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Twelve female rats weighing approximately 150 g received in the submaxillary gland a pellet capable of releasing 3.5 microg GHRH/h for 60 days. Another eight sex- and weight-matched animals received placebo pellets in the same place. After two months the animals were killed, heart blood was collected and pituitary and submaxillary glands were carefully dissected. Pituitary GH content in both placebo- and GHRH-treated animals showed similar values, but plasma GH and IGF-I levels were significantly lower in the animals carrying GHRH pellets (P<0.03); these animals also had a significantly higher GH content in the submaxillary gland (19.2+/-8 ng/mg protein) compared with the placebo-treated group (1.1+/-0.3 ng/mg protein). GH mRNA was present only in the submaxillary gland of GHRH-treated rats as determined by PCR-Southern blot and by in situ hybridization methods. It is concluded that high local GHRH levels are capable of inducing transdifferentiation in submaxillary gland cells to synthesize GH.
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High local GH-releasing hormone (GHRH) levels are capable of inducing transdifferentiation in salivary cells to synthesize GH. However, the factors implicated in this process remain unknown. To study this subject, normal and Ames dwarf mice were implanted in the submaxillary gland with a slow release pellet releasing 21 microgram GHRH (1-29)-NH(2)/day for 2 months. Control animals received placebo pellets at the same site. After 60 days, heart blood was collected and submaxillary glands were removed. Circulating levels of GH and IGF-I were significantly decreased (P<0.05) in dwarf mice in comparison with controls, and GHRH treatment did not modify either of these two parameters. Controls carrying GHRH pellets showed a significantly higher GH content (P<0.05) in the submaxillary gland than the placebo-treated normal mice. There were no differences between the IGF-I concentrations of placebo- and GHRH-treated salivary tissue from normal mice. Analysis of GH mRNA by RT-PCR followed by Southern blot revealed that GH transcripts were present in the salivary gland samples carrying the placebo pellets in both normal and dwarf mice. The expression of GH was significantly (P<0.05) increased by the GHRH pellets in salivary tissue from normal mice, but not in submaxillary glands from dwarf mice. Pit-1 mRNA was not detected in the GHRH-treated glands of normal and dwarf mice by RT-PCR or by Southern blot. Using these highly sensitive methods, we have been able to detect the transcription of both GH and Pit-1 in pituitaries from Pit-1-deficient Ames dwarf mice. The present experiment demonstrates that salivary tissue synthesizes GH when it is exposed to the influence of GHRH. Both basal and GHRH-induced salivary GH expression appear to be independent of Pit-1.
