Relationship between NGF-mediated volume increase and "priming effect" in fast and slow reacting clones of PC12 pheochromocytoma cells. Role of cAMP.

Nerve Growth Factor (NGF)-mediated fiber outgrowth in pheochromocytoma PC12 cells is a slow process, developing over a period of several days. However, if these cells are pre-exposed to NGF for 7-10 days, renewed NGF treatment of the subcultured cells elicits fiber outgrowth within 24 h, comparable to the rate of response of physiological target cells to NGF. The present experiments demonstrated that this effect, previously termed "priming", was accompanied by a 60% increase in the volume of the PC12 cells, and that the dose-response curves for NGF-mediated induction of fiber outgrowth and for the increase in cell volume were very similar. Furthermore, the rates of NGF-mediated fiber outgrowth and of cell volume increase were both much slower in conventional PC12 cells (slow-reacting) compared to a newly-selected, fast-responding (FR)subclone of PC12 cells. These results suggested a possible causal relationship between the increase in cell volume and the induction of fiber outgrowth. However, when the cells were pre-exposed for 7 days to dibutyryl-cAMP (db-cAMP), the increase in cell volume was 3-fold higher than that effected by NGF. Nevertheless, db-cAMP had only a very limited ability to "prime" the cells for a subsequent response to NGF. Thus, the induction of cell volume increase and the increased availability of structural elements is not sufficient to explain the "priming" effect of NGF. The effects of db-cAMP are discussed in the context of a possible role of cAMP as a second messenger in the action of NGF.