Abstract
Calcium (Ca 2+) modulates several of the enzymatic pathways that mediate phototransduction in the outer segments of vertebrate rod photoreceptors. Ca 2+ enters the rod outer segment through cationic channels kept open by cyclic GMP (cGMP) and is pumped out by a Na +/Ca 2+,K + exchanger. Light initiates a biochemical cascade, which leads to closure of the cGMP-gated channels, and a concomitant decline in the concentration of Ca 2+. 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 Ca 2+ concentration propagates through the rod outer segment depends on the Ca 2+ diffusion coefficient. We have used the fluorescent Ca 2+ indicator fluo-3 and confocal microscopy to measure the profile of the Ca 2+ concentration after stimulation of the rod photoreceptor by light. From these measurements, we have obtained a value of 15 ± 1 μm 2s −1 for the radial Ca 2+ 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 Ca 2+. Also, because of baffling by the disks, the longitudinal Ca 2+ diffusion coefficient will be in the order of 2 μm 2s −1, which is much smaller than the longitudinal cGMP diffusion coefficient (30–60 μm 2s −1; Y. Koutalos, K. Nakatani, and K.-W. Yau, 1995, Biophys. J. 68:373–382). Therefore, the longitudinal decline of Ca 2+ lags behind the longitudinal spread of excitation by cGMP.
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