Histamine-induced Ca2+ signaling in human valvular myofibroblasts

In this study, we examined histamine-induced calcium signaling in cultured human valvular myofibroblasts (hVMFs), which are the most prominent interstitial cells in cardiac valves mediating valvular contraction, extracellular matrix secretion, and wound repair. Despite the functional importance of VMFs in cardiac valves, the cellular-signaling pathways, especially those mediated by Ca2+, are still poorly understood. Using fluorescence imaging microscopy, we measured intracellular Ca2+ ([Ca2+]i) levels in fura-2-loaded hVMFs. Activation of H1 receptors released Ca2+ from one compartment of the endoplasmic reticulum (ER) of hVMFs, but did not induce Ca2+ entry. This histamine-induced Ca2+ release was oscillatory and dependent on Ca2+ re-uptake into the ER by sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA). Application of the reversible SERCA blocker, cyclopiazonic acid (CPA), after depletion of the histamine-sensitive Ca2+ store revealed the presence of a second, smaller histamine-insensitive Ca2+ store in the ER. The Ca2+ content ratio of the histamine-sensitive and histamine-insensitive Ca2+ stores in the ER was found to be approximately 1.15:1. Another effect of CPA in hVMFs was the activation of store-operated Ca2+ channels, as demonstrated by maintained [Ca2+]i elevation as well as accelerated Mn2+ entry. In conclusion, this study establishes for the first time an agonist-induced Ca2+-signaling pathway in hVMFs.