LONG-TERM CALORIE RESTRICTION PROTECTS RAT PITUITARY GROWTH HORMONE-RELEASING HORMONE-BINDING SITES FROM AGE-RELATED ALTERATIONS

In mammals, middle age and late adulthood is characterized by a decrea se of growth hormone (GH) secretion and insulin-like growth factor 1 ( IGF-I) serum levels, contributing to tissue and organ atrophy. This co ndition is related, at least in part, to alterations of pituitary GH-r eleasing hormone (GHRH) receptor-binding sites. Prevention of age-rela ted deterioration of tissues and organs, retardation of the onset or p rogression of a wide range of age-related diseases and extension of bo th mean and maximum life span can be achieved through life-long modera te calorie restriction (CR). Because CR has been reported to positivel y modulate the somatotropic axis resulting in the maintenance of a you thful GH secretory pattern in aged rats, we investigated whether or no t benefits of a long-term (10 months) 40% CR, started in 8-month-old m ale Sprague-Dawley rats, was accomplished by preventing age-related al terations of pituitary GHRH receptor binding sites. We also studied wh ether or not a short-term (50 days) 40% CR, started in 16-month-old ra ts, could revert them. Potential hormonal and metabolic modulators of the GHRH receptors were investigated as well. GHRH binding parameters were derived from saturation studies performed in pituitary homogenate s with [I-125-Tyr(10)]hGHRH (1-44)NH2. As previously reported, the hig h affinity GHRH receptor-binding sites were blunted in 18-month-old ad libitum-fed rats and the apparent concentration of total binding site s was reduced. Short-term CR neither restored high affinity GHRH bindi ng sites nor increased the apparent concentration of total binding sit es. On the contrary, long-term calorie-restricted 18-month-old rats ex hibited high and low affinity GHRH binding sites(K-d1: 1.73 +/- 0.35 n M K-d2: 310 +/-: 41 nM;B-max1: 183 +/- 55 fmol/mg protein; B-max2: 30 +/- 3 pmol/mg protein) as found in 2-month-old rats (K-d1: 0.68 +/- 0. 15 nM; K-d2: 350 +/- 47 nM B-max1: 219 +/- 53 fmol/mg protein; B-max2: 84 +/- 9 pmol/mg protein). Our results imply that CR must be implemen ted before age-related alterations of GHRH receptor-binding sites beco me too severe or that CR has to be carried out for a long period of ti me, independently from the age at which it begins. Protection of pitui tary GHRH binding sites from age-related alterations could not be attr ibuted to changes in circulating levels of total or free T-4 or free f atty acids. Finally, the anti-aging effect of a long-term CR observed at the level of pituitary GHRH receptors does not result in a signific ant increase of total IGF-1 circulating levels. Identification of mole cular and cellular mechanisms responsible for these actions will deser ve attention in order to identify centrally and/or peripherally active classes of molecules that could preserve, in aging mammals, the funct ionality of the somatotropic axis through selective regulation of pitu itary GHRH receptors.