LIGHT-INDUCED MN2+ INFLUX IN LIMULUS VENTRAL PHOTORECEPTORS

In contrast to insect species, light-activated influx of divalent ions into Limulus ventral photoreceptors has proven difficult to demonstra te. We used the quench of the fluorescent indicator dye, fura-2, to me asure Mn2+ influx. Limulus ventral photoreceptors were injected with f ura-2 and excited at 360 nm. When the photoreceptors were bathed in 1 mmol.l(-1) Mn2+, an approximately 1% per 10 s decline in the fura-2 fl uorescence during intervals between 50-ms flashes was taken as a measu re of Mn2+ entry in darkness. Fluorescence decline during 10-s flashes was used to monitor Mn2+ entry during the photoresponse. During the 1 0-s flashes we observed a small rapid decline of the fura-2 fluorescen ce even in the absence of Mn2+. This reflected a contamination of the fluorescence signal arising from light-induced release of intracellula r calcium stores. A subsequent slower decline in fluorescence during t he 10-s flash, amounting to approximately 9% per 10 s, was only observ ed in the presence of extracellular Mn2+ and was attributed to Mn2+ in flux. This light-activated influx was not through voltage-gated calciu m channels since it persisted under voltage clamp, was not stimulated by depolarizing current injections, nor blocked by NiCl2. Depletion of internal calcium stores by cyclopiazonic acid treatment did not accel erate Mn2+ influx.