Ultraviolet light activates blocking actions of dantrolene on intracellular Ca2+ release in bullfrog sympathetic...

Effects of dantrolene, a blocker of intracellular Ca2+ release, on the oscillation of the intracellular Ca2+ ([Ca2+]i) induced by caffeine were studied in bullfrog sympathetic ganglion cells, using a Fura-2 fluorescence technique. Dantrolene blocked the Ca2+ oscillation only in the cell illuminated by ultraviolet light (335-385 nm). Likewise, the blocking effects on rhythmic Ca(2+)-dependent hyperpolarizations, representing Ca2+ oscillations via activation of Ca(2+)-dependent K+ channel, occurred only under the illumination with ultraviolet light (335-385 nm), but not with visible light (404-417 nm). This wavelength dependence differs from the absorbance spectrum of dantrolene. On the other hand, dantrolene preirradiated with ultraviolet light under dark condition or ultraviolet light itself did not affect the [Ca2+]i oscillation. The blocking action was not prevented by the pretreatment of the cells with reducing agents. These results indicate that illumination of the Ca2+ release channel or dantrolene itself with ultraviolet light (possibly the former) is necessary for the drug to exert its blocking effect. Furthermore, dantrolene was found to decrease Fura-2 fluorescence and to increase cell autofluorescence, leading sometimes to a false decrease in the basal [Ca2+]i.