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.