Synchronized oscillation of Ca2+ entry and Ca2+ release in agonist-stimulated AR42J cells

Calcium signalling in platelets and other cells

Reviewed are new concepts and models of Ca(2+) signalling originating from work with various animal cells, as well as the applicability of these models to the signalling systems used by blood platelets. The following processes and mechanisms are discussed: Ca(2+) oscillations and waves; Ca(2+) -induced Ca(2+) release; involvement of InsP(3)-receptors and quanta1 release of Ca(2+); different pathways of phospholipase C activation; heterogeneity in the intracellular Ca(2+) stores; store-and receptor-regulated Ca(2+) entry. Additionally, some typical aspects of Ca(2+) signalling in platelets are reviewed: involvement of protein serine/threonine and tyrosine kinases in the regulation of signal transduction; possible functions of platelet glycoproteins; and the importance of Ca(2+) for the exocytotic and procoagulant responses.

Methods for studying store-operated calcium entry

Activation of surface membrane receptors coupled to phospholipase C results in the generation of cytoplasmic Ca2+ signals comprised of both intracellular Ca2+ release, and enhanced entry of Ca2+ across the plasma membrane. A primary mechanism for this Ca2+ entry process is attributed to store-operated Ca2+ entry, a process that is activated by depletion of Ca2+ ions from an intracellular store by inositol 1,4,5-trisphosphate. Our understanding of the mechanisms underlying both Ca2+ release and store-operated Ca2+ entry have evolved from experimental approaches that include the use of fluorescent Ca2+ indicators and electrophysiological techniques. Pharmacological manipulation of this Ca2+ signaling process has been somewhat limited; but recent identification of key molecular players, STIM and Orai family proteins, has provided new approaches. Here we describe practical methods involving fluorescent Ca2+ indicators and electrophysiological approaches for dissecting the observed intracellular Ca2+ signal to reveal characteristics of store-operated Ca2+ entry, highlighting the advantages, and limitations, of these approaches.

STIM1 heteromultimerizes TRPC channels to determine their function as store-operated channels

Stromal interacting molecule 1 (STIM1) is a Ca(2+) sensor that conveys the Ca(2+) load of the endoplasmic reticulum to store-operated channels (SOCs) at the plasma membrane. Here, we report that STIM1 binds TRPC1, TRPC4 and TRPC5 and determines their function as SOCs. Inhibition of STIM1 function inhibits activation of TRPC5 by receptor stimulation, but not by La(3+), suggesting that STIM1 is obligatory for activation of TRPC channels by agonists, but STIM1 is not essential for channel function. Through a distinct mechanism, STIM1 also regulates TRPC3 and TRPC6. STIM1 does not bind TRPC3 and TRPC6, and regulates their function indirectly by mediating the heteromultimerization of TRPC3 with TRPC1 and TRPC6 with TRPC4. TRPC7 is not regulated by STIM1. We propose a new definition of SOCs, as channels that are regulated by STIM1 and require the store depletion-mediated clustering of STIM1. By this definition, all TRPC channels, except TRPC7, function as SOCs.

Ca2+-dependent potentiation of muscarinic receptor-mediated Ca2+ elevation

Muscarinic receptor-mediated increases in Ca(2+) in SH-SY5Y neuroblastoma cells consist of an initial fast and transient phase followed by a sustained phase. Activation of voltage-gated Ca(2+) channels prior to muscarinic stimulation resulted in a several-fold potentiation of the fast phase. Unlike the muscarinic response under control conditions, this potentiated elevation of intracellular Ca(2+) was to a large extent dependent on extracellular Ca(2+). In potentiated cells, muscarinic stimulation also activated a rapid Mn(2+) entry. By using known organic and inorganic blockers of cation channels, this influx pathway was easily separated from the known Ca(2+) influx pathways, the store-operated pathway and the voltage-gated Ca(2+) channels. In addition to the Ca(2+) influx, both IP(3) production and Ca(2+) release were also enhanced during the potentiated response. The results suggest that a small increase in intracellular Ca(2+) amplifies the muscarinic Ca(2+) response at several stages, most notably by unravelling an apparently novel receptor-activated influx pathway.

Capacitative Ca2+ entry is graded with degree of intracellular Ca2+ store depletion in bovine vascular endothelial cells

1. In endothelial cells, release of Ca2+ from endoplasmic reticulum (ER) Ca2+ stores activates Ca2+ influx via the capacitative Ca2+ entry (CCE) pathway. In cultured bovine pulmonary artery endothelial cells, we investigated the relationship between intracellular Ca2+ store load and CCE activity, as well as the kinetics of CCE activation and deactivation, by simultaneously measuring changes in [Ca2+]i and unidirectional manganese (Mn2+) entry through the CCE pathway. 2. Submaximal concentrations of ATP caused quantal release of Ca2+ from the ER, resulting in a dose-dependent depletion of Ca2+ stores and acceleration of Mn2+ entry. Mn2+ entry rate, as a measure of CCE activity, was graded with the amount of released Ca2+. Maximal activation of CCE did not require complete store depletion. 3. Slow depletion of the ER by exposure to the ER Ca2+ pump inhibitor cyclopiazonic acid resulted in a delayed activation of CCE, revealing a temporal dissociation between release of Ca2+ from intracellular stores and activation of CCE. 4. During [Ca2+]i oscillations, at frequencies higher than 0.5 spikes min-1, each Ca2+ spike resulted in a progressive acceleration of CCE without leading to oscillations of Ca2+ entry. In contrast, low frequency [Ca2+]i oscillations were paralleled by transient CCE that was activated and deactivated with each Ca2+ spike, resulting in an oscillatory pattern of Ca2+ entry. 5. It is concluded that CCE is a rapidly activating process which is graded with store depletion and becomes fully activated before complete depletion. The duration of CCE activation correlates with the degree of store depletion and the time that is required to refill depleted stores. Overall, a mechanism of graded CCE prevents exhaustion of intracellular Ca2+ reserves and provides an efficient way to respond to variable degrees of intracellular store depletion.

Characterization of a plasma membrane calcium oscillator in rat pituitary somatotrophs

In excitable cells, oscillations in intracellular free calcium concentrations ([Ca(2+)](i)) can arise from action-potential-driven Ca(2+) influx, and such signals can have either a localized or global form, depending on the coupling of voltage-gated Ca(2+) influx to intracellular Ca(2+) release pathway. Here we show that rat pituitary somatotrophs generate spontaneous [Ca(2+)](i) oscillations, which rise from fluctuations in the influx of external Ca(2+) and propagate within the cytoplasm and nucleus. The addition of caffeine and ryanodine, modulators of ryanodine-receptor channels, and the depletion of intracellular Ca(2+) stores by thapsigargin and ionomycin did not affect the global nature of spontaneous [Ca(2+)](i) signals. Bay K 8644, an L-type Ca(2+) channel agonist, initiated [Ca(2+)](i) signaling in quiescent cells, increased the amplitude of [Ca(2+)](i) spikes in spontaneously active cells, and stimulated growth hormone secretion in perifused pituitary cells. Nifedipine, a blocker of L-type Ca(2+) channels, decreased the amplitude of spikes and basal growth hormone secretion, whereas Ni(2+), a blocker of T-type Ca(2+) channels, abolished spontaneous [Ca(2+)](i) oscillations. Spiking was also abolished by the removal of extracellular Na(+) and by the addition of 10 mM Ca(2+), Mg(2+), or Sr(2+), the blockers of cyclic nucleotide-gated channels. Reverse transcriptase-polymerase chain reaction and Southern blot analyses indicated the expression of mRNAs for these channels in mixed pituitary cells and purified somatotrophs. Growth hormone-releasing hormone, an agonist that stimulated cAMP and cGMP productions in a dose-dependent manner, initiated spiking in quiescent cells and increased the frequency of spiking in spontaneously active cells. These results indicate that in somatotrophs a cyclic nucleotide-controlled plasma membrane Ca(2+) oscillator is capable of generating global Ca(2+) signals spontaneously and in response to agonist stimulation. The Ca(2+)-signaling activity of this oscillator is dependent on voltage-gated Ca(2+) influx but not on Ca(2+) release from intracellular stores.

Agonist-dependent phosphorylation of the inositol 1,4,5-trisphosphate receptor: A possible mechanism for agonist-specific calcium oscillations in pancreatic acinar cells

The properties of inositol 1,4,5-trisphosphate (IP3)-dependent intracellular calcium oscillations in pancreatic acinar cells depend crucially on the agonist used to stimulate them. Acetylcholine or carbachol (CCh) cause high-frequency (10-12-s period) calcium oscillations that are superimposed on a raised baseline, while cholecystokinin (CCK) causes long-period (>100-s period) baseline spiking. We show that physiological concentrations of CCK induce rapid phosphorylation of the IP3 receptor, which is not true of physiological concentrations of CCh. Based on this and other experimental data, we construct a mathematical model of agonist-specific intracellular calcium oscillations in pancreatic acinar cells. Model simulations agree with previous experimental work on the rates of activation and inactivation of the IP3 receptor by calcium (DuFour, J.-F., I.M. Arias, and T.J. Turner. 1997. J. Biol. Chem. 272:2675-2681), and reproduce both short-period, raised baseline oscillations, and long-period baseline spiking. The steady state open probability curve of the model IP3 receptor is an increasing function of calcium concentration, as found for type-III IP3 receptors by Hagar et al. (Hagar, R.E., A.D. Burgstahler, M.H. Nathanson, and B.E. Ehrlich. 1998. Nature. 396:81-84). We use the model to predict the effect of the removal of external calcium, and this prediction is confirmed experimentally. We also predict that, for type-III IP3 receptors, the steady state open probability curve will shift to lower calcium concentrations as the background IP3 concentration increases. We conclude that the differences between CCh- and CCK-induced calcium oscillations in pancreatic acinar cells can be explained by two principal mechanisms: (a) CCK causes more phosphorylation of the IP3 receptor than does CCh, and the phosphorylated receptor cannot pass calcium current; and (b) the rate of calcium ATPase pumping and the rate of calcium influx from the outside the cell are greater in the presence of CCh than in the presence of CCK.

Characterization of the rat mesangial cell type 2 sulfonylurea receptor

BACKGROUND

Sulfonylurea receptors are classified as either high-affinity type 1 (SUR1) or low-affinity type 2 receptors (SUR2), and the gene expression of SURs has recently been demonstrated in kidney. However, functional data regarding a renal SUR are lacking. We previously demonstrated that mesangial cell (MC) gene and protein expression of extracellular matrix components were up-regulated by the sulfonylurea, tolazamide. After noting this biological response, we next sought to investigate the presence of a sulfonylurea receptor in rat MCs.

METHODS

Equilibrium binding studies employing [3H]glibenclamide as a ligand were performed on crude MC membrane preparations. Gene expression for SUR was explored by Northern analysis of cultured MCs and whole kidney tissue. The effect of sulfonylurea on intracellular Ca2+ in MCs was assayed by spectrofluorometry, and glibenclamide-induced changes in the contractility of MCs were assessed.

RESULTS

MCs bound [3H]glibenclamide with a KD of 2.6 microM and a Bmax of 30.4 pmol/mg protein as determined by Scatchard analysis. Three SUR2 transcripts were detected in MCs. A major transcript was detected at 5.5 kb and minor transcripts at 7.5 and 8.6 kb. Following sulfonylurea treatment of MCs, real-time videomicroscopy revealed intense MC contraction, coinciding with oscillatory increments of intracellular Ca2+ concentration. Further evidence of sulfonylurea-induced MC contraction was demonstrated by glibenclamide-induced deformation of a silicone rubber substrate.

CONCLUSIONS

These results demonstrate that SUR2 resides on MCs. Functional activation of this receptor by sulfonylurea induces Ca2+ transients that result in MC contraction.

Calcium store depletion activates two distinct calcium entry pathways in secretory cells of the blowfly salivary gland

Ca2+ influx into secretory cells of the intact salivary gland of the blowfly Calliphora erythrocephala elicited by the agonist 5-hydroxytryptamine (5-HT) or the Ca2+ uptake inhibitor thapsigargin was studied by using Fura-2 and digital fluorescence imaging and by recordings of the transepithelial potential. Application of saturating [5-HT] in the absence of Ca2+ (Ca2+o) from the bathing saline did not affect the initial Ca2+ transient but greatly attenuated the subsequent sustained Ca2+ elevation observed in the presence of Ca2+o demonstrating that the latter component of the [Ca2+]i response is largely dependent on Ca2+ entry across the baso-lateral plasma membrane. La3+ or Gd3+ (10 microM) mimicked the effects of the withdrawal of Ca2+o. Experimental attempts temporally to uncouple 5-HT stimulation and Ca2+ influx by withdrawal of Ca2+o during agonist application revealed a second Ca2+ entry pathway. This pathway was insensitive to 10 microM La3+ and produced transient [Ca2+]i increases whose amplitudes were a function of the [5-HT] during the preceding stimulation and that were selectively suppressed by 50 microM SK&F 96365. Both (10 microM) La(3+)-insensitive [Ca2+]i transients and (10 microM) La3+ inhabitable tonic [Ca2+]i increases could be sequentially activated in the presence of 5-HT or thapsigargin (1 microM). These results indicate that Ca2+ store depletion by 5-HT or thapsigargin activates two distinct store-operated Ca2+ entry pathways, one of which supports tonic [Ca2+]i increases. The other is transiently activated, even under conditions that prohibit store refilling and does not significantly contribute to the [Ca2+]i responses evoked by saturating 5-HT concentrations.

Intracellular calcium oscillations in cell populations of ras-transfected I-7 subline of human HaCaT keratinocytes

We have observed oscillations of intracellular Ca2+ (Ca[i]) concentration in populations of ras-transfected HaCaT keratinocytes of I-7 subline. In postconfluent monolayers of I-7 keratinocytes, an increase in extracellular Ca2+ (Ca[o]) concentration to 0.25-0.5 mM induced sinusoidal Ca(i) oscillations, which persisted longer than 1 h with amplitudes of 50-150 nM and periods of 5-10 min. Thapsigargin, which depletes internal Ca2+ stores, did not prevent Ca(o)-induced Ca(i) oscillations, and it also induced Ca(i) oscillations in the ras-transfected I-7 line. Removal of extracellular Ca2+ or addition of Ca2+-entry blocker La3+ or SK&F 96365 inhibited Ca(i) oscillations, suggesting that Ca(i) oscillations in ras-transfected HaCaT keratinocytes were dependent on Ca2+ influx across the plasma membrane. Because the Ca(o)-induced Ca(i) oscillations have been observed only in ras-transfected I-7 subline and not in its nontransfected parental HaCaT line, this may provide a partial explanation for the divergent responses of ras-transfected and nontransfected keratinocytes to Ca(o) signal for control of growth and differentiation.

Uncoupling of calcium mobilization and entry pathways in endothelin-stimulated pituitary lactotrophs

In cells expressing Ca2+-mobilizing receptors, InsP3-induced Ca2+ release from intracellular stores is commonly associated with extracellular Ca2+ influx. Operation of these two Ca2+ signaling pathways mediates thyrotropin-releasing hormone (TRH) and angiotensin II (AII)-induced prolactin secretion from rat pituitary lactotrophs. After an initial hyperpolarization induced by Ca2+ mobilization from the endoplasmic reticulum (ER), these agonists generated an increase in the steady-state firing of action potentials, further facilitating extracellular Ca2+ influx and prolactin release. Like TRH and AII, endothelin-1 (ET-1) also induced a rapid release of Ca2+ from the ER and a concomitant spike prolactin secretion during the first 3-5 min of stimulation. However, unlike TRH and AII actions, Ca2+ mobilization was not coupled to Ca2+ influx during sustained ET-1 stimulation, as ET-1 induced a long-lasting abolition of action potential firing. This lead to a depletion of the ER Ca2+ pool, a prolonged decrease in [Ca2+]i, and sustained inhibition of prolactin release. ET-1-induced inhibition and TRH/AII-induced stimulation of Ca2+ influx and hormone secretion were reduced in the presence of the L-type Ca2+ channel blocker, nifedipine. Basal [Ca2+]i and prolactin release were also reduced in the presence of nifedipine. Furthermore, TRH-induced Ca2+ influx and secretion were abolished by ET-1, as TRH was unable to reactivate Ca2+ influx and prolactin release in ET-1-stimulated cells. Depolarization of the cells during sustained inhibitory action of ET-1, however, increased [Ca2+]i and prolactin release. These results indicate that L-type Ca2+ channel represents a common Ca2+ influx pathway that controls basal [Ca2+]i and secretion and is regulated by TRH/AII and ET-1 in an opposite manner. Thus, the receptor-mediated uncoupling of Ca2+ entry from Ca2+ mobilization provides an effective control mechanism in terminating the stimulatory action of ET-1. Moreover, it makes electrically active lactotrophs quiescent and unresponsive to other calcium-mobilizing agonists.

Spacial compartmentalization of Ca2+ signaling complexes in pancreatic acini

Imaging [Ca2+]i at high temporal resolution and measuring the properties of Ca2+ signaling in streptolysin O (SLO)-permeabilized cells were used to study the spacial organization of signaling complexes. Sequential stimulation of single cells within pancreatic acini with several Ca2+-mobilizing agonists revealed an agonist-specific pattern and propagation rate of Ca2+ waves in the same cells, with CCK8 stimulating the fastest and bombesin the slowest waves. More importantly, each agonist initiated the wave in a different region of the same cell. On the other hand, repetitive stimulation with the same agonist induced Ca2+ waves of the same pattern that were initiated from the same region of the cell. The agonist-specific Ca2+ signaling does not appear to be the result of coupling to different G proteins as infusion of an anti-Galphaq antibody into the cells through a patch pipette equally inhibited Ca2+ signaling by all agonists. Further evidence for compartmentalization of signaling complexes was developed in permeabilized cells. The time-dependent loss of Ca2+ signaling due to SLO permeabilization occurred in an agonist-specific manner in the sequence cabachol > bombesin > cholecystokinin. Signaling by all agonists could be completely restored with as low as 2 micro guanosine 5'-3-O-(thio)triphosphate (GTPgammaS). At this low concentration GTPgammaS recoupled inositol 1,4,5-trisphosphate production and Ca2+ release, rather than enhancing phospholipase C activity. Priming of Ca2+ signaling by GTPgammaS was agonist-specific. Guanosine 5'-O-(thio)diphosphate (GDPbetaS) uncoupled the ability of signaling complexes to release Ca2+ much better than stimulating inositol 1,4,5-trisphosphate production. The uncoupling of Ca2+ signaling by GDPbetaS was also agonist-specific. The combined findings of agonist-specific initiation sites of the Ca2+ wave and differential access of guanine nucleotides to signaling complexes suggest spacial compartmentalization of Ca2+ signaling complexes. Each complex must include a receptor, G protein, and phospholipase C that are coupled to a specific portion of the Ca2+ pool.

Expression of purinergic receptor channels and their role in calcium signaling and hormone release in pituitary gonadotrophs. Integration of P2 channels in plasma membrane- and endoplasmic reticulum-derived calcium oscillations

The role of ATP as a positive feedback element in Ca2+ signaling and secretion was examined in female rat pituitary gonadotrophs. ATP and ADP, but not AMP or adenosine, induced a dose- and extracellular Ca2+-dependent rise in [Ca2+]i in identified gonadotrophs in a Mg2+- and suramin-sensitive manner. ATP, adenosine-5'-O-(3-thiotriphosphate), adenosine-5'-O-(1-thiotriphosphate), 2-methylthio-ATP, and 3'-O-(4-benzoyl)benzoyl-ATP were roughly equipotent in rising [Ca2+]i in gonadotrophs, while ADP was effective only at submillimolar concentration range, and none of these compounds permeabilized the cells. On the other hand, alpha,beta-methylene-ATP, beta,gamma-methylene-ATP, and UTP were unable to induce any rise in [Ca2+]i. This pharmacological profile is consistent with expression of P2X2 and/or P2X5 purinergic receptor channels. Patch-clamp experiments showed that ATP induced an inward depolarizing current in gonadotrophs clamped at -90 mV, associated with an increase in [Ca2+]i. The ATP-induced [Ca2+]i response was partially inhibited by nifedipine, a blocker of voltage-sensitive Ca2+ channels (VSCC), but was not affected by tetrodotoxin, a blocker of voltage-sensitive Na+ channels. Thus, the P2-depolarizing current itself drives Ca2+ into the cell, but also activates Ca2+ entry through VSCC. In accord with this, low [ATP] induced plasma membrane-dependent [Ca2+]i oscillations in quiescent cells, and increased the frequency of spiking in spontaneously active cells. ATP-induced Ca2+ influx also affected agonist-induced and InsP3-dependent [Ca2+]i oscillations by increasing the frequency, base line, and duration of Ca2+ spiking. In addition, ATP stimulated gonadotropin secretion and enhanced agonist-induced gonadotropin release. ATP was found to be secreted by pituitary cells during agonist stimulation and was promptly degraded by ectonucleotidase to adenosine. These observations indicate that ATP represents a paracrine/autocrine factor in the regulation of Ca2+ signaling and secretion in gonadotrophs, and that these actions are mediated by P2 receptor channels.

Gbetagamma transduces [Ca2+]i oscillations and Galphaq a sustained response during stimulation of pancreatic acinar cells with [Ca2+]i-mobilizing agonists

A central unresolved question in agonist-evoked [Ca2+]i signaling is the pathway by which [Ca2+]i oscillations and a sustained response are transduced. We show here that activation of Gbetagamma signal [Ca2+]i oscillations and activation of Galphaq signal a sustained response during stimulation by a number of Ca2+-mobilizing agonists. Thus, infusion of purified Gbetagamma into pancreatic acinar cells through a patch pipette evokes [Ca2+]i oscillations by Ca2+ release from internal stores, which were inhibited by two independent scavengers of Gbetagamma, the beta-adrenergic receptor kinase fragment, and a mutated Galphai1G203A. These proteins, as well as an inhibitory antibody against Galphaq/11, prevent [Ca2+]i oscillations and the sustained response when applied before cell stimulation, possibly by preventing the dissociation of Gq into its subunits. After cell stimulation and dissociation of Gq into Gbetagamma and Galphaq, scavenging Gbetagamma stabilized the sustained response and inhibited reassociation of the subunits on termination of cell stimulation with antagonist, whereas scavenging Galphaq inhibited the sustained response and uncovered the Gbetagamma-dependent oscillations. These findings provide a general mechanism by which Ca2+-mobilizing agonists can control the type of [Ca2+]i signal to be transduced to the cell interior.

Evidence for the existence of inositol (1,4,5)-trisphosphate- and ryanodine-sensitive pools in bovine endothelial cells. Ca2+ releases in cells with different basal level of intracellular Ca2+

In single bovine aortic endothelial (BAE) cells pre-loaded with Fura-2, Ca2+ transients in a Ca2+-free medium have been revealed, which evidently reflects Ca2+ release from intracellular stores. In cells with different levels of resting basal cytoplasmic Ca2+ ([Ca2+]i) from about 50 to 110 nM, a biphasic dependence of the Ca2+ transients on resting [Ca2+]i was shown and spontaneous Ca2+ oscillations were observed. At a [Ca2+]i level over 110 nM, a pronounced rise in Ca2+ transients occurred and only single transients were observed. Ryanodine (10 microM) produced a transient [Ca2+]i elevation, suggesting the presence of ryanodine receptors in intracellular store membranes. The results imply that both inositol 1,4,5-trisphosphate-sensitive Ca2+ release (IICR) and Ca2+-sensitive Ca2+ release (CICR) take place in BAE cells. Only IICR seems to be sufficient for generating baseline Ca2+ oscillations in BAE cells, whereas the ATP-induced (5-100 microM) Ca2+ response involves the CICR set in motion by an oscillatory IICR of high frequency. The completion of both the spontaneous and ATP-induced Ca2+ transients was associated with a [Ca2+]i decrease to a level below the initial resting [Ca2+]i (undershoot). Its depth biphasically depended on the resting [Ca2+]i from 50 to 110 nM, suggesting that the lack of a Ca2+ leak from inositol 1,4,5-trisphosphate-sensitive stores is responsible for the undershoot in this range. The Ca2+ leak is concluded to play a key role in the initiation and termination of regenerative IICR both in spontaneous oscillations and in ATP-induced transients.

A direct measurement of increased divalent cation influx in fertilised mouse oocytes

On fertilisation of mouse oocytes, the fusing spermatozoon triggers a series of repetitive calcium (Ca2+) spikes. The Ca2+ spikes seem to be necessary for successful progression through the cell cycle and are regulated in a cell-cycle-dependent manner. The spikes appear to require the linkage of continuous Ca2+ influx to the periodic release of Ca2+ from intracellular stores by a process of Ca(2+)-induced Ca2+ release. The precise role of Ca2+ influx was explored using the manganese (Mn2+)-quench technique to monitor unidirectional cation influx into single mouse oocytes. There was a marked stimulation of cation influx associated closely with the upsweep of the first and subsequent fertilisation Ca2+ spikes. A smaller but significant increase in the rate of cation influx persisted in the interspike period in fertilised oocytes. Spike-associated entry was not as apparent in oocytes stimulated to spike repetitively by thimerosal or acetylcholine application. Instead, there was a continuous increase in cation influx underlying Ca2+ spiking which commenced with the onset of the first spike. Using the specific microsomal inhibitor thapsigargin and the Ca2+ ionophore ionomycin, we found evidence for a capacitative entry mechanism in mouse oocytes. We propose that the persistent influx of Ca2+ observed in response to all stimuli examined is controlled by a capacitative mechanism and sets the frequency of spiking by determining the time taken to refill the internal stores to a point where they are again sensitive enough to initiate the next spike.

Ca2+ oscillations and intercellular Ca2+ waves in ATP-stimulated articular chondrocytes

Cytosolic Ca2+ oscillations are known to occur in many cell types stimulated with agonists linked to the phosphoinositide signaling pathway. Trains of repetitive short-lasting Ca2+ spikes could be induced in articular chondrocytes by extracellular ATP, an agonist potently effective in stimulating cartilage resorption. The mechanism of these Ca2+ oscillations was studied by computerized video imaging on primary cultures of articular chondrocytes. Few cycles of oscillatory activity could be evoked in the absence of extracellular Ca2+, while, for oscillations to be sustained, Ca2+ influx was required. Thapsigargin irreversibly blocked Ca2+ oscillations, thus demonstrating the crucial involvement of intracellular stores in triggering the rhythmic activity. Apart from activating intracellular Ca2+ release, extracellular ATP also induced a noncapacitive Ca2+ influx in these cells. This ATP-mediated influx modulates both the oscillation frequency and intracellular stores refilling. In monolayers of confluent cells, Ca2+ oscillations spread from cell to cell in the form of intercellular waves. Propagating waves could also be observed in the absence of extracellular Ca2+, demonstrating that Ca2+ itself is not required for signal coordination. These results demonstrate that complex spatiotemporal pathways of Ca2+ oscillations and intercellular Ca2+ waves could be activated in articular chondrocytes during degenerative diseases.

Oscillations of cytosolic free calcium in bombesin-stimulated HIT-T15 cells

The mechanism underlying the generation of cytosolic free Ca2+ ([Ca2+]i) oscillations by bombesin, a receptor agonist activating phospholipase C, in insulin secreting HIT-T15 cells was investigated. At 25 microM, 61% of cells displayed [Ca2+]i oscillations with variable patterns. The bombesin-induced [Ca2+]i oscillations could last more than 1 h and glucose was required for maintaining these [Ca2+]i fluctuations. Bombesin-evoked [Ca2+]i oscillations were dependent on extracellular Ca2+ entry and were attenuated by membrane hyperpolarization or by L-type Ca2+ channel blockers. These [Ca2+]i oscillations were apparently not associated with fluctuations in plasma membrane Ca2+ permeability as monitored by the Mn2+ quenching technique. 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBuBHQ) and 4-chloro-m-cresol, which interfere with intracellular Ca2+ stores, respectively, by inhibiting Ca(2+)-ATPase of endoplasmic reticulum and by affecting Ca(2+)-induced Ca2+ release, disrupted bombesin-induced [Ca2+]i oscillations. 4-chloro-m-cresol raised [Ca2+]i by mobilizing an intracellular Ca2+ pool, an effect not altered by ryanodine. Caffeine exerted complex actions on [Ca2+]i. It raised [Ca2+]i by promoting Ca2+ entry while inhibiting bombesin-elicited [Ca2+]i oscillations. Our results suggest that in bombesin-elicited [Ca2+]i oscillations in HIT-T15 cells: (i) the oscillations originate primarily from intracellular Ca2+ stores; and (ii) the Ca2+ influx required for maintaining the oscillations is in part membrane potential-sensitive and not coordinated with [Ca2+]i oscillations. The interplay between intracellular Ca2+ stores and voltage-sensitive and voltage-insensitive extracellular Ca2+ entry determines the [Ca2+]i oscillations evoked by bombesin.

Differential regulation of Ca2+ release-activated Ca2+ influx by heterotrimeric G proteins

The least understood aspect of the agonist-induced Ca2+ signal is the activation and regulation of the Ca2+ release-activated Ca2+ influx (CRAC) across the plasma membrane. To explore the possible role of heterotrimeric G proteins in the various regulatory mechanisms of CRAC, continuous renal epithelial cell lines stably expressing alpha 13 and the constitutively active alpha qQ209L were isolated and used to measure CRAC activity by the Mn2+ quench technique. Release of intracellular Ca2+ by agonist stimulation or thapsigargin was required for activation of CRAC in all cells. Although the size of the internal stores was similar in all cells, CRAC was 2-3-fold higher in alpha 13- and alpha qQ209L-expressing cells. However, the channel was differentially regulated in the two cell types. Incubation at low [Ca2+]i, inhibition of the NOS pathway, or inhibition of tyrosine kinase inhibited CRAC activity in alpha 13 but not alpha qQ209L cells. Treatment with okadaic acid prevented inhibition of the channel by low [Ca2+]i and the protein kinase inhibitors in alpha 13 cells. These results suggest that expression of alpha qQ209L dominantly activates CRAC by stabilizing a phosphorylated state, whereas expression of alpha 13 makes CRAC activation completely dependent on phosphorylation by several kinases. G proteins may also modulate CRAC activity independently of the phosphorylation/dephosphorylation state of the pathway to increase maximal CRAC activity. Furthermore, our results suggest a general mechanism for regulation of CRAC that depends on coupling of receptors to specific G proteins.

Ca2+ influx drives agonist-activated [Ca2+]i oscillations in an exocrine cell

In current models describing agonist-induced oscillations in [Ca2+]i, Ca2+ entry is generally assumed to have a simple sustaining role, replenishing Ca2+ lost from the cell and recharging intracellular Ca2+ stores. In cells from the avian nasal gland, a model exocrine cell, we show that inhibition of Ca2+ entry by La3+, SK&F 96365, or by membrane depolarization, rapidly blocks [Ca2+]i oscillations but does so without detectable depletion of agonist-sensitive Ca2+ stores. As the rate of Mn2+ quenching during [Ca2+]i oscillations is constant, Ca2+ entry is not directly contributing to the [Ca2+]i changes and, instead, appears to be involved in inducing the repetitive release of Ca2+ from internal stores. Together, these data contradict current models in that (i) at the low agonist concentrations where [Ca2+]i oscillations are seen, generated levels of Ins(1,4,5)P3 are themselves inadequate to result in a regenerative [Ca2+]i signal, and (ii) Ca2+ entry is necessary to actually drive the intrinsic oscillatory mechanism.

Temporal relationships between Ca2+ store mobilization and Ca2+ entry in an exocrine cell

Consideration of the principal current models for agonist-induced activation of Ca2+ entry in electrically non-excitable cells suggests that it may be possible to distinguish between them on the basis of predicted differences in the temporal relationship(s) between intracellular Ca2+ release and the activation of Ca2+ entry. Measurements of changes in [Ca2+]i and Mn2+ quench in individual exocrine cells from the avian nasal gland indicate that, whereas Ins(1,4,5)P3-induced release of intracellular Ca2+ occurs within 3-5 s, the increase in Mn2+ quench is delayed by some 20-30 s. Mn2+ quench rate is similarly increased by thapsigargin, and is blocked by SK&F 96365, indicating that the increased Mn2+ quench observed genuinely reflects agonist-enhanced activity of the divalent cation entry pathway normally traversed by Ca2+. Additional experiments indicate that the observed delay is not due to inhibition of this pathway by elevated [Ca2+]i. Furthermore, the delay cannot be explained by the time required for Ins(1,3,4,5)P4 generation, which is essentially maximal within 10 s of agonist addition. It is concluded that the observed delay in the activation of the Ca2+ entry pathway is best explained by 'capacitative' models where increased entry requires the generation, and transmission to the plasma membrane, of an unknown messenger as a direct result of the depletion of intracellular Ca2+ stores.

Induction of Ca2+ oscillations by selective, U73122-mediated, depletion of inositol-trisphosphate-sensitive Ca2+ stores in rabbit pancreatic acinar cells

The effect of the putative inhibitor of phospholipase C activity, U73122, on the Ca2+ sequestering and releasing properties of internal Ca2+ stores was studied in both permeabilized and intact rabbit pancreatic acinar cells. U73122 dose dependently inhibited ATP-dependent Ca2+ uptake in the inositol (1,4,5)-trisphosphate-[Ins(1,4,5)P3]-sensitive, but not the Ins(1,4,5)P3-insensitive, Ca2+ store in acinar cells permeabilized by saponin treatment. In a suspension of intact acinar cells, loaded with the fluorescent Ca2+ indicator, Fura-2, U73122 alone evoked a transient increase in average free cytosolic Ca2+ concentration ([Ca2+]i,av), which was largely independent of external Ca2+. Addition of U73122 to cell suspensions prestimulated with either cholecystokinin octapeptide or JMV-180 revealed an inverse relationship in size between the U73122- and the agonist-evoked [Ca2+]i,av transient. Moreover, thapsigargin-induced inhibition of intracellular Ca(2+)-ATPase activity resulted in a [Ca2+]i,av transient, the size of which was not different following maximal prestimulation with either U73122 or agonist. These observations suggest that U73122 selectively affects the Ins(1,4,5)P3- casu quo agonist-sensitive internal Ca2+ store, whereas thapsigargin affects both the Ins(1,4,5)P3-sensitive and -insensitive Ca2+ store. Digital-imaging microscopy of Fura-2-loaded acinar cells demonstrated that U73122, in contrast to thapsigargin, evoked sustained oscillatory changes in [Ca2+]i. The U73122-evoked oscillations were abolished in the absence of external Ca2+. The ability of U73122 to generate external Ca(2+)-dependent Ca2+ oscillations suggests that depletion of the agonist-sensitive store leads to an increase in Ca2+ permeability of the plasma membrane and that the Ins(1,4,5)P3-insensitive Ca2+ pool is necessary for the Ca2+ oscillations.

Multiple pathways for entry of calcium and other divalent cations in a vascular smooth muscle cell line (A7r5)

The influx of calcium in response to vasopressin receptor stimulation is an important component of excitation-contraction coupling. We have examined the routes by which Ca2+ and other divalent cations enter vascular smooth muscle cells using a cultured vascular smooth muscle cell line (A7r5). Confluent A7r5 cells were loaded with Fura-2 to permit measurement of intracellular divalent cation concentration (Ca2+, Ba2+, Mn2+). Combinations of excitation wavelengths (340/380, 340/356, 356/380 and 340/370) were used depending on the divalent cation being studied. Emission was measured at 510 nm for all studies. Ca2+, Ba2+ and Mn2+ permeated unstimulated A7r5 cells. Vasopressin increased intracellular Ca2+ in cells both in the presence and absence of extracellular Ca2+, although responses in the absence of extracellular Ca2+ were smaller and had no sustained component. Amlodipine, a voltage-dependent calcium channel blocker, had no effect on Ca2+ entry, but Ni2+ did block Ca2+ influx. Vasopressin-induced elevations of intracellular Ca2+ in Ca(2+)-free physiological saline were abolished by ionomycin and thapsigargin. In the presence of extracellular Ba2+ vasopressin increased intracellular Ca2+ transiently and caused a small sustained increase in intracellular Ba2+ concentration. Ionomycin and thapsigargin increased intracellular Ca2+ but had no effect on Ba2+ influx. In contrast vasopressin, ionomycin and thapsigargin had no effect on Mn2+ influx. Econazole and SKF 96365, imidazoles reported to be blockers of receptor-induced cation entry, increased intracellular Ca2+ by releasing intracellular Ca2+ from a different site to that mobilized by vasopressin or thapsigargin in A7r5 cells. Econazole and SKF 96365 partially inhibited passive influx of Ca2+ and Ba2+ but did not inhibit passive influx of Mn2+, or vasopressin-induced influx of Ba2+. U73122, a putative inhibitor of phospholipase C partially inhibited passive entry of Ca2+ but not passive entry of Mn2+ and Ba2+. U73122 also inhibited vasopressin-induced release of intracellular Ca2+ and agonist-induced Ca2+ influx but did not block vasopressin-induced Ba2+ influx. Divalent cations enter A7r5 cells by a number of routes - 'passive' pathway(s) that admit Ca2+, Ba2+ and Mn2+ and receptor-operated pathway(s) that are permeable to Ca2+, Ba2+ but not Mn2+. On the basis of ionic permeabilities and the effect of various blocking agents, there appear to be two distinct passive influx routes. One is permeable to Ca2+ and Ba2+ and is blocked by econazole or SKF 96365. The other is permeable to Mn2+ and is blocked by Ni2+. There also appear to be two different routes of divalent cation entry involved in responses to receptor activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium signalling in platelets and other nonexcitable cells

By virtue of their biological simplicity and widespread availability, platelets frequently have been used as a model system to study signal transduction. Such studies have revealed that changes in intracellular free calcium concentration are central to platelet functioning. The following article reviews current concepts of platelet structure and function, with particular emphasis on the mechanisms involved in platelet Ca2+ signalling.

Intracellular calcium waves generated by Ins(1,4,5)P3-dependent mechanisms

Cellular oscillations of cytosolic free Ca2+ ([Ca2+]i) have been observed in many cell types in response to cell surface receptor agonists acting through inositol 1,4,5-trisphosphate (InsP3). In a number of cases where appropriate spatial and temporal resolution have been used to examine these [Ca2+]i oscillations, they have been found to be organized as repetitive waves of Ca2+ increase that propagate through the cytosol of individual cells. In some cases Ca2+ waves also occur as a single pass through stimulated cells. This review discusses the factors underlying the spatial organization of [Ca2+]i signals in the form of Ca2+ waves. In addition, potential mechanisms for the initiation and subsequent propagation of these Ca2+ waves are described.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Receptor-evoked Ca2+ mobilization in pancreatic acinar cells: evidence for a regulatory role of protein kinase C by a mechanism involving the transition of high-affinity receptors to a low-affinity state

In order to establish a regulatory role for phosphoproteins in the process of receptor-stimulated Ca2+ mobilization, isolated pancreatic acinar cells, loaded with fura-2, were stimulated with cholecystokinin-octapeptide (CCK8) in the presence of either staurosporine, a general inhibitor of protein kinase activity, or 12-O-tetradecanoylphorbol 13-acetate (TPA), an activator of protein kinase C. Staurosporine alone did not affect the average free cytosolic Ca2+ concentration ([Ca2+]i,av) in a suspension of acinar cells. However, in the presence of 1.0 microM staurosporine the stimulatory effect of submaximal concentrations of CCK8 was significantly enhanced. The potentiating effect of the inhibitor was paralleled by the increased production of inositol 1,4,5-trisphosphate. In addition, staurosporine evoked a transient increase in [Ca2+]i,av in cells prestimulated with a submaximal concentration of CCK8. The data obtained with staurosporine indicate that CCK8-stimulated phosphorylations exert a negative feedback role in the process of receptor-mediated Ca2+ mobilization. The involvement of protein kinase C was investigated by studying the effects of TPA on CCK8-induced Ca2+ mobilization. The phorbol ester induced a rightward shift of the dose/response curve for the CCK8-evoked increase in [Ca2+]i,av, which, in contrast to the unlimited shift obtained with the receptor antagonist D-lorglumide, reached a maximum of approximately one order of a magnitude at 10 nM TPA. The inhibitory effect of TPA was completely overcome by CCK8 at concentrations at or beyond 10 nM. This observation has led to the hypothesis that protein kinase C, directly or indirectly, converts the CCK receptor from a high-affinity state to a low-affinity state. Substantial evidence in favour of this hypothesis was provided by the observation that the increase in [Ca2+]i,av evoked by the CCK8 analogue JMV-180, which acts as an agonist at the high-affinity receptor, was completely blocked by TPA pretreatment. TPA also evoked a rightward shift of the dose/response curve for the carbachol-induced increase in [Ca2+]i,av, indicating that the protein-kinase-C-mediated transition of the affinity state of receptors is a more general phenomenon. In the presence of submaximal CCK8 concentrations, TPA dose-dependently decreased the poststimulatory elevated [Ca2+]i,av to the prestimulatory level, indicating that protein kinase C also inhibits the process of sustained Ca2+ mobilization. The effects of TPA were counteracted by staurosporine, suggesting that the effects of the inhibitor itself were indeed due to inhibition of the receptor-mediated activation of protein kinase C.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple mechanisms of manganese-induced quenching of fura-2 fluorescence in rat mast cells

Whole-cell patch-clamp recordings of membrane currents and fura-2 measurements of free intracellular calcium concentration ([Ca2+]i) were used to study Mn2+ influx in rat peritoneal mast cells. The calcium-selective current, activated by depletion of intracellular calcium stores (ICRAC for calcium release-activated calcium current), supports a small but measurable Mn2+ current. In the presence of intracellular BAPTA, a Mn2+ current through ICRAC was recorded in isotonic MnCl2 (100 mM) without a significant quenching of fura-2 fluorescence. Its amplitude was 10% of that measured in physiological solution containing 10 mM Ca2+. However, following store depletion, a significant quenching of fura-2 fluorescence could be measured only when intracellular BAPTA was omitted, so that all the incoming Mn2+ could be captured by the fluorescent dye. Two other ionic currents activated by receptor stimulation also induced Mn2+ quenching of fura-2 fluorescence: a small current through non-specific cation channels of 50-pS unitary conductance and a distinct cationic current of large amplitude. In addition to these influx mechanisms, Mn2+ was taken up into calcium stores and was subsequently co-released with Ca2+ by Ca(2+)-mobilizing agonists.

Phase-dependent contributions from Ca2+ entry and Ca2+ release to caffeine-induced [Ca2+]i oscillations in bullfrog sympathetic neurons

Sympathetic neurons display robust [Ca2+]i oscillations in response to caffeine and mild depolarization. Oscillations occur at constant membrane potential, ruling out voltage-dependent changes in plasma membrane conductance. They are terminated by ryanodine, implicating Ca(2+)-induced Ca2+ release. Ca2+ entry is necessary for sustained oscillatory activity, but its importance varies within the oscillatory cycle: the slow interspike rise in [Ca2+]i requires Ca2+ entry, but the rapid upstroke does not, indicating that it reflects internal Ca2+ release. Sudden alterations in [Ca2+]o, [K+]o, or [caffeine]o produce immediate changes in d[Ca2+]i/dt and provide information about the relative rates of surface membrane Ca2+ transport as well as uptake and release by internal stores. Based on our results, [Ca2+]i oscillations can be explained in terms of coordinated changes in Ca2+ fluxes across surface and store membranes.

Ion selectivity of Ba2+ inward current oscillations in ras-transformed fibroblasts that elicit cytoplasmic Ca2+ oscillations by bradykinin

Ion selectivity of divalent cations on Ba2+ inward current oscillations was examined by voltage-clamp recording in v-Ki-ras-transformed NIH/3T3 (DT) fibroblasts where repetitive transient increases in cytoplasmic Ca2+ concentration were evoked by bradykinin. Application of bradykinin onto DT cells in 50 mM Ba2+ solution initiated Ba2+ inward current oscillations. The inward currents were inhibited in equimolar Sr2+ or Ca2+ solutions. Ba2+ current oscillations were dependent upon extracellular Ba2+ concentration. The results suggest that inward current oscillations are highly selective to Ba2+.

Different protein kinase C isozymes could modulate bradykinin-induced extracellular calcium-dependent and -independent increases in osteoblast-like MC3T3-E1 cells

Effects of protein kinase C (PKC) on bradykinin (BK)-induced intracellular calcium mobilization, consisting of rapid Ca2+ release from internal stores and a subsequent sustained Ca2+ inflow, were examined in Fura-2-loaded osteoblast-like MC3T3-E1 cells. The sustained Ca2+ inflow as inferred with Mn2+ quench method was blocked by Ni2+ and a receptor-operated Ca2+ channel blocker SK&F 96365, but not by nifedipine. The short-term pretreatment with phorbol 12-myristate 13-acetate (PMA), inhibited BK-stimulated Ca2+ inflow, and the prior treatment with PKC inhibitors, H-7 or staurosporine, enhanced the initial internal release and reversed the PMA effect. Moreover, 6 h pretreatment with PMA caused similar effect on the BK-induced inflow to that obtained with PKC inhibitors, whereas 24 h pretreatment was necessary to affect the internal release. On the other hand, the translocation and down-regulation of PKC isozymes were examined after PMA treatment of MC3T3-E1 cells by immunoblot analyses of PKCs with the isozyme-specific antibodies. 6 h treatment with PMA induced down-regulation of PKC beta, whereas longer treatment was needed for down-regulation of PKC alpha. Taken together, it was suggested that the BK-induced initial Ca2+ peak and the sustained Ca2+ inflow through the activation of a receptor-operated Ca2+ channel, are differentially regulated by PKC isozymes alpha and beta, respectively, in osteoblast-like MC3T3-E1 cells.

Regulation of agonist-evoked [Ca2+]i oscillation by intracellular Ca2+ and Ba2+ in AR42J cells

Measurements of intracellular Ca2+ ([Ca2+]i) and intracellular Ba2+ ([Ba2+]i) in single AR42J cells were used to evaluate the effect of [Ca2+]i and [Ba2+]i on agonist-evoked [Ca2+]i oscillations. Variations in [Ca2+]i and [Ba2+]i were imposed by gradual activation of entry through voltage-activated Ca2+ channels (VACC) present in the plasma membrane of these cells. Activation of high K+ was followed by partial inactivation of the channels and stabilization of [Ca2+]i at a new steady-state level depending on the extent of depolarization. Activation by BAY K 8644 was followed by complete inactivation and return of [Ca2+]i to resting levels. Ba2+ activated the channels and entered the cells but could not be removed from the cytosol by cellular Ca2+ pumps. The use of channel blockers and the ability to increase [Ca2+]i and [Ba2+]i by channel activation during [Ca2+]i oscillations showed that VACC do not contribute to or are activated during agonist-stimulated Ca2+ oscillation in this cell type. Graded activation of VACC showed that an increase in [Ca2+]i between the spikes to below 200 nM increased the frequency of the oscillation. Further increase in [Ca2+]i caused gradual reduction in the frequency. At [Ca2+]i above 500 nM, [Ca2+]i oscillations were inhibited. The inhibitory but not the stimulatory effects of [Ca2+]i on the oscillations can be mimicked by [Ba2+]i. These observations suggest that [Ca2+]i levels between the spikes play an important role in regulating the oscillations.

Mechanisms of activated Ca2+ entry in the rat pancreatoma cell line, AR4-2J

The characteristics of Ca2+ entry activated by surface receptor agonists and membrane depolarization were studied in the rat pancreatoma cell line, AR4-2J. Ca2+ mobilization activated by substance P, bombesin, or muscarinic receptor stimulation was found to involve both Ca2+ release and entry. In addition, depolarization of the surface membrane of AR4-2J cells with elevated concentrations of K+ activated Ca2+ entry. Ca2+ entry induced by membrane depolarization was inhibited by the L-channel antagonist, nimodipine, while that due to surface receptor agonists was not inhibited by this agent. The microsomal Ca(2+)-ATPase inhibitor, thapsigargin, caused both depletion of the agonist-sensitive intracellular Ca2+ pool and sustained Ca2+ influx indistinguishable from that produced by bombesin or methacholine. These results confirm that, unlike the pancreatic acinar cells from which they are presumably derived, AR4-2J cells express voltage-sensitive, dihydropyridine-inhibitable Ca2+ channels. However, in contrast to previous reports with this cell line, in the AR4-2J cells in use in our laboratory, and under our experimental conditions, surface receptor agonists (including substance P) do not cause Ca2+ influx through voltage-sensitive Ca2+ channels. Instead, we conclude that agonist-activated Ca2+ mobilization is initiated by (1,4,5)IP3-mediated intracellular Ca2+ release and that Ca2+ influx is regulated primarily, if not exclusively, by the state of depletion of the (1,4,5)IP3-sensitive intracellular Ca2+ pool.

Ba2+ current oscillations modulated by cyclic AMP and phorbol esters in ras-transformed fibroblasts

An oscillatory influx of divalent cations was measured as Ba2+ inward currents (Ba2+ current oscillations) by voltage-clamp recording in v-Ki-ras-transformed NIH/3T3 (DT) fibroblasts after activation with bradykinin or serum. Application of forskolin or dibutyryl cyclic AMP onto DT cells initiated Ba2+ current oscillations. Increasing intracellular cyclic AMP reduced the amplitude but increased the frequency of the Ba2+ current oscillations. Activation of protein kinase C by phorbol esters terminated Ba2+ current oscillations. No inhibition of Ba2+ current oscillations by phorbol esters was observed in down-regulated cells that had been pretreated with phorbol esters for 24 hrs. The results suggest that Ba2+ current oscillations are regulated by intracellular second messengers.

Thyrotropin-releasing hormone-induced cytosolic calcium transients: characterisation of store refilling in bovine anterior pituitary cells

The intracellular calcium ion concentration ([Ca2+]i) in individual bovine anterior pituitary cells was measured using fura-2 and ratiometric imaging. Addition of thyrotropin-releasing hormone (TRH) in the presence of external calcium ion ([Ca2+]e; 1 mM) caused a rapid transient increase in [Ca2+]i falling to a plateau which remained above pre-stimulation levels in the continued presence of TRH. Decreasing [Ca2+]e to 0.1 microM decreased [Ca2+]i. At 0.1 microM [Ca2+]e, the first TRH addition caused the rapid transient rise in [Ca2+]i but no plateau phase and a second addition of TRH did not cause a second transient rise. However, the second application of TRH in 0.1 microM [Ca2+]e caused a rise in [Ca2+]i if it was preceded by transient exposure of the cells to 2 mM [Ca2+]e. The presence of nitrendipine, 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBHQ), or TRH during the re-exposure to external calcium blocked this recovery of subsequent responses to TRH in the presence of only 0.1 microM [Ca2+]e. We conclude that refilling of the calcium stores depleted by TRH occurred only after the removal of agonist, used a tBHQ-sensitive uptake mechanism, and was mainly sustained by voltage-gated calcium entry into the cells.