The calcium dependence of pigment translocation in freshwater shrimp red ovarian chromatophores
McNamara, J.C.; Ribeiro, M.R. (2000). The calcium dependence of pigment translocation in freshwater shrimp red ovarian chromatophores. Biol. Bull. 198: 357-366
In: The Biological Bulletin. Marine Biological Laboratory: Lancaster. ISSN 0006-3185; e-ISSN 1939-8697, more
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| Authors | | Top |
- McNamara, J.C.
- Ribeiro, M.R.
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| Abstract |
The roles of calcium in cell signaling consequent to chromatophorotropin action and as an activator of mechanochemical transport proteins responsible for pigment granule translocation were investigated in the red ovarian chromatosomes of the freshwater shrimp Macrobrachium olfersii. Chromatosomes were perfused with known concentrations of free Ca 11 (10 23 to 10 29 M) prepared in Mg 11 -EGTA-buffered physiological saline after selectively permeabilizing with 25 mM calcium ionophore A23187 or with 10 28 M red pigment concentrating hormone (RPCH). The degree of pigment aggregation and the translocation velocity of the leading edges of the pigment mass were recorded in individual chromatosomes during aggregation induced by RPCH or A23187 and dispersion induced by low Ca 11 . Aggregation is Ca 11 dependent, showing a dual extracellular and intracellular requirement. After perfusion with reduced Ca 11 (10 24 to 10 29 M), RPCH triggers partial aggregation ('65%), although the maximum translocation velocities ('16.5 mm/min) and velocity profiles are unaffected. After aggregation induced at or below 10 25 M Ca 11 , spontaneous pigment dispersion ensues, suggesting a Ca 11 requirement for RPCH coupling to its receptor, or a concentration-dependent, Ca 11 -induced Ca 11 -release mechanism. The Ca 11 -channel blockers Mn 11 (5 mM) and verapamil (50 mM) have no effect on RPCH-triggered aggregation. An intracellular Ca 11 requirement for aggregation was demonstrated in chromatosomes in which the Ca 11 gradient across the cell membrane was dissipated with A23187. At free [Ca 11 ] above 10 23 M, aggregation is complete; at 10 24 M, aggregation is partial, followed by spontaneous dispersion; below 10 25 M Ca 11 , pigments do not aggregate but disperse slightly. Aggregation velocities diminish from 11.6 6 1.2 mm/min at 5.5 mM Ca 11 to 7.4 6 1.3 mm/min at 10 24 M Ca 11 . Half-maximum aggregation occurs at 3.2 3 10 25 M Ca 11 and half-maximum translocation velocity at 4.8 3 10 25 M Ca 11 . Pigment redispersion after 5.5 mM Ca 11 -A23187- induced aggregation is initiated by reducing extracellular Ca 11 : slight dispersion begins at 10 27 M, complete dispersion being attained at 10 29 M Ca 11 . Dispersion velocities increase from 0.6 6 0.2 to 3.1 6 0.5 mm/min. Half-maximum dispersion occurs at 7.6 3 10 29 M Ca 11 and half-maximum trans-location velocity at 2.9 3 10 29 M Ca 11 . These data reveal an extracellular and an intracellular Ca 11 requirement for RPCH action, and demonstrate that the centripetal or centrifugal direction of pigment movement, the translocation velocity, and the degree of pigment aggregation or dispersion attained are calcium-dependent properties of the granule translocation apparatus. |
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