Insect elementary calcium release events measured in honeybee muscle cells: Calcium Signaling and Excitation–Contraction in Cardiac, Skeletal and Smooth Muscle

Subcellular calcium variations are involved in physiological and pathological mechanisms. Whereas elementary calcium release events (CREs) have been known for almost three decades in intact muscle cells isolated from vertebrates, they remained not characterized in invertebrates until recently. Dynamic confocal imaging was used on intact skeletal muscle cells isolated enzymatically from the adult honeybee legs to characterize spatio-temporal features of subcellular CREs. The frequency of these insect CREs, measured in x–y time lapse series, was higher than frequencies usually described in vertebrates. Spatial spread at half maximum was larger than in vertebrates and had a slightly ellipsoidal shape, two characteristics that may be related to ultrastructural features specific to invertebrate cells. In line-scan experiments, the histogram of CREs’ duration followed a bimodal distribution, supporting the existence of both sparks and embers. Unlike in vertebrates, embers and sparks had similar amplitudes, a difference that could be related to genomic differences and/or excitation–contraction coupling specificities in honeybee skeletal muscle fibers. Arthropods muscle cells show strong genomic, ultrastructural and physiological differences with vertebrates and a comparative analysis may help to better understanding the roles and regulations of CREs. From a toxicological point of view, such a comparison will lead to better anticipating the myotoxicity of new insecticides targeting ryanodine receptors. Recent studies described the effects of these insecticides on macroscopic calcium homeostasis in bee neurons and muscle cells. Here, cyantraniliprole, the most recently approved anthranilic diamide in Europe, triggers calcium transients in bee muscle cell as well. Cyantraniliprole effects on Ca²⁺ sparks are currently under study.