Clinical Applications of the Renin-Angiotensin-Aldosterone-Vasopressin Axis in the Horse and Future Directions for Research

The renin–angiotensin–aldosterone–vasopressin axis (RAAV) is known to play significant roles in preserving hemodynamic stability in response to changes in blood volume, blood pressure, electrolytes, and water. In the previous years, some studies focused on RAAV in the horse, showing substantial future clinical and research applications. Early studies assessed the response of this axis to different types of exercise (increasing intensity exercise vs. endurance). Aldosterone and vasopressin concentrations were measured in horses with mild dehydration induced by endurance exercise in comparison with horses suffering from exhaustion and metabolic problems after prolonged exercise. More recently, the neurohumoral response to heart failure, mainly in congestive states, has received special attention. The activation of the RAAV in congestive heart failure has important prognosis applications and inhibitors of this axis have been clinically studied, as the angiotensin-converting enzyme inhibitors. However, inhibitors of renin, aldosterone, and vasopressin have not been studied in horses. Additionally, the investigation of the changes in blood pressure during dehydration, sepsis, and endotoxemia in foals and adult horses might show important applications in the treatment of states that induce modifications in blood pressure. Inappropriately low RAAV activity has been detected in human patients with prolonged vasodilatory shock, and exogenously administered vasopressin could be used as a potent vasopressor agent to stabilize cardio-circulatory function. Similarly, it might be administered to septic foals for similar purposes. Finally, the RAAV axis also has important implications in the development as well as in the treatment of the chronic renal failure, although it has not been investigated in horses.