Heart valve regurgitation is a common problem affecting horses. The valves between the chambers of the heart do not form a tight seal and some blood moves backward rather than forward which over time can worsen and have important consequences for both horse and rider. Mechanisms of why this occurs and how quickly it progresses in horses are largely unknown and require further investigation. This study proposes looking at the differences in normal and regurgitant valves with cell culture. Cell culture involves removal of the valves from a donor deceased horse and growing the different cell populations in a lab setting. Here we propose that serotonin, a molecule that can be found in the blood, may affect the valve cells and contribute to the formation of thickening/irregular valves, similar to what is seen in humans and dogs. By investigating this in a laboratory setting, we can closely monitor the change in cells, receptors within cells and overall growth in a controlled environment and at a significantly cheaper cost than if using live horses. This approach will improve our understanding of equine valvular heart disease and provide a new technique to be used to study valve disease and investigate potential treatment options.