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cells per animal, 0 ppm: 0.60 (10), 1 ppm: 0.36 (7), 10 ppm: 0.40 (4), 100 ppm: 0.67 (6), 1000 ppm: 4.20* (8) (% abnormalities excluding gaps, number of animals in parentheses, * t test significance at < 0.05).

The positive results obtained by analysis of 50 cells per animal appear to be due to an inadequate sample size. Upon re-analysis using 200 cells per animal only the positive result at 1000 ppm remains and this is enhanced. These results suggest that while a sample size of 50 cells per animal is adequate for the detection of powerful clastogens or high doses of weak clastogens, a sample size of 200 cells is required to define a dose response from chemicals with low activity.

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Richold, M. 1, E. Jones t, R. Rodford 2 and D.E.W, Clarke 2, 1 Department of Mutagenesis, Huntingdon Research Centre, Huntingdon, Cambridgeshire, and 2 Rank Xerox, CPSG, Welwyn Garden City, Hertfordshire (Great Britain)

Health and safety in the office environment: use of the Ames test as a biological screen

The mutagenic activity of certain office toners and the successful reduction in impurity level of the associated carbon black reported in 1980, highlights the possibility that other sources of mutagenic activity exist in products destined for non-industrial work environments. The mutagenicity of typewriter ribbons has been examined using the Ames assay. Weak positive responses have been obtained with correctable typewriter ribbon films, but frequently only after using a sequential extraction technique. If a saline wash is followed by hexane, acetone, ethanol and DMSO extractions the removal of toxic elements in the hexane phase enables the demonstration of a much larger positive response in the acetone phase. The response occurs with and without $9 mix.

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Ricordy, R., and P. Perticone, Centro di Genetica Evoluzionistica, Universith di Roma, Rome (Italy)

Effect of TPA on induction of SCE in mammalian cells in vitro

An increase of 30-40% of the incidence of sister-chromatid exchanges was observed in Chinese hamster ovary cells treated with 12-O-tetradecanoyl-phorbol- 13-acetate (TPA) for 2 cell cycles in the presence of BrUdr, but there is no potentiation by TPA on MMC-induced SCE, as already observed by several authors. Instead we found that the frequency of MMC-induced SCE in human lymphocytes stimulated by PHA, is increased by TPA. This difference between Chinese hamster

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cells and normal human lymphocytes may be related to the difference in their capacity of DNA repair.

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Roberfroid, M.B., Unit6 de Biochimie Toxicologique et Canc6rologique, Universit6 Catholique de Louvain, Louvain (Belgium)

Mechanisms of promotion in chemically initiated hepatocarcinogenesis

It is becoming increasingly evident that chemical carcinogenesis is a multistep process implying at least two distinct events; initiation and promotion. Such a multistep process first demonstrated to occur in the skin has also been identified in the neoplastic transformation of many other tissues such as mammary glands, urinary bladder, colon mucosa, lung and liver.

The present lecture will discuss the mechanisms of promotion in chemically initiated carcinogenesis with special reference to hepatocarcinogenesis.

A new experimental model will be presented which allows the rapid production of preneoplastic nodules in the liver of rats given one initiating dose of a chemical followed by a selection and then submitted to chronic promotion hy phenobarbital.

That model was applied to the sequential histochemical analysis of the promotion step. Cells isolated from the well characterized hyperplastic nodules were further- more biochemically analysed.

Since promotion of chemically initiated cells could play a major role in the etiology of human cancers, the problem of identifying potential promoters will also be discussed.

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Ronchi, Nuti V., G. Turchi and S. Bonatti, Istituto di Mutagenesi e Dif- ferenziamento CNR, Via Svezia, 10, 56100-Pisa (Italy)

Cytogenetic activity of 4-epoxyethyl-l,2-epoxycyclohexane (VCH-diepoxide)

In the present study, the cytogenetic activity of VCH-diepoxide was investigated in Vicia faba. An increasing mitotic delay was found with increasing doses.

VCH-diepoxide induced chromatid and chromosome aberrations although a clear dose-effect relationship was not found in the dose range used (10-40 mM). This cytogenetic effect was cell-cycle independent.

Feulgen positive bodies, clearly different from typical micronuclei, were also induced at higher frequencies than micronuclei. Treatment with DNAase and incorporation of [3H]TdR indicated DNA as the major component of these bodies.

A non-random distribution of chromosome aberrations was also found by