What do we really know about oxidative stress? Facing the problems with current oxidative stress studies in passerine birds.

Abstract

Oxidative stress occurs in organisms when there are not enough antioxidants to satiate reactive oxygen species formed during oxidative phosphorylation within the mitochondria. The excess in reactive oxygen species can cause oxidative damage to lipids, proteins, and DNA. This damage has proposed involvement in numerous biological processes, including certain diseases, immune response, and energetically demanding life-history traits, such as reproduction and longevity. The proposed importance necessitates that investigators determine how to best measure oxidative damage and what causes damage to differ between species and individuals. Recently, methods for quantifying oxidative stress as well as the relationships proposed between certain biological processes and oxidative damage have been questioned. Herein, I investigate the value of erythrocytes and plasma as indicators of oxidative damage present within pectoralis, heart, and liver tissues of Northern Cardinals, as well as measure the effect of energy expenditure on oxidative damage in pectoralis and liver tissues of House Finches. The preferred method for evaluating the products of oxidative damage in evolutionary and ecological studies is through plasma or erythrocyte samples. This is assumed to be indicative of oxidative damage occurring throughout the body, yet variations between organ generation of oxidative damage and rate of oxidative damage by-product transfer into circulation make this questionable. Some mammalian studies have garnered support for this assumption, however it has never been investigated in birds. This relationship is further confounded in birds due to the presence of functioning mitochondria within avian erythrocytes. To determine if erythrocytes and plasma provide valuable insight into oxidative damage throughout the body of birds, in chapter 1 I measured two different markers of oxidative damage, 4-hydroxynonenal and protein carbonyls, as well as antioxidant potential in erythrocytes, plasma, pectoralis, liver, and heart of 18 wild Northern Cardinals. I found no evidence to support the use of either erythrocytes or plasma as valuable indicators of oxidative damage to other tissues. When investigating the effects of oxidative damage on life-history traits, it is often assumed that reactive oxygen species production is positively correlated with an animal’s metabolic rate as more electrons must be moved through the electron transport system to accommodate increased energy demands. Yet the effect of energy expenditure on oxidative damage has never been directly evaluated in birds. In chapter 2, I measured levels of oxidative damage and antioxidants in pectoralis and liver tissues of 24 wild caught House Finches that spent at least two weeks in one of three cage sizes to promote low-, medium-, and high-energy expenditure. I found no significant evidence supporting an increase in oxidative damage relative to an increase in energy expenditure. The results of these two chapters indicate that the relationships surrounding oxidative damage are more complicated than originally proposed, and that further investigations need to be made into establishing more reliable methods of oxidative damage measurement.