Intracellular Ca2+ accumulation is strain-dependent and correlates with apoptosis in aortic valve fibroblasts

Aortic valve (AV) disease is often characterized by the formation of calcific nodules within AV leaflets that alter functional biomechanics. In vitro, formation of these nodules is associated with osteogenic differentiation and/or increased contraction and apoptosis of AV interstitial cells (AVICs), leading to growth of calcium phosphate crystal structures. In several other cell types, increased intracellular Ca2+ has been shown to be an important part in activation of osteogenic differentiability. However, elevated intracellular Ca2+ is known to mediate cell contraction, and has also been shown to lead to apoptosis in many cell types. Therefore, a rise in intracellular Ca2+ may precede cellular changes that lead to calcification, and fibroblasts similar to AVICs have been shown to exhibit increases in intracellular Ca2+ in response to mechanical strain. In this study, we hypothesized that strain induces intracellular Ca2+ accumulation through stretch-activated calcium channels. We were also interested in assessing possible correlations between intracellular Ca2+ increases and apoptosis in AVICs. To test our hypothesis, cultured porcine AVICs were used to assess correlates between strain, intracellular Ca2+, and apoptosis. Ca2+ sensitive fluorescent dyes were utilized to measure real-time intracellular Ca2+ changes in strained AVICs. Ca2+ changes were then correlated with AVIC apoptosis using flow cytometric Annexin V apoptosis assays. These data indicate that strain-dependent accumulation of intracellular Ca2+ is correlated with apoptosis in AVICs. We believe that these findings indicate early mechanotransductive events that may initiate AV calcification pathways.