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Am J Physiol Cell Physiol 276: C558-C565, 1999;
0363-6143/99 $5.00
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Vol. 276, Issue 3, C558-C565, March 1999

Direct evidence of Na+/Ca2+ exchange in squid rhabdomeric membranes

Paul J. Bauer1, Heike Schauf1, Andreas Schwarzer1, and Joel E. Brown2

1 Institut für Biologische Informationsverarbeitung, Forschungszentrum Jülich, D-52425 Jülich, Germany; and 2 Department of Ophthalmology and Visual Sciences, Albert Einstein College of Medicine, Bronx, New York 10461

Na+/Ca2+ exchange has been investigated in squid (Loligo pealei) rhabdomeric membranes. Ca2+-containing vesicles have been prepared from purified rhabdomeric membranes by extrusion through polycarbonate filters of 1-µm pore size. After removal of external Ca2+, up to 90% of the entrapped Ca2+ could be specifically released by the addition of Na+; this finding indicates that most of the vesicles contained Na+/Ca2+ exchanger. The Na+-induced Ca2+ efflux had a half-maximum value (K1/2) of ~44 mM and a Hill coefficient of ~1.7. The maximal Na+-induced Ca2+ efflux was ~0.6 nmol Ca2+ · s-1 · mg protein-1. Similar Na+-induced Ca2+ effluxes were measured if K+ was replaced with Li+ or Cs+. Vesicles loaded with Ca2+ by Na+/Ca2+ exchange also released this Ca2+ by Na+/Ca2+ exchange, suggesting that Na+/Ca2+ exchange operated in both forward and reverse modes. Limited proteolysis by trypsin resulted in a rate of Ca2+ efflux enhanced by approximately fivefold when efflux was activated with 95 mM NaCl. For vesicles subjected to limited proteolysis by trypsin, Na+/Ca2+ exchange was characterized by a K1/2 of ~25 mM and a Hill coefficient of 1.6. For these vesicles, the maximal Na+-induced Ca2+ efflux was about twice as great as in control vesicles. We conclude that Na+/Ca2+ exchange proteins localized in rhabdomeric membranes mediate Ca2+ extrusion in squid photoreceptors.

photoreceptor; invertebrate; calcium extrusion; calcium homeostasis


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