Nakatani, K., C. Chen and Y. Koutalos (2002)
Calcium Diffusion Coefficient in Rod Photoreceptor Outer Segments.
Biophys. J., 82:728-739

[ABSTRACT]

Calcium (Ca²⁺) modulates several of the enzymatic pathways that mediate phototransduction in the outer segments of vertebrate rod photoreceptors. Ca²⁺ enters the rod outer segment through cationic channels kept open by cyclic GMP (cGMP) and is pumped out by a Na⁺/Ca²⁺,K⁺ exchanger. Light initiates a biochemical cascade, which leads to closure of the cGMP-gated channels, and a concomitant decline in the concentration of Ca²⁺. This decline mediates the recovery from stimulation by light and underlies the adaptation of the cell to background light. The speed with which the decline in the Ca²⁺ concentration propagates through the rod outer segment depends on the Ca²⁺ diffusion coefficient. We have used the fluorescent Ca²⁺ indicator fluo-3 and confocal microscopy to measure the profile of the Ca²⁺ concentration after stimulation of the rod photoreceptor by light. From these measurements, we have obtained a value of 15 ± 1 μm²s^-1 for the radial Ca²⁺ diffusion coefficient. This value is consistent with the effect of a low-affinity, immobile buffer reported to be present in the rod outer segment (L. Lagnado, L. Cervetto, and P. A. McNaughton, 1992, J. Physiol. 455:111-142) and with a buffering capacity of ~20 for rods in darkness (S. Nikonov, N. Engheta, and E. N. Pugh, Jr., 1998, J. Gen. Physiol. 111:7-37). This value suggests that diffusion provides a significant delay for the radial propagation of the decline in the concentration of Ca²⁺. Also, because of baffling by the disks, the longitudinal Ca²⁺ diffusion coefficient will be in the order of 2 μm²s^-1, which is much smaller than the longitudinal cGMP diffusion coefficient (30-60 μm²s^-1; Y. Koutalos, K. Nakatani, and K.-W. Yau, 1995, Biophys. J. 68:373-382). Therefore, the longitudinal decline of Ca²⁺ lags behind the longitudinal spread of excitation by cGMP.