Cellular Mechanisms of Calcium Transport in Crustaceans

Abstract

Exchanges of calcium and other ions of biological significance in crustaceans take place between the hemolymph and environment across epithelial cell layers of the integument, gills, gut, and antennal glands (kidneys). During most of the life cycle these exchanges are modest and are largely osmoregulatory, but during periodic molting large movements of ions, particularly calcium, occur between the organism and its environment; these movements must be closely regulated in order for the animal to grow and strengthen its new exoskeleton. This article describes, for the first time, the brush border and basolateral epithelial calcium transport processes located in the hepatopancreas of the American lobster, Homarus americanus, and discusses their potential roles in calcium balance during the molt cycle. Three independent brush border membrane transport proteins control the uptake of calcium from the gastrointestinal lumen: (1) an amiloride-sensitive, electrogenic cation antiporter exchanging 1 Ca²⁺ (or 2 Na⁺) for a single intracellular monovalent cation (H⁺ or Na⁺), (2) an amiloride-insensitive, probably electroneutral, cation antiporter exchanging 1 Ca²⁺ for 2 Na⁺ (or 2 H⁺), and (3) a verapamil- and amiloride-inhibited Ca²⁺ channel. Likewise, the epithelial basolateral membrane of this digestive organ possesses three calcium transfer processes for cation exchange between cytoplasm and blood: (1) a Ca²⁺/Na⁺ antiporter of unknown stoichiometry but of relatively low apparent Ca²⁺ binding affinity, (2) a high-afinity Ca²⁺-ATPase for cellular efflux, and (3) a verapamil-inhibited Ca²⁺ channel for possible calcium uptake from the blood during premolt. The kinetic properties of these six hepatopancreatic transporters and their potential general distribution to other ion regulatory epithelia in crustaceans are discussed in this article.