Epithelial calcium transport in crustaceans: Adaptation to intrinsic and extrinsic stressors

Since the classical studies of Ussing employing a nonmammalian isolated epithelium (frog skin) to explore the basic principles of ion transport, physiologists have adopted increasingly reductionist approaches to dissect the biophysical mechanisms undergirding biological transport. In vitro characterization has employed isolated perfused organs, isolated epithelia, and reconstituted vesicle studies. Depth of resolution has been further enhanced by the emerging molecular revolution. Following years of deconstruction, physiologists are now engaging in reconstruction, namely putting the genes back into the organism. This contribution attempts such an integrative approach for a single electrolyte, calcium (Ca²⁺), in a nonmammalian epithelium, the crayfish antennal gland (kidney). Two collaborating laboratories have archived an inventory of Ca²⁺ associated proteins believed to play a role in transcellular Ca²⁺ movement. Using the basic building blocks (expression profiles of key Ca²⁺ associated proteins and their regulators), the authors attempt to reconstruct a whole cell model for Ca²⁺ regulation in transporting epithelium (compared with a nonepithelial tissue) under stressors that perturb Ca²⁺ homeostasis which originate either intrinsically (the postmolt stage of the molting cycle) or extrinsically (unanticipated cold acclimation). Through horizontal integration of expression profiles of seven target Ca²⁺ associated proteins in epithelial and nonepithelial tissue under two contrasting experimental conditions, emergent themes inform the physiological complexity of Ca²⁺ homeostasis. Integration at the next level will require placing the epithelium in the context of organismic Ca²⁺ balance. The unique Ca²⁺ handling capabilities of the freshwater crayfish make it an excellent nonmammalian model for those studies.