Energized Ca²⁺ Transport by Hepatopancreatic Basolateral Plasma Membranes of Homarus Americanus Available to Purchase

Ca²⁺ transport by hepatopancreatic basolateral mem-brane 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⁺ (K_t=65.28±14.39 nmol l⁻¹; J_max=1.07±0.06 pmol µg⁻¹ pro-tein 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⁺, K_t=14.57±5.02 µmol l⁻¹; with Li⁺, K_t=20.17±6.99 µmol l⁻¹), but the maximal Ca²⁺ transport velocity was reduced by a factor of three (with Na⁺, J_max=2.72±0.23 pmol µg⁻¹ pro-tein 8 s⁻¹; with Li⁺, J_max=1.03±0.10 pmol µg⁻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.