (Na+ + K+)-ATPase were also assessed. Of the four Ca2+ entry blockers tested, only verapamil and diltiazem specifically inhibited the calmodulin-stimulated (Ca2+ + Mg2+)-ATPase activity, the basal enzyme activity being unaltered by these drugs. Other membrane-associated ATPases were not affected. Calmodulin concentration effect curves showed the inhibition by verapamil (10(-3) M) and diltiazem (10(-3) M) to be non-competitive. This concentration inhibited the calmodulin-dependent increment (5.1 nM calmodulin) of the ATPase activity by 35 and 36% respectively. Similarly, both drugs inhibited the Ca2+-activation process of calmodulin-stimulated activity in a non-competitive manner, decreasing Vmax by 23 and 17% respectively. Basal (Ca2+ + Mg2+)-ATPase activity was not affected by verapamil or diltiazem at any calcium concentration. In contrast, cinnarizine non-specifically inhibited all four membrane ATPases including calmodulin-stimulated (Ca2+ + Mg2+)-ATPase activity at concentrations above 3 X 10(-6) M. Nifedipine was without effect on any of the four membrane ATPases. From this we conclude that certain calcium channel entry blockers can inhibit calmodulin-regulated plasma membrane Ca2+-pump ATPase. Therefore, this identifies an additional functional low affinity receptor in the plasma membrane for some of the calcium channel entry blockers.

Dissociation of insulin binding from insulin stimulation of 2-deoxyglucose transport by ruthenium red

Ruthenium red increased specific insulin binding to isolated adipocytes 5.4 fold and 2.6 fold over binding determined in the absence and presence of Ca2+ and Mg2+. The increase in insulin binding was not accompanied by an increase in insulin sensitivity. The lack of effect of ruthenium red on insulin action argued strongly against an increase in intracellular Ca2+ as a potential messenger/transducer of insulin action and suggested that the enhancing effect of Ca2+ on insulin action was a result of increased receptor affinity.

The effect of ETH 1001 on ion fluxes across red blood cell membranes

The calcium selective ionophore, ETH 1001, and the divalent cation ionophore, A23187, promoted Ca2+ flux across RBC membranes under various experimental conditions. ETH 1001 did not promote the passive movement of Mg2+ whereas A23187 did. The results confirm the potential application of ETH 1001 as a Ca2+ selective ionophore for biological membranes.

On the failure of calmodulin to activate Ca2+ pump ATPase of dog red blood cells

Ca2+-pump ATPase activities of membranes isolated from human and dog RBCs were compared under a variety of conditions. Specific activity of the dog enzyme was less than that of human. Unlike the human enzyme, the dog Ca2+-pump ATPase was not stimulated by exogenously added calmodulin (CaM) or oleate. The Ca2+ dependence of the dog Ca2+-pump ATPase resembled that of the CaM-activated form of the human enzyme. Cross-linking of Azido-125I-CaM to dog RBC membranes did not label a Ca2+-pump ATPase of molecular weight similar to that found in human RBC membranes. It is suggested that the Ca2+-pump ATPase in isolated dog RBC membranes exists in an activated state, not due to endogenous CaM, but possibly due to partial proteolysis.

A Ca2+-stimulated adenosine triphosphatase in Golgi-enriched membranes of lactating murine mammary tissue

A membrane fraction isolated from lactating murine mammary tissue and enriched for the Golgi membrane marker enzyme galactosyltransferase exhibited Ca2+-stimulated ATPase activity (Ca-ATPase) in 20 microM-free Mg2+ and 10 microM-MgATP, with an apparent Km for Ca2+ of 0.8 microM. Exogenous calmodulin did not enhance Ca2+ stimulation, nor could Ca-ATPase activities be detected in millimolar total Mg2+ and ATP. When assayed with micromolar Mg2+ and MgATP the Ca-ATPases of skeletal-muscle sarcoplasmic reticulum and of calmodulin-enriched red blood cell plasma membranes were half-maximally activated by 0.1 microM- and 0.6 microM-Ca2+ respectively. All three Ca-ATPases were inhibited by similar micromolar concentrations of trifluoperazine, but the Golgi activity was unaffected by quercetin in concentrations which completely inhibited both the sarcoplasmic-reticulum and red-blood-cell enzymes. The results are consistent with the hypothesis that the high-affinity Ca-ATPase is responsible for the ATP-dependent Ca2+ transport exhibited by Golgi-enriched vesicles derived from lactating mammary gland [Neville, Selker, Semple & Watters (1981) J. Membr. Biol. 61, 97-105; West (1981) Biochim. Biophys. Acta 673, 374-386].

Effect of trifluoperazine, compound 48/80, TMB-8 and verapamil on ionophore A23187 mediated calcium uptake in ATP depleted human red cells

The A23187 induced calcium uptake in ATP depleted cells was determined at pH 6.9 in the presence of trifluoperazine (TFP, 0.30 mM), compound 48/80 (0.89 mg/ml), 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8, 2.13 mM) and verapamil (1.81 mM). Apart from verapamil the drugs all increased the maximum rate of ionophore-mediated calcium flux by 50-60 per cent. After the ionophore addition some time elapsed before the calcium flux attained the maximum value, and this time dependence could be interpreted as a slow uptake of A23187 into the membrane: five seconds after the addition of A23187 half of the added ionophore was able to transport calcium through the membrane. The effect of pH on the ionophore-mediated calcium uptake was determined in the absence and presence of TFP. At pH 7.4 the maximum rate of calcium flux in the absence of TFP was two to three times higher than that at pH 6.9 and TFP increased the uptake rate by 98 per cent.

Nematodirus battus: permeability changes, calcium binding, and phosphorylation of the eggshell during hatching

Eggshells of Nematodirus battus leaked trehalose 4 hr after being stimulated to hatch, and became permeable to trypan blue at their poles; 80% of eggs were stained blue 24 hr later. Exogenous application of ruthenium red significantly inhibited chill- and sodium fluoride-stimulated hatching, 50% hatch inhibition occurring in 44.67 +/- 2.2 and 8.5 +/- 1.5 microM, respectively. Lanthanum chloride, however, was not as inhibitory as ruthenium red on fluoride-stimulated hatching, 50% occurring at 31.60 +/- 1.25 microM. A Scatchard plot of the competitive binding of ruthenium red to eggshells demonstrated a high-affinity binding site for calcium, KCa' = 1.92 microM and a second, low-affinity site, KCa" = 1169.60 microM. Ruthenium red binding was significantly reduced by several enzymes, e.g., EGTA-buffered trypsin reduced binding by 73%. Radioiodinated concanavalin A also bound competitively to the eggshells in the presence of alpha-D-glucosyl-alpha-D-glucopyranoside and alpha-methyl-D-mannopyranoside. Eggshells incorporated phosphorus-32 from ATP after chilling or on exposure to sodium fluoride; gel filtration of solubilized homogenates of these samples showed that two proteins were radiolabelled with molecular weights of 38 X 10(3) and 8 X 10(3) Da, respectively. This phosphorylation was inhibited by N-ethylmaleimide, which also prevented hatching.

Effect of trifluoperazine, compound 48/80, TMB-8 and verapamil on the rate of calmodulin binding to erythrocyte Ca2+-ATPase

The erythrocyte Ca2+-ATPase shifts reversibly between two states, the calmodulin-deficient A-state and the calmodulin-saturated B-state, dependent on calcium and calmodulin. The effects on this system of the four drugs, trifluoperazine, compound 48/80, TMB-8 and verapamil were studied. All four drugs inhibited the maximum activity of the B -state Ca2+-ATPase and, in addition, trifluoperazine and compound 48/80 in higher doses inhibited the A-state. Furthermore, the four drugs decreased the calmodulin sensitivity of the Ca2+-ATPase in the order of decreasing effect: trifluoperazine greater than compound 48/80 greater than TMB-8 greater than verapamil. In the same order of decreasing effect the drugs increased the time required for full calmodulin activation of the A-state of Ca2+-ATPase, whereas the drugs had only small effects on the rate of deactivation of the B-state, caused by dissociation of calmodulin from the enzyme. It is discussed whether the effects on calmodulin activation were caused by a reduction of free calmodulin due to the formation of drug-calmodulin complexes or whether the drugs, especially trifluoperazine, compound 48/80 and TMB-8, by binding to the Ca2+-ATPase, decreased the rate constants for association of calmodulin and enzyme.

Trifluoperazine, a calmodulin antagonist, inhibits muscle cell fusion

We investigated the effect of trifluoperazine (TFP), a calmodulin antagonist, on the fusion of chick skeletal myoblasts in culture. TFP was found to inhibit myoblast fusion. This effect occurs at concentrations that have been reported to inhibit Ca2+-calmodulin in vitro, and is reversed upon removal of TFP. In addition, other calmodulin antagonists, including chlorpromazine, N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide (W7), and N-(6-aminohexyl)-1-naphthalene-sulfonamide (W5), inhibit fusion at doses that correspond closely to the antagonistic effects of these drugs on calmodulin. The expression of surface acetylcholine receptor, a characteristic aspect of muscle differentiation, is not impaired in TFP-arrested myoblasts. Myoblasts inhibited from fusion by 10 microM TFP display impaired alignment. In the presence of the Ca2+ ionophore A23187, the fusion block by 10 microM TFP is partially reversed and myoblast alignment is restored. The presence and distribution of calmodulin in both prefusional myoblasts and fused muscle cells was established by immunofluorescence. We observed an apparent redistribution of calmodulin staining that is temporally correlated with the onset of myoblast fusion. Our findings suggest a possible role for calmodulin in the regulation of myoblast fusion.

Inducement of wound motility in intact giant algal cells

The cytoplasm (with its organelles) of intact cells of Ernodesmis verticillata (Chlorophyta) can be induced to contract in the presence of calcium ionophores and phenothiazine antipsychotics. The cell contents mimic wound-healing contraction if a combination of 10 microM A23187 and 10 microM chlorpromazine (or trifluoperazine) is present in a Ca2+-containing medium. The average incubation time is approx. 50 min for contraction. It is imperative that only fresh solutions of ionophores and phenothiazines are used, because stock solutions only 3 h old virtually double the response time of these cells. Separately, neither the ionophore nor the phenothiazines will induce contraction. The addition of 1.0 mM La3+ completely prevents induction of motility. With trifluoperazine, equimolar X-537A will substitute for A23187, but it takes three times as long to induce contraction. Motility cannot be induced in Ca2+-free media (containing 5.0 mM EGTA). It therefore appears that Ca2+ fluxes are responsible for triggering wound contraction in these giant algal cells. An influx of calcium ions from the external medium is suggested as being at least partially involved.

Presynaptic action of trifluoperazine at the frog neuromuscular junction

Treatment of frog neuromuscular preparations bathed in basic frog saline (1.8 mM Ca2+) with trifluoperazine (25 microM) caused an increase in MEPP frequency in 6 out of 10 preparations tested. The mean normalised MEPP frequency after 15 min of treatment was approximately 1.5. 10 microM trifluoperazine had a similar effect. In salines containing low concentrations of Ca2+ (50 microM Ca2+, 2 mM Mg2+ or 0 Ca2+, 1 mM EGTA) the stimulatory action of trifluoperazine was more marked and occurred in a higher proportion of the preparations tested (11 out of 14). When evoked release of transmitter was reduced to very low levels by Mg2+-containing salines treatment with trifluoperazine (2.5-25 microM) caused an increase in quantal content of 20-60%. Depolarisation of preparations bathed in standard frog saline by increasing [K+]o to 10 mM resulted in a 10-fold increase in MEPP frequency. This response was inhibited by about 25% in 10 microM trifluoperazine and by about 45% in 25 microM trifluoperazine. Pre-treatment of preparations with trifluoperazine (25 microM) caused a marked reduction in the response of MEPP frequency to tetanic stimulation (50 Hz) both in the presence of an inward electrochemical gradient for Ca2+ (50 microM Ca2+, 2 mM Mg2+) and in a Ca2+-free saline (0 Ca2+, 1 mM EGTA). The effects of trifluoperazine on tetanic enhancement of MEPP frequency are compared to those of other agents and it is shown that the results are inconsistent with an effect of the drug on Ca2+-fluxes at the plasma membrane. It is concluded that trifluoperazine has both stimulatory and inhibitory effects on transmitter release at the frog neuromuscular junction and that the inhibitory effect is probably due to inhibition of excitation-secretion coupling at a point subsequent to Ca2+ mobilization.

The Mg2+-dependent ATPase from the erythrocyte plasma membrane of the flounder Platichthys flesus L. General properties and some observations on the steady state kinetics

1. The plasma membrane of the flounder erythrocyte contains a Mg2+-dependent ATPase which is insensitive to ouabain. Mg2+ is part of the substrate, Mg-ATP, and Mg2+ also functions as a nonessential activator. 2. Ca2+, Mn2+ and Co2+ can replace Mg2+ as an activator of ATP hydrolysis. Cu2+ and Zn2+ abolish the Mg-dependent activity. It is shown that Ca2+ and Mg2+ activate the same enzyme and that Mg-ATP and Ca-ATP are mutually competitive. 3. The hydrolysis of ATP obeys Michaelis-Menten kinetics whether or not the Mg2+-ATPase is fully activated by Mg2+. The KM values for Mg-ATP were found to be 0.13 and 0.43 mM respectively. 4. Free ATP acts as a competitive inhibitor towards Mg-ATP and the dissociation constant for the enzyme-ATP complex was determined to be about 0.55 mM. 5. The Mg2+ -ATPase has a low specificity and reacts with the common nucleoside triphosphates GTP, ITP, UTP and CTP. 6. The enzyme has a broad pH optimum ranging from 6.5 to 7.2 and an energy of activation of 13.5 kcal/mol between 0 and 30 degrees C. 7. The effect of some activators and inhibitors of membrane-bound ATPases are reported.

Calcium-dependent K+ efflux from rat submandibular gland. The effects of trifluoperazine and quinidine

The Ca2+-dependent K+ efflux from rat submandibular gland was studied using a K+-sensitive electrode. A K+ efflux was induced by either adrenalin or by using the divalent cation ionophore A23187 plus added Ca2+ to bypass the receptor mechanism. Trifluoperazine, which was used to investigate the role of calmodulin, was found to block the adrenalin-induced K+ efflux but not the A23187/Ca2+-induced K+ efflux. The adrenalin-induced K+ efflux was abolished by quinidine and the A23187/Ca2+-induced K+ efflux was significantly reduced by quinidine. In other experiments, the presence of indomethacin did not inhibit the adrenalin-induced K+ efflux, and exogenously added arachidonic acid did not induce a K+ efflux. It is concluded that neither prostaglandin synthesis, nor a cytosolic Ca2+-calmodulin complex is involved in the agonist-induced K+ efflux from rat submandibular gland. A similarity between the Ca2+-dependent K+ efflux mechanism of erythrocyte ghosts and submandibular tissue is indicated by their common response to quinidine.

Pharmacological modification of the Ca2+-pump ATPase activity of human erythrocytes

The Ca2+-pump ATPase of human RBC membranes appears to be exquisitely sensitive to a variety of amphipathic molecules. The acidic protein calmodulin (CaM) activates the enzyme some three- to fivefold with an apparent Kd of approximately 1-5 nM. A variety of other amphipathic anions, such as acidic phospholipids, free fatty acids, and anionic detergents, are less potent and in some cases less efficacious than CaM, but also activate the enzyme. Similar results have been observed for other CaM-dependent enzymes, and it is suggested that these agents mimic CaM in a general, but rather nonspecific, fashion. Activation of the human RBC Ca2+-pump ATPase by CaM or other amphipathic anions can be selectively antagonized by a wide variety (structurally and pharmacologically) of amphipathic cations. There is no simple relationship between antagonism of CaM in vitro and the general systemic pharmacology of these drugs. The only common feature of such drugs is that they are amphipathic cations. Neutral molecules such as saponin exerted neither CaM-like activity nor CaM antagonism. Great caution is urged in the inferential use of presumed anti-CaM drugs to study biological systems.

Ca2+-Mg2+-ATPase activity in brain coated microvesicles purified on immunosorbents

Coated microvesicle fractions isolated from ox forebrain cortex by the ultracentrifugation procedure of Pearse (1) and by the modified, less time consuming method of Keen et al (2) had comparable Ca2+ +Mg2+ dependent ATPase activities (about 9 mumol/h per mg protein). The Na+ +K+ +Mg2+ dependent ATPase activity was 3.2 mumol/h per mg (+/- 1.0, S.D., n = 3) when microvesicles were prepared according to (1) and 1.5 mumol/h per mg (+/- 1.0, S.D., n = 3) when prepared according to (2). Oligomycin, ruthenium red, and trifluoperazine, inhibitors of Ca2+ transport in mitochondria and erythrocyte membranes had no effect on Ca2+ +Mg2+ dependent ATPase from any of the preparations. As demonstrated both by ATPase assays and electron microscopy, coated microvesicles could be bound to immunosorbents prepared with poly-specific antibodies against a coated microvesicle fraction obtained by the method of Pearse (1). The binding could be inhibited by dissolved coat protein using partially purified clathrin. The fraction of coated vesicles eluted from the immunosorbent was purified relative to the starting material as judged by electron microscopy. The Ca2+ +Mg2+ ATPase activity and calmodulin content was copurified with the coated microvesicles and the specific activity of Na+ +K+ +Mg2+ ATPase was decreased. Na+ +K+ +Mg2+ dependent ATPase activity in the coated microvesicle fraction could be ascribed to membranes with the appearance of microsomes. These membranes were also bound to the immunosorbents, but the binding was not influenced by clathrin. The capacity of the immunosorbents for these membranes was less than for the coated microvesicles, resulting in a decrease of Na+ +K+ +Mg2+ dependent ATPase activity in the eluted coated microvesicle fraction. It was concluded that Ca2+ +Mg2+ ATPase activity is not a contamination from plasma membrane vesicles or mitochondrial membranes but seems to be an integral part of the coated vesicle membrane.

Thyrotropin-releasing hormone mobilizes Ca2+ from endoplasmic reticulum and mitochondria of GH3 pituitary cells: characterization of cellular Ca2+ pools by a method based on digitonin permeabilization

Treatment of 45Ca2+-loaded GH3 pituitary cells with various concentrations of digitonin revealed discrete pools (I and II) of cellular 45Ca2+ defined by differing detergent sensitivities. Markers for cytosol and intracellular organelles indicated that the two 45Ca2+ pools were correlated with the two major cellular Ca2+-sequestering organelles, endoplasmic reticulum (I) and mitochondria (II). Studies with various inhibitors were consistent with these assignments. Mitochondrial uncouplers preferentially depleted 45Ca2+ pool II while trifluoperazine selectively depleted 45Ca2+ pool I. Control experiments indicated that translocation of in situ organellar 45Ca2+ during and after permeabilization was negligible. We used the digitonin-permeabilization method to examine the effect of thyrotropin-releasing hormone (TRH) treatment on intracellular Ca2+ pools of GH3 pituitary cells. TRH was found to rapidly deplete both endoplasmic reticulum and mitochondrial exchangeable Ca2+ by 25-30%. The 45Ca2+ loss from both pools was maximal by 1 min after TRH addition and was followed by a recovery phase; mitochondrial 45Ca2+ content returned to control levels by 30 min. Previous treatment of cells with the mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxy-phenylhydrazone blocked TRH-induced 45Ca2+ efflux from mitochondria, while previous treatment with valinomycin, an agent that depleted both 45Ca2+ pools, blocked any additional effect of TRH on these pools. We conclude that TRH rapidly promotes a net loss of exchangeable Ca2+ from GH3 cells as a result of hormone-induced mobilization of Ca2+ from endoplasmic reticulum and mitochondria.

ATP-dependent calcium transport and its correlation with Ca2+ -ATPase activity in basolateral plasma membranes of rat duodenum

Isolated basolateral plasma membrane vesicles from rat duodenum epithelial cells exhibit ATP-dependent calcium-accumulation and Ca2+ -dependent ATPase activity. Calcium accumulation stimulated by ATP is prevented by the calcium ionophore A23187, inhibited 80% by 0.1 mM orthovanadate but is not effected by oligomycin. Calcium accumulation is not observed with the substrate beta-gamma-(CH2)-ATP, ADP and p-nitrophenyl phosphate. Kinetic studies reveal an apparent Km of 0.2 microM Ca2+ and a Vmax of 5.3 nmol Ca2+/min per mg protein for the ATP-dependent calcium-uptake system. Calmodulin and phenothiazines have no effect on calcium accumulation in freshly prepared membranes, but small effects are inducible after a wash with a 5 mM EGTA. The kinetic parameters of Ca2+ -ATPase are: Km = 0.25 microM Ca2+ and Vmax = 19.2 nmol Pi/min per mg protein. Three techniques, osmotic shock, treatment with Triton X-100 or the channel-forming peptide alamethicin, reveal that about 40% of the vesicles are resealed. Assuming that half of the resealed vesicles have an inside-out orientation, the Vmax of ATP-dependent calcium uptake amounts to 25 nmol Ca2+/min per mg protein and of the Ca2+ -ATPase to 23 nmol Pi/min per mg protein. The close correlation between kinetic parameters of Ca2+ -ATPase and ATP-dependent calcium-transport strongly suggests that both systems are expressions of a Ca2+ -pump located in duodenal basolateral plasma membranes.

Purification and characterization of actinogelin, a calcium-sensitive actin-accessory protein, from rat liver

Although cell-free extracts prepared from several types of free-living cells, including Ehrlich tumor cells, macrophages and sea-urchin eggs, readily form gels under low Ca2+ conditions, no such ability to induce actin-related gel has been detected in tissue-cell extracts. Ca2+ -insensitive gelation activity was discovered, however, in several tissue-cell extracts, including liver and brain, provided that the extracts were supplemented with skeletal muscle actin. Based on sodium dodecylsulfate/polyacrylamide gel electrophoretic analysis of the gel, these extracts seem to contain both a Ca2+ -insensitive gelation factor and Ca2+ -sensitive one, actinogelin. A procedure for purification of actinogelin from rat liver was developed, and the properties of actinogelin thus purified were compared with those of Ehrlich tumor cell actinogelin. No appreciable difference was found in these two proteins, and Ca2+ sensitivity (50% inhibition of gelation at 1 microM) was very similar. Some of the molecular characteristics are described, and the importance of the presence of actinogelin in tissue cells is discussed.

Effects of trifluoperazine and mitogenic lectins on calcium ATPase activity and calcium transport by human lymphocyte plasma membrane vesicles

The phenothiazine, trifluoperazine, and the mitogenic lectins, phytohemagglutinin (PHA) and Concanavalin A (Con A), were tested for their effects on human lymphocyte plasma membrane Ca-activated Mg-ATPase and ATP-dependent calcium uptake. Trifluoperazine completely inhibited Ca-uptake when present from the start of the assay at concentrations of 100 microM or more. When added during measurement of calcium uptake, trifluoperazine reduced the rate of vesicular calcium accumulation but was unlike the calcium ionophore, A23187, which caused a rapid release of accumulated calcium from the vesicles. Trifluoperazine also inhibited membrane vesicle Ca-ATPase activity, but this inhibition was non-specific since the Mg-ATPase and Na,K-ATPase activities were inhibited to similar extents at the same concentration of the phenothiazine. In contrast, concentrations of PHA and Con A, which are mitogenic for lymphocytes, did not cause any change in Ca-uptake when added to suspensions of membrane vesicles. Con A had no effect and PHA had a weak inhibitory effect on Ca-ATPase activity.

Possible role of calcium uptake and calmodulin in adrenal glomerulosa cells: effects of verapamil and trifluoperazine

The effects of verapamil and trifluoperazine were examined on isolated rat adrenal glomerulosa cells so as to assess the role of calcium ion influx and calmodulin in the function of this cell population. Verapamil (10(-5) and 10(-4) moles/1) slightly reduced the basal production rate of aldosterone and strongly inhibited the response to angiotensin II, potassium ions, corticotrophin (ACTH) and dibutyryl cyclic AMP (db-cAMP). The concentration of verapamil required to reduce the response to these agonists by 50% varied between 2 and 6 mumoles/1. Trifluoperazine (30 mumoles/1) slightly increased the basal production rate of aldosterone. The response to angiotensin and potassium was variably antagonized by 3 mumoles/1 trifluoperazine and completely inhibited by the drug at 30 mumoles/1. The antagonist at a concentration of 3 mumoles/1 exerted either a facilitatory or inhibitory effect on the response to ACTH and db-cAMP, depending on the concentration of the agonist. Trifluoperazine at a concentration of 30 mumoles/1 reduced the response to both agonists to a level which was 2-3 fold higher than that observed in appropriate control samples. The present results indicate that (1) calcium influx is an essential event in the aldosterone stimulating action of angiotensin II, potassium ions, ACTH and cyclic AMP; (2) stimulation by angiotensin II and potassium ions are completely dependent on calmodulin; (3) stimulation by ACTH and cyclic AMP is mediated by calmodulin-dependent and independent mechanisms.

Calcium ions, drug action and the red cell membrane

Intracellular calcium regulates a number of membrane functions in the erythrocyte, including control of shape, membrane lipid composition and cation permeability. Measurement of total red cell calcium has yielded values between 5 and 15 nmol/ml cells, and these low values in part reflect the absence of Ca2+ -containing organelles. Most intracellular Ca2+ is bound and the low cell ionized Ca2+ concentration (approximately 0.2 microM) is maintained by a combination of low membrane permeability and a powerful Ca2+ -pump. This pump has been identified with a (Ca2+ + Mg2+)-stimulated ATPase, and both Ca2+ transport and ATP splitting are stimulated by calmodulin, a low molecular weight protein which binds Ca2+ avidly and activates many Ca2+ -dependent enzymes. Both high and low affinity kinetics for Ca2+ pumping have been demonstrated, depending on the extent of binding of calmodulin to the pump. A stoichiometry of either 1 or 2 Ca2+ ions pumped per ATP molecule split has been shown, and the value may vary with the level of intracellular Ca2+. Phenothiazines, such as chlorpromazine inhibit the Ca2+ -pump by antagonizing the increment in activity produced by calmodulin. The passive inward leak of Ca2+ into erythrocytes can be quantitated by 45Ca2+ uptake into red cells whose Ca2+ -pump has been inhibited. Estimates of the Ca2+ permeability, based on unidirectional influx, yield values many orders of magnitude lower than for nucleated cells. Influx of Ca2+ into human erythrocytes occurs by a facilitated diffusion process, which can be inhibited by phenothiazines and the cinchona alkaloids. Calcium affects many membrane functions including cation permeability, lipid composition and some cytoskeletal interactions which may determine cell shape. Any rise in intracellular Ca2+ activates a specific K+ channel which normally makes little contribution to K+ fluxes. Kinetic studies of this process demonstrate either high or low affinity Ca2+ -activation of K+ efflux, with low affinity of the channel to Ca2+ being the probable state in vivo. Propranolol is the best known activator of Ca2+ -stimulated K+ efflux, although the mechanism of stimulation is unclear. Like other tissues, red cells possess a Ca2+ -activated phosphoinositol phosphodiesterase. Although it has been suggested that the echinocytic shape change induced by Ca2+ is due to the hydrolysis of polyphosphoinositides, it seems more likely that this shape change results from an effect of Ca2+ on the macromolecular interactions of the cytoskeleton. Abnormal Ca2+ permeability may contribute to red cell destruction in a variety of diseases. For example, in sickle cell anemia a large Ca2+ influx occurs when cells are sickled under deoxy conditions, and moreover, the ability of the Ca2+ -pump to extrude the increment of cell Ca2+ is impaired. Thus, red cell Ca2+ is increased 3-7-fold above normal and this may contribute to the short survival of sickle red cells...

Calmodulin pharmacology

Calmodulin (CaM) is a major intracellular receptor for Ca2+. CaM is thus a crucial receptor to consider in pharmacological modification of cellular activity. Potential mechanisms by which drugs may modify CaM effectiveness are considered in the context of its interaction with Ca2+ and in turn with its various effectors. Some examples of established drug mechanisms are considered. A wide range of chemical compounds representing diverse pharmacological classes are anti-CaM under some conditions. No simple relationships have been established between molecular level events and therapeutic applicability of anti-CaM compounds.