Year

2011

Degree Name

Doctor of Philosophy

Department

School of Biological Sciences

Abstract

Birds, as a group, are long-living. Both for mammals and birds, the maximum lifespan potential (MLSP) of a species is correlated with body size, but birds live, on average, twice as long as mammals. Differences in longevity also exist within the group of birds, again as a correlation with body size, but there is also considerable mass-independent variation in MLSP. For example, Psittaciformes (parrots) are extremely long-living with an average MLSP of 25 years, and with some species living more than 100 years. In contrast, similar-sized Galliformes (fowl) are short-living, and the comparison of these bird orders might give considerable insights into the mechanisms of aging.

Early attempts to understand the mechanisms determining maximum longevity were carried out in mammals and implicated differences in metabolic rate. Thus, the longevity differences between mammals and birds, as well as within birds, were surprising as the "rate of living" of birds (i.e. their metabolic rate) is generally higher than that of mammals of the same size, and birds usually have much higher resting body temperatures than mammals. Furthermore, similar-sized birds with extreme differences in MLSP (such as parrots and fowl) have very similar metabolic rates.

While differences in metabolic rate per se cannot fully explain longevity differences among animals, there does appear to be some link between the „rate of living‟ and the „length of life‟. Oxygen-derived radicals (nowadays called reactive oxygen species; ROS) are produced as a normal by-product of mitochondrial respiration. These ROS cause oxidative damage to biological molecules and the accumulated damage, in turn, results in the breakdown of homeostatic regulatory systems, eventually causing an animal's death and consequently determining the characteristic maximum longevity of the particular species. This “oxidative stress theory of aging” is currently the most widely accepted explanation of an animal‟s maximum lifespan, and can be divided into its functional components: (i) the mitochondrial production of ROS during normal respiration, (ii) the countervailing influence of an array of antioxidant systems (both enzymatic and non-enzymatic), and (iii) oxidative damage to a wide variety of bio-molecules.

This study examined the biochemical mechanisms underlying the longevity differences, on the one hand between long-living pigeons (MLSP 35y) and short-living rats (MLSP 5y), and on the other hand between three species of long-living parrots (average MLSP 27y) and two species of short-living quails (average MLSP 5.5y). A number of functional aspects of the oxidative stress theory were investigated in the two species-comparisons.

In the rat-pigeon comparison, total antioxidant status and non-enzymatic antioxidants are essentially the same in rats and pigeons. The enzymatic antioxidants (especially mitochondrial) suggest that the rats experience a much greater degree of oxidative stress in vivo than do pigeons. This is especially the case for the peroxidases (glutathione peroxidase and catalase). However, the results from in vitro measurements of mitochondrial ROS production (superoxide and hydrogen peroxide) do not support this interpretation. It is only heart mitochondria that suggest a greater oxidative stress in rats compared to pigeons, and only with succinate as a substrate.

There was no convincing evidence of rat tissues having consistently higher in vivo mitochondrial superoxide production than tissues in pigeons. Yet, the higher enzymatic antioxidant content of rats compared to pigeons suggests that rat tissues in vivo may experience greater oxidative stress than pigeon tissues. Is this apparent contradiction real? In this respect it is of interest that the most consistent difference that was observed between rats and pigeons was in the peroxidation index of membrane lipids, with the rats having a significantly higher membrane susceptibility to oxidative damage than pigeons. Fatty acid peroxidation leads to the formation of harmful secondary lipid-based ROS. These secondary ROS might be as important as mitochondria-derived primary ROS in the determination of an animal‟s lifespan. Biomarkers of lipid peroxidation, as well as markers of protein and mitochondrial DNA damage, were determined for the same individuals. No differences were found between rats and pigeons which may be due to rapid repair and removal rates.

A similar approach was used in the parrot-quail comparison, with the difference that all birds in this comparison were fed the same diet for two months prior to the beginning of the experiments, to exclude dietary effects on all variables examined. ROS production was determined in intact cells (erythrocytes), as well as in vitro in isolated mitochondria. Both methodological approaches revealed that parrots and quails have very similar levels of ROS generation. Mitochondrial ROS production, therefore, may not account for their longevity differences.

Glutathione peroxidase (GPx) and glutathione (GSH) levels are higher in the long-living parrots and suggest higher protection against the harmful effects of hydroperoxides which might be important for parrot longevity. Parrots have a higher total antioxidant capacity, but only on a „per g tissue‟ basis. All other antioxidants show either no association, or a negative correlation with MLSP. Despite indications of higher protection against some aspects of oxidative stress in the parrots, overall antioxidant defence mechanisms do not account for their longevity.

Besides maximum lifespan, basal metabolic rate (BMR) also varies with body size in mammals and birds and it has been suggested that both mass-related variations are mediated through differences in membrane fatty acid composition. BMR, the tissue phospholipid fatty acid composition of seven tissues, and fatty acid composition of mitochondrial membranes from two tissues, were evaluated in all parrots and quails. Whereas neither BMR nor the membrane susceptibilities to oxidative damage corresponded with the long MLSP of parrots, there was consistent demonstration that (i) all birds exclude n-3 polyunsaturated fatty acids from their mitochondria, and that (ii) independent of the mode of locomotion (flight vs. non-flight muscles) both pectoral and leg muscle have an almost identical membrane fatty acid composition in all birds.

In agreement with the absence of differences in ROS production, antioxidants and membrane composition, the tissue levels of oxidative damage (mitochondrial DNA, protein and lipid damage) were similar in parrots and quails.

The rat-pigeon comparison and the parrot-quail comparison can be regarded as two separate studies, with one investigating the mechanisms underlying the long MLSP of birds in general (in comparison to mammals), and the other examining the basis of lifespan differences within birds. Mitochondrial primary ROS generation is commonly regarded as an important determinant of longevity differences between species. However, both studies showed equally that mitochondria-derived ROS might not be as important as generally assumed. Instead, secondary lipid-based ROS, produced during lipid peroxidation of membrane fatty acids, might be the more important ROS with respect to oxidative stress. A low secondary lipid-based ROS production can to some extent explain the long lifespan of pigeons compared to rats due to much higher membrane susceptibility to oxidative damage in the rats. It can also to some extent explain the long lifespan of parrots because of their high GPx and GSH levels, which protect against hydroperoxides formed during lipid peroxidation of membrane fatty acids.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.