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In the present work in vitro GH pituitary responsiveness to GHRH in short-term (STO) and long-term orchidectomized (LTO) male rats was compared. In agreement with previous data obtained in vivo, pituitaries from STO rats showed reduced GH release after GHRH stimulation while LTO male pituitaries presented responses similar to those from control animals after maximal GHRH (10^-6 M) stimulation. This suggests that compensatory mechanisms have taken place, probably at the pituitary level, in order to restore GH pituitary responsiveness to high doses of GHRH. However, LTO male rats showed a reduced sensitivity to GHRH relative to intact males, as indicated by a higher EC₅₀ vs controls (40·82 ± 12·03 nM vs 0·35 ± 0·09 nM in intact males). We aimed to investigate further the events involved in the compensatory mechanisms that take place in LTO rats. For this purpose, we compared in vitro GH secretion by pituitaries from intact and LTO male rats after stimulation with specific activators of the signal transduction pathways related to GH release. Forskolin and dibutyryl cyclic-adenosine 3',5'-monophosphate were more effective in eliciting GH secretion (expressed in terms of percent increment over basal GH release) in LTO males, whereas phorbol 12-myristate 13-acetate was completely ineffective in stimulating GH release in this group. Thus, our results clearly showed that long-term orchidectomy enhances the effectiveness of the cAMP pathway in inducing GH release while it completely blunts that of the protein kinase C pathway. In conclusion, orchidectomy decreased the effectiveness of GHRH in eliciting GH release in vitro. However, long-term orchidectomy activated compensatory mechanisms that restored complete GH pituitary responsiveness to maximal GHRH stimulation. These mechanisms seem not to operate in STO rats. An increased effectiveness of the cAMP pathway in eliciting GH release in LTO rats is probably involved in the aforementioned compensatory mechanisms.

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