Histamine-induced Ca(2+) 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 Ca(2+), are still poorly understood. Using fluorescence imaging microscopy, we measured intracellular Ca(2+) ([Ca(2+)](i)) levels in fura-2-loaded hVMFs. Activation of H(1) receptors released Ca(2+) from one compartment of the endoplasmic reticulum (ER) of hVMFs, but did not induce Ca(2+) entry. This histamine-induced Ca(2+) release was oscillatory and dependent on Ca(2+) re-uptake into the ER by sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA). Application of the reversible SERCA blocker, cyclopiazonic acid (CPA), after depletion of the histamine-sensitive Ca(2+) store revealed the presence of a second, smaller histamine-insensitive Ca(2+) store in the ER. The Ca(2+) content ratio of the histamine-sensitive and histamine-insensitive Ca(2+) stores in the ER was found to be approximately 1.15:1. Another effect of CPA in hVMFs was the activation of store-operated Ca(2+) channels, as demonstrated by maintained [Ca(2+)](i) elevation as well as accelerated Mn(2+) entry. In conclusion, this study establishes for the first time an agonist-induced Ca(2+)-signaling pathway in hVMFs.