β-escin activates ALDH and prevents cigarette smoke-induced cell death

BACKGROUND

The tobacco use is one of the biggest public health threats worldwide. Cigarette smoke contains over 7000 chemicals among other aldehydes, regarded as priority toxicants. β-escin (a mixture of triterpenoid saponins extracted from the Aesculus hippocastanum. L) is a potent activator of aldehyde dehydrogenase (ALDH) - an enzyme catalyzing oxidation of aldehydes to non-toxic carboxylic acids.

PURPOSE

The aim of this study was to evaluate the effect of β-escin on ALDH activity, ALDH isoforms mRNA expression and cytotoxicity in nasal epithelial cells exposed to cigarette smoke extract (CSE).

METHODS

Nasal epithelial cells from healthy non-smokers were treated with β-escin (1 µM) and exposed to 5% CSE. After 6- or 24-hours of stimulation cell viability, DNA damage, ALDH activity and mRNA expression of ALDH isoforms were examined.

RESULTS

24 h β-escin stimulation revised CSE induced cytotoxicity and DNA damage. Cells cultured with β-escin or exposed to CSE responded with strong increase in ALDH activity. This effect was more pronounced in cultures treated with combination of β-escin and CSE. The strongest stimulatory effect on ALDH isoform mRNA expression was observed in cells cultured simultaneously with β-escin and CSE: at 6 h for ALDH1A1 and ALDH3A1, and at 24 h for ALDH1A3, ALDH3A2, ALDH3B1, and ALDH18A1. Combined β-escin and CSE treatment prevented the CSE-induced inhibition of ALDH2 expression at 24 h.

CONCLUSIONS

β-escin is an effective ALDH stimulatory and cytoprotective agent and might be useful in the prevention or supportive treatment of tobacco smoke-related diseases.

Effect of Escin Alone or in Combination with Antifungal Agents on Resistant Candida glabrata Biofilms: Mechanisms of Action

Nowadays, the increase in antimicrobial-resistant fungi (AMR) is certainly a major health concern, and the development of alternative therapeutic strategies has become crucial. Natural products have been used to treat various infections, and their chemical properties contribute to the performance of their biological activities, such as antifungal action. The various virulence factors and mechanisms of resistance to antifungals contribute to making Candida glabrata one of the most frequent agents of candidiasis. Here we investigate the in vitro and in vivo activity of β-escin against Candida glabrata. The β-escin MICs were determined for a reference strain and two clinical isolates of C. glabrata. Furthermore, growth kinetics assays and biofilm inhibition/eradication assays (crystal violet) were performed. The differences in the expression of some anti-biofilm-associated genes were analyzed during biofilm inhibition treatment so that reactive oxygen species could be detected. The efficacy of β-escin was evaluated in combination with fluconazole, ketoconazole, and itraconazole. In addition, a Galleria mellonella infection model was used for in vivo treatment assays. Results have shown that β-escin had no toxicity in vitro or in vivo and was able to inhibit or destroy biofilm formation by downregulating some important genes, inducing ROS activity and affecting the membrane integrity of C. glabrata cells. Furthermore, our study suggests that the combination with azoles can have synergistic effects against C. glabrata biofilm. In summary, the discovery of new antifungal drugs against these resistant fungi is crucial and could potentially lead to the development of future treatment strategies.

β-Escin overcomes trastuzumab resistance in HER2-positive breast cancer by targeting cancer stem-like features

Background

The emergence of de novo or intrinsic trastuzumab resistance is exceedingly high in breast cancer that is HER2 positive and correlates with an abundant cancer stem cell (CSC)-like population. We sought to examine the capacity of β-escin, an anti-inflammatory drug, to address trastuzumab resistance in HER2-positive breast cancer cells.

Methods

The effect of β-escin on trastuzumab-resistant and -sensitive cell lines in vitro was evaluated for apoptosis, expression of HER2 family members, and impact on CSC-like properties. An in vivo model of trastuzumab-resistant JIMT-1 was used to examine the efficacy and toxicity of β-escin.

Results

β-escin induced mitochondrial-mediated apoptosis accompanied by reactive oxygen species (ROS) production and increased active p18Bax fragmentation, leading to caspase-3/-7 activation. Attenuation of CSC-related features by β-escin challenge was accompanied by marked reductions in CD44^high/CD24^low stem-like cells and aldehyde dehydrogenase 1 (ALDH1) activity as well as hindrance of mammosphere formation. β-escin administration also significantly retarded tumor growth and angiogenesis in a trastuzumab-resistant JIMT-1 xenograft model via downregulation of CSC-associated markers and intracellular domain HER2. Importantly, β-escin selectively inhibited malignant cells and was less toxic to normal mammary cells, and no toxic effects were found in liver and kidney function in animals.

Conclusions

Taken together, our findings highlight β-escin as a promising candidate for the treatment of trastuzumab-resistant HER2-positive breast cancers.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-022-02713-9.

Beneficial effects of β-escin on muscle regeneration in rat model of skeletal muscle injury

BACKGROUND

Recent advancements in understanding β-escin action provide basis for new therapeutic claims for the drug. β-escin-evoked attenuation of NF-κB-dependent signaling, increase in MMP-14 and decrease in COUP-TFII content and a rise in cholesterol biosynthesis could be beneficial in alleviating muscle-damaging processes.

PURPOSE

The aim of this study was to investigate the effect of β-escin on skeletal muscle regeneration.

METHODS

Rat model of cardiotoxin-induced injury of fast-twich extensor digitorum longus (EDL) and slow-twich soleus (SOL) muscles and C2C12 myoblast cells were used in the study. We evaluated muscles obtained on day 3 and 14 post-injury by histological analyses of muscle fibers, connective tissue, and mononuclear infiltrate, by immunolocalization of macrophages and by qPCR to quantify the expression of muscle regeneration-related genes. Mechanism of drug action was investigated in vitro by assessing cell viability, NF-κB activation, MMP-2 and MMP-9 secretion, and ALDH activity.

RESULTS

In rat model, β-escin rescues regenerating muscles from atrophy. The drug reduces inflammatory infiltration, increases the number of muscle fibers and decreases fibrosis. β-escin reduces macrophage infiltration into injured muscles and promotes their M2 polarization. It also alters transcription of muscle regeneration-related genes: Myf5, Myh2, Myh3, Myh8, Myod1, Pax3 and Pax7, and Pcna. In C2C12 myoblasts in vitro, β-escin inhibits TNF-α-induced activation of NF-κB, reduces secretion of MMP-9 and increases ALDH activity.

CONCLUSIONS

The data reveal beneficial role of β-escin in muscle regeneration, particularly in poorly regenerating slow-twitch muscles. The findings provide rationale for further studies on β-escin repositioning into conditions associated with muscle damage such as strenuous exercise, drug-induced myotoxicity or age-related disuse atrophy.

The Natural Product β-Escin Targets Cancer and Stromal Cells of the Tumor Microenvironment to Inhibit Ovarian Cancer Metastasis

Simple Summary

β-escin, a component of horse chestnut seed extract, was first identified as an inhibitor of ovarian cancer (OvCa) adhesion/invasion in our high-throughput screening program using a three-dimensional organotypic model assembled from primary human cells and extracellular matrix. The goal of the study presented here is to determine if β-escin and structurally-similar compounds have a therapeutic potential against OvCa metastasis. β-escin and cardiac glycosides inhibit ovarian cancer adhesion/invasion to the omental microenvironment in vivo, and β-escin inhibits ovarian cancer metastasis in the prevention and intervention setting. Additionally, β-escin was found to decrease the stemness of ovarian cancer cells, inhibit extracellular matrix production in the tumor microenvironment, and inhibit HIF1α stability in ovarian cancer cells and the tumor microenvironment. This study reveals that the natural compound β-escin has therapeutic potential because of its ability to prevent OvCa dissemination by targeting both cancer and stromal cells in the OvCa tumor microenvironment.

Abstract

The high mortality of OvCa is caused by the wide dissemination of cancer within the abdominal cavity. OvCa cells metastasize to the peritoneum, which is covered by mesothelial cells, and invade into the underlying stroma, composed of extracellular matrices (ECM) and stromal cells. In a study using a three-dimensional quantitative high-throughput screening platform (3D-qHTS), we found that β-escin, a component of horse chestnut seed extract, inhibited OvCa adhesion/invasion. Here, we determine whether β-escin and structurally similar compounds have a therapeutic potential against OvCa metastasis. Different sources of β-escin and horse chestnut seed extract inhibited OvCa cell adhesion/invasion, both in vitro and in vivo. From a collection of 160 structurally similar compounds to β-escin, we found that cardiac glycosides inhibited OvCa cell adhesion/invasion and proliferation in vitro, and inhibited adhesion/invasion and metastasis in vivo. Mechanistically, β-escin and the cardiac glycosides inhibited ECM production in mesothelial cells and fibroblasts. The oral administration of β-escin inhibited metastasis in both OvCa prevention and intervention mouse models. Specifically, β-escin inhibited ECM production in the omental tumors. Additionally, the production of HIF1α-targeted proteins, lactate dehydrogenase A, and hexokinase 2 in omental tumors was blocked by β-escin. This study reveals that the natural compound β-escin has a therapeutic potential because of its ability to prevent OvCa dissemination by targeting both cancer and stromal cells in the OvCa tumor microenvironment.

β-Escin alleviates cobalt chloride-induced hypoxia-mediated apoptotic resistance and invasion via ROS-dependent HIF-1α/TGF-β/MMPs in A549 cells

Hypoxia is contributed in various pathophysiological conditions including obesity, cardiovascular diseases, and cancer. In cancer, hypoxia is a salient phenomenon and has been correlated with tumor progression, metastasis, and provoke resistance to therapies in cancer patients, which exert with stabilization of main effector, hypoxia inducible factor-1 alpha (HIF-1α). Therefore, therapeutic targeting of hypoxic responses in cancer is the potential approach to improve the better treatment efficacy. In the present study, we evaluated the effect of β-Escin (β-Es) on hypoxia-induced resistance to apoptosis and metastasis in human non-small-cell lung cancer cells. The MTT assay revealed that β-Es treatment decreased the A549 cells viability under cobalt chloride-induced hypoxia. Apoptotic proteins were analyzed by western blot that showed cancer cells treated with β-Es induced cell death in hypoxia condition as proteins compared with normoxia. Moreover, we observed that cobalt chloride induced hypoxia through the generation of intracellular reactive oxygen species and stabilized the transcriptional factor HIF-1α, which leads to cancer metastasis. This notion was supported by the migration, invasion, and adhesion assays. Furthermore, hypoxia increased the expression of transforming growth factor-β, and the activation of matrix metalloproteinases were suppressed by the treatment of β-Es as well as pretreatment with N-acetylcysteine (NAC). Therefore, we demonstrate that a concurrent activation of HIF-1α, transforming growth factor-β, and matrix metalloproteinases participate in hypoxia-induced metastasis and that β-Es prevent A549 cells metastasis by inhibition of reactive oxygen species.

Influence of Acetylated Annealed Starch on the Release of β-Escin from the Anionic and Non-Ionic Hydrophilic Gels

Naturally sourced products introduced to human nutrition and rediscovered for therapy include polysaccharides from potatoes. The starch may obtain unique properties via acetylation with acetic anhydride at 13 cm³/100 g of starch as the basic dose of reagent used in industrial conditions. The hydrogel formulation was applied as a carrier for escin included in the dry extract of Aesculus hippocastanum. Six hydrogels were evaluated (methylcellulose, polyacrylic acid-Carbopol 980 NF and polyacrylate crosspolymer 11—Aristoflex Velvet) with various concentrations of the modified starch. The kinetic studies of in vitro β-escin release were carried out in purified water at 37 ± 0.5 °C using a paddle apparatus at 50 rpm and a time period of 7 h. The criterion for the most suitable model was based on a high correlation coefficient of evaluated release profiles. The addition of modified annealed acetylated potato starch resulted in prolongation of β-escin release.

The Biosurfactant β-Aescin: A Review on the Physico-Chemical Properties and Its Interaction with Lipid Model Membranes and Langmuir Monolayers

This review discusses recent progress in physicochemical understanding of the action of the saponin β-aescin (also called β-escin), the biologically active component in the seeds of the horse chestnut tree Aesculus hippocastanum. β-Aescin is used in pharmacological and cosmetic applications showing strong surface activity. In this review, we outline the most important findings describing the behavior of β-aescin in solution (e.g., critical micelle concentration (cmc) and micelle shape) and special physicochemical properties of adsorbed β-aescin monolayers at the air–water and oil–water interface. Such monolayers were found to posses very special viscoelastic properties. The presentation of the experimental findings is complemented by discussing recent molecular dynamics simulations. These simulations do not only quantify the predominant interactions in adsorbed monolayers but also highlight the different behavior of neutral and ionized β-aescin molecules. The review concludes on the interaction of β-aescin with phospholipid model membranes in the form of bilayers and Langmuir monolayers. The interaction of β-aescin with lipid bilayers was found to strongly depend on its cmc. At concentrations below the cmc, membrane parameters are modified whereas above the cmc, complete solubilization of the bilayers occurs, depending on lipid phase state and concentration. In the presence of gel-phase phospholipids, discoidal bicelles form; these are tunable in size by composition. The phase behavior of β-aescin with lipid membranes can also be modified by addition of other molecules such as cholesterol or drug molecules. The lipid phase state also determines the penetration rate of β-aescin molecules into lipid monolayers. The strongest interaction was always found in the presence of gel-phase phospholipid molecules.

Activation of p53 Gene Expression and Synergistic Antiproliferative Effects of 5-Fluorouracil and β-escin on MCF7 Cells

One of the most common malignancies in women is breast cancer. β-escin has pharmacological anticancer effects. 5-fluorouracil (5-FU) has antimetabolite and antiproliferative properties. The purpose of this study was to investigate the combined effects of 5-FU and β-escin on apoptosis, colony formation, Bcl-2 signaling protein, and p53 gene expression in MCF7 breast cancer cell line. The cytotoxic effects, the number of colonies, apoptosis, p53 gene expression, and Bcl-2 signaling protein of the combined 5-FU and β-escin on MCF7 cells were determined using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, clonogenic assay, flow cytometry, real-time quantitative polymerase chain reaction, and western blotting methods, respectively. Half-maximal inhibitory concentration values of β-escin and 5-FU were 80 μg/ml and 2 μM, respectively. The combination of 5-FU and β-escin on MCF7 cell viability showed a combination index equal to 0.5. The expression of p53 and apoptosis increased in the combination of 5-FU and β-escin on MCF7 cells compared to that of control group (P < 0.05). In addition, the number of colonies and Bcl-2 signaling protein in combination of 5-FU and β-escin decreased with respect to untreated control cells or single treatment of 5-FU and β-escin. The combination of 5-FU and β-escin not only has synergistic effects by increasing cell apoptosis and p53 gene expression but also decreases Bcl-2 signaling protein in MCF7 cell lines.

Inhibition of Migration and Invasion in Melanoma Cells by β-Escin via the ERK/NF-κB Signaling Pathway

β-Escin, a natural triterpene saponin was extracted from Aesculus hippocastanum seeds, which have been widely used to treat inflammation in traditional medicine. In an effort to study the possible anti-tumor effects of β-escin, we performed wound healing, invasion, and adhesion assays to examine the effects of β-escin on cell migration, invasion, and angiogenesis. Our results revealed that β-escin inhibits cell migration as well as motility in B16F10 and SK-MEL5 cells in a dose-dependent manner. RT-PCR and Western blot analysis showed that β-escin increased TIMP-1, -2 while significantly downregulated phosphorylated extracellular signal-regulated kinase (p-ERK) expression, and suppressing nuclear factor-kappa B (NF-κB) and inhibitor of nuclear factor-kappa B (IκB) expression. Overall, the data from the current study suggest that β-escin has the potential for inhibiting both metastatic and angiogenic activities, and are the earliest evidence for the involvement of the NF-κB/IκB signaling in β-escin-induced anti-tumor effects.

Virucidal, antiviral and immunomodulatory activities of β-escin and Aesculus hippocastanum extract

OBJECTIVES

β-Escin, one of the constituents of Aesculus hippocastanum L. (Hippocastanaceae) seed extract (AH), inhibits NF-κB activation, which plays an important role in HSV-1 replication. The aim was to examine the antiherpetic activity of β-escin and AH, as well as their effect on the activation of NF-κB and AP-1 and cytokine secretion in epithelial cells and macrophages.

METHODS

Cell viability was evaluated using MTT assay, and antiviral and virucidal activity was determined by plaque assay. The effect on NF-κB and AP-1 signalling pathways activation was determined by a luciferase reporter assay, and cytokine production was measured by ELISA.

KEY FINDINGS

β-Escin and AH had virucidal and anti-HSV-1 activities, and the antiviral activity was discovered for other enveloped viruses (VSV and Dengue). Moreover, β-escin and AH significantly reduced NF-κB and AP-1 activation and cytokine production in macrophages stimulated with HSV-1 and TLRs ligands. However, an enhanced activation of these pathways and an increase in the levels of pro-inflammatory cytokines in β-escin and AH-treated HSV-1-infected epithelial cells were found.

CONCLUSIONS

This study demonstrates virucidal and broad-spectrum antiviral activities for β escin and AH. Besides, β-escin and AH modulate cytokine production depending on the stimuli (viral or non-viral) and the cell type under study.

β-Escin Effectively Modulates HUVECS Proliferation and Tube Formation

In the present study we evaluated the anti-angiogenic activities of β-escin (the major active compound of Aesculus hippocastanum L. seeds). Human umbilical-vein endothelial cells (HUVECs) were used as an in vitro model for studying the molecular mechanism underlying the anti-angiogenic effect of β-escin. We investigated the in vitro effects on proliferation, migration, and tube formation of HUVECs and in vivo anti-angiogenic activity was evaluated in a chick chorioallantoic membrane (CAM) angiogenesis assay. Moreover, the effect on gene expressions was determined by the RT2 ProfilerTM human angiogenesis PCR Array. It was found that β-escin exerts inhibitory effect on the basic fibroblast growth factor (bFGF)-induced proliferation, migration and tube formation, as well as CAM angiogenesis in vivo. The inhibition of critical steps of angiogenic process observed with β-escin could be partially explained by suppression of Akt activation in response to bFGF. Moreover, the anti-angiogenic effects of β-escin could also be mediated via inhibition of EFNB2 and FGF-1 gene expressions in endothelial cells. In conclusion, β-escin affects endothelial cells as a negative mediator of angiogenesis in vitro and in vivo and may therefore be considered as a promising candidate for further research elucidating its underlying mechanism of action.

by capillary zone electrophoresis

INTRODUCTION

Seeds of Aesculus hippocastanum L. are used in European phytotherapy to treat inflammatory and vascular problems, and also to help in the regulation of the microcirculation. Thus, the quality control of herbal medicines using this species is important.

OBJECTIVE

To develop and to optimise a capillary zone electrophoresis method to determine total β-escin in different extracts of A. hippocastanum L.

METHODS

The optimal condition found through chemometric approach was: 25 mmol/L of bicarbonate-carbonate buffer, pH 10.3; +20 kV of voltage; 20°C of cartridge temperature; direct ultraviolet detection at 226 nm; 13 mbar injection for 5 s and analysis time within 6 min.

RESULTS

Repeatability, coefficient of variation (CV; %) = 3.19, 3.07 and 1.89 (n = 12), and intermediate precision, CV (%) = 3.05, 3.53 and 2.99 (n = 24) for dry, hydroalcoholic and hydroglycolic extracts, respectively were achieved. The accuracy was evaluated through recovery tests in concentration levels of 100, 150 and 200 g/L, ranging from 98.17 to 104.68%. The proposed method exhibited linearity (r = 0.9983) in the concentration range from 101.4 to 907.2 g/L and limits of detection and quantification equal to 11.63 and 38.76 g/L respectively.

CONCLUSION

A fast and reliable methodology for determination of total β-escin was successfully validated and applied on extracts of A. hippocastanum L. demonstrating its usefulness to quality control of medicines containing this plant species.

Xestospongin C, a novel blocker of IP3 receptor, attenuates the increase in cytosolic calcium level and degranulation that is induced by antigen in RBL-2H3 mast cells

1. We evaluated the role of the cross-linking of Fc epsilon RI-mediated inositol 1,4,5-triphosphate (IP(3)) in the increase in cytosolic Ca(2+) level ([Ca(2+)](i)) using xestospongin C, a selective membrane permeable blocker of IP(3) receptor, in RBL-2H3 mast cells. 2. In the cells sensitized with anti-dinitrophenol (DNP) IgE, DNP-human serum albumin (DNP-HSA) and thapsigargin induced degranulation of beta-hexosaminidase and a sustained increase in [Ca(2+)](i). Xestospongin C (3 - 10 microM) inhibited both of these changes that were induced by DNP-HSA without changing those induced by thapsigargin. 3. In the absence of external Ca(2+), DNP-HSA induced a transient increase in [Ca(2+)](i). Xestospongin C (3 - 10 microM) inhibited this increase in [Ca(2+)](i). 4. In the cells permeabilized with beta-escin, the application of IP(3) decreased Ca(2+) in the endoplasmic reticulum (ER) as evaluated by mag-fura-2. Xestospongin C (3 - 10 microM) inhibited the effect of IP(3). 5. After the depletion of Ca(2+) stores due to stimulation with DNP-HSA or thapsigargin, the addition of Ca(2+) induced capacitative calcium entry (CCE). Xestospongin C (3 - 10 microM) inhibited the DNP-HSA-induced CCE, whereas it did not affect the thapsigargin-induced CCE. 6. These results suggest that Fc epsilon RI-mediated generation of IP(3) contributes to Ca(2+) release not only in the initial phase but also in the sustained phase of the increase in [Ca(2+)](i), resulting in prolonged Ca(2+) depletion in the ER. The ER Ca(2+) depletion may subsequently activate CCE to achieve a continuous [Ca(2+)](i) increase, which is necessary for degranulation in the RBL-2H3 mast cells. Xestospongin C may inhibit Ca(2+) release and consequently may attenuate degranulation.

Inhibition of capacitative calcium entry is not obligatory for relaxation of the mouse anococcygeus by the NO/cyclic GMP signalling pathway

1. The object of this study was to determine whether inhibition of capacitative calcium entry is essential for relaxation of the mouse anococcygeus via the NO/cyclic GMP signalling pathway. 2. In intact muscles, thapsigargin (Tg; 100 nM)-induced tone was relaxed by NO, sodium nitroprusside (SNP), 8-Br-cyclic GMP, and nitrergic field stimulation. The relaxations were similar in magnitude to those observed against carbachol (50 microM) tone and, with the exception of those to 8-Br-cyclic GMP, were reduced by the soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxodiazolo[4,3-a]quinoxalin-1-one (ODQ, 5 microM). 3. In single smooth muscle cells, loaded with Fura-2, both carbachol and Tg produced sustained elevations in cytoplasmic calcium levels ([Ca2+]i). SNP inhibited the rise in [Ca2+]i produced by carbachol, an effect attenuated by ODQ. In contrast, neither SNP nor 8-Br-cyclic GMP reduced the elevated [Ca2+]i associated with Tg. 4. In beta-escin skinned preparations, NO had no effect on tone induced by calcium (1 microM in the presence of 100 microM GTP). Carbachol and Tg produced further increases in calcium/GTP-induced tone and, in both cases, this additional tone was relaxed by NO and 8-Br-cyclic GMP. 5. The results support the hypothesis that the NO/cyclic GMP pathway inhibits capacitative calcium entry by refilling the internal stores, since reduction in [Ca2+]i was not observed in the presence of Tg. However, as muscle relaxation was still observed, impairment of capacitative calcium entry cannot be considered obligatory for relaxation. Results from skinned tissues suggest that inhibition of calcium sensitization processes, perhaps associated with store-depletion, may be an important mechanism of NO/cyclic GMP-induced relaxation.

Augmented acetylcholine-induced, Rho-mediated Ca2+ sensitization of bronchial smooth muscle contraction in antigen-induced airway hyperresponsive rats

Treatment with acetylcholine (ACh) of a beta-escin-permeabilized intrapulmonary bronchial smooth muscle of the rat induced force when the Ca2+ concentration was clamped at 1 microM. The ACh-induced Ca2+ sensitization of myofilaments was significantly greater in antigen-induced airway hyperresponsive rats than in control rats. The ACh-induced Ca2+ sensitization was completely blocked by treatment with Clostridium botulinum C3 exoenzyme, an inactivator of Rho family of proteins. Moreover, the protein level of RhoA in the intrapulmonary bronchi was significantly increased in the airway hyperresponsive rats. Thus, increased airway smooth muscle contractility observed in asthmatics may be related to augmented agonist-induced, Rho-mediated Ca2+ sensitization of myofilaments.