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First published online April 23, 2004
Journal of Experimental Biology 207, 1759-1769 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00955

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The role of muscarinic receptors and intracellular Ca²⁺ in the spectral reflectivity changes of squid iridophores

Lydia M. Mäthger^1,2,*, Toby F. T. Collins^1,2, and Pedro A. Lima^1,

¹ The Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill, Plymouth PL1 2PB, UK
² Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK

View larger version (65K): [in a new window]   Fig. 1. (A) Diagram of a squid iridophore, showing iridosomes (is.), which are made up of groups of iridophore plates (ir.p.); N, nucleus (drawn using information given in Cloney and Brocco, 1983; Mirow, 1972). The iridophore cells are oriented with their long axes parallel to the plane of the skin, whereas the iridophore plates within the iridosomes can be oriented at a variety of angles (see Mirow, 1972). (B) Iridophore stripes of the squid Alloteuthis subulata. The names are given according to the colour reflected when the stripes are viewed in white light at near-normal incidence. The figure also shows a cross-section perpendicular to the long axis of a squid's mantle, in the region of the fins (d, dorsal; v, ventral). For each stripe, solid lines indicate the iridophores; dot-dashed lines, normal incidence to the iridophores; dotted lines, horizontal (adapted from Mäthger and Denton, 2001). (C) Diagram showing in cross-section the location in an iridophore of the different reflective stripes and the orientation of their reflective plates (not to scale). ir., iridophore; ir.p., iridophore plates. Shown also are electron micrographs of the iridophore plates with the respective stripes shown in diagram. The electron micrographs are oriented in the figure so that the skin surface in each micrograph (off scale, not shown) coincides with the orientation of the skin surface in the diagram. Scale bars, 1 µm.

View larger version (71K): [in a new window]   Fig. 2. (A) Iridophores of the `red' stripe of A. subulata superfused with 50 µmol l-1 acetylcholine (ACh). (a) Before superfusion, (b) 20 s after start of superfusion, (c) after 1 min. The `red' stripe iridophores change from non-reflective to reflective upon perfusion with ACh and become non-reflective after washing with ASW. (d) 5 min after washing, (e) 9 min after washing, (f) 12 min after washing. (B) Reflective `red' stripe iridophores of A. subulata at 45° incidence. Reflectivity is best in the green parts of the spectrum. Photos are taken at 1 min intervals. Reflections gradually shift from green (a) to red (f) before switching off completely (not shown). (C) Video images of A. subulata at an angle of approximately 45° to the `red' stripe. (a) Shortly after capture the `red' stripe was strongly reflective in the blue-green. (b) After approximately 15 min the reflectance changed to green. (c) After 30 min the `red' stripe reflected yellow-green light, with some red in the ventral parts of the stripe. (D) The `blue' stripe of A. subulata superfused with 50 µmol l-1 ACh. (a) Before superfusion the `blue' stripe is reflective. Following ACh superfusion it becomes gradually non-reflective. (b) After 15 s, (c) after 1 min, (d) after 1.5 min, (e) after 2 min, (f) after 3 min. (E) The iridophores of the ventral side during ACh superfusion (50 µmol l-1). (a) Before superfusion the iridophores are weakly red reflective. In response to ACh, reflections shift gradually towards the orange and yellow parts of the spectrum. (b) After 1 min of perfusion, (c) after 1.5 min, (d) after 2 min.

View larger version (91K): [in a new window]   Fig. 3. (A,B) Electron micrographs of enzymatically isolated iridophore cells. Scale bars, 5 µm (A); 10 µm (B). (C) An isolated iridophore cell filled with Lucifer Yellow (scale bars, 50 µm). Inset shows the iridophore cell before being filled with Lucifer Yellow.

View larger version (28K): [in a new window]   Fig. 4. Fluorescence ratio (F340/F380) measurements of cytoplasmic Ca2+ concentration of an isolated iridophore cell (concentrations of drugs given below; see text for details). Cytoplasmic [Ca2+] increases transiently in response to (A) potassium chloride (KCl), (B) caffeine and (C) ACh, but not nicotine. (D) Muscarine evokes a transient increase in cytoplasmic Ca2+, which is blocked by atropine. After washing for 15 min, the muscarine response is regained. (E) Ca2+ increases transiently in response to ACh and caffeine in the absence of Ca2+ in the external solution (EGTA Ca2+ free ASW). (F) Changing the osmolarity of the external solution has no effect on cytoplasmic Ca2+ concentration, although ACh evokes a strong increase in cytoplasmic [Ca2+]. s-ASW, ASW + 50 mmol l-1 sorbitol (1112 mOsm kg-1); 75% ASW, 769 mOsm kg-1. Concentrations of drugs: KCl, 20 mmol l-1; caffeine, 5 mmol l-1; Ach, 50 µmol l-1; nicotine, 2 mmol l-1; muscarine, 50 µmol l-1; atropine, 50 µmol l-1.