Calcium imaging of mechanically induced fluxes in tissue-cultured chick heart: Role of stretch-activated ion channels

Heart rate and contractility are sensitive to stretch. To better understand the origin of these effects, we have studied the effect of mechanical stimuli on a model system of tissue-cultured heart cells. Gently prodding cells with a pipette produced a Ca²⁺ influx that often led to waves of calcium-induced calcium release (CICR) spreading from the site of stimulation. Ca²⁺ release could also be produced by pulling on neighboring cells. The response was blocked by removing extracellular Ca²⁺ or by adding 20 μM Gd³⁺ to normal saline. The mechanical sensitivity probably arose from stretch-activated ion channels (SACs) based on several lines of evidence. Chick heart cells contain nonselective cation SACs that pass Ca²⁺ as well as Na⁺ and K⁺. Both the SACs and the fluorescence response are blocked by 20 μM Gd³⁺. Removal of Ca²⁺ from the extracellular medium blocked the fluorescent response. Cultures without SACs (grown in the absence of embryo extract) had no mechanically induced fluxes. These data contradict the recent claim that SAC activity is a patch-clamp artifact (C.E. Morris and R. Horn, Science Wash. DC 256: 1246-1249, 1991). The SACs had a density of ~1/μm² and were expected to pass <20 fA of Ca²⁺ current under physiological conditions. The change in intracellular concentration of Ca²⁺ ([Ca²⁺](i)) resulting from activation of SACs may be too small to induce CICR unless the channels pass current into a restricted space (N. LeBlanc and J.R. Hume, Science Wash. DC 248: 372, 1990). Because of their effects on cell Ca²⁺, SACs in the heart may serve to improve load sharing among fibers within the complex, interwoven, fibrillar structure of the heart and, via their net inward current, may serve to provide feedback to the pacemaker potential from the level of atrial filling.