PITUITARY AND HYPOTHALAMIC INSULIN-LIKE GROWTH FACTOR-I (IGF-I) AND IGF-I RECEPTOR EXPRESSION IN FOOD-DEPRIVED RATS

The present study was designed to evaluate a possible role for the ins ulin-like growth factor-I (IGF-I) system in mediating the suppression of growth hormone (GH) secretion observed in food-deprived rats by mea suring IGF-I mRNA, receptor concentration and receptor mRNA in neuroen docrine tissues (hypothalamus and pituitary). Rats were deprived of fo od (food-deprived) for 72 h or had free access to food (fed). Tissues were processed for measurement of steady-state levels of: (a) IGF-I an d IGF-I receptor mRNA (by solution hybridization/RNase protection assa y); (b) IGF-I in serum and tissue extracts (by RIA) and (c) IGF-I disp lacable [I-125]IGF-I binding to plasma membrane preparations. Food dep rivation resulted in decreased serum and liver levels of IGF-I. Kidney IGF-I mRNA levels were reduced 80% in food-deprived rats with a conco mitant increase in IGF-I receptor concentration and mRNA levels. Refee ding of food-deprived rats fully normalized these perturbations. Pitui tary IGF-I content was reduced 50% in food-deprived rats while IGF-I m RNA levels were unaffected. A modest increase was seen in pituitary IG F-I receptor concentration; however, IGF-I receptor mRNA levels were n ot changed. Hypothalamic IGF-I mRNA content was reduced in 72 h food-d eprived rats while IGF-I receptor binding capacity and mRNA were unaff ected. In conclusion, IGF-I mRNA levels are decreased in liver, kidney and hypothalamus together with a reduction in plasma IGF-I in food-de prived rats but is unaffected in anterior pituitary. IGF-I receptor ge ne expression and binding capacity are coordinately regulated in kidne y and hypothalamus, but not in the pituitary. Thus, nutrient signals a ppear to differentially influence the pituitary compared to peripheral tissues, suggesting that IGF-I may play a unique role in pituitary GH regulation.