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The purpose of this study was to determine whether the cells of the ovine pars tuberalis (PT) secrete a factor(s) that can influence the activity of cells in the pars distalis (PD). By Northern blotting of total RNA isolated from PD cells that had been stimulated in the presence of cycloheximide (10 micrograms/ml), PT cell-conditioned medium was shown to induce a significant increase in the expression of the early response gene, c-fos, above both PD cell-conditioned and nonconditioned medium control levels (P &lt; 0.05). Although forskolin (5 microM) induced a weak increase in c-fos expression in PD cells, the effect of PT medium conditioned in the presence of forskolin enhanced this expression more than additively (P &lt; 0.05); furthermore, this effect was reversed by melatonin. These results are consistent with the release of a factor(s) from the PT, which for simplicity we have called tuberalin. This factor was released from PT cells in a time-dependent and cycloheximide-sensitive manner and was resistant to heating at 100 C for 10 min. Tuberalin activity could be size-fractionated using molecular size cut-off filters to produce activity in both the 1- to 10-kDa and more than 10-kDa size ranges. The activities in both of these fractions were sensitive to trypsin degradation and, therefore, appeared to be peptidergic. However, it was not clarified whether the biological activities were due to one or two components. Tuberalin also induced c-fos expression in other cell types, including GH3 and NIH3T3 cells. Dual labelling of PD cells by in situ hybridization using riboprobes for c-fos and PRL demonstrated that both the less than and more than 10-kDa fractions of tuberalin activated c-fos expression in some, but not all, lactotrophs in PD cell cultures, suggesting that a primary function of the PT is to regulate the activity of lactotrophs. This was supported further by enhanced secretion of PRL from PD cells in the presence of either PT-conditioned medium or PT cells in coculture. In addition, PT-conditioned medium was found to increase c-fos in a second cell type, which did not hybridize positively for PRL, indicating the existence of other endocrine interactions between the PT and PD.

The ovine pars tuberalis secretes a factor(s) that regulates gene expression in both lactotropic and nonlactotropic pituitary cells.