Ca(2+)-dependent and Ca2+ release-dependent excitation in leech photoreceptors: evidence from a novel "inside-out" cell model

We have developed a novel, electrophysiologically intact and light-sensitive "inside-out" cell model (IOCM) of microvillar photoreceptors of the leech Hirudo medicinalis. Light responses recorded from the IOCM with sharp microelectrodes are depolarizations with amplitudes of up to 50-60 mV. In darkness, graded elevations of the free Ca(2+) concentration in the "intracellular medium" (ICM) reversibly increase the conductance of the microvillar membrane leading to Ca(2+)-induced graded voltage changes up to approximately 50 mV. The threshold for Ca(2+)-induced voltage changes is approximately 0.06 microM, EC(50) is approximately 1.2 microM, and saturation occurs at approximately 20 microM free Ca(2+). Small Ca(2+) elevations (<0.6 microM) produce discrete waves of depolarization resembling quantum bumps. Stimulating IOCMs with short (20-ms) and long (5-s) light stimuli produces transient light responses (repolarization within ca. 200 ms) in an ICM containing only 10nM free Ca(2+). At 0.44 microM free Ca(2+) in the ICM, the microvillar membrane depolarizes by 10-20 mV and responses to 5-s light steps have an initial transient component and a plateau component, similar to responses in intact cells. Generation of the plateau component in IOCMs is suppressed by heparin and cyclopiazonic acid (CPA), agents that block inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3))-induced Ca(2+) release from and Ca(2+) uptake into the endoplasmic reticulum (ER). These results indicate that there is a Ca(2+)-dependent conductance in the microvillar membrane and that the light-induced Ins(1,4,5)P(3)- and Ca(2+) release-mediated intracellular Ca(2+) elevation in leech photoreceptors contributes to the generation of the receptor potential, particularly the plateau component of responses to long steps of light.