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.