Manipulation of intracellular calcium in NCB-20 cells

Direct demonstration of discrete Ca2+ microdomains associated with different isoforms of adenylyl cyclase

Ca(2+)-sensitive adenylyl cyclases (ACs) orchestrate dynamic interplay between Ca(2+) and cAMP that is a crucial feature of cellular homeostasis. Significantly, these ACs are highly selective for capacitative Ca(2+) entry (CCE) over other modes of Ca(2+) increase. To directly address the possibility that these ACs reside in discrete Ca(2+) microdomains, we tethered a Ca(2+) sensor, GCaMP2, to the N-terminus of Ca(2+)-stimulated AC8. GCaMP2-AC8 measurements were compared with global, plasma membrane (PM)-targeted or Ca(2+)-insensitive AC2-targeted GCaMP2. In intact cells, GCaMP2-AC8 responded rapidly to CCE, but was largely unresponsive to other types of Ca(2+) rise. The global GCaMP2, PM-targeted GCaMP2 and GCaMP2-AC2 sensors reported large Ca(2+) fluxes during Ca(2+) mobilization and non-specific Ca(2+) entry, but were less responsive to CCE than GCaMP2-AC8. Our data reveal that different AC isoforms localize to distinct Ca(2+)-microdomains within the plasma membrane. AC2, which is regulated via protein kinase C, resides in a microdomain that is exposed to a range of widespread Ca(2+) signals seen throughout the cytosol. By contrast, a unique Ca(2+) microdomain surrounds AC8 that promotes selectivity for Ca(2+) signals arising from CCE, and optimizes CCE-mediated cAMP synthesis. This direct demonstration of discrete compartmentalized Ca(2+) signals associated with specific signalling proteins provides a remarkable insight into the functional organization of signalling microdomains.

Calcium and cAMP signaling induced by gamma-hydroxybutyrate receptor(s) stimulation in NCB-20 neurons

The NCB-20 neurohybridoma cells differentiated with dibutyryl-cyclic-AMP represent an interesting model to study several components of the gamma-hydroxybutyrate (GHB) system in brain. In particular, an active Na(+)-dependent uptake and a depolarization-evoked release of GHB is expressed by these cells, together with high affinity specific binding sites for this substance. However, only little is known about cellular mechanisms following GHB receptor(s) stimulation in these neurons. Electrophysiological data indicate that GHB can differently affect Ca(2+) currents. L-type calcium channels were typically inhibited by GHB when NCB-20 cells were depolarized. In contrast, when NCB-20 cells were at resting potential, GHB induced a specific Ca(2+) entry through T-type calcium channels. In this study, we investigated the effect induced on cytosolic free Ca(2+) level and cAMP production by GHB receptor(s) stimulated with micromolar concentrations of GHB or structural analogues of GHB. Ca(2+) movements studied by cellular imaging were dose-dependently increased but disappeared for GHB concentrations >25 microM. In addition, nanomolar doses of GHB inhibited forskolin-stimulated adenylate cyclase. This effect was also rapidly desensitized at higher GHB concentrations. Acting as an antagonist, NCS-382 decreased GHB receptor(s) mediated cAMP and calcium signals. The agonist NCS-356 mimicked GHB effects which were not affected by the GABA(B) receptor antagonist CGP-55-845. Our results reveal the occurrence of Ca(2+)-dependent adenylate cyclase inhibition in NCB-20 neurons after GHB receptor(s) stimulation by GHB concentrations <50 microM. Above this dose, GHB effects were inactivated. In addition, at GHB concentrations exceeding 50 microM, GTP-gammaS binding was also reduced, confirming the desensitization of GHB receptor(s). Taken together, these results support the existence in NCB-20 neurons of GHB receptors belonging to GPCR family that may recruit various G protein subtypes.

Localized Na+/H+ exchanger 1 expression protects Ca2+-regulated adenylyl cyclases from changes in intracellular pH

The Ca2+-sensitive adenylyl cyclases (ACs) are exclusively regulated by capacitative Ca2+ entry (CCE) in nonexcitable cells. The present study investigates whether this Ca2+-dependent modulation of AC activity is further regulated by local pH changes that can arise beneath the plasma membrane as a consequence of cellular activity. Ca2+ stimulation of AC8 expressed in HEK 293 cells and inhibition of endogenous AC6 in C6-2B glioma cells exhibited clear sensitivity to modest pH changes in vitro. Acid pH (pH 7.14) reduced the Ca2+ sensitivity of both ACs, whereas alkaline pH (pH 7.85) enhanced the responsiveness of the enzymes to Ca2+, compared with controls (pH 7.50). Surprisingly, in the intact cell, the response of AC8 and AC6 to CCE was largely unperturbed by similar changes in intracellular pH (pH(i)), imposed using a weak acid (propionate) or weak base (trimethylamine). A range of hypotheses were tested to identify the mechanism(s) that could underlie this lack of pH effect in the intact cell. The pH sensitivity of CCE in HEK 293 cells is likely to dampen the effects of pH(i) on Ca2+-regulated ACs and may partly explain the discrepancy between in vitro and in vivo data. However, we have found that the Na+/H+ exchanger (NHE), NHE1, is functionally active in these cells, and like AC8 (and AC6) it resides in lipid rafts or caveolae, which may create cellular microdomains where pH(i) is tightly regulated. An abundance of NHE1 in these cellular subdomains may generate a privileged environment that protects the Ca2+-sensitive ACs and other caveolar proteins from local acid shifts.

Inhibition of bradykinin-induced phosphoinositide hydrolysis and Ca2+ mobilisation by phorbol ester in rat cultured vascular smooth muscle cells

Regulation of the increase in inositol phosphate (IP) production and intracellular Ca2+ concentration ([Ca2+]i by protein kinase C (PKC) was investigated in cultured rat vascular smooth muscle cells (VSMCs). Pretreatment of VSMCs with phorbol 12-myristate 14-acetate (PMA, 1 microM) for 30 min almost abolished the BK-induced IP formation and Ca2+ mobilisation. This inhibition was reduced after incubating the cells with PMA for 4 h, and within 24 h the BK-induced responses were greater than those of control cells. The concentrations of PMA giving a half-maximal (pEC50) and maximal inhibition of BK induced an increase in [Ca2+]i, were 7.8 +/- 0.3 M and 1 microM, n = 8, respectively. Prior treatment of VSMCs with staurosporine (1 microM), a PKC inhibitor, inhibited the ability of PMA to attenuate BK-induced responses, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. Paralleling the effect of PMA on the BK-induced IP formation and Ca2+ mobilisation, the translocation and downregulation of PKC isozymes were determined by Western blotting with antibodies against different PKC isozymes. The results revealed that treatment of the cells with PMA for various times, translocation of PKC-alpha, betaI, betaII, delta, epsilon, and zeta isozymes from the cytosol to the membrane were seen after 5 min, 30 min, 2 h, and 4 h of treatment. However, 24-h treatment caused a partial downregulation of these PKC isozymes in both fractions. Treatment of VSMCs with 1 microM PMA for either 1 or 24 h did not significantly change the K(D) and Bmax of the BK receptor for binding (control: K(D) = 1.7 +/- 0.2 nM; Bmax = 47.3 +/- 4.4 fmol/mg protein), indicating that BK receptors are not a site for the inhibitory effect of PMA on BK-induced responses. In conclusion, these results demonstrate that translocation of PKC-alpha, betaI, betaII, delta, epsilon, and zeta induced by PMA caused an attenuation of BK-induced IPs accumulation and Ca2+ mobilisation in VSMCs.

Uncoupling of bradykinin-induced phosphoinositide hydrolysis and Ca2+ mobilization by phorbol ester in canine cultured tracheal epithelial cells

1. Regulation of the increase in inositol phosphates (IPs) production and intracellular Ca2+ concentration ([Ca2+]i by protein kinase C (PKC) was investigated in canine cultured tracheal epithelial cells (TECs). Stimulation of TECs by bradykinin (BK) led to IPs formation and caused an initial transient [Ca2+]i peak in a concentration-dependent manner. 2. Pretreatment of TECs with phorbol 12-myristate 13-acetate (PMA, 1 microM) for 30 min attenuated the BK-induced IPs formation and Ca2+ mobilization. The maximal inhibition occurred after incubating the cells with PMA for 2 h. 3. The concentrations of PMA that gave half-maximal (pEC50) inhibition of BK-induced IPs accumulation and an increase in [Ca2+]i were 7.07 M and 7.11 M, respectively. Inactive phorbol ester, 4alpha-phorbol 12,13-didecanoate at 1 microM, did not inhibit these responses. Prior treatment of TECs with staurosporine (1 microM), a PKC inhibitor, inhibited the ability of PMA to attenuate BK-induced responses, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. 4. In parallel with the effect of PMA on the BK-induced IPs formation and Ca2+ mobilization, the translocation and down-regulation of PKC isozymes were determined. Analysis of cell extracts by Western blotting with antibodies against different PKC isozymes revealed that TECs expressed PKC-alpha, betaI, betaII, gamma, delta, epsilon, theta and zeta. With PMA treatment of the cells for various times, translocation of PKC-alpha, betaI, betaII, gamma, delta, epsilon and theta from cytosol to the membrane was seen after 5 min, 30 min, 2 h, and 4 h treatment. However, 6 h treatment caused a partial down-regulation of these PKC isozymes. PKC-zeta was not significantly translocated and down-regulated at any of the times tested. 5. Treatment of TECs with 1 microM PMA for either 30 min or 6 h did not significantly change the KD, and Bmax receptor for BK binding (control: KD=1.7+/-0.3 nM; Bmax=50.5+/-4.9 fmol/mg protein), indicating that BK receptors are not a site for the inhibitory effect of PMA on BK-induced responses. 6. In conclusion, these results suggest that activation of PKC may inhibit the phosphoinositide hydrolysis and consequently attenuate the [Ca2+]i increase or inhibit independently both responses to BK. The translocation of pKC-alpha, betaI, betaII, delta, epsilon, gamma, and theta induced by PMA caused an attenuation of BK-induced IPs accumulation and Ca2+ mobilization in TECs.

Intracellular calcium signals in response to bradykinin in individual neuroblastoma cells

The Ca indicator fura 2 was used to study the modulation of cytoplasmic Ca by bradykinin (Bk) in single N1E-115 murine neuroblastoma cells. Increases in cytoplasmic Ca in response to Bk were mediated by the B2 receptor subtype. Responses to high concentrations of Bk (1-100 nM) were homogeneous and characterized by a rapidly rising transient that decayed to baseline in the continued presence of agonist, with a half-time of 15 s. Responses to low concentrations of Bk (100-500 pM) were more heterogeneous, with longer latencies and often with oscillations. Pretreatment with thapsigargin for 20 min prevented the Ca response, showing that the Ca change results from intracellular Ca release. Removal of external Ca had little effect on the response to Bk, indicating that the agonist does not activate Ca influx. The extent of Ca release and refilling after Bk was tested with ionomycin. A saturating dose of Bk (20 nM) mobilizes > 90% of stored Ca within 30 s, and this is replaced slowly. Replacement of external Na by N-methyl-D-glucamine to block Na/Ca exchange affected the Ca response, causing decreases in latency and in the period of Ca oscillations and increases in overall duration and peak amplitude of the response.

Inhibitory effect of phorbol ester on bradykinin-induced phosphoinositide hydrolysis and calcium mobilization in cultured canine tracheal smooth muscle cells

Regulation of the increase in inositol 1,4,5-trisphosphate (IP3) production and intracellular Ca2+ concentration ([Ca2+]i) by protein kinase C (PKC) was investigated in cultured canine tracheal smooth muscle cells (TSMCs). Stimulation of TSMCs by bradykinin (BK) led to IP3 formation and caused an initial transient peak followed by a sustained elevation of [Ca2+]i in a concentration-dependent manner. Pretreatment of TSMCs with phorbol 12-myristate 13-acetate (PMA, 1 microM) for 30 min blocked the BK-induced IP3 formation and Ca2+ mobilization. However, this inhibition was reduced after incubating the cells for 4 h with PMA. Inactive phorbol ester, 4 alpha-phorbol 12,13-didecanoate at 1 microM, did not inhibit these responses to BK. Prior treatment with staurosporine (1 microM), a PKC inhibitor, inhibited the effect of PMA on the BK-induced response, suggesting that the effect of PMA is mediated by the activation of PKC. In parallel experiments, a change of PKC activity was observed. PMA rapidly decreased PKC activity in the cytosol of TSMCs, while increasing it transiently in the cell membranes within 30 min. Thereafter the membrane-associated PKC activity decreased and persisted for at least 24 h of PMA treatment. Moreover, treatment with 1 microM PMA for 2 and 24 h did not significantly change the KD and Bmax of the BK receptor for [H]BK binding (control: KD = 2.3 +/- 0.3 nM, Bmax = 25.2 +/- 1.4 fmol/mg protein). These results suggest that activation of PKC inhibit IP3 accumulation and consequently attenuate [Ca2+]i increase or inhibit independently both responses. The PMA-induced inhibition of responses to BK was associated with an increase in membranous PKC activity.

Capacitative Ca2+ entry exclusively inhibits cAMP synthesis in C6-2B glioma cells. Evidence that physiologically evoked Ca2+ entry regulates Ca(2+)-inhibitable adenylyl cyclase in non-excitable cells

Elevation of cytosolic free Ca2+ inhibits the type VI adenylyl cyclase that predominates in C6-2B cells. However, it is not known whether there is any selective requirement for Ca2+ entry or release for inhibition of cAMP accumulation to occur. In the present study, the effectiveness of intracellular Ca2+ release evoked by three independent methods (thapsigargin, ionomycin, and UTP) was compared with the capacitative Ca2+ entry that was triggered by these treatments. In each situation, only Ca2+ entry could inhibit cAMP accumulation (La3+ ions blocked the effect); Ca2+ release, which was substantial in some cases, was without effect. A moderate inhibition, as was elicited by a modest degree of Ca2+ entry, could be rendered substantial in the absence of phosphodiesterase inhibitors. Such conditions more closely mimic the physiological situation of normal cells. These results are particularly significant, in demonstrating not only that Ca2+ entry mediates the inhibitory effects of Ca2+ on cAMP accumulation, but also that diffuse elevations in [Ca2+]i are ineffective in modulating cAMP synthesis. This property suggests that, as with certain Ca(2+)-sensitive ion channels, Ca(2+)-sensitive adenylyl cyclases may be functionally colocalized with Ca2+ entry channels.

Bradykinin-stimulated calcium mobilization in cultured canine tracheal smooth muscle cells

Bradykinin (BDK)-induced increases in intracellular Ca2+ concentration ([Ca2+]i) were monitored in cultured canine tracheal smooth muscle cells (TSMCs) using a fluorescent Ca2+ indicator, Fura-2. BDK and kallidin caused an initial transient peak followed by a sustained elevation of [Ca2+]i in a concentration-dependent manner, with half-maximal stimulation (log EC50) obtained at -8.10 M and -8.04 M, respectively. The BDK-induced rise in [Ca2+]i was not affected by the BDK B1 receptor antagonist, des-Arg9[Leu8]-BDK (10 microM). However, the BDK B2 receptor antagonists des-Arg[Hyp3, Thi5,8, D-Phe7]-BDK and Hoe 140 had high affinity in antagonizing BDK with pKB values of 7.5 +/- 0.3 and 8.7 +/- 0.3, respectively. The sustained phase of the rise in [Ca2+]i was dependent on the presence of external Ca2+, as evidenced by a decline to the resting level on addition of EGTA. In the absence of external Ca2+, only an initial transient peak was seen which then declined to the resting level; a sustained elevation of [Ca2+]i could then be evoked by addition of 1.8 mM Ca2+ in the continued presence of BDK. Ca2+ influx was required for the changes in [Ca2+]i, since Ca(2+)-channel blockers, diltiazem, verapamil, and Ni2+, decreased both the initial and sustained elevation of [Ca2+]i in response to BDK. In conclusion, these findings indicate that the initial increase in [Ca2+]i stimulated by BDK acting on BDK B2 receptors is due to the release of Ca2+ from internal stores, followed by the influx of external Ca2+ into the cells. The influx of extracellular Ca2+ partially involves a diltiazem- and verapamil-sensitive Ca2+ channel.

Loss of responses to bradykinin, ATP or carbachol follows depletion of a shared pool of calcium ions

Treatment of NCB-20 neuronal hybrid cells in culture with 100 microM ATP, 100 microM carbachol or 1 microM bradykinin is followed by a transient rise in intracellular calcium ion concentration ([Ca2+]i). Exposure of these cells to any one of these three agonists (ATP, carbachol or bradykinin) is also followed by heterologous desensitization of responses mediated by either of the other two classes of agonist. The heterologous desensitization is not inhibited by Ro-31-2880 (a selective protein kinase C inhibitor), neither is it accompanied by a reduction in the receptor-dependent increase in inositol trisphosphate. Cells were pre-treated in the absence or presence of extracellular Ca2+ with 1 microM bradykinin, 100 microM carbachol or carrier alone, and thereafter exposed to 1 microM ionomycin. The results showed that pre-treatment with bradykinin or carbachol reduced the maximum increase in [Ca2+]i triggered by ionomycin. Exposure of cells to 1 microM ionomycin in the absence of extracellular Ca2+ totally eliminated the subsequent increase in [Ca2+]i mediated by bradykinin, ATP or carbachol. The results indicate that the heterologous desensitization that follows exposure to bradykinin, ATP or carbachol in NCB-20 cells is mediated by a reduction in a shared pool of intracellular calcium ions.

Selective expression of one Ca(2+)-inhibitable adenylyl cyclase in dopaminergically innervated rat brain regions

Type I adenylyl cyclase, which can be stimulated by elevated cellular levels of Ca2+, has been proposed to provide a positive coincidence signal detection system, which can integrate signals arising via Gs- and Ca(2+)-mediated pathways. The occurrence of this adenylyl cyclase in brain regions implicated with associative learning in invertebrates and with the mammalian model of plasticity--hippocampal long-term potentiation, supports the notion that the ability of this species of adenylyl cyclase to detect two signals simultaneously may play a role in this neuronal function. In the present study, two recently cloned, closely-related adenylyl cyclases (Types V and VI), are shown to be inhibited by physiological elevation in [Ca2+]i. As a first step towards probing the neuronal significance of Ca(2+)-inhibitable adenylyl cyclases, their distribution was evaluated by in situ hybridization analysis of the rat brain. Strikingly distinct patterns of gene expression were found, ranging from a highly selective distribution of Type V mRNA within the striatum, nucleus accumbens and olfactory tubercle, to a weak and ubiquitous distribution of Type VI mRNA. Type V AC mRNA is expressed exclusively in medium-sized striatal neurons, which also express D1-dopaminergic (Gs-linked) and M1-muscarinic cholinergic (Ca(2+)-linked) receptors. Thus the adenylyl cyclase is primed for simultaneous detection of opposing regulatory influences. The utility of this novel mode of signal detection to dopaminergic function remains to be established.

Substance P-evoked calcium mobilization and ionic current activation in the human astrocytoma cell line U 373 MG: pharmacological characterization

In the human astrocytoma cell line U 373 MG, application of substance P (SP) leads to a transient increase in cytosolic calcium concentration and to a biphasic current response in voltage-clamped cells. Using these two functional assays we have characterized pharmacologically the SP response in U 373 MG cells. SP and [L-Pro9]SP displayed high potencies in both assays with EC50 values of 2.5 x 10(-9) M and 1 x 10(-9) M on calcium responses and 1 x 10(-9) M and 5 x 10(-9) M on ion current responses, respectively. The high potency of SP and [L-Pro9]SP as well as the low potency of [Lys5,MeLeu9,N-Leu10]neurokinin A(4-10) and the inactivity of senktide demonstrate the NK1-type pharmacology of these responses. Furthermore, the NK1 antagonists (+/-)-CP 96,345, its chloro analogue, (+/-)-cis-3-(2-chlorobenzylamino)-2-benzhydrylquinuclidine, and RP 67580 were potent antagonists of both SP responses. For the calcium mobilization induced by SP (10(-7) M), the IC50 values for the three antagonists were 4 x 10(-10) M, 4 x 10(-9) M, and 9 x 10(-9) M, respectively, whereas on the current response evoked by SP (10(-8) M), the IC50 values were 8 x 10(-9) M, 2.4 x 10(-8) M, and 1.2 x 10(-7) M, respectively. Despite differences in the absolute IC50 values obtained with both techniques, the relative potencies of the three antagonists correlate fairly well.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of elevated cyclic AMP levels on histamine-H1-receptor-stimulated inositol phospholipid hydrolysis and calcium mobilization in the smooth-muscle cell line DDT1MF-2

The effects of raising cyclic AMP levels, by forskolin stimulation, beta-adrenoceptor activation or cyclic AMP phosphodiesterase inhibition, on inositol phospholipid hydrolysis and increases in intracellular free [Ca2+] ([Ca2+]i) elicited by a range of agonists have been investigated in the hamster vas deferens smooth-muscle cell line DDT1MF-2. Isoprenaline (log [EC50 (M)] = -7.7 +/- 0.2), forskolin and the type IV cyclic AMP phosphodiesterase inhibitor rolipram elicited significant increases in the accumulation of cyclic [3H]AMP. Pretreatment with forskolin (10 microM) attenuated histamine (100 microM)- and N6-cyclopentyladenosine (CPA; 300 nM)-induced release of intracellular Ca2+, observed when cells are stimulated in Ca(2+)-free buffer containing 0.1 mM EGTA. Forskolin had no effect on ATP (100 microM)- or bradykinin (1 microM)-stimulated release of intracellular Ca2+. Histamine-induced intracellular Ca2+ release was also inhibited by pretreatment with rolipram (100 microM) or the membrane-permeant cyclic AMP analogue (Sp)-adenosine 3',5'-monophosphothioate (100 microM). Isoprenaline (1 microM) pretreatment (in the presence of 10 microM rolipram, a concentration which on its own did not decrease the histamine response) attenuated histamine-induced intracellular Ca2+ release. Forskolin inhibited histamine (100 microM)- and CPA (100 nM) stimulated accumulation of [3H]-inositol phosphates, but was without effect on ATP or bradykinin responses. Addition of forskolin (in the presence of 100 microM rolipram) after the cells had been stimulated with histamine (in experiments initiated in Ca(2+)-free buffer) inhibited the rise in [Ca2+]i observed when extracellular Ca2+ (2 mM) was re-applied (owing to receptor-mediated Ca2+ influx). Finally, the refilling of intracellular Ca2+ stores (after receptor-mediated Ca2+ influx is blocked by mepyramine) can be demonstrated in the presence of raised cyclic AMP levels.

Inhibition of cyclic AMP accumulation in intact NCB-20 cells as a direct result of elevation of cytosolic Ca2+

Earlier studies established that adenylyl cyclase in NCB-20 cell plasma membranes is inhibited by concentrations of Ca2+ that are achieved in intact cells. The present studies were undertaken to prove that agents such as bradykinin and ATP, which elevate the cytosolic Ca2+ concentration ([Ca2+]i) from internal stores in NCB-20 cells, could inhibit cyclic AMP (cAMP) accumulation as a result of their mobilization of [Ca2+]i and not by other mechanisms. Both bradykinin and ATP transiently inhibited [3H]cAMP accumulation in parallel with their transient mobilization of [Ca2+]i. The [Ca2+]i rise stimulated by bradykinin could be blocked by treatment with thapsigargin; this thapsigargin treatment precluded the inhibition of cAMP accumulation mediated by bradykinin (and ATP). A rapid rise in [Ca2+]i, as elicited by bradykinin, rather than the slow rise evoked by thapsigargin was required for inhibition of [3H]cAMP accumulation. Desensitization of protein kinase C did not modify the inhibitory action of bradykinin on [3H]cAMP. Effects of Ca2+ on phosphodiesterase were also excluded in the present studies. The accumulated data are consistent with the hypothesis that hormonal mobilization of [Ca2+]i leads directly to the inhibition of cAMP accumulation in these cells and presumably in other cells that express the Ca(2+)-inhibitable form of adenylyl cyclase.