Light activation of phospholipase A2 in the photoreceptor of the crayfish (Procambarus clarkii)

Abstract

Retinal lipids of crayfish, kept at 4°C under continuous darkness for 3 weeks, consisted mainly of phosphatidylcholine (PC) and phosphatidylethanolamine (PE); sphingomyelin (SM), phosphatidylinositol (PI) and phosphatidylserine (PS) were minor contributors. PI, involved in the phototransduction cascade, never reached greater concentrations than 7% of the total. High concentrations of polyunsaturated fatty acids (PUF A) such as 20:4n-6, 20:5n-3 and 22:6n-3 (DHA, docosahexaenoic acid) were present in PC, PE and PS, but scarce in SM and PI. In retinae of crayfish kept at 4°C in darkness for 3 weeks and then exposed to white light (6 h; ca. 4500 lx), SM and PS remained seemingly unaffected. PC, however, significantly decreased within 10 min to 65% of the initial value and 50% at 180 min. To study the reduction of PC, lipids of retinae suspended in physiological solution with/without phospholipase C (PLC) and phospholipase A₂ (PLA₂) inhibitors such as DMDA (=DEDA), manoalide, ET-18-0CH₃, and U-73122 were measured. Only free fatty acids (FFA) of retinae with inhibitors of PLA₂ like DMDA and manoalide decreased. Retinae irradiated by white light for 3 h displayed a significant reduction of PC, compared with those that had remained in continuous darkness. However, the PC of retinae with Pl.A₂-inhibitors was not decreased by light. Our results provide evidence that not only photoreceptor cell PLC, but also PLA₂ is activated by light.