The use of cultured skeletal muscle cells in testing for acute systemic toxicity

A test system using primary cultured, spontaneously contracting, rat skeletal muscle cells has been developed to detect chemical interactions with excitable membranes. Taking into account some basic toxicokinetic differences between the in vivo and in vitro conditions, results obtained with the first 30 MEIC reference chemicals were quantitatively compared with in vivo acute toxicity data. The results show that the muscle cell test system is suitable for use in acute toxicity assessment and, for instance, for toxicity classification of chemicals.

Cytotoxic potency of trialkyltins to C6 glioma cells in vitro: impact of exposure conditions

Because of a possible role of astrocytes in trialkyltin-induced neurotoxicity in vivo various studies have been performed using cultures of astrocytes or glioma cells in vitro. With respect to cytotoxic potencies of trialkyltins these studies gave rather divergent results. Therefore the aim of the present study was to clarify whether variations of experimental conditions could be responsible for the differences of the cytotoxic activities of trimethyltin (TMT), triethyltin (TET) and tributyltin (TBT). Experiments were performed with rat C6 glioma cells. Toxicity was determined by measuring the reduction of the cell protein content. Cultures of proliferating and growth-arrested cells did not differ in their sensitivity. Exposure duration (1-72 h) had a strong but differing influence on the cytotoxic potency of the trialkyltins. After short exposure times the potencies differed largely (TMT < TET < TBT), whereas they became more and more similar with increasing exposure duration. The potency-time relationships for TMT and TET could be described by the equation: EC50 = k x t(-n), while for TBT an incipient value (EC50, infinity) had to be included: EC50 = EC50, infinity + k x t(-n). Addition of serum albumin to the culture medium decreased the cytotoxic potency of the trialkyltins. However, the impact of protein binding on their bioavailability was relatively low. The cytotoxic potency of the alkyltins was not dependent on the concentration of C6 cells. Taken together, neither differences in exposure conditions nor in the proliferative status of the cells are sufficient to account for the discrepancies in published results for trialkyltin cytotoxicity to astrocytes. Instead they may--at least partially--be explained by differing sensitivities of the endpoints used. Furthermore, C6 glioma cells respond considerably more sensitively to trialkytins than primary astrocytes, which questions their applicability as models for astrocyte toxicity.

Factors influencing nominal effective concentrations of chemical compounds in vitro: medium protein concentration

Quantitative data used to characterise biological activities of chemicals in vitro (e. g. EC50 values) are generally based on nominal concentrations and thus depend on factors influencing the availability of a compound. In this study, the impact of protein binding on the availability of chemicals in vitro is theoretically investigated and experimentally examined using a bovine sperm cell assay to measure the cytotoxic potency of selected compounds at different medium protein concentrations. In agreement with theoretical considerations, linear correlations between EC50 values and medium albumin concentrations were determined with 2,4-dichlorophenol, pentachlorophenol, p,p'-DDT and mercuric chloride. Ratios of EC50 values measured in the presence and absence of 4% (w/v) albumin varied between 500 (hexachlorophene), 258 (pentachlorophenol) and almost 1 (potassium cyanide, dextropropoxyphene). Calculated molar ratios of substance bound to albumin ranged from 0.05 (arsenic trioxide) and 0.1 (potassium cyanide) to 2.5 and 4.7 moles/mole for malathion and xylene, respectively. The fractions bound at 4% albumin varied between 11 and 15% for dextropropoxyphene and potassium cyanide, respectively, and more than 99% for hexachlorophene, pentachlorophenol and mercuric chloride. The results clearly demonstrate that the differing impact of protein binding on the bioavailability of chemicals considerably influences their nominal and relative potencies in the presence of albumin.

Toxicity of organotin compounds in primary cultures of rat cortical astrocytes

The neurotoxic organotin compounds trimethyl (TMT) and triethyltin (TET) are known to induce astrogliosis in vivo, which is indicated by an increased synthesis of glial fibrillary acidic protein (GFAP) in astrocytes. In contrast, tributyltin (TBT) does not induce astrogliosis. The aim of this study was to investigate whether trialkyltin derivatives can induce an increased GFAP synthesis in astrocyte cultures in the absence of neurons and whether differences between the action of TMT, TET, and TBT can be detected. Primary cultures of rat cortical astrocytes from 2-day-old rats were grown in 96-well plates until confluency and then exposed to various concentrations of TMT, TET, and TBT for 40 h. Effects on basal cell functions were measured by colorimetric determination of cell protein contents and by assessment of viability by means of the MTT assay. An indirect sandwich ELISA for 96-well plates was used for quantitative measurements of the GFAP content of the cells. All three compounds induced a concentration-dependent cytotoxicity indicated by parallel decreases of protein contents and MTT reduction. Half-maximum cytotoxic concentrations were 3 micromol/L (TBT), 30 micromol/L (TET), and 800 micromol/L (TMT). Cellular GFAP contents were reduced in parallel to cytotoxic action but no increase in GFAP expression at subcytotoxic concentrations could be observed. Thus, the astrocytes were not able to respond to TMT or TET exposure by an increased synthesis of GFAP in the absence of neuronal signals.

Factors influencing nominal effective concentrations of chemical compounds in vitro: cell concentration

In vitro potency data (e.g. EC(50) values), used to characterise the biological activity of chemicals, are generally based on nominal effective concentrations and thus depend on any factor influencing the availability of a compound. In this study the significance of cell binding for the availability of chemicals in vitro is (i) theoretically investigated by means of a simple equilibrium distribution model and (ii) experimentally examined using a bull sperm assay to measure the cytotoxic potency of selected compounds at different cell concentrations. Compounds were selected either to cover a wide range of hydrophobicity (log K(ow)=2.52-5.69) or to represent modes of cell binding other than partitioning into cellular lipids. With the exception of xylene, the EC(50) values increased with increasing cell concentration. The ratios of EC(50) values determined at about 120 x 10(6) and 15 x 10(6) cells/ml were: pentachlorophenol. 1.2, 1-nitronaphthalene: 1.9, thioridazine: 2.7, dieldrin: 4.1, hexachlorophene: 4.1, digitonin: 5.1, methylmercury chloride: 7.9, antimycin A: 10.1 and p,p'-dichlorodiphenyl dichloroethylene (p,p'-DDE): >19.1. The influence of partitioning into cell lipids was rather well predicted by the equilibrium distribution model, except for p,p'-DDE. The results show that cell binding can significantly affect the availability of compounds in vitro and thus toxic potencies and toxic equivalency factors.

Effects of toxin isolated from the sea anemone Bolocera tuediae on electrical properties of isolated rat skeletal muscle and cultured myotubes

Electrophysiological effects of a polypeptide toxin isolated from the sea anemone Bolocera tuediae (BTTX II) on isolated fast and slow rat skeletal muscle and on cultured rat myotubes are described in this paper. Nanomolar concentrations of BTTX II cause a concentration-dependent depolarization. This effect is prevented by the presence of tetrodotoxin. Action potential duration is prolonged in all preparations investigated, with the fast muscle being the least sensitive. The positive overshoot and the maximum rate of rise of the action potential is decreased by BTTX II. Cultured myotubes which are less susceptible to the sodium channel blocking activity of tetrodotoxin have the highest sensitivity to all actions of BTTX II. It is further shown that the proportion of spontaneously contracting myotubes is diminished and the frequency of contractions is increased by the toxin. The effects of Bolocera tuediae toxin II resemble those observed with Anemonia sulcata toxin II on mammalian skeletal muscle. From the results it is concluded, that BTTX II may interfere with both the activation and inactivation of sodium channels of the muscle membrane.

Cultured myotubes from skeletal muscle of adult rats. Characterization and action of Anemonia sulcata toxin II

Mononucleated myogenic cells (satellite cells) were isolated from skeletal muscle of adult rats and grown in culture. These cells replicated and, beginning with the 6th day in culture, they fused and differentiated into multinucleated myotubes, which accumulated creatine kinase and developed cross striation and spontaneous contractions. The differentiation of the excitable membrane and the action of sea anemone toxin ATX II were investigated with microelectrode techniques. Mature myotubes reached a stable membrane potential of -47.3 mV (+/- 6.5 mV) with the 11th day in culture. Action potentials could be generated in all myotubes. During maturation they became faster (increasing rate of rise) and shorter in duration. In spontaneously contracting myotubes spontaneous action potentials were recorded, which were often associated with subthreshold oscillations of membrane potential. ATX II reduced the membrane potential and prolonged the action potential duration with the lowest effective concentrations being 1 nmol/l and 0.5 nmol/l, respectively. Furthermore, ATX II induced electrical activity in quiescent myotubes. After fusion the development of the membrane electrical properties of satellite cell derived muscle cells followed essentially the same pattern as in primary cultures of embryonic myotubes. Electrophysiologically and with respect to their sensitivity to ATX II the mature myotubes resemble denervated muscle fibres.

Restitution of the thymus in lethally irradiated mice after transplantation of syngeneic or allogeneic bone marrow

The early restitution of the thymus of bone marrow chimeras was investigated by the immunoperoxidase technique using monoclonal antibodies against Thy-1 and Lyt-1, Lyt-2, Lyt-3. Within two weeks, normal thymus histology was restored in mice which received untreated syngeneic BM or syngeneic or allogeneic BM pretreated with SAL (specificed antilymphocytic serum). Irradiation depleted the thymic cortex of small Thy-1+, Lyt-1+2+3+ cells but did not affect a medullary population of medium sized weakly stained Thy-1+, strongly stained Lyt-1+ cells. Preceded by the appearance of an increasing number of large Thy-1+, Lyt-1- blasts (days 2 and 4), the thymic cortex was repopulated (beginning on day 6) by smaller Thy-1+ cells which acquired Lyt-1, Lyt-2 and Lyt-3 though, obviously not in a strictly sequential manner. Simultaneously, the medullary radioresistant cells disappeared, nd the medulla was subsequently repopulated (beginning on day 8) by thymocytes of a mature phenotype. Early restitution of the thymus in radiation control mice was similar to the bone marrow chimeras. The results indicate that the histological restitution of the thymus originates substantially from radioresistant precursors of host origin. Graft-versus-host reaction induced by untreated allogeneic bone marrow cells prevented normal thymic restitution. A delayed localized cortical repopulation with small Thy-1+, Lyt-1+2+3+ cells, progressive destruction of thymic architecture and almost no restoration of the medullary immunocompetent thymocytes were noted. T cell differentiation obviously was seriously affected by the injuries to the thymic microenvironment due to alloreactive T cells.