Evidence that Ca2+ cycling by the plasma membrane Ca2+-ATPase increases the `excitability' of the extracellular Ca2+-sensing receptor

Abstract

IntroductionThe extracellular Ca 2+ -sensing receptor (CaR) is a cell surface receptor for divalent cations whose existence was originally suspected in the parathyroid gland (Brown et al., 1987;Nemeth and Scarpa, 1987;Shoback et al., 1988). Cloning of the receptor by Brown and colleagues (Brown et al., 1993) confirmed the essential role of CaR as a Ca 2+ sensor in the parathyroid, and also set the stage for the identification of the receptor in a wide variety of tissues not involved in systemic Ca 2+ homeostasis . This seventransmembrane-spanning receptor couples, via heterotrimeric G-proteins, to several different signaling cascades, including the phospholipase C (PLC)/phosphoinositide/Ca 2+ pathway (Brown and MacLeod, 2001). The binding of Ca 2+ to the human parathyroid receptor is known to be a highly cooperative process, making CaR a good Ca 2+ 'increment detector'. Although half-maximal activation of the receptor occurs around 3.5 mM, with maximal stimulation at 5.5 mM (Conigrave et al., 2000a;Quinn et al., 1997), CaR is able to sense small [Ca 2+ ] changes (±0.2 mM) in the physiological range (around 1.8 mM Ca 2+ ).It is important to note that CaR is also stimulated by Mg 2+ and certain polycations [including some endogenous polyamines such as spermine and spermidine (Quinn et al., 1997)]. Synthetic small molecule agonists of the receptor, such as the allosteric modulator NPS R-467 (Nemeth et al., 1998), have also been prepared. Brown and colleagues recently showed that the CaR is activated by physiological concentrations of amino acids [particularly aromatic and small aliphatic L-amino acids (Conigrave et al., 2000b)], and that it is modulated by ionic strength . It is therefore likely that multiple agonists act in concert to stimulate the receptor physiologically.We noted previously that stimulation of CaR with maximal doses of CaR agonists was frequently manifested by an oscillatory Ca 2+ signal in HEK293 cells transfected with the human extracellular Ca 2+ -sensing receptor (HEK CaR cells) (Hofer et al., 2000). The same observation was made in CaRtransfected HEK cells by Breitwieser and Gama (Breitwieser and Gama, 2001), who also reported that Ca 2+ spiking was acutely sensitive to small incremental increases in extracellular [Ca 2+ ]. Such oscillations were sustained for up to 40 minutes, until they gradually diminished in amplitude. Young and Rozengurt recently extended these findings, showing that aromatic amino acids can sensitize the receptor to small incremental increases in external [Ca 2+ ] in HEK CaR cells, producing distinctive patterns of oscillations (Young and Rozengurt, 2002). Native tissues that express CaR endogenously, including parathyroid cells (Miki et al., 1995)

Cell signaling, beyond cytosolic calcium in eukaryotes

“…In clusters of cultured HEK-293 cells, various extracellular stimuli give rise to oscillations in [Ca 2+ ] cyt that correlate with oscillations in [Ca 2+ ] ext ; it is possible that CaSR is involved in generating and in decoding calcium oscillations [36].…”

Another dimension to calcium signaling: a look at extracellular calcium

“…In co-cultures of HEK CaR-bearing 'sensor' cells and other 'donor' cells (e.g. fibroblasts) grown under an artificial polypropylene mesh to minimize the volume of extracellular fluid, stimulation of donor cells with a Ca 2+ -mobilizing agonist that does not bind to the sensor cells activates adjacent HEK CaR-bearing sensor cells (Hofer et al, 2000;Thomas, 2000 (Belan et al, 1998;De Luisi and Hofer, 2003), and this might repetitively stimulate CaR on the same or a neighboring cell (Gracheva and Gunton, 2003). Since intracellular [Ca 2+ ] oscillates vigorously in HEK CaR cells during CaR activation (Breitwieser and Gama, 2001;De Luisi and Hofer, 2003;Young and Rozengurt, 2002) (Fig.…”

Pharmacological Modulation of Sarcoplasmic Reticulum Function in Smooth Muscle

“…However, it produces an endothelium-dependent relaxation in intact rings of rat aorta (0.34 mM) (Chaudhary et al, 2001). Caloxin does not inhibit Mg 2ϩ -ATPase or Na ϩ /K ϩ -ATPase (Holmes et al, 2003) and has been shown to also inhibit PMCA activity in human mesenchymal stem cells (Kawano et al, 2003) and human embryonic kidney (HEK) cells (De Luisi and Hofer, 2003 ATPases, which occur predominantly in the Golgi compartment of eukaryotic cells (reviewed by Wuytack et al, , 2003. The rise of intracellular Ca 2ϩ is vital to cell function, but Ca 2ϩ availability must occur rapidly and in sufficient concentration at required intracellular targets.…”