Gap Junction Formation in Heart Valves in Response to Mechanical Loading
Valvular interstitial cells (VICs) are responsible for the maintenance of heart valve leaflet structure, however their responses to mechanical loading are not fully understood. Further characterization of VIC responses with regards to phenotype (quiescent or activated via ?-smooth muscle actin [?-SMA]) and communication (through gap junction proteins connexins 43 and 26) were studied. Tissue strips from porcine aortic, pulmonary, and mitral valves were cyclically stretched in the circumferential direction at normal and above normal membrane tensions for 48 hours at 1 Hz, 37°C, and 5% CO2. Unloaded tissues were statically incubated concurrently with loaded tissues, and fresh tissue controls were collected immediately. VIC phenotype was identified by ?- SMA via immunohistochemical staining and cell enumeration, as well as by gene expression via RT-PCR. Gap junction protein Cx43 was also evaluated via immunohistochemical staining and cell enumeration and by gene expression via RT-PCR, whereas Cx26 was evaluated using immunohistochemical staining and cell enumeration only. Within the range tested, it was found that mechanical loading did not affect ?-SMA or gap junction protein levels, nor were any differences in responses noted between valve types. However, the ?-SMA gene expression level was significantly lower in the mitral valve compared to the aortic and pulmonary valves. This may indicate a difference in the genetic response pathways among the valves, but not in the functional outcomes. This difference may be explained by embryological origins, since the mitral valve, unlike the aortic and pulmonary valves, contains only VICs and no neural crest cells.