Somatotroph response to periodical IGF-I administration to male rats

Insulin-like growth factor I (IGF-I) downregulates growth hormone (GH) expression in pituitary cell cultures. However, in vivo different results were found depending on the experimental protocol used. We determined the kinetics of changes of pituitary and serum GH concentrations after subcutaneous I...

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Bibliographic Details
Published in:Acta histochemica 2000, Vol.102 (4), p.439-451
Main Authors: Pellizas, Claudia G., Bonaterra, Mónica, de Paul, Ana L., Aoki, Agustín, Coleoni, Aldo H., Torres, Alicia I.
Format: Article
Language:English
Subjects:
Animals
Blood - drug effects
Cytoplasm - metabolism
Down-Regulation
GH secretion
Growth Hormone - metabolism
IGF-I
immunocytochemistry
Immunohistochemistry
Insulin-Like Growth Factor I - pharmacology
Kinetics
Male
Microscopy, Electron
Pituitary Gland - drug effects
Pituitary Gland - ultrastructure
Radioimmunoassay
Rats
Rats, Wistar
RIA
somatotroph
Time Factors
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Summary:Insulin-like growth factor I (IGF-I) downregulates growth hormone (GH) expression in pituitary cell cultures. However, in vivo different results were found depending on the experimental protocol used. We determined the kinetics of changes of pituitary and serum GH concentrations after subcutaneous IGF-I administration (240µg/100 g body weight) to rats every 12h for various periods. These parameters were correlated with changes in the somatotroph cell population. A significant increase in serum GH was registered at 6h after IGF-I injection. At this time, some somatotroph cells exhibited ultrastructurally signs of high secretory activity, whereas adjacent somatotroph cells showed a quiescent appearance with sizeable stores of secretory granules. In contrast, serum GH levels remained unchanged at 1, 2 and 12h after each IGF-I injection. Pituitary GH concentrations were comparable to control levels during the first 48h and declined significantly at 72h and 96h of IGF-I treatment. After these prolonged periods of time of treatment, the size and extension of organelles involved in protein synthesis decreased and mature secretory granules in the cytoplasm increased significantly in GH-secreting cells. The somatotroph cell density remained unchanged even at 96h of treatment. In conclusion, our results suggest that periodical IGF-I administration to rats does not inhibit GH secretion. Interestingly, IGF-I injections induced early and significant increases in serum GH levels. This result may be a consequence of a temporary stimulatory action on somatotroph cells concurrent with increased secretory activity.Key words:
ISSN:0065-1281
1618-0372
DOI:10.1078/0065-1281-00570