acta histochem. 102, 439±451 (2000) Ó Urban & Fischer Verlag Somatotroph response to periodical IGF-I administration to male rats Claudia G. Pellizas 1 , Mo  nica Bonaterra 1 , Ana L. De Paul 2 , AgustõÂn Aoki 1 , Aldo H. Coleoni 2 , and Alicia I. Torres 1 1 Centro de MicroscopõÂa Electro  nica, Facultad de Ciencias Me  dicas and 2 Departamento de BioquõÂmica ClõÂnica, Facultad de Ciencias Quõ Âmicas, Universidad Nacional de Co  rdoba, Co  rdoba, Argentina Received 17 March 2000 and in revised form 21 June 2000; accepted 28 June 2000 Summary Insulin-like growth factor I (IGF-I) downregulates growth hormone (GH) ex- pression in pituitary cell cultures. However, in vivo different results were found depending on the experimental protocol used. We determined the ki- netics of changes of pituitary and serum GH concentrations after subcuta- neous IGF-I administration (240 lg/100 g body weight) to rats every 12 h for various periods. These parameters were correlated with changes in the soma- totroph cell population. A significant increase in serum GH was registered at 6 h after IGF-I injection. At this time, some somatotroph cells exhibited ultra- structurally signs of high secretory activity, whereas adjacent somatotroph cells showed a quiescent appearance with sizeable stores of secretory gran- ules. In contrast, serum GH levels remained unchanged at 1, 2 and 12 h after each IGF-I injection. Pituitary GH concentrations were comparable to control levels during the first 48 h and declined significantly at 72 h and 96 h of IGF-I treatment. After these prolonged periods of time of treatment, the size and ex- tension of organelles involved in protein synthesis decreased and mature se- cretory granules in the cytoplasm increased significantly in GH-secreting cells. The somatotroph cell density remained unchanged even at 96 h of treat- ment. In conclusion, our results suggest that periodical IGF-I administration to rats does not inhibit GH secretion. Interestingly, IGF-I injections induced Correspondence to: Prof. Dr. A. I. Torres, Centro de MicroscopõÂa Electro  nica, Facultad de Ciencias Me  di- cas, Universidad Nacional de Co  rdoba, Casilla Postal 362, 5000 Co  rdoba, Argentina; tel/fax: +54 3 51 4 33 30 21; e-mail: atorres@cmefcm.uncor.edu http://www.urbanfischer.de/journals/actahist 0065±1281/00/102/4±439 $ 15.00/0