Characteristics of agonist-induced Ca2+ responses in diseased human valvular myofibroblasts.

Valvular myofibroblasts (VMFs) are present in large numbers in the cardiac valve. Although the functional roles of VMFs in vivo remain to be determined, evidence suggests the cells' ability to contract in vitro. Since Ca2+ is important in the contractility of many cell types, we examined Ca2+ responses induced by different agonists in normal and rheumatic VMFs. The agonists used were histamine, adenosine triphosphate (ATP) and 5-hydroxytryptamine (5-HT), all of which are important mediators in cardiac function. Cytosolic Ca2+ concentrations ([Ca2+]i) in fura-2-loaded VMFs were measured with ratiometric fluorescence microscopy. VMFs were challenged with a single concentration of each agonist in either Ca2+-containing (+Ca) or Ca2+-free (-Ca) physiological salt solution (PSS). From the resulting Ca2+ response, area under curve (AUC) was calculated from the point of drug addition (i.e., baseline) until the response reached the first peak (i.e., maximum). Our data show that more Ca2+ was mobilized in normal than in rheumatic VMFs, suggesting possible Ca2+-mobilizing dysfunction in the initial phase of a response under disease conditions. The most prominent difference was observed with 5-HT stimulation in +Ca PSS, where normal VMFs showed significantly greater AUC than rheumatic VMFs. The investigation of agonist-induced Ca2+ signaling characteristics in VMFs may provide information pertaining to Ca2+-associated changes and their consequences in cardiac valvular diseases.