Effects of calmodulin antagonists on the active Ca2+ uptake by rat liver mitochondria

Antibacterial properties of phenothiazine derivatives against multidrug-resistant Acinetobacter baumannii strains

AIM

As options to treat recalcitrant bacterial infections which are increasingly limited due to multidrug-resistant strains, searching for new, effective antibacterial compounds is necessary. One strategy is to generate treatment alternatives by drug repurposing.

METHODS AND RESULTS

In this work, phenotypic microarrays were used for the screening of miscellaneous compounds against the growth and biofilm formation of Acinetobacter baumannii, an important emergent multidrug-resistant opportunistic pathogen. The results showed that the phenothiazine derivatives, such as promethazine, trifluoperazine, thioridazine, and chlorpromazine, inhibited the growth of antibiotic-sensitive and multidrug-resistant strains (showing minimal inhibitory concentrations ranging from 0·05 to 0·6 g l^-1 and minimal bactericidal concentrations ranging from 0·1 to 2·5 g l^-1 ). All phenothiazine derivatives were active against biofilm cells (with minimal biofilm eradication concentrations ranging from 0·5 to >3 g l^-1 ). Chlorpromazine promoted reactive oxigen species (ROS) production, and cell membrane and DNA damage. Chlorpromazine showed synergy with antibiotics such as ceftazidime, meropenem, and colistin and was an effective treatment for experimentally infected Galleria mellonella when combined with ceftazidime.

CONCLUSIONS

It was demonstrated that phenothiazine derivatives, especially chlorpromazine, are drugs with attractive antibacterial properties against nosocomial MDR strains of A. baumannii, by generating ROS and cell membrane and DNA damage.

SIGNIFICANCE AND IMPACT OF THE STUDY

The present study indicates that repurposing phenothiazine derivatives for treating recalcitrant infections by A. baumannii could be promising.

Evaluation of putative inhibitors of mitochondrial permeability transition for brain disorders--specificity vs. toxicity

Inhibition of mitochondrial permeability transition (mPT) has emerged as a promising approach for neuroprotection and development of well-tolerated mPT inhibitors with favorable blood-brain barrier penetration is highly warranted. In a recent study, 28 clinically available drugs with a common heterocyclic structure were identified as mPT inhibitors e.g. trifluoperazine, promethazine and nortriptyline. In addition, neuroprotection by structurally unrelated drugs e.g. neurosteroids, 4-hydroxy-tamoxifen and trimetazidine has been attributed to direct inhibition of mPT. The regulation of mPT is complex and highly dependent on the prevailing experimental conditions. Several features of mPT, such as swelling, depolarization or NADH oxidation, can also occur independently of the mPT phenomenon. Here, in isolated rodent brain-derived and human liver mitochondria, we re-evaluate drugs promoted as potent mPT inhibitors. We address the definition of an mPT inhibitor and present strategies to reliably detect mPT inhibition in vitro. Surprisingly, none of the 12 compounds tested displayed convincing mPT inhibition or effects comparable to cyclophilin D inhibition by the non-immunosuppressive cyclophilin inhibitor D-MeAla(3)-EtVal(4)-Cyclosporin (Debio 025). Propofol and 2-aminoethoxydiphenyl borate (2-APB) inhibited swelling in de-energized mitochondria but did not increase calcium retention capacity (CRC). Progesterone, trifluoperazine, allopregnanolone and 4-hydroxy-tamoxifen dose-dependently reduced CRC and respiratory control and were thus toxic rather than beneficial to mitochondrial function. Interestingly, topiramate increased CRC at high concentrations likely by a mechanism separate from direct mPT inhibition. We conclude that a clinically relevant mPT inhibitor should have a mitochondrial target and increase mitochondrial calcium retention at concentrations which can be translated to human use.

Regucalcin increases Ca2+-ATPase activity in the mitochondria of brain tissues of normal and transgenic rats

The role of regucalcin, which is a regulatory protein in intracellular signaling, in the regulation of Ca(2+)-ATPase activity in the mitochondria of brain tissues was investigated. The addition of regucalcin (10(-10) to 10(-8) M), which is a physiologic concentration in rat brain tissues, into the enzyme reaction mixture containing 25 microM calcium chloride caused a significant increase in Ca(2+)-ATPase activity, while it did not significantly change in Mg(2+)-ATPase activity. The effect of regucalcin (10(-9) M) in increasing mitochondrial Ca(2+)-ATPase activity was completely inhibited in the presence of ruthenium red (10(-7) M) or lanthanum chloride (10(-7) M), both of which are inhibitors of mitochondrial uniporter activity. Whether the effect of regucalcin is modulated in the presence of calmodulin or dibutyryl cyclic AMP (DcAMP) was examined. The effect of regucalcin (10(-9) M) in increasing Ca(2+)-ATPase activity was not significantly enhanced in the presence of calmodulin (2.5 microg/ml) which significantly increased the enzyme activity. DcAMP (10(-6) to 10(-4) M) did not have a significant effect on Ca(2+)-ATPase activity. The effect of regucalcin (10(-9) M) in increasing Ca(2+)-ATPase activity was not seen in the presence of DcAMP (10(-4) M). Regucalcin levels were significantly increased in the brain tissues or the mitochondria obtained from regucalcin transgenic (RC TG) rats. The mitochondrial Ca(2+)-ATPase activity was significantly increased in RC TG rats as compared with that of wild-type rats. This study demonstrates that regucalcin has a role in the regulation of Ca(2+)-ATPase activity in the brain mitochondria of rats.

Stimulatory effect of regucalcin on mitochondrial ATP-dependent calcium uptake activity in rat kidney cortex

The effect of regucalcin, which is a regulatory protein of Ca(2+) signaling, on Ca(2+)-ATPase activity in isolated rat renal cortex mitochondria was investigated. The presence of regucalcin (50, 100, and 250 nM) in the enzyme reaction mixture led to a significant increase in Ca(2+)-ATPase activity. Regucalcin significantly stimulated ATP-dependent (45)Ca(2+) uptake by the mitochondria. Ruthenium red (10(-6) M) or lanthunum chloride (10(-6) M), an inhibitor of mitochondrial Ca(2+) uptake, markedly inhibited regucalcin (100 nM)-increased mitochondrial Ca(2+)-ATPase activity and (45)Ca(2+) uptake. The effect of regucalcin (100 nM) in elevating Ca(2+)-ATPase activity was completely prevented by the presence of digitonin (10(-2)%), a solubilizing reagent of membranous lipids, vanadate, an inhibitor of phosphorylation of ATPase, or dithiothreitol (50 mM), a protecting reagent of the sulfhydryl (SH) group of the enzyme. The activating effect of regucalcin (100 nM) on Ca(2+)-ATPase activity was not further enhanced by calmodulin (0.30 microM) or dibutyryl cyclic AMP (10(-4) M), which could increase Ca(2+)-ATPase activity. Trifluoperazine (TFP; 50 microM), an antagonist of calmodulin, significantly decreased Ca(2+)-ATPase activity. The activating effect of regucalcin on the enzyme was also seen in the presence of TFP, indicating that regucalcin's effect is not involved in mitochondrial calmodulin. The present study demonstrates that regucalcin can stimulate Ca(2+)-pump activity in rat renal cortex mitochondria, and that the protein may act on an active site (SH group) related to phosphorylation of mitochondrial Ca(2+)-ATPase.

Can trifluoperazine protect mitochondria against reactive oxygen species-induced damage?

Trifluoperazine (TFP) (35 microM) prevents mitochondrial transmembrane potential (delta psi) collapse and swelling induced by 10 microM Ca2+ plus oxyradicals generated from delta-aminolevulinic acid autoxidation. In contrast with EGTA, TFP cannot restore the totally collapsed delta psi. So, TFP might not remove Ca2+ from its 'harmful site', but could impair the ROS-driven cross-linking between membrane-SH proteins. Our data are correlated with the protective uses of TFP against oxidative processes promoted by oxyradicals plus Ca2+.

Metabolic effects of trifluoperazine in the liver and the influence of calcium

The effects of trifluoperazine on hepatic cell metabolism were investigated using isolated perfused rat liver. The following effects of trifluoperazine were found: (1) trifluoperazine inhibited oxygen uptake, the site of action being the mitochondria. Half-maximal inhibition occurred at concentrations around 50 microM; with 100 microM trifluoperazine the effect was already maximal. When Ca2+ was withdrawn from the perfusion medium and the intracellular Ca2+ pools were exhausted, the inhibitory action on respiration was no longer observable. The reintroduction of Ca2+ restored inhibition. (2) Glycogenolysis and glycolysis were not significantly affected during the infusion of trifluoperazine. After stopping trifluoperazine infusion, however, glycogenolysis (glucose release) experienced a transitory stimulation. (3) Gluconeogenesis from lactate as the carbon source was inhibited by trifluoperazine. This inhibition was approximately proportional to the inhibition of oxygen uptake. Withdrawal of Ca2+ diminished, but it did not eliminate, inhibition of gluconeogenesis. (4) Ketogenesis was also inhibited in parallel with the inhibition of oxygen uptake. Withdrawal of Ca2+ from the perfusion fluid also abolished this action. (5) The effects of trifluoperazine were reverted very slowly when its infusion was stopped. The recovery of oxygen uptake at 50 min after cessation of the infusion was only 30%. Uptake of the substance was very fast. Absence of Ca2+ did not affect uptake. It was concluded that inhibition of mitochondrial energy metabolism is one of the most prominent effects of trifluoperazine in the liver. The fact that this inhibition depends on Ca2+ is unique.

Influence of some biological response modifiers on swelling of rat liver mitochondria in vitro

In order to understand any involvement of altered calcium functions in peroxidative membrane damage, the effect of a few chemicals, known to modify specific biological responses involving calcium related functions on mitochondrial swelling in vitro was studied. Histamine caused swelling, whereas antihistamines reduced calcium induced swelling. Anti-inflammatory agents aspirin and indomethacin did not affect the initial rapid phase of swelling but reduced the swelling during the later phase. The uncouplers of oxidative phosphorylation and electron transport chain blockers such as dinitrophenol (DNP), antimycin-A and rotenone reduced swelling and the respiratory inhibitors KCN and sodium azide completely abolished it. Trifluoperazine, an anti-calmodulin agent did not influence the initial phase of calcium induced swelling but in the subsequent phase swelling was reduced. c-AMP as well as calcium ionophores, calcimycin and lasalocid acid, potentiated swelling. Thus agents capable of modulating calcium functions could influence the in vitro swelling of mitochondria.

Inhibition of mitochondrial translation by calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide

The possible role of calmodulin in mitochondrial functions was investigated in Ehrlich ascites tumor cell and mouse liver mitochondria employing sulfonamide compounds as calmodulin indicators. N-[6-Aminohexyl)-5-chloro-1-naphthalenesulfonamide (W7), the most potent of the sulfonamide compounds, inhibited mitochondrial protein synthesis and oxidative phosphorylation. The inhibitors had no significant effect on mitochondrial cytochrome c oxidase, oligomycin-sensitive ATPase and NADH dehydrogenase activities. Depletion of endogenous ATP pool seemed to be the main mechanism of inhibition of mitochondrial translation by sulfonamides. The results also show that mitochondria from hepatic tissues are relatively less sensitive to sulfonamide drugs as compared to the Ehrlich ascites tumor cell mitochondria. Results of Ca2+ autoradiography revealed 2-3-fold higher levels of calmodulin-like Ca2+ binding protein in extracts from Ehrlich ascites tumor cell mitoplasts as compared to mitoplasts from mouse liver. These results suggest cell and tissue specific variations in Ca(2+)-dependent processes in the mitochondrial compartment.

Protective effect of trifluoperazine on the mitochondrial damage induced by Ca2+ plus prooxidants

Isolated rat liver mitochondria undergo extensive swelling and disruption of membrane potential when they accumulate Ca2+ in the presence of a prooxidant such as diamide or t-butylhydroperoxide. The phenothiazinic drug trifluoperazine, at concentrations (15-35 microM) which do not inhibit respiration or the influx of Ca2+ into mitochondria, significantly protected mitochondria against the deleterious effects of Ca2+ plus a prooxidant. In contrast, at concentrations higher than 100 microM the drug potentiated these deleterious effects of Ca2+ and prooxidants and had a damaging effect per se on the inner mitochondrial membrane. It is proposed that the protection conferred by the drug is mediated by changes in membrane protein structure that decrease the production of protein thiol cross-linkings which occur when mitochondria accumulate calcium under oxidant stress conditions.

Disruption of mitochondrial function as the basis of the trypanocidal effect of trifluoperazine on Trypanosoma cruzi

The tricyclic anti-calmodulin drug trifluoperazine (TFP) inhibited growth and motility of epimastigotes of Trypanosoma cruzi, at concentrations lower than 100 microM, and motility and infectivity of the bloodstream trypomastigote form at 200 microM. Electron microscopy of TFP-treated epimastigotes showed that the major effect was at the mitochondrial level, with gross swelling and disorganization. The oligomycin-sensitive, mitochondrial ATPase was completely inhibited by 20 microM TFP, and the same drug concentration caused a 60% decrease in intracellular ATP content. The results suggest that the trypanocidal effect of TFP may be related more to mitochondrial damage than to the well-known anticalmodulin effect of the drug.

A calmodulin inhibitor with high specificity, compound 48/80, inhibits axonal transport in frog nerves without disruption of axonal microtubules

The calmodulin inhibitor compound 48/80 has previously been shown to arrest axonal transport in vitro in the regenerating frog sciatic nerve. The inhibition was limited to the outgrowth region of nerves, which had been allowed to regenerate in vivo for 6 days after a crush lesion, before they were incubated with or without drugs in vitro overnight. The effects of compound 48/80 on the regenerating nerve were further investigated. A concentration of compound 48/80 (50 micrograms ml-1), which effectively inhibits axonal transport, did not cause observable changes of the microtubules of regenerating axons in the outgrowth region as judged by electron microscopy. Furthermore, it was shown that also a lower concentration (25 micrograms ml-1) inhibited axonal transport. As a measure of possible metabolic effects, the level of ATP was assessed in the regenerating nerve after exposure to compound 48/80. Compound 48/80 at 25 micrograms ml-1 did not change the level of ATP in the nerve. The assembly of bovine brain microtubule proteins in a cell-free system was unaffected by 25 micrograms ml-1 of compound 48/80 and slightly inhibited by 50 micrograms ml-1. At higher concentrations (greater than 100 micrograms ml-1) assembly of microtubules appeared stimulated, and microtubule spirals as well as closely aligned microtubules could be seen. These effects appeared to be unrelated to the transport effects. The present results indicate that compound 48/80 arrests axonal transport via mechanisms other than destruction of axonal microtubules or interference with the energy metabolism. It is possible that these mechanisms involve inhibition of calmodulin-regulated events essential to the transport.

Effects of chronic chlorpromazine treatment on peripheral benzodiazepine binding sites in heart, kidney, and cerebral cortex of rats

Daily intraperitoneal administration to rats of 5 mg/kg of chlorpromazine (CPZ) for 21 days induced a significant up-regulation (51%) of peripheral benzodiazepine binding sites (PBSs) in cerebral cortex and a down-regulation of PBSs in the heart (25%) and kidney (14%), whereas no alteration in [3H]flunitrazepam binding in cerebral cortex was observed. [3H]PK 11195 binding to cerebral cortex returned to normal following 5 days of CPZ withdrawal, whereas the density of PBSs in the heart and kidney remained reduced. The affinity of PBSs for the ligand [3H]PK 11195 in the cerebral cortex and heart was not affected by the drug treatment or withdrawal. The CPZ-induced alterations in PBSs may be relevant to the effects of the drug on CNS and/or peripheral organs.

Trifluoperazine inhibits ovarian mitochondrial side-chain cleavage enzyme activity in the absence of calcium

In a previous report we described the inhibitory effect of trifluoperazine (TFP) on steroidogenesis in avian granulosa cells. To clarify the possible site of action of TFP we measured the cholesterol side-chain cleavage enzyme (CSCC) activity in a mitochondrial preparation of granulosa cells isolated from mature and developing ovarian follicles. Using a calcium free medium, TFP inhibited CSCC in a dose related manner with an IC50 of 50 microM. Kinetic parameters (apparent Km and Vmax) obtained in the presence of TFP are indicative of uncompetitive inhibition of CSCC. Moreover, enzyme activity increased during follicular maturation while the efficacy of TFP was similar in both young and mature follicles. Because the inhibitory effects of TFP were manifest in medium from which calcium was omitted, it is suggested that the drug acts independently of the calcium-calmodulin system to suppress CSCC activity.

Effect of chlorpromazine and trifluoperazine on cytoskeletal components and mitochondria in cultured mammalian cells

Antipsychotic drugs such as chlorpromazine and trifluoperazine have been implicated to mediate their action by inhibiting calmodulin, the general calcium regulatory protein in eukaryotic cells. We observed that both these drugs were cytotoxic to different mammalian cell types at concentrations two- to three-fold lower than those required to inhibit calmodulin-dependent phosphodiesterase activity. These drugs also caused shrinkage and rounding of chicken embryo fibroblast cells without affecting any of the cytoskeletal components, viz. microtubules, microfilaments and intermediate filaments. However, at physiological concentrations of these drugs, a major change was observed in mitochondria which assumed rounded and swollen shapes and concentrated towards the perinuclear region of cells. These studies provide evidence that in contrast to earlier reports, cytoskeletal components are not the primary targets of these drugs. It is suggested that mitochondria may be one of the first structures to be affected by these drugs and the consequent energy depletion may lead to the other observed effects.

Effect of calmidazolium (R24571) on histamine release from isolated rat mast cells

The effects of selected calmodulin-antagonists, i.e. calmidazolium (R24571), trifluoperazine, cis- and transflupenthixol, chlorpromazine, and imipramine, on rat mast cells and on mast cell histamine release were investigated. The drugs induced histamine release, apparently by cytotoxic effects, with a rank order of potency in accordance with their lipid solubility and with maximal release at calmidazolium (5 mumol/l), trifluoperazine (40 mumol/l), cis- and trans-flupenthixol (50 mumol/l), chlorpromazine (100 mumol/l), and imipramine (500 mumol/l). Inhibition of the histamine release induced by antigen, compound 48/80, and the ionophore A23187 was only observed with some of the drugs tested, with maximal inhibition at calmidazolium (2 mumol/l), chlorpromazine (25-50 mumol/l), and imipramine (100-250 mumol/l). The concentration-response curve for histamine release induced by calmidazolium was significantly shifted to the right by antigen (i.e. horse serum) in the medium and the addition of antigen was capable of immediately stopping the release induced by calmidazolium, indicating binding of calmidazolium by antigen. Similar effects on the actions of calmidazolium were observed with phosphatidylserine. The inhibition by calmidazolium of the histamine release induced by antigen, compound 48/80, and the ionophore A23187 was significantly counteracted by glucose in the medium. The findings do not confirm an involvement of calmodulin in the histamine release process in rat mast cells. The ability of calmidazolium to bind to proteins and phospholipids in the medium indicates multiple cellular targets for calmidazolium, and the observations with glucose suggest an impaired mitochondrial function to be of major significance.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of phenothiazine drugs on the active Ca2+ transport by sarcoplasmic reticulum

The effect of phenothiazines (trifluoperazine and chlorpromazine) on the activity of the sarcoplasmic reticulum Ca2+ pump was investigated. These drugs have a biphasic action on the ATPase activity. They inhibit the enzyme at high concentrations, but below 150 microM, they promote a significant stimulation of the ATP hydrolysis, which is accompanied by a slight increase of the Ca2+ transport. Leaking of the membrane occurs only at drug concentrations above 150 microM. The phenothiazine stimulatory effect was not found in the isolated enzyme whose activity was inhibited at all concentrations studied. These results indicate that low concentrations of phenothiazines uncouple the sarcoplasmic reticulum Ca2+ pump without disrupting the membrane and that the drug stimulatory effect on the ATPase is not due to a direct interaction with the enzyme. It is suggested that a coupling factor or a specific microenvironment surrounding the enzyme regulates the association between the Ca2+ transport and the ATP hydrolysis by sarcoplasmic reticulum of the skeletal muscle cell.

Ca2+-binding properties of a unique ATPase inhibitor protein isolated from mitochondria of bovine heart and rat skeletal muscle

Previous studies showed that Ca2+ induced monomer to active dimer interconversion of a mitochondrial ATPase inhibitor protein from bovine heart or rat skeletal muscle (Yamada, E.W., Huzel, N.J. and Dickison, J.C. (1981) J. Biol. Chem. 256, 10203-10207). Initial equilibrium dialysis measurements of Ca2+ binding showed that this unique protein possesses three binding sites of high affinity with a maximum of one mol of Ca2+ bound/mol of protein monomer. Magnesium (1 mM) did not affect the first association constant but increased the second and third by about 1.2 and 1.5 fold, respectively. That the apparent association constants varied with concentration of protein monomer was in agreement with the self-associating nature of the protein. Scatchard plots at three concentrations of protein intersected at a molar ratio of about 0.5 (Ca2+/monomer). Ka1 and Ka2 values of 4.2 microM and 12.1 microM, respectively, were estimated by extra-polation of apparent constants to infinite dilution of protein. Ka3 (51.3 microM) was estimated by extrapolation of double reciprocal plots of apparent constants versus protein concentration to infinite levels of protein. A model for Ca2+ binding by this self-associating protein is described. Trifluoperazine had no effect on the activity of the inhibitor protein from either tissue.

Activation of liver cytosol phosphoenolpyruvate carboxykinase by Ca2+ through intracellular redistribution of Mn2+

Calcium has no known direct effect on phosphoenolpyruvate carboxykinase from rat liver cytosol. However, addition of calcium salts to liver postnuclear supernatant led to an increase in assayable enzyme activity in cytosols. This indicates that mitochondria and microsomes present in postnuclear supernatant can participate in observed enzyme activation. The stimulation of phosphoenolpyruvate carboxykinase was prevented by the manganese complexion 1-(2-pyridylazo)-2-naphthol, was not additive with activation by MnCl2 and was inhibited by La3+, Sr2+ and ruthenium red. These data indicate that manganese and mitochondrial or microsomal calcium carriers participate in the mechanism of indirect calcium effect. Measuring of manganese content in cytosols directly, by atomic absorption spectrometry, has provided evidence that there is a pool of manganese associated with mitochondrial and microsomal fraction of rat liver that can be mobilized to the cytosol by calcium ions. The direct addition of this pool of manganese to the cytosol caused the stimulation of phosphoenolpyruvate carboxykinase activity to the same levels as did calcium ions in the postnuclear supernatant. It is postulated that calcium can effect enzyme activity indirectly by releasing manganese from specific cellular compartments into the cytosol.

The inhibitor peptide of the mitochondrial F1.F0-ATPase interacts with calmodulin and stimulates the calmodulin-dependent Ca2+-ATPase of erythrocytes

The binding of calmodulin to the mitochondrial F1.F0-ATPase has been studied. [125I]Iodoazidocalmodulin binds to the epsilon-subunit and to the endogeneous ATPase inhibitor peptide in a Ca2+-dependent reaction. The effect of the mitochondrial ATPase inhibitor peptide on the purified Ca2+-ATPase of erythrocytes has also been analyzed. The inhibitor peptide stimulates the ATPase when pre-incubated with the enzyme. The activation of the Ca2+-ATPase by calmodulin is not influenced by the inhibitor peptide, indicating that the two mechanisms of activation are different. These in vitro effects of the two regulatory proteins may reflect a common origin of the two ATPases considered and/or of the regulatory proteins.

Calmodulin inhibitors potentiate hyperthermic cell killing

The role of calmodulin (CaM) in cellular heat injury of neuroblastoma N2A and hepatoma H35 cells has been investigated, using specific calmodulin-inhibiting drugs (Trifluoperazine, Compound 48/80 and Calmidazolium). These CaM-specific drugs potentiate hyperthermia-induced cell killing, suggesting CaM to be involved in processes aimed on the repair of heat injury. The CaM inhibitors also prevent hyperthermia-induced cytoskeletal alterations in the cell types studied. The action of CaM inhibitors was dose dependent, and seems to be confined to the first period of the hyperthermic treatment. Neither production of heat shock proteins in heat-shocked cultures, nor the rate of protein synthesis in control cultures were affected by the CaM inhibitors. It was concluded that an inverse correlation exists between hyperthermic cell killing and cytoskeletal alterations. Activation of CaM is suggested to be a fundamental aspect of the cellular heat shock response.

Effect of Ca2+-antagonists on virally-induced cell-permeability changes

Sendai virus-mediated permeability changes in cells are affected by extracellular Ca2+ or Mn2+ as follows: the lag period to onset of permeability changes is lengthened and the subsequent extent of leakage is reduced. Drugs that block Ca2+ action in excitable cells, such as verapamil and prenylamine, and drugs that inhibit the action of calmodulin, such as trifluoperazine and R24571, have an effect opposite to that of Ca2+: lag is shortened and extent of leakage is increased. The concentration at which either type of drug shows 50% of maximal effect is similar to the concentration at which 50% of binding by drug to calmodulin is achieved. It is concluded that calmodulin may be involved in protecting cells against virally-mediated membrane damage; alternatively the action of calmodulin-binding drugs may not be as specific as currently thought.