Résumé : When quiescent dog thyroid epithelial cells in primary culture are stimulated for 48 h with thyrotropin (TSH), forskolin acting through cAMP, or with cAMP-independent mitogens including epidermal growth factor (EGF), hepatocyte growth factor (HGF), and a tumor promoting phorbol ester (TPA), only 30-60% of cells progress through the cell cycle. A more general growth response requires the combination of EGF and TSH or forskolin. In this study we ask whether this intercellular heterogeneity in mitogen sensitivity could depend on a similar heterogeneity at early stages of the mitogenic stimulation process, i.e., at the levels of p42/p44 MAP kinase nuclear translocation and c-Fos protein appearance. We used indirect immunofluorescence microscopy with photometric quantitation and corroborated data using Western blotting. We analyzed the double staining of c-Fos and p42/p44 MAP kinases, since the nuclear translocation of these MAP kinases has been suggested as a key step for the stimulation of c-fos transcription. (i) EGF and HGF induced c-Fos accumulation and MAP kinase translocation in variable fractions of the cell population that corresponded to their relative potency as mitogens. c-Fos appearance and MAP kinase translocation poorly correlated in individual cells. Many cells accumulated c-Fos without any detectable p42/p44 MAP kinase translocation. The heterogeneity of proliferative responses to EGF could be due to the lack of c-Fos or MAP kinase responsiveness of many cells. (ii) TPA induced c-Fos accumulation and MAP kinase translocation within the whole cell population, which did not explain the heterogeneity of the growth response to this factor and showed that these events are not sufficient to elicit DNA synthesis, (iii) TSH and forskolin induced a weak c-Fos accumulation in only a minority of cells but, as previously shown, no p42/p44 MAP kinase phosphorylation and translocation. An important c-Fos expression was thus dispensable for the strong DNA synthesis stimulation exerted by cAMP-dependent mitogens. (iv) Forskolin potentiated the EGF effect on c-Fos expression but not on p42/p44 MAP kinase phosphorylation and translocation. This reflected the fact that EGF induced c-Fos accumulation in 90% of cells in the presence of forskolin but in 30-50% of cells in its absence. This kind of potentiation, which specifically implies an increase in the fraction of responding cells, is termed "generalization" in the present study.(ABSTRACT TRUNCATED AT 400 WORDS)