Does Ca2+ Reach Millimolar Concentrations after Single Photon Absorption in Drosophila Photoreceptor Microvilli?

The quantum bump, the elementary event of fly phototransduction induced by the absorption of a single photon, is a small, transient current due to the opening of cation-channels permeable to Ca2+. These channels are located in small, tube-like protrusions of the cell membrane, the microvilli. Using a modeling approach, we calculate the changes of free Ca2+ concentration inside the microvilli, taking into account influx and diffusion of Ca2+. Independent of permeability ratios and Ca2+ buffering, we find that the free Ca2+ concentrations rise to millimolar values, as long as we assume that all activated channels are located in a single microvillus. When we assume that as much as 25 microvilli participate in a single bump, the free Ca2+ concentration still reaches values higher than 80 µM. These very high concentrations show that the microvilli of fly photoreceptors are unique structures in which the Ca2+ signaling is even more extreme than in calcium concentration microdomains very close to Ca2+ channels.