Background: Pulmonary hypertension (PH) is a common complication of degenerative mitral valve disease (DMVD), the most common cardiovascular disease in dogs. Serotonin has been suspected to play a role in the pathogenesis of PH, so this study aimed to investigate the differences in platelet and plasma serotonin between normal, DMVD and DMVD with PH (DMVD+PH) dogs.Materials and Methods: Sixty-two small-breed dogs were enrolled to the study and divided into the normal (n = 22), DMVD (n = 20), and DMVD+PH (n = 20) groups. The platelet and plasma serotonin concentrations were measured by the competitive ELISA.Results: The Kruskal–Wallis revealed the difference among the four groups of normal (179.73 [102.37–352.24] ng/109 platelets), DMVD (325.99 [96.84–407.66] ng/109 platelets), DMVD with intermediate probability of PH (291.11 [106.69–400.84] ng/109 platelets) and DMVD with high probability of PH (35.82 [2.69–126.35] ng/109 platelets) (p = 0.014). The Dunn's post-hoc test showed a decrease in the platelet serotonin concentration of the DMVD dogs with high probability of PH compared to the DMVD group (p = 0.008). The plasma serotonin concentration was not different between normal, DMVD, and DMVD+PH dogs.Conclusion: In conclusion, a decrease in platelet serotonin concentration, which is associated with a degree of PH probability was found in DMVD dogs with PH. Further studies investigating roles of platelet serotonin in PH secondary to DMVD should be performed.

Pulmonary hypertension (PH) is defined as an increase in pulmonary vascular pressure. It is one of the most common complications that occur as a result of degenerative mitral valve disease (DMVD) in dogs. Serotonin (5-HT) can trigger the development of PH. Accordingly, this study investigated the changes in the expression of genes and proteins associated with local 5-HT signaling in the lungs and pulmonary arteries (PA) of dogs with PH secondary to DMVD. Lung and PA tissue samples were collected from the cadavers of fourteen small-breed dogs and divided into normal (n = 4), DMVD (n = 5) and DMVD with PH (n = 5) groups. Gene expression (tph1, slc6a4 and htr2a) was analyzed by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). The expression of proteins (TPH-1, SERT, 5-HTR2A, ERK1/2 and pERK1/2) was examined by Western blot analysis and immunohistochemical staining. The results showed that the expression of genes and proteins evaluated by qRT-PCR and Western blot analysis in lung and PA tissues did not differ among groups. However, the expression of proteins related to 5-HT signaling tended to be upregulated in PA tissues from DMVD dogs with and without PH. Immunohistochemical examination revealed the overexpression of these proteins in the DMVD and DMVD with PH groups in lung tissue. These findings suggest a local effect of 5-HT signaling in DMVD dogs with and without PH.

Pulmonary hypertension (PH) is an abnormal increase in pulmonary vascular pressure. In dogs, PH is commonly found secondary to degenerative mitral valve disease (DMVD), especially in small breed dogs. Serotonin, a biogenic amine playing an essential role in both physiology and abnormalities of several organs, is a vasoactive substance that has been one of the suspicious mediators for the development of PH. Both local and circulating effects of serotonin have been investigated to discover the involvement of serotonin and the pathogenesis of PH in both humans and animals. The present study aimed to investigate the local serotonin signaling in lung and pulmonary arteries (PA) and the source and differences of serotonin in platelets and plasma of dogs with PH secondary to DMVD compared to healthy dogs and DMVD dogs without PH. The lung and PA tissues of fourteen small-breed dogs were collected and divided into the control (n = 4), DMVD (n = 5) and DMVD+PH (n = 5) groups. The quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and Western blot were used to assess the expression of genes and proteins associating with the serotonin signaling pathway. To measure the platelet and plasma serotonin concentrations, whole blood was collected from sixty-two small-breed dogs divided into the control (n = 22), DMVD (n = 20) and DMVD+PH (n = 20) groups. The blood samples were prepared and measured by enzyme-linked immunosorbent assay (ELISA). The tendency of upregulated TPH-1, SERT, 5-HTR2A, ERK1/2 and pERK1/2 protein in PA tissues were found in DMVD dogs with and without PH, whereas the gene and protein expression in lung tissues was varied. The concentration of platelet serotonin of dogs with DMVD and high probability of PH (35.82 [2.69 - 126.35] ng/109 platelets) was decreased compared to DMVD dogs without PH (325.99 [96.84 - 407.66] ng/109 platelets) (p = 0.008). The concentration of plasma serotonin did not differ among all groups. These findings revealed that proteins related to the serotonin signaling pathway increased in dogs affected with DMVD with and without PH suggesting the local effect of serotonin in PA in dogs affected with DMVD with and without PH. In circulation, the degree of PH probability of dogs with PH secondary to DMVD is correlated with a decrease in platelet serotonin concentration. Roles of serotonin in PA and platelet serotonin should be investigated to elucidate the involvement of the local and systemic effect of the serotonin signaling pathway in dogs with naturally occurring PH due to DMVD.