This body of work concerns studies investigating aspects of ACE activity in the horse. Since very little information is available in the existing literature regarding this topic, basic research included assay validation, comparison of methods, and sample stability testing. Further investigations included the influence of environmental factors on circulating ACE activity, including diurnal and seasonal variations and the effects of training and acute exercise. This research sought to investigate whether there was an association between ACE phenotype and performance for different equestrian modalities, namely flat racing and endurance. Subsequently, aspects of the ACE genotype, ACE activity in tissues and whether there was an association between ACE activity and diseases of metabolic or pulmonary origins were investigated.

Assay testing demonstrated that different methods are not comparable, therefore caution is required when interpreting data obtained with different protocols for ACE measurement. Also, sample stability testing provided evidence that equine ACE is more labile than its human counterpart, requiring strict handling and storage to achieve repeatable and consistent results.

Data demonstrated that equine ACE has a seasonal variation, peaking during spring of the Southern Hemisphere, whereas no significant circadian rhythm of the enzyme was observed.

Results demonstrate that greater ACE activity is associated with improved performance indices in maiden Thoroughbreds, and is also associated with distance successfully raced by Thoroughbred racehorses, both under flat racing conditions. ACE activity in equine plasma is responsive to both acute and chronic exercise, with increased levels proportional to workload. This has important implications with regard to selection of horses suitable for particular types of event, and also in the assessment of fitness.

In relation to clinical conditions, although ACE activity was not associated with the systemic response to an experimental endotoxic challenge, horses admitted to a referral centre with colic had improved likelihood of being discharged if their ACE activity in plasma was higher at admission. ACE activity was not a good indicator of pulmonary disease diagnosed via endoscopy only, but there is an indication that ACE activity might be correlated to severity of chronic lung disease.

ACE activity in equine lungs is higher than in the kidneys and the heart, and data suggests that acute lung injury may cause a reduction in both plasma and lung tissue ACE activity, whereas chronic pulmonary disease leads to an elevation in plasma ACE with reduced lung tissue ACE activity.

ACE genotyping demonstrated that horses carrying a single copy of the H6 haplotype had lower levels of circulating ACE activity, but since only 9.5% of animals in the sample had the H6, the association was poor. ACE activity in equine plasma was demonstrated to be a better tool for assessing suitability for athletic events or training than ACE genotype, and to have potential as an accessory tool to monitor performance and as an indicator of disease course in some clinical situations.

This thesis was completed at the University of Melbourne and the record in their archive can be found here:

https://minerva-access.unimelb.edu.au/handle/11343/35448