Amir S. Khan

Growth hormone, insulin-like growth factor-1 and the aging cardiovascular system

There is a large body of evidence that biological aging is related to a series of long-term catabolic processes resulting in decreased function and structural integrity of several physiological systems, among which is the cardiovascular system. These changes in the aging phenotype are correlated with a decline in the amplitude of pulsatile growth hormone secretion and the resulting decrease in plasma levels of its anabolic mediator, insulin like growth factor-1 (IGF-1). The relationship between growth hormone and biological aging is supported by studies demonstrating that growth hormone administration to old animals and humans raises plasma IGF-1 and results in increases in skeletal muscle and lean body mass, a decrease in adiposity, increased immune function, improvements in learning and memory, and increases in cardiovascular function. Since growth hormone and IGF-1 exert potent effects on the heart and vasculature, the relationship between age-related changes in cardiovascular function and the decline in growth hormone levels with age have become of interest. Among the age-related changes in the cardiovascular system are decreases in myocyte number, accumulation of fibrosis and collagen, decreases in stress-induced cardiac function through deterioration of the myocardial conduction system and beta-adrenergic receptor function, decreases in exercise capacity, vessel rarefaction, decreased arterial compliance and endothelial dysfunction leading to alterations in blood flow. Growth hormone has been found to exert potent effects on cardiovascular function in young animals and reverses many of the deficits in cardiovascular function in aged animals and humans. Nevertheless, it has been difficult to separate the effects of growth hormone deficiency from age-related diseases and associated pathologies. The development of novel animal models and additional research are required in order to elucidate the specific effects of growth hormone deficiency and assess its contribution to cardiovascular impairments and biological aging.

The effects of growth hormone and IGF-1 deficiency on cerebrovascular and brain ageing

Research studies clearly indicate that age‐related changes in cellular and tissue function are linked to decreases in the anabolic hormones, growth hormone and insulin‐like growth factor (IGF)‐1. Although there has been extensive research on the effects of these hormones on bone and muscle mass, their effect on cerebrovascular and brain ageing has received little attention. We have also observed that in response to moderate calorie restriction (a treatment that increases mean and maximal lifespan by 30–40%), age‐related decreases in growth hormone secretion are ameliorated (despite a decline in plasma levels of IGF‐1) suggesting that some of the effects of calorie restriction are mediated by modifying the regulation of the growth hormone/IGF‐1 axis. Recently, we have observed that microvascular density on the surface of the brain decreases with age and that these vascular changes are ameliorated by moderate calorie restriction. Analysis of cerebral blood flow paralleled the changes in vasculature in both groups. Administration of growth hormone for 28 d was also found to increase microvascular density in aged animals and further analysis indicated that the cerebral vasculature is an important paracrine source of IGF‐1 for the brain. In subsequent studies, administration of GHRH (to increase endogenous release of growth hormone) or direct administration of IGF‐1 was shown to reverse the age‐related decline in spatial working and reference memory. Similarly, antagonism of IGF‐1 action in the brains of young animals impaired both learning and reference memory. Investigation of the mechanisms of action of IGF‐1 suggested that this hormone regulates age‐related alterations in NMDA receptor subtypes (e.g. NMDAR2A and R2B). The beneficial role of growth hormone and IGF‐1 in ameliorating vascular and brain ageing are counterbalanced by their well‐recognised roles in age‐related pathogenesis. Although research in this area is still evolving, our results suggest that decreases in growth hormone and IGF‐1 with age have both beneficial and deleterious effects. Furthermore, part of the actions of moderate calorie restriction on tissue function and lifespan may be mediated through alterations in the growth hormone/IGF‐1 axis.

Age and insulin-like growth factor-1 modulate N-methyl-D-aspartate receptor subtype expression in rats.

N-Methyl-D-aspartate (NMDA) receptors have been reported to have an important role in synaptic plasticity and neurodegeneration. Two major subtypes of these receptors, NMDAR1 and NMDAR2, are present in brain and heterogeneity of these receptors have been reported to define specific functional responses. In this study, the effects of age and chronic insulin-like growth factor-1 (IGF-1) administration on NMDA receptor density and subtype expression were investigated in frontal cortex, CA1, CA2/3 and the dentate gyrus of the hippocampus of young (10 months), middle-aged (21 months) and old (30 months) male Fisher 344xBrown Norway (F1) rats. No age-related changes in (125)I-MK-801 binding or NMDAR1 protein expression were observed in hippocampus or frontal cortex. However, analysis of NMDAR2A and NMDAR2B protein expression in hippocampus indicated a significant decrease between 21 and 30 months of age and administration of IGF-1 increased these receptor subtypes. In cortex, NMDAR2A and NMDAR2B protein expression were not influenced by age or IGF-1 treatment, although NMDAR2C protein expression decreased with age and this decline was not ameliorated by IGF-1 administration. These data demonstrate that NMDA receptor subtypes are altered with age in a regional and subtype specific manner. We conclude that both age and IGF-1 regulate the expression of NMDA receptor subtypes and suggest that age-related changes in NMDA receptor heterogeneity may result in functional changes in the receptor that have relevance for aging.

Alterations in Insulin-like Growth Factor-1 Gene and Protein Expression and Type 1 Insulin-like Growth Factor Receptors in the Brains of Ageing Rats

Ageing in mammals is characterized by a decline in plasma levels of insulin-like growth factor-1 that appears to contribute to both structural and functional changes in a number of tissues. Although insulin-like growth factor-1 has been shown to provide trophic support for neurons and administration of insulin-like growth factor-1 to ageing animals reverses some aspects of brain ageing, age-related changes in insulin-like growth factor-1 or type 1 insulin-like growth factor receptors in brain have not been well documented. In this series of studies, insulin-like growth factor-1 messenger RNA and protein concentrations, and type 1 insulin-like growth factor receptor levels were analysed in young (three to four- and 10-12-month-old), middle-aged (19-20-month-old) and old (29-32-month-old) Fisher 344 x Brown Norway rats. Localization of insulin-like growth factor-1 messenger RNA throughout the lifespan revealed that expression was greatest in arteries, arterioles, and arteriolar anastomoses with greater than 80% of these vessels producing insulin-like growth factor-1 messenger RNA. High levels of expression were also noted in the meninges. No age-related changes were detected by either in situ hybridization or quantitative dot blot analysis of cortical tissue. However, analysis of insulin-like growth factor-1 protein levels in cortex analysed after saline perfusion indicated a 36.5% decrease between 11 and 32 months-of-age (P<0.05). Similarly, analysis of type 1 insulin-like growth factor receptor messenger RNA revealed no changes with age but levels of type 1 insulin-like growth factor receptors indicated a substantial decrease with age (31% in hippocampus and 20.8 and 27.3% in cortical layers II/III and V/VI, respectively). Our results indicate that (i) vasculature and meninges are an important source of insulin-like growth factor-1 for the brain and that expression continues throughout life, (ii) there are no changes in insulin-like growth factor-1 gene expression with age but insulin-like growth factor-1 protein levels decrease suggesting that translational deficiencies or deficits in the transport of insulin-like growth factor-1 through the blood-brain barrier contribute to the decline in brain insulin-like growth factor-1 with age, and (iii) type 1 insulin-like growth factor receptor messenger RNA is unchanged with age but type 1 insulin-like growth factor receptors decrease in several brain regions. We conclude that significant perturbations occur in the insulin-like growth factor-1 axis with age. Since other studies suggest that i.c.v. administration of insulin-like growth factor-1 reverses functional and cognitive deficiencies with age, alterations within the insulin-like growth factor-1 axis may be an important contributing factor in brain ageing.

Effects of Moderate Caloric Restriction on Cortical Microvascular Density and Local Cerebral Blood Flow in Aged Rats

The present study was designed to assess the impact of moderate caloric restriction (60% of ad libitum fed animals) on cerebral vascular density and local cerebral blood flow. Vascular density was assessed in male Brown-Norway rats from 7-35 months of age using a cranial window technique. Arteriolar density, arteriole-arteriole anastomoses, and venular density decreased with age and these effects were attenuated by moderate caloric restriction. Analysis of local cerebral blood using [14C]iodoantipyrine indicated that basal blood flow decreased with age in CA1, CA3 and dentate gyrus of hippocampus; similar trends were evident in cingulate, retrosplenal, and motor cortex. Basal blood flow was increased in all brain regions of moderate caloric restricted old animals (compared to old ad libitum fed animals) and no differences were observed between ad libitum fed young and caloric restricted older animals. In response to a CO2 challenge to maximally dilate vessels, blood flow increased in young and old ad libitum fed animals, but a similar increase was not observed in caloric restricted old animals. We conclude that a decrease in cerebral vasculature is an important contributing factor in the reduction in blood flow with age. Nevertheless, vessels from young and old animals have the capacity to dilate in response to a CO2 challenge and, after CO2, no differences are observed between the two age-groups. These results are consistent with the hypothesis that aged animals fail to adequately regulate local cerebral blood flow in response to physiological stimuli. Moderate caloric restriction increases microvascular density and cerebral blood flow in aged animals but tissues exhibit little or no increase in blood flow in response to CO2 challenge. The cause of this deficient response may indicate that vessels are maximally dilated in aged calorically restricted animals or that they fail to exhibit normal regulatory control.