Extracellular calcium acts as a "third messenger" to regulate enzyme and alkaline secretion (original) (raw)
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The EMBO Journal, 2001
We recently proposed that extracellular Ca 2+ ions participate in a novel form of intercellular communication involving the extracellular Ca 2+ -sensing receptor (CaR). Here, using Ca 2+ -selective microelectrodes, we directly measured the pro®le of agonist-induced [Ca 2+ ] ext changes in restricted domains near the basolateral or luminal membranes of polarized gastric acid-secreting cells. The Ca 2+ -mobilizing agonist carbachol elicited a transient, La 3+ -sensitive decrease in basolateral [Ca 2+ ] (average »250 mM, but as large as 530 mM). Conversely, carbachol evoked an HgCl 2sensitive increase in [Ca 2+ ] (average »400 mM, but as large as 520 mM) in the lumen of single gastric glands. Both responses were signi®cantly reduced by pretreatment with sarco-endoplasmic reticulum Ca 2+ ATPase (SERCA) pump inhibitors or with the intracellular Ca 2+ chelator BAPTA-AM. Immuno¯uorescence experiments demonstrated an asymmetric localization of plasma membrane Ca 2+ ATPase (PMCA), which appeared to be partially co-localized with CaR and the gastric H + /K + -ATPase in the apical membrane of the acid-secreting cells. Our data indicate that agonist stimulation results in local¯uctuations in [Ca 2+ ] ext that would be suf®cient to modulate the activity of the CaR on neighboring cells.
The Journal of Physiology, 2006
2+ ] i ) is a key factor controlling secretion from various cell types. We investigated how different patterns of [Ca 2+ ] i signals evoke salt secretion via ion transport mechanisms and mucin secretion via exocytosis in dog pancreatic duct epithelial cells (PDEC). Activation of epithelial P2Y 2 receptors by UTP generated two patterns of [Ca 2+ ] i change: 2-10 μM UTP induced [Ca 2+ ] i oscillations, whereas 100 μM UTP induced a sustained [Ca 2+ ] i increase, both in the micromolar range. As monitored by carbon-fibre amperometry, the sustained [Ca 2+ ] i increase stimulated a larger increase in exocytosis than [Ca 2+ ] i oscillations, despite their similar amplitude. In contrast, patch-clamp recordings revealed that [Ca 2+ ] i oscillations synchronously activated a K + current as efficiently as the sustained [Ca 2+ ] i increase. This K + current was mediated by intermediate-conductance Ca 2+ -activated K + channels (32 pS at −100 mV) which were sensitive to charybdotoxin and resistant to TEA. Activation of these Ca 2+ -dependent K + channels hyperpolarized the plasma membrane from a resting potential of −40 mV to −90 mV, as monitored in perforated whole-cell configuration, in turn enhancing Na + -independent, Cl − -dependent and DIDS-sensitive HCO 3 − secretion, as monitored through changes in intracellular pH. PDEC therefore encode concentrations of purinergic agonists as different patterns of [Ca 2+ ] i changes, which differentially stimulate K + channels, the Cl − -HCO 3 − exchanger, and exocytosis. Thus, in addition to amplitude, the temporal pattern of [Ca 2+ ] i increases is an important mechanism for transducing extracellular stimuli into different physiological effects.
Intracellular Ca2+ signalling in secretory cells
The Journal of Experimental Biology, 1997
The secretion of ions and fluid plays a critical role in a variety of physiological activities that are vital to homeostatic mechanisms in animals. Control of such secretory activity is achieved by a range of neurotransmitters and hormones many of which act intracellularly by generating the second messenger inositol 1,4,5-trisphosphate (InsP3) and increasing cytosolic free calcium ion concentrations ([Ca 2+ ]i). These increases are achieved by a combination of the InsP3-induced release of Ca 2+ from specific intracellular stores and the activation of Ca 2+ entry from the extracellular environment. The [Ca 2+ ]i signal represents a balance between the adequate activation of components of the secretory mechanism and the avoidance of [Ca 2+ ]i levels that are toxic to the cell. Resting [Ca 2+ ]i is maintained low by the action of Ca 2+ pumps on the intracellular stores and plasma membrane, with the result that gradients for Ca 2+ movement into the cytosol from either of these two sources are very large and there is considerable potential for achieving rapid increases in [Ca 2+ ]i. Consequently, for successful Ca 2+ signalling, it is imperative that these two mechanisms of raising [Ca 2+ ]i (i.e. Ca 2+ release and Ca 2+ entry) are closely integrated. Current models emphasize the activation of Ca 2+ entry as a downstream result of the emptying of the intracellular stores ('capacitative' model). Whilst this may be true for situations of maximal stimulation, recent experiments on the oscillatory [Ca 2+ ]i responses typical of more physiological levels of stimulation indicate a previously unsuspected, independent activation of Ca 2+ entry involving arachidonic acid. This arachidonic-acid-activated entry plays a key role, along with InsP3, in inducing the repetitive release of Ca 2+ from the stores to produce the [Ca 2+ ]i oscillations. In this way, the two components responsible for the elevation of [Ca 2+ ]i are intimately related and their dual effects closely coordinated, resulting in the finely tuned control of agonistinduced changes in [Ca 2+ ]i.
Evidence for agonist-induced export of intracellular Ca2+ in epithelial cells
Pfl�gers Archiv European Journal of Physiology, 1993
There is increasing evidence that some agonists not only induce intracellular Ca 2+ increases, due to store release and transmembranous influx, but also that they stimulate Ca 2+ effiux. We have investigated the agonist-stimulated response on the intracellular Ca 2+ activity ([Ca2+]i) in the presence of thapsigargin (10 -8 mol/1, TG) in HT29 and CFPAC-1 cells. For CFPAC-I the agonists ATP (10-7-10 -3 tool/l, n = 9), carbachol (10 6_ 10 -3 mol/1, n = 5) and neurotensin (10-1~ -7 mol/1, n = 6) all induced a concentration-dependent decrease in [Ca2+]i in the presence of TG. Similar results were obtained with HT29 ceils. This decrease of [Ca2+]i could be caused by a reduced Ca 2+ inflUX, either due to a reduced driving force for Ca 2+ in the presence of depolarizing agonists or due to agonist-regulated decrease in Ca 2+ permeability. Using the fura-2 Mn 2+ quenching technique we demonstrated that ATP did not slow the TG-induced Mn 2+ quench. This indicates that the agonist-induced [Ca2+]i decrease in the presence of TG was not due to a reduced influx of Ca 2+ into the cell, but rather due to stimulation of Ca 2+ export. We used the cell attached nystatin patch clamp technique in CFPAC-1 cells to examine whether, in the presence of TG, the above agonists still led to the previously described electrical changes. The cells had a mean membrane voltage of -49 + 3.6 mV (n = 9). Within the first 3 rain ATP was still able to induce a depolarization which could be attributed to an increase in C1-conductance. This was expected, since at this time after TG stimulation all Ca 2+ agonists still liberated some [Ca2+]~. When TG incubation was prolonged, agonist application led to strongly attenuated or to no electrical responses. Therefore, the agonist-stimulated [Ca2+]~ decrease cannot be explained by the reduction of the driving force for Ca > into the cell. In the same cells hypotonic swelling (160 mosmol/1, n = 15) still induced a further [Ca2+]~ increase in the presence of TG and concomitantly induced C1-and K + conductances. We conclude that the agonist-induced decrease of [Ca2+]~ in the presence of TG probably unmasks a stimulation of [Ca2+]~ export.
Role of Ca++ in stimulus-secretion coupling in the gastric oxyntic cell: Effect of A23187
Cell Calcium, 1981
The effects of Ca ++ ionophore A23187 on H + secretion and histamine release were studied in the isolated gastric mucosa of the toad Bufo marinus. A23187 added from the mucosal side stimulated H+ secretion. At high concentrations, A23187 also caused histamine release. This histamine was not sufficient to explain the effects of A23187 on H+ secre tion. Metiamide, only partially inhibited the effect ofionophore. There was summation and/or potentiation of effects between A23187 and histamine. The results are consistent with the hypothesis that Ca++ acts as a second messenger in stimulus-secretion coupling in the oxyntic cell. It is possible that Ca++ and CAMP may interact as parallel second messengers in the control of gastric H+ secretion.
Membrane ion channels as physiological targets for local Ca2+ signalling
Journal of Microscopy, 1999
Ionized calcium plays a central role as a second messenger in a number of physiologically important processes determining smooth muscle function. To regulate a wide range of cellular activities the mechanisms of subcellular calcium signalling should be very diverse. Recent progress in development of visible light-excitable¯uorescent dyes with high af®nity for Ca 2 (such as oregon green 488 BAPTA indicators,¯uo-3 and fura red) and confocal laser scanning microscopy provides an opportunity for direct visualization of subcellular Ca 2 signalling and reveals that many cell function are regulated by the microenvironment within small regions of the cytoplasm (`local control' concept). Here confocal imaging is used to measure and locate changes in [Ca 2 ] i on a subcellular level in response to receptor stimulation in visceral myocytes. We show that stimulation of muscarinic receptors in ileal myocytes with carbachol leading to activation of inositol 1,4,5-trisphosphate receptors (IP 3 Rs) accelerates the frequency of spontaneous calcium sparks (discharged via ryanodine receptors, RyRs) and gives rise to periodic propagating Ca 2 waves oscillating with a frequency similar to that of carbacholactivated cationic current oscillations. Furthermore, by combining the whole-cell patch clamp technique with simultaneous confocal imaging of [Ca 2 ] i in voltage-clamped vascular myocytes we demonstrate that calcium sparks may lead to the opening of either Ca 2-activated Cl À channels or Ca 2-activated K channels, and the discharge of a spontaneous transient inward current (STIC) or a spontaneous transient outward current (STOC), respectively.