Flash-Induced Depression of the ERP under the Ca^<2+>-Free Condition in the Octopus Retina : Physiology

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Effects of Ca^<2+> on the octopus photoreceptor cells were investigated, taking advantage of the early receptor potential (ERP) closely concerned with conformation of the photoreceptor rhabdomeric membrane. In Ca^<2+>-free artificial sea water (ASW), the ERP evoked by the first test flash was normal but those by the following test flashes were almost depressed (flash-induced depression, FID). Recovery usually took 30-60min in the dark, but was greatly enhanced within 5min by an addition of Ca^<2+> of more than 5 × 10^<-4>M. If Na^+, in addition to Ca^<2+>, was freed from the ASW after previously loading Ca^<2+> into the photoreceptor cells by irradiating with test flashes in Na^+-free ASW, however, the FID was extremely weakened, suggesting weak (or almost no) Na-Ca exchange due to a lack of counter ion, Na^+. Alkali earth metals, Sr^<2+> and Ba^<2+> could substitute for Ca^<2+> regarding the FID. Higher concentrations of Zn^<2+> could also prevent the retina from an FID, but even in normal ASW it strongly suppressed the late receptor potential (LRP), while Sr^<2+> and Ba^<2+> showed no particular inhibitory effect on the LRP. Imipramine (or decipramine), an antidepressant and N-ethyl maleimaid, an SH-reagent, also strongly suppressed the LRP. Such agents, including Zn^<2+>, also abolished the FID under a Ca^<2+>-free condition, suggesting that there might be some common mechanisms between the FID and the LRP.
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