Serotonin Concentrations in Platelets, Plasma, Mitral Valve Leaflet, and Left Ventricular Myocardial Tissue in Dogs with Myxomatous Mitral Valve Disease

Hypothesis/Objectives Altered serotonin (5‐hydroxytryptamine, 5 HT ) signaling is postulated in development and progression of canine myxomatous mitral valve disease ( MMVD ). Little is known regarding platelet, plasma, valvular, or myocardial 5 HT concentration ([5 HT ]) in affected dogs. We quantified [5HT] in platelet‐rich plasma ( PRP ), platelet‐poor plasma ( PPP ), mitral valve leaflets ( MV ), and left ventricular myocardium ( LV ). Animals Forty‐five dogs comprised 4 plasma groups of Cavalier King Charles Spaniels ( CKCS ) or non‐ CKCS , either healthy ( CON ) or MMVD affected: CKCS CON (n = 12); non‐ CKCS CON (n = 8); CKCS MMVD (n = 14); non‐ CKCS MMVD (n = 11). Twenty‐four dogs comprised 3 tissue groups: MMVD (n = 8); other‐ HD (heart disease) (n = 7); non‐ HD , extracardiac disease (n = 9). Methods High‐performance liquid chromatography measured PRP , PPP , MV , and LV [5 HT ]. Results Platelet‐rich plasma platelet [5 HT ] was greater in CKCS CON (1.83 femtograms/platelet [fg/plt]; range, 0.20–4.76; P  = .002), CKCS MMVD (1.58 fg/plt; range, 0.70–4.03; P  = .005), and non‐ CKCS MMVD (1.72 fg/plt; range, 0.85–4.44; P  = .003) versus non‐ CKCS CON (0.92 fg/plt; range, 0.63–1.30). There was no group difference in PPP [5 HT ]. MV [5 HT ] was significantly higher in MMVD (32.4 ng/mg; range, 8.4–106.7) versus non‐ HD (3.6 ng/mg; range, 0–28.3; P  = .01) and LV [5 HT ] was significantly higher in MMVD (11.9 ng/mg; range, 4.0–104.8) versus other‐ HD (0.9 ng/mg; range, 0–10.1; P  = .011) and non‐ HD (2.5 ng/mg; range, 0–6.9; P  = .001). Conclusions and Clinical Importance Platelet [5 HT ] was highest in healthy CKCS and both MMVD groups, but plasma [5 HT ] showed no group differences. Tissue [5 HT ] was highest in MV and LV of MMVD ‐affected dogs, suggesting altered 5 HT signaling as a potential feature of MMVD . Interactions of platelet, valvular, and myocardial 5 HT signaling warrant further investigation.