Myxomatous mitral valve disease (MMVD) is a common cause of congestive heart failure in small-breed dogs, but the cause is unknown. Serotonin signaling is thought to play a role in canine MMVD development; however, the source of serotonin is unclear. Normally, serotonin is stored in platelets and circulating plasma levels of serotonin are low. Serotonin is released into circulation after platelet adhesion and activation.
This study compared serotonin concentration in plasma and platelets of normal, healthy small-breed dogs predisposed to MMVD and of dogs with naturally occurring MMVD. Small-breed, client-owned dogs weighing <10 kg and ≥6 years of age (n = 43) were matched by age, breed, and size and divided into a healthy control group and a group with echocardiographic evidence of MMVD. Median plasma serotonin and platelet serotonin concentrations were not significantly different between healthy dogs and dogs with MMVD.
Although male dogs had higher levels of plasma serotonin concentrations than did female dogs, there was no correlation between plasma or platelet serotonin concentration and age, echocardiographic indices, or platelet count. Results showed low levels of plasma serotonin concentration in normal dogs and dogs with MMVD; this suggests that circulating plasma serotonin is an unlikely source of serotonin contributing to MMVD pathogenesis. It has been suggested that platelet activation on damaged mitral valves may induce serotonin release into the circulation. However, more evidence is needed.