Immunocytological localization of endogenous calcitonin-like molecules in the crustacean Orchestia

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 Ca(2+) movement to achieve organismal Ca homeostasis. Specifically, periods of net Ca(2+) influx (postmolt) have been compared with periods of net Ca(2+) balance (intermolt). The broader goal is to understand the paradox facing epithelial cells of maintaining low cytosolic Ca(2+)in the face of mass Ca(2+)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 Ca(2+) flux in a selected epithelium, the antennal gland (kidney analog) specifically during apical to basolateral Ca(2+) conveyance. Target proteins were selected as representative of key "stages" in the transcellular transfer of Ca(2+): import (epithelial Ca(2+) channel, ECaC); storage (sarco/endoplasmic reticulum Ca(2+) ATPase, SERCA); buffering (sarcoplasmic Ca(2+) binding protein, SCP); and export (plasma membrane Ca(2+) ATPase, PMCA and Na(+)/Ca(2+) exchanger, NCX). The purpose of this review is to assess coordination of expression of these target proteins at times of high Ca(2+) 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 Ca(2+) handling proteins involved in transcellular transport is fundamental to investigating their endocrine regulation.

Calcium balance in crustaceans: nutritional aspects of physiological regulation

Calcium homeostasis in crustaceans is influenced by their natural molting cycle that periodically requires replacement of the calcified exoskeleton in order for growth to occur. Whole body Ca balance transitions from intermolt (zero net flux) to premolt (net efflux) and postmolt (net influx at the rate of 2 mmol kg(-1)h(-1)). As such, molting provides a convenient model to study up- and down-regulation of epithelial Ca transporting proteins (such as Ca pumps and exchangers), the genes that encode them, and the steroid hormone (ecdysone) that putatively regulates the genes. Species residing in either freshwater or in terrestrial environments are more limited in their Ca availability than are marine species. Further the advance towards terrestriality is accompanied by decreased reliance upon branchial Ca uptake and increased reliance upon digestive uptake. This review will correlate Ca handling strategies with environment in semi-terrestrial and terrestrial crabs through examining environmental sources of Ca uptake. Ca homeostasis will also be discussed at the whole animal level, cellular, subcellular and molecular levels of regulation.

Calcitropic peptides: neural perspectives

In mammals and higher vertebrates, calcitropic peptides are produced by peripheral endocrine glands: the parathyroid gland (PTH), thyroid or ultimobranchial gland (calcitonin) and the anterior pituitary gland (growth hormone and prolactin). These hormones are, however, also found in the neural tissues of lower vertebrates and invertebrates that lack these endocrine organs, suggesting that neural tissue may be an ancestral site of calcitropic peptide synthesis. Indeed, the demonstration of CNS receptors for these calcitropic peptides and their induction of neurological actions suggest that these hormones arose as neuropeptides. Neural and neuroendocrine roles of some of these calcitropic hormones (calcitonin and parathyroid hormone) and related peptides (calcitonin gene related peptide, stanniocalcin and parathyroid hormone related peptide) are thus the focus of this review.