A physiological role for protoporphyrin IX photodynamic action in the rat Harderian gland?

Aims: The lipid‐secreting exocrine Harderian gland contains a large amount of porphyrins (mainly protoporphyrin IX, PPIX) in the glandular cells, the physiological significance of which is rather poorly understood. Methods: In the present study, the possibility of using Fura‐2 to measure intracellular calcium ([Ca²⁺]_c) changes in these cells was assessed. Results: It was found that when Fura‐2‐loaded cells were excited by light at 340/380 nm, [Ca²⁺]_c increased spontaneously, indicating a photodynamic action powered by light at 340/380 nm. In contrast, with the visible spectrum calcium probe Fluo‐3 (λ_ex = 475 nm), carbachol at 10 μm induced [Ca²⁺]_c increase; [Ca²⁺]_c did not change without carbachol stimulation. Brief illumination with light at 340/380 nm induced a large [Ca²⁺]_c increase in Fluo‐3‐loaded cells. Photodynamic stimulation of [Ca²⁺]_c increase was confirmed with an exogenous photosensitizer sulphonated aluminium phthalocyanine (SALPC) and visible light (>580 nm). The wavelength‐dependence of the [Ca²⁺]_c increase correlates well with the excitation spectrum of the isolated Harderian glandular cells. Conclusion: These data suggest that PPIX present in rat Harderian glandular cells plays the role of a photosensitizer which upon activation by UVA and blue components of daylight and subsequent singlet oxygen generation, triggers [Ca²⁺]_c increase and secretory response. The PPIX photodynamic action may also play a potential role in photic entrainment of the central circadian clock.