Energized Ca2+ transport by hepatopancreatic basolateral plasma membranes of Homarus americanus.
Ca2+ transport by hepatopancreatic basolateral membrane vesicles of Atlantic lobster (Homarus americanus) occurred by at least two independent processes: (1) an ATP-dependent carrier transport system, and (2) a Na+-gradient-dependent carrier mechanism. The sensitivity of ATP-dependent Ca2+ transport to vanadate indicated that it was probably due to a P-type ATPase. This system exhibited an extremely high apparent affinity for Ca2+ (Kt=65.28+/-14.39 nmol l-1; Jmax=1. 07+/-0.06 pmol microg-1 protein 8 s-1). The Na+-gradient-dependent carrier transport system exhibited the properties of a Ca2+/Na+ antiporter capable of exchanging external Ca2+ with intravesicular Na+ or Li+. Kinetic analysis of the Na+-dependence of the antiport indicated that at least three Na+ were exchanged with each Ca2+ (n=2. 91+/-0.22). When Li+ replaced Na+ in exchange for 45Ca2+, the apparent affinity for Ca2+ influx was not significantly affected (with Na+, Kt=14.57+/-5.02 micromol l-1; with Li+, Kt=20.17+/-6.99 micromol l-1), but the maximal Ca2+ transport velocity was reduced by a factor of three (with Na+, Jmax=2.72+/-0.23 pmol microg-1 protein 8 s-1; with Li+, Jmax=1.03+/-0.10 pmol microg-1 protein 8 s-1). It is concluded that Ca2+ leaves hepatopancreatic epithelial cells across the basolateral membrane by way of a high-affinity, vanadate-sensitive Ca2+-ATPase and by way of a low-affinity Ca2+/Na+ antiporter with an apparent 3:1 exchange stoichiometry. The roles of these transporters in Ca2+ balance during the molt cycle are discussed.
Zhuang
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Department of Zoology, University of Hawaii at Manoa, Honolulu, HI 96822, USA.
Ahearn
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GA
D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002462:Cell Membrane / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D004064:Digestive System / Q000166:cytology / Q000201:enzymology / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D008121:Nephropidae / Q000378:metabolism*; D009994:Osmolar Concentration; D019831:Sodium-Calcium Exchanger / Q000378:metabolism
D002135:Calcium-Binding Proteins; D019831:Sodium-Calcium Exchanger; D000001:Calcimycin; D004533:Egtazic Acid; D000252:Calcium-Transporting ATPases; D002118:Calcium