Paradox of epithelial cell calcium homeostasis during vectorial transfer in crayfish kidney

The molting cycle of the freshwater crayfish, Procambarus clarkii, has been used as a model to study the cellular physiology and molecular biology of Ca "supply" proteins that effect transcellular vectorial Ca²⁺ movement to achieve organismal Ca homeostasis. Specifically, periods of net Ca²⁺ influx (postmolt) have been compared with periods of net Ca²⁺ balance (intermolt). The broader goal is to understand the paradox facing epithelial cells of maintaining low cytosolic Ca²⁺ in the face of mass Ca²⁺ transit across epithelial cells. This mini-review compares mRNA and protein expression profiles for a series of proteins that are of strategic importance in effecting transcellular Ca²⁺ flux in a selected epithelium, the antennal gland (kidney analog) specifically during apical to basolateral Ca²⁺ conveyance. Target proteins were selected as representative of key "stages" in the transcellular transfer of Ca²⁺: import (epithelial Ca²⁺ channel, ECaC); storage (sarco/endoplasmic reticulum Ca²⁺ ATPase, SERCA); buffering (sarcoplasmic Ca²⁺ binding protein, SCP); and export (plasma membrane Ca²⁺ ATPase, PMCA and Na⁺/Ca²⁺ exchanger, NCX). The purpose of this review is to assess coordination of expression of these target proteins at times of high Ca²⁺ demand (premolt and postmolt) compared to low Ca demand (intermolt) as a function of cellular location (apical vs. basolateral; endomembranes vs. plasma membranes) and relative abundance within different regions of the antennal gland. Understanding the spatiotemporal regulation of Ca²⁺ handling proteins involved in transcellular transport is fundamental to investigating their endocrine regulation.