Calcium waves in colonic myocytes produced by mechanical and receptor-mediated stimulation

The mechanisms underlying intracellular Ca2+ waves induced by either mechan ical or receptor-mediated stimulation of myocytes isolated from the longitu dinal muscle layer of the rabbit distal colon were compared using fura 2 an d fluorescence videomicroscopy. Light focal mechanical deformation of the p lasma membrane or focal application of substance P resulted in localized in tracellular Ca2+ concentration ([Ca2+](i)) transients that propagated throu ghout the cell. In both cases, the Ca2+ response consisted of a transient p eak response followed by a delayed-phase response. Substance P-mediated [Ca 2+](i) responses involved generation of inositol 1,4,5-trisphosphate and re lease of Ca2+ from thapsigargin-sensitive stores, whereas mechanically indu ced responses were partially (29%) dependent on La3+-sensitive influx of ex tracellular Ca2+ and partially on release of intracellular Ca2+ from thapsi gargin-insensitive stores gated by ryanodine receptors. The delayed-phase r esponse in both cases was dependent on extracellular Ca2+. However although the response to substance P was sensitive to La3+, that after mechanical s timulation was not. In the later case, the underlying mechanism may involve capacitative Ca2+ entry channels that are activated after mechanical stimu lation but not by substance P.