Real time measurements of water flow in amphibian gastric glands: modulation via the extracellular Ca2+-sensing receptor

Aquaporin-1 Facilitates Transmesothelial Water Permeability: In Vitro and Ex Vivo Evidence and Possible Implications in Peritoneal Dialysis

We previously showed that mesothelial cells in human peritoneum express the water channel aquaporin 1 (AQP1) at the plasma membrane, suggesting that, although in a non-physiological context, it may facilitate osmotic water exchange during peritoneal dialysis (PD). According to the three-pore model that predicts the transport of water during PD, the endothelium of peritoneal capillaries is the major limiting barrier to water transport across peritoneum, assuming the functional role of the mesothelium, as a semipermeable barrier, to be negligible. We hypothesized that an intact mesothelial layer is poorly permeable to water unless AQP1 is expressed at the plasma membrane. To demonstrate that, we characterized an immortalized cell line of human mesothelium (HMC) and measured the osmotically-driven transmesothelial water flux in the absence or in the presence of AQP1. The presence of tight junctions between HMC was investigated by immunofluorescence. Bioelectrical parameters of HMC monolayers were studied by Ussing Chambers and transepithelial water transport was investigated by an electrophysiological approach based on measurements of TEA⁺ dilution in the apical bathing solution, through TEA⁺-sensitive microelectrodes. HMCs express Zo-1 and occludin at the tight junctions and a transepithelial vectorial Na⁺ transport. Real-time transmesothelial water flux, in response to an increase of osmolarity in the apical solution, indicated that, in the presence of AQP1, the rate of TEA⁺ dilution was up to four-fold higher than in its absence. Of note, we confirmed our data in isolated mouse mesentery patches, where we measured an AQP1-dependent transmesothelial osmotic water transport. These results suggest that the mesothelium may represent an additional selective barrier regulating water transport in PD through functional expression of the water channel AQP1.

The Different Facets of Extracellular Calcium Sensors: Old and New Concepts in Calcium-Sensing Receptor Signalling and Pharmacology

The current interest of the scientific community for research in the field of calcium sensing in general and on the calcium-sensing Receptor (CaR) in particular is demonstrated by the still increasing number of papers published on this topic. The extracellular calcium-sensing receptor is the best-known G-protein-coupled receptor (GPCR) able to sense external Ca2+ changes. Widely recognized as a fundamental player in systemic Ca2+ homeostasis, the CaR is ubiquitously expressed in the human body where it activates multiple signalling pathways. In this review, old and new notions regarding the mechanisms by which extracellular Ca2+ microdomains are created and the tools available to measure them are analyzed. After a survey of the main signalling pathways triggered by the CaR, a special attention is reserved for the emerging concepts regarding CaR function in the heart, CaR trafficking and pharmacology. Finally, an overview on other Ca2+ sensors is provided.

Importance of Ca(2+) in gastric epithelial restitution-new views revealed by real-time in vivo measurements

It has been a few decades since Ca(2+) was identified as one of the important factors that can accelerate gastric wound repair as well as contribute to epithelial homeostasis and regulation of gastric secretions. The mechanistic basis has remained largely unexplored in vivo because it was not possible to track in real time either intracellular Ca(2+) mobilization or wound repair in living tissues. Recent advances in technology, such as combining high resolution light microscopy and genetically encoded Ca(2+) reporters in mice, now allow the monitoring of Ca(2+) mobilization during gastric epithelial cell restitution. Ca(2+) is a ubiquitous second messenger that influences numerous cellular processes, including gastric acid/bicarbonate secretion, mucus secretion, and cell migration. We have demonstrated that cytosolic Ca(2+) mobilization within the restituting gastric epithelial cells is a central signal driving small wound repair. However, extracellular Ca(2+) is also mobilized in the juxtamucosal luminal space above a wound, and evidence suggests extracellular Ca(2+) is a third messenger that also promotes gastric epithelial restitution. Interplay between intracellular and extracellular Ca(2+) is necessary for efficient gastric epithelial restitution.

Glucose increases extracellular [Ca2+] in rat insulinoma (INS-1E) pseudoislets as measured with Ca2+-sensitive microelectrodes

Secretory granules of pancreatic β-cells contain high concentrations of Ca2+ ions that are co-released with insulin in the extracellular milieu upon activation of exocytosis. As a consequence, an increase in the extracellular Ca2+ concentration ([Ca2+]ext) in the microenvironment immediately surrounding β-cells should be expected following the exocytotic event. Using Ca2+-selective microelectrodes we show here that both high glucose and non-nutrient insulinotropic agents elicit a reversible increase of [Ca2+]ext within rat insulinoma (INS-1E) β-cells pseudoislets. The glucose-induced increases in [Ca2+]ext are blocked by pretreatment with different Ca2+ channel blockers. Physiological agonists acting as positive or negative modulators of the insulin secretion and drugs known to intersect the secretory machinery at different levels also induce [Ca2+]ext changes as predicted on the basis of their described action on insulin secretion. Finally, the glucose-induced [Ca2+]ext increase is strongly inhibited after disruption of the actin web, indicating that the dynamic [Ca2+]ext changes recorded in INS-1E pseudoislets by Ca2+-selective microelectrodes occur mainly as a consequence of exocytosis of Ca2+-rich granules. In conclusion, our data directly demonstrate that the extracellular spaces surrounding β-cells constitute a restricted domain where Ca2+ is co-released during insulin exocytosis, creating the basis for an autocrine/paracrine cell-to-cell communication system via extracellular Ca2+ sensors.

The source of carbon dioxide for gastric acid production

The source of carbon dioxide for the chemical reaction leading to the production of gastric acid is unknown. The decarboxylation of an amino acid releases carbon dioxide. Pepsinogens provide a rich source of the amino acid arginine. Both the source of carbon dioxide, arginine, and the consequence of arginine decarboxylation, agmatine, have been studied. The site of carbon dioxide production has been related to the survival of the parietal cell. An immunohistochemical study has been carried out on glycol methacrylate embedded gastric biopsies from the normal stomach of 38 adult patients. The sections have been stained using polyclonal antibody to pepsinogen II, polyclonal antibody to agmatine, and polyclonal antibody to Helicobacter pylori. Pepsinogen II and agmatine are found in the parietal cell canaliculi. This is consistent with the production of carbon dioxide from arginine in the parietal cell canaliculi. Evidence is presented for the decarboxylation of arginine derived from the activation segment of pepsinogen as the source of carbon dioxide for the production of gastric acid. The production of carbon dioxide by the decarboxylation of arginine in the parietal cell canaliculus enables the extracellular hydration of carbon dioxide at the known site of carbonic anhydrase activity. The extracellular production of acid in the canaliculus together with the presence of agmatine helps to explain why the parietal cells are not destroyed during the formation of gastric acid. Agmatine is found in the mucus secreting cells of the stomach and its role in acid protection of the stomach is discussed. Anat Rec, 2009. (c) 2008 Wiley-Liss, Inc.

Extracellular calcium-sensing receptor distribution in osmoregulatory and endocrine tissues of the tilapia

The extracellular calcium-sensing receptor (CaSR) serves an important detector function in vertebrate Ca(2+) homeostasis. In this study, we surveyed using immunohistochemistry the tissue and cellular distribution of the CaSR protein in the Mozambique tilapia (Oreochromis mossambicus) and the Japanese eel (Anguilla japonica). Specifically, we examined receptor expression in ion-transporting barrier tissues that may be directly responsive to extracellular Ca(2+) levels, and in tissues that are implicated in endocrine signaling to homeostatic effectors such as Ca(2+)-transporting epithelia. In tilapia osmoregulatory tissues, CaSR protein is strongly expressed in proximal segments of renal tubule, but not in distal segments (where Na(+),K(+)-ATPase is prominently expressed) or in glomeruli. The receptor was also localized in the ion-transporting mitochondria-rich cells of gill and in ion- and nutrient-transporting epithelia of middle and posterior intestine. Consistent with our earlier RT-PCR assessment of mRNA expression in tilapia, CaSR protein expression was salinity dependent in some osmoregulatory tissues. In tilapia pituitary gland, CaSR expression was observed in the rostral pars distalis (containing prolactin-secreting cells, and in the pars intermedia (containing somatolactin-secreting and melanocyte-stimulating hormone-secreting cells), with notably greater expression in the latter. In the eel, weak immunostaining was seen in the stanniocalcin-secreting cells of the corpuscles of Stannius. Olfactory lobe CaSR expression suggests an environment-sensing role for the receptor. Altogether, these findings support the involvement of CaSR in piscine Ca(2+) homeostasis at the levels of environmental sensing, of integrative endocrine signaling through both hypercalcemic (prolactin, and perhaps somatolactin) and hypocalcemic (stanniocalcin) hormones, and of direct local regulation of Ca(2+)-transporting tissues.

Extracellular calcium and cAMP: second messengers as "third messengers"?

Calcium and cyclic AMP are familiar second messengers that typically become elevated inside cells on activation of cell surface receptors. This article will explore emerging evidence that transport of these signaling molecules across the plasma membrane allows them to be recycled as "third messengers," extending their ability to convey information in a domain outside the cell.