title: Calcium diffusion coefficient in rod photoreceptor outer segments.
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 microm(2)s(-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 approximately 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 microm(2)s(-1), which is much smaller than the longitudinal cGMP diffusion coefficient (30-60 microm(2)s(-1); ). Therefore, the longitudinal decline of Ca(2+) lags behind the longitudinal spread of excitation by cGMP.
authors.affiliation: Institute of Biological Sciences, University of Tsukuba, Tsukuba, Ibaraki 305, Japan.
authors.forename: Kei
authors.initials: K
authors.lastname: Nakatani
authors.forename: Chunhe
authors.initials: C
authors.lastname: Chen
authors.forename: Yiannis
authors.initials: Y
authors.lastname: Koutalos
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000814:Aniline Compounds / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004058:Diffusion; D005456:Fluorescent Dyes / Q000494:pharmacology; D007700:Kinetics; D018613:Microscopy, Confocal; D008954:Models, Biological; D008962:Models, Theoretical; D011188:Potassium / Q000378:metabolism; D017948:Retinal Rod Photoreceptor Cells / Q000737:chemistry* / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D013997:Time Factors; D014562:Caudata; D014966:Xanthenes / Q000494:pharmacology
chemical_list: D000814:Aniline Compounds; D005456:Fluorescent Dyes; D014966:Xanthenes; C059715:Fluo-3; D000255:Adenosine Triphosphate; D012964:Sodium; D011188:Potassium; D002118:Calcium