Long-Term Calorie Restriction Protects Rat Pituitary Growth Hormone-Releasing Hormone Binding Sites from Age-Related Alterations

Abstract

In mammals, middle age and late adulthood is characterized by a decrease of growth hormone (GH) secretion and insulin-like growth factor 1 (IGF-1) serum levels, contributing to tissue and organ atrophy. This condition is related, at least in part, to alterations of pituitary GH-releasing hormone (GHRH) receptor-binding sites. Prevention of age-related deterioration of tissues and organs, retardation of the onset or progression of a wide range of age-related diseases and extension of both mean and maximum life span can be achieved through life-long moderate calorie restriction (CR). Because CR has been reported to positively modulate the somatotropic axis resulting in the maintenance of a youthful GH secretory pattern in aged rats, we investigated whether or not benefits of a long-term (10 months) 40% CR, started in 8-month-old male Sprague-Dawley rats, was accomplished by preventing age-related alterations of pituitary GHRH receptor binding sites. We also studied whether or not a short-term (50 days) 40% CR, started in 16-month-old rats, 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 homogenates with [¹²⁵I-Tyr¹⁰]hGHRH (1-44)NH₂. As previously reported, the high affinity GHRH receptor-binding sites were blunted in 18-month-old ad libitum-fed rats and the apparent concentration of total binding sites was reduced. Short-term CR neither restored high affinity GHRH binding sites nor increased the apparent concentration of total binding sites. On the contrary, long-term calorie-restricted 18-month-old rats exhibited high and low affinity GHRH binding sites (K_d1: 1.73 ± 0.35 nM; 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 implemented before age-related alterations of GHRH receptor-binding sites become too severe or that CR has to be carried out for a long period of time, independently from the age at which it begins. Protection of pituitary GHRH binding sites from age-related alterations could not be attributed to changes in circulating levels of total or free T₄ or free fatty acids. Finally, the anti-aging effect of a long-term CR observed at the level of pituitary GHRH receptors does not result in a significant increase of total IGF-1 circulating levels. Identification of molecular and cellular mechanisms responsible for these actions will deserve attention in order to identify centrally and/or peripherally active classes of molecules that could preserve, in aging mammals, the functionality of the somatotropic axis through selective regulation of pituitary GHRH receptors.

Aging and Diets Regulate the Rat Anterior Pituitary and Hypothalamic Transcriptome

“…This effect appears to be selective since other G-protein-coupled receptor genes such Srif and ghrelin receptors were not affected. These results support previous data showing a beneficial effect of LTMCR on GHRH-R expression [22,23]. The increase of GHRH-R mRNA levels in 18- to 19-month-old calorie-restricted rats was associated with the maintenance of youthful GHRH-binding parameters and GHRH-induced cAMP production [23].…”

“…Phenotypic characteristics of the old AL-fed and calorie-restricted rats were in agreement with our previous studies [22,23]. Interestingly, changing the source of dietary protein from casein to soy did not affect the anthropometric and hormonal parameters tested, suggesting that reduction of energy intakes is a stronger determinant of gene expression regulation in the aging anterior pituitary and hypothalamus, and likely in other tissues.…”

“…Two-month-old male Sprague-Dawley rats (Charles River Canada, St-Constant, Que., Canada) were housed individually. The rats were fed AL a chow diet containing either 20% casein (CA group) or soy (SA group) as the source of protein, prepared according to the Teklad control diet TD89248 (Harland Laboratories Inc., Madison, Wisc., USA) [22]. At 8 months, the casein-fed rats were randomized in two groups: AL or calorie-restricted.…”

“…The decline in circulating growth hormone (GH) appears to be due, at least in part, to a diminution of functional growth-hormone-releasing hormone receptors (GHRH-R), leading to a decrease of somatotroph sensitivity to GHRH [21]. We have shown that LTMCR maintains youthful levels of functional GHRH-R in the anterior pituitary of old rats [22,23]. …”

Delayed Age-Associated Decrease in Growth Hormone Pulsatile Secretion and Increased Orexigenic Peptide Expression in the Lou C/Jall Rat

“…A more conventional way to increase longevity is caloric restriction which also delays the age-associated decline in GH secretion [14]and in pituitary GH-releasing hormone (GHRH) binding sites [15]in Sprague-Dawley rats. It also increases the life span in GH-deficient dwarf mice [10, 16]as well as in Tg/– dwarf rats [17], suggesting that pathways for extending the life span by GH deficiency and caloric restriction are not totally identical despite the close relationships between GH secretion and food intake regulations [6, 17].…”