Modeling and analysis of spatio-temporal change in [Ca2+]i in a retinal rod outer segment

The change in [Ca2+]i, in a retinal rod outer segment to flash and step stimuli was simulated. The present model included inward and outward calcium fluxes through cation channels and an Na-Ca-K exchanger, respectively, calcium buffers, and diffusion through the interdiskal space of cytoplasm and incisures. Under control conditions (with the diffusion constant for calcium (DCa) of 10(-10) m2/s and the total concentration ([B]t) of 240 microM) the decrease in [Ca2+]i to flash stimuli was found to be localized around the edge of a disk irrespective of the presence or absence of incisures. Homogeneous but limited degree of decrease in [Ca2+]i was seen under a larger DCa of 10(-9) m2/s with no incisure. For the step illumination by which all the cation channels on a plasma membrane were closed, the decrease in [Ca2+]i around the edge of a disk was quick, while that at the center of an interdiskal space was slow (50% of the resting level at 5 s after the onset of the step illumination). These results indicate that the calcium feedback on guanylate cyclase and possibly on S-modulin in response to flash stimuli proceeds only around the edge of disk membranes or on the plasma membrane. This implicates localized mechanisms for the signal transduction and early phase of adaptation within rod outer segments.