The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

https://www.physiology.org/doi/pdf/10.1152/physrev.1999.79.3.763

Sodium/Calcium Exchange: Its Physiological Implications

The cloning of a G protein-coupled extracellular Ca(2+) (Ca(o)(2+))-sensing receptor (CaR) has elucidated the molecular basis for many of the previously recognized effects of Ca(o)(2+) on tissues that maintain systemic Ca(o)(2+) homeostasis, especially parathyroid chief cells and several cells in the kidney. The availability of the cloned CaR enabled the development of DNA and antibody probes for identifying the CaR's mRNA and protein, respectively, within these and other tissues. It also permitted the identification of human diseases resulting from inactivating or activating mutations of the CaR gene and the subsequent generation of mice with targeted disruption of the CaR gene. The characteristic alterations in parathyroid and renal function in these patients and in the mice with "knockout" of the CaR gene have provided valuable information on the CaR's physiological roles in these tissues participating in mineral ion homeostasis. Nevertheless, relatively little is known about how the CaR regulates other tissues involved in systemic Ca(o)(2+) homeostasis, particularly bone and intestine. Moreover, there is evidence that additional Ca(o)(2+) sensors may exist in bone cells that mediate some or even all of the known effects of Ca(o)(2+) on these cells. Even more remains to be learned about the CaR's function in the rapidly growing list of cells that express it but are uninvolved in systemic Ca(o)(2+) metabolism. Available data suggest that the receptor serves numerous roles outside of systemic mineral ion homeostasis, ranging from the regulation of hormonal secretion and the activities of various ion channels to the longer term control of gene expression, programmed cell death (apoptosis), and cellular proliferation. In some cases, the CaR on these "nonhomeostatic" cells responds to local changes in Ca(o)(2+) taking place within compartments of the extracellular fluid (ECF) that communicate with the outside environment (e.g., the gastrointestinal tract). In others, localized changes in Ca(o)(2+) within the ECF can originate from several mechanisms, including fluxes of calcium ions into or out of cellular or extracellular stores or across epithelium that absorb or secrete Ca(2+). In any event, the CaR and other receptors/sensors for Ca(o)(2+) and probably for other extracellular ions represent versatile regulators of numerous cellular functions and may serve as important therapeutic targets.

Extracellular Calcium Sensing and Extracellular Calcium Signaling

Proteinase-activated receptors

Geigy Scientific Tables

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ/cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

/pdf/cellular-distribution-and-functions-of-p2-receptor-subtypes-32555c8g2p.pdf

Cellular distribution and functions of P2 receptor subtypes in different systems.

Ion analyses of human cataractous lenses.

Purpose To study the role of TGF-beta2 in posterior capsule opacification (PCO) and to determine whether CAT-152 (lerdelimumab), a fully human monoclonal antibody that neutralizes the effect of TGF-beta2, can also provide therapeutic benefit for PCO. Methods In vitro capsular bags were prepared from human donor eyes and maintained in a 5% CO(2) atmosphere at 35 degrees C. To investigate expression of active TGF-beta2, capsular bags were incubated in serum-free EMEM for 2, 28, or more than 100 days and analyzed by ELISA (n > or = 4 at each time point). To study underlying mechanisms, match-pair experiments were also performed, so that the medium was supplemented with 0, 1 or 10 ng/mL TGF-beta2 with or without 10 microg/mL CAT-152 (n = 4 in all cases). On-going observations were by phase-contrast microscopy. In addition, donor material from patients who had undergone cataract surgery was analyzed. Cellular architecture was examined by fluorescence cytochemistry. Expression of matrix metalloproteinase (MMP)-2 and -9 was assessed by gelatin zymography. Results Analysis of capsular bags from donor eyes that had received an intraocular lens (IOL) revealed the presence of endogenous active TGF-beta2, matrix wrinkling, and expression of transdifferentiation markers alphaSMA and fibronectin. When cultured in vitro, donor bags also showed sustained release of MMP-2 and -9. Culture of capsular bags prepared in vitro from whole lenses showed that TGF-beta2 (1-10 ng/mL) stimulated transdifferentiation and contraction of the capsular bag, resulting in light scatter. TGF-beta2 also induced sustained release of MMP-2 and -9. Active TGF-beta2 was detected in these cultures. The human monoclonal anti-TGF-beta2 antibody CAT-152 (10 microg/mL) effectively inhibited all TGF-beta2-induced effects. Conclusions Addition of TGF-beta2 accelerates transdifferentiation and contraction of the capsular bag, resulting in light scatter. CAT-152 inhibited all the effects of TGF-beta2 that were examined and therefore has the potential to suppress development of PCO and provide potential therapeutic benefit to cataract patients.

TGF-beta2-induced matrix modification and cell transdifferentiation in the human lens capsular bag.

Purpose. After intraocular lens (IOL) implant surger for cataracty , cell growth on the posteriorcapsule is responsible for renewed visual impairmen in approximatelt y 30% of patients. Theauthors have, therefore, develope a humad n lens capsule system to study this growth in vitro.Methods. Sham cataract surgery, including anterior capsulorhexis, nucleus hydroexpression,and aspiratio onf lens fibers, was performed on donor eyes I.n some cases, a polymethylmeth-acrylate IOL implant was place idn th capsulae r bag Th. e capsular bag was dissected free,pinned flat o an plastic culture dish, covered with Eagle's minimum essential medium supple-mented with 10% fetal calf seru anmd observed by phase-contrast and dark-field microscopyfor as long as 100 days. At the end-point, capsules were examined by fluorescence microscopyfor actin, vimentin. , and chromatin.Results. Withi 2n4 hours, there was evidenc oef cell growth in the equatorial region. Afte 2rto 3 days, cells were normally observed growin thg froe rhexims onto the posterior capsuleand acros ths e anterior surface o f the IOL i,f present. Growth proceeded rapidl so thayt theposterior capsule fo,r example, was totally covere byd a confluent monolaye of cellr s at 5.8± 0.6 days and 7.2 ± 0.7 days for capsules aged 60 years, respectively. Totalcover of the anterior IOL surface generally followe 4 to 5 dayds behind tha otf the capsule.Capsular wrinkles became increasingly apparent as time progressed, causin a markegd rise inlight scatter A. n increase in capsular tension also occurred and th, e actin filaments becamemore polarized nea thre wrinkles.Conclusions. The model presented her foer posterior capsule opacification shows o manf theychanges seen in vivo, including rapid lens cell growth, wrinkling, tensioning, and light scatterin the posterior capsule It wil. l b e possibl teo develop strategie fors inhibitin g cell growthwith this system. Invest Ophthalmol Vi 1996s Sci 37:906-914;. .An modern extracapsular cataract surgery, to restorethe best possible vision, a small artificial lens is im-planted into the remaining lens capsular bag fromwhich most of the fiber mass has been removed. Thisapproach is not without subsequent complications,however, and within 2 years as many as 35% of patientsexperience a significant loss of visual acuity.

A study of human lens cell growth in vitro : A model for posterior capsule opacification

It is today considered as crystal clear that calcium plays an essential role in the regulation of insulin release by the pancreatic B-cell. Some of the major issues concerning such a role are as follows: ( i ) what is the detailed mechanism by which secretagogues are susceptible to influence the handling of calcium in the B-cell; (ii) what is the nature and location of the critical pool of calcium that controls insulin release; (iii) what is the relative and respective contribution of calcium influx, efflux, and subcellar distribution in the regulation of such a pool; and (iv) how does calcium influence the process by which secretory granules migrate to the cell boundary and are extruded via exocytosis in the interstitial fluid.’ In the present report, for the sake of clarity, we will restrict the discussion of these questions to the process of glucose-induced insulin release, with the main emphasis on the possible significance of passive ionophoretic movements. The process by which glucose provokes insulin release can be viewed as a sequence of three major events, namely: ( i ) the recognition or identification of glucose by the B-cell; (ii) the subsequent remodelling of cationic fluxes; and (iii) the activation by calcium of an effector system controlling the migration and exocytosis of secretory granules.2 The role of calcium in insulin release is here considered within the framework of such a sequential view.

Regulation of calcium fluxes and their regulatory roles in pancreatic islets

PURPOSE. Cortical cataract in humans is associated with Ca2+ overload and protein loss, and although animal models of cataract have implicated Ca2+-activated proteases in this process, it remains to be determined whether the human lens responds in this manner to conditions of Ca2+ overload. The purpose of these experiments was to investigate Ca2+-induced opacification and proteolysis in the organ-cultured human lens. METHODS. Donor human lenses were cultured in Eagle's minimum essential medium (EMEM) for up to 14 days. The Ca2+ ionophore ionomycin was used to induce a Ca2+ overload. Lenses were loaded with [H-3]-amino acids for 48 hours. After a 24-hour control efflux period, lenses were cultured in control EMEM (Ca2+ 1.8 mM), EMEM + 5 mu M ionomycin, or EMEM + 5 mu M ionomycin + 5 mM EGTA (Ca2+

George Duncan

Papers

Ion analyses of human cataractous lenses.

TGF-beta2-induced matrix modification and cell transdifferentiation in the human lens capsular bag.

A study of human lens cell growth in vitro : A model for posterior capsule opacification

Regulation of calcium fluxes and their regulatory roles in pancreatic islets

A human lens model of cortical cataract: Ca2+-induced protein loss, vimentin cleavage and opacification