Light stimulation releases calcium from intracellular stores in the microvillar lobe of Lima photoreceptors. (A) Simultaneous recording of membrane current under voltage clamp (top traces) and fluorescence of the indicator Calcium Green 5N, which was dialyzed through the patch pipette at a concentration of 63 µM (bottom traces). A large increase in fluorescence above the initial level occurred shortly after activating the epi-fluorescence beam, coincident with the beginning of the photocurrent. A similar Ca²⁺ transient was observed in a cell superfused for 8 min with nominally calcium-free solution (right). (B) The Ca²⁺ transient originates in the microvillar lobe and precedes the opening of the light-dependent channels. The fluorescence was recorded from a small window positioned either on the microvillar lobe (Rhabdomere) or on the cell body (Soma), as indicated in C. The signal from the rhabdomere slightly preceded the membrane current (inverted for display purposes and normalized), whereas that from the cell body lagged behind the electrical response by many milliseconds. (D) Pooled data of the temporal shift between current and fluorescence, averaged for several cells tested in the two arrangements.