Not all sires have sperm suitable for chilled or frozen storage, and success in artificial insemination (AI) varies highly among individual dogs and breeds. Fertilizing potential is further complicated as sperm quality declines with the aging process. The overall objective of this work was to study the effects of age and mitochondrial function on sperm quality and bioenergetics using traditional methods assessed in the clinical setting, as well as emerging methods of investigating metabolic flexibility and the heterogenous nature of sperm. This dissertation explores the physiology and pathophysiology of canine sperm through the study of a breeding colony of highly fertile Labrador retrievers, and a rapidly aging purebred population of North American Great Danes. This was accomplished in three chapters. Chapter 1, Comparative Oxidative Metabolism in Mammalian Sperm, provides a review of the literature and current understanding of mitochondrial physiology and metabolic control of essential sperm functions in mammalian species of veterinary importance. We discuss the differences in fundamental oxygen and ATP substrate balance in stallions, bulls, and dogs, with an emphasis on the maintenance of sperm motility, electron transport chain (ETC) function, reactive oxygen species (ROS) production, and the balance of glycolytic and oxidative phosphorylation production of ATP in sperm.
In Chapter 2, Sperm Parameters in the Great Dane: Influence of Age on Semen Quality, I characterized the distribution of sperm quality parameters within a purebred population of actively showing North American Great Danes, a rapidly aging breed currently suffering declining fertility. I identified progressive declines in sperm quality that accompany aging, detailed the relationship between ROS production and sperm motility and morphology, and distinguished sources of variation in sperm quality within the breed. Age was negatively associated with several sperm motility parameters associated with sperm fertility such as total and progressive motility, and amplitude of lateral head displacement (p<.05), with a predicted -9.9%, -9.0%, and +8.3% change per year of age. Sperm of younger GD dogs aged 12 ≤ x <24 months had significantly higher total and progressive motility (TM, PM), amplitude of lateral head displacement (ALH), and nonlinear motility (p<.05) than older dogs (x ≥ 48 months). Results support anecdotal reports of decline of the fertility with the advance of age in this breed, and indicate that age and ROS have significant influences on sperm parameters in the GD. The influence of selection for breed specific phenotypes could help explain the functional significance of the diversity among GD males.
In Chapter 3, Effects of Substrate Availability and Mitochondrial Disruption on Oxidative Metabolism and Sperm Motility in Fertile Dogs, I investigated the adaptability of canine sperm bioenergetics under differing nutrient conditions and the effects of mitochondrial dysregulation on sperm motility. With simultaneous measurement of mitochondrial oxygen consumption (MITOX) and sperm kinematics, I observed a high adaptability and metabolic flexibility of high-quality canine sperm. While all energetic substrates tested successfully maintained sperm motility, significant differences were found in MITOX and mitochondrial spare respiratory capacity (SRC), the strength with which mitochondrial energy production may rise to meet energetic challenge. In addition, the ability to maintain sperm kinematics when supplemented with various energetic substrates differed significantly with mitochondrial disruption by mitochondrial effectors. A population of Labrador retrievers from a highly fertile breeding colony managed by Guide Dogs for the Blind was chosen for this study due to their high semen quality, good health, and known fertility.
