Differing temporal roles of Ca2+ and cAMP in nicotine-elicited elevation of tyrosine hydroxylase mRNA

The involvement of cAMP- andCa²⁺-mediated pathways in theactivation of tyrosine hydroxylase (TH) gene expression by nicotine wasexamined in PC-12 cells. ExtracellularCa²⁺ and elevations inintracellular Ca²⁺ concentration([Ca²⁺]_i)were required for nicotine to increase TH mRNA. The nicotine-elicited rapid rise in[Ca²⁺]_iwas inhibited by blockers of either L-type or N-type, and to a lesserextent P/Q-, but not T-type, voltage-gatedCa²⁺ channels. With continualnicotine treatment,[Ca²⁺]_ireturned to basal levels within 3-4 min. After a lag of~5-10 min, there was a smaller elevation in[Ca²⁺]_ithat persisted for 6 h and displayed different responsiveness toCa²⁺ channel blockers. This secondphase of elevated[Ca²⁺]_iwas blocked by an inhibitor of store-operatedCa²⁺ channels, consistent with theobserved generation of inositol trisphosphate.1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM (BAPTA-AM), when added before or 2 h after nicotine,prevented elevation of TH mRNA. Nicotine treatment significantly raised cAMP levels. Addition of the adenylyl cyclase inhibitor2',5'-dideoxyadenosine (DDA) prevented thenicotine-elicited phosphorylation of cAMP response element bindingprotein. DDA also blocked the elevation of TH mRNA only when addedafter the initial transient rise in [Ca²⁺]_iand not after 1 h. This study reveals that several temporal phases areinvolved in the induction of TH gene expression by nicotine, each ofthem with differing requirements forCa²⁺ and cAMP.

     

Intracellular signal transduction during gastrin-induced histamine secretion in rat gastric ECL cells

Activation of G_qprotein-coupled receptors usually causes a biphasic increase inintracellular calcium concentration ([Ca²⁺]_i)that is crucial for secretion in nonexcitable cells. In gastric enterochromaffin-like (ECL) cells, stimulation with gastrin leads to aprompt biphasic calcium response followed by histamine secretion. Thisstudy investigates the underlying signaling events in this neuroendocrine cell type. In ECL cells, RT-PCR suggested the presence of inositol 1,4,5-trisphosphate receptor (IP₃R) subtypes1-3. The IP₃R antagonist 2-aminoethoxydiphenyl borateabolished both gastrin-induced elevation of[Ca²⁺]_i and histamine release. Thapsigarginincreased [Ca²⁺]_i, however, without inducinghistamine secretion. In thapsigargin-pretreated cells, gastrinincreased [Ca²⁺]_i through calcium influxacross the plasma membrane. Both nimodipine and SKF-96365 inhibitedgastrin-induced histamine release. The protein kinase C (PKC) activatorphorbol 12-myristate 13-acetate induced histamine secretion, an effectthat was prevented by nimodipine. In summary, gastrin-stimulatedhistamine release depends on IP₃R activation andplasmalemmal calcium entry. Gastrin-induced calcium influx wasmediated by dihydropyridine-sensitive calcium channels that appear tobe L-type channels activated through a pathway involving activation of PKC.

     

[Ca2+]i-reducing action of cAMP in rat pancreatic beta -cells: involvement of thapsigargin-sensitive stores

In the presentstudy, we examined the ability of adenosine 3',5'-cyclicmonophosphate (cAMP) to reduce elevated levels of cytosolicCa²⁺ concentration([Ca²⁺]_i)in pancreatic -cells.[Ca²⁺]_iand reduced pyridine nucleotide, NAD(P)H, were measured in rat single-cells by fura 2 and autofluorescence microfluorometry. Sustained[Ca²⁺]_ielevation, induced by high KCl (25 mM) at a basal glucose concentration (2.8 mM), was substantially reduced by cAMP-increasing agents, dibutyryl cAMP (DBcAMP, 5 mM), an adenylyl cyclase activatorforskolin (10 µM), and an incretin glucagon-likepeptide-1-(7-36) amide (10⁹ M), as well as byglucose (16.7 mM). The[Ca²⁺]_i-reducingeffects of cAMP were greater at elevated glucose (8.3-16.7 mM)than at basal glucose (2.8 mM). An inhibitor of protein kinase A (PKA),H-89, counteracted[Ca²⁺]_i-reducingeffects of cAMP but not those of glucose. Okadaic acid, a phosphataseinhibitor, at 10-100 nM also reduced sustained [Ca²⁺]_ielevation in a concentration-dependent manner. Glucose, but not DBcAMP,increased NAD(P)H in -cells.[Ca²⁺]_i-reducingeffects of cAMP were inhibited by 0.3 µM thapsigargin, an inhibitorof the endoplasmic reticulum (ER)Ca²⁺ pump. In contrast,[Ca²⁺]_i-reducingeffects of cAMP were not altered by ryanodine, an ERCa²⁺-release inhibitor,Na⁺-free conditions, or diazoxide,an ATP-sensitive K⁺ channelopener. In conclusion, the cAMP-PKA pathway reduces[Ca²⁺]_ielevation by sequestering Ca²⁺ inthapsigargin-sensitive stores. This process does not involve, but ispotentiated by, activation of -cell metabolism. Together with theknown[Ca²⁺]_i-increasingaction of cAMP, our results reveal dual regulation of -cell[Ca²⁺]_iby the cAMP-signaling pathway and by a physiological incretin.

     

Differential Ca2+ signaling by thrombin and protease-activated receptor-1-activating peptide in human brain microvascular endothelial cells

Thrombin and related protease-activated receptors 1, 2, 3, and 4 (PAR1–4) play a multifunctional role in many types of cells including endothelial cells. Here, using RT-PCR and immunofluorescence staining, we showed for the first time that PAR1–4 are expressed on primary human brain microvascular endothelial cells (HBMEC). Digital fluorescence microscopy and fura 2 were used to monitor intracellular Ca²⁺ concentration ([Ca²⁺]_i) changes in response to thrombin and PAR1-activating peptide (PAR1-AP) SFFLRN. Both thrombin and PAR1-AP induced a dose-dependent [Ca²⁺]_i rise that was inhibited by pretreatment of HBMEC with the phospholipase C inhibitor U-73122 and the sarco(endo)plasmic reticulum Ca²⁺-ATPase inhibitor thapsigargin. Thrombin induced transient [Ca²⁺]_i increase, whereas PAR1-AP exhibited sustained [Ca²⁺]_i rise. The PAR1-AP-induced sustained [Ca²⁺]_i rise was significantly reduced in the absence of extracellular calcium or in the presence of an inhibitor of store-operated calcium channels, SKF-96365. Restoration of extracellular Ca²⁺ to the cells that were initially activated by PAR1-AP in the absence of extracellular Ca²⁺ resulted in significant [Ca²⁺]_i rise; however, this effect was not observed after thrombin stimulation. Pretreatment of the cells with a low thrombin concentration (0.1 nM) prevented [Ca²⁺]_i rise in response to high thrombin concentration (10 nM), but pretreatment with PAR1-AP did not prevent subsequent [Ca²⁺]_i rise to high PAR1-AP concentration. Additionally, treatment with thrombin decreased transendothelial electrical resistance in HBMEC, whereas PAR1-AP was without significant effect. These findings suggest that, in contrast to thrombin, stimulation of PAR1 by untethered peptide SFFLRN results in stimulation of store-operated Ca²⁺ influx without significantly affecting brain endothelial barrier functions. store-operated calcium influx; desensitization; transendothelial electrical resistance; digital imaging     

Ca2+]i regulates trafficking of Cav1.3 (alpha1D Ca2+ channel) in insulin-secreting cells

Chronic exposure of pancreatic -cells to high concentrations of glucose impairs the insulin secretory response to further glucose stimulation. This phenomenon is referred to as glucose desensitization. It has been shown that glucose desensitization is associated with abnormal elevation of -cell basal intracellular free Ca²⁺ concentration ([Ca²⁺]_i). We have investigated the relationship between the basal intracellular free Ca²⁺ and the L-type (Ca_v1.3) Ca²⁺ channel translocation in insulin-secreting cells. Glucose stimulation or membrane depolarization induced a nifedipine-sensitive Ca²⁺ influx, which was attenuated when the basal [Ca²⁺]_i was elevated. Using voltage-clamp techniques, we found that changing [Ca²⁺]_i could regulate the amplitude of the Ca²⁺ current. This effect was attenuated by drugs that interfere with the cytoskeleton. Immunofluorescent labeling of Ca_v1.3 showed an increase in the cytoplasmic distribution of the channels under high [Ca²⁺]_i conditions by deconvolution microscopy. The [Ca²⁺]_i-dependent translocation of Ca_v1.3 channel was also demonstrated by Western blot analysis of biotinylation/NeutrAvidin-bead-eluted surface proteins in cells preincubated at various [Ca²⁺]_i. These results suggest that Ca_v1.3 channel trafficking is involved in glucose desensitization of pancreatic -cells. internalization; intracellular free calcium; glucose desensitization     

Mitochondrial reactive oxygen species and Ca2+ signaling

Mitochondria are an important source of reactive oxygen species (ROS) formed as a side product of oxidative phosphorylation. The main sites of oxidant production are complex I and complex III, where electrons flowing from reduced substrates are occasionally transferred to oxygen to form superoxide anion and derived products. These highly reactive compounds have a well-known role in pathological states and in some cellular responses. However, although their link with Ca²⁺ is well studied in cell death, it has been hardly investigated in normal cytosolic calcium concentration ([Ca²⁺]_i) signals. Several Ca²⁺ transport systems are modulated by oxidation. Oxidation increases the activity of inositol 1,4,5-trisphosphate and ryanodine receptors, the main channels releasing Ca²⁺ from intracellular stores in response to cellular stimulation. On the other hand, mitochondria are known to control [Ca²⁺]_i signals by Ca²⁺ uptake and release during cytosolic calcium mobilization, specially in mitochondria situated close to Ca²⁺ release channels. Mitochondrial inhibitors modify calcium signals in numerous cell types, including oscillations evoked by physiological stimulus. Although these inhibitors reduce mitochondrial Ca²⁺ uptake, they also impair ROS production in several systems. In keeping with this effect, recent reports show that antioxidants or oxidant scavengers also inhibit physiological calcium signals. Furthermore, there is evidence that mitochondria generate ROS in response to cell stimulation, an effect suppressed by mitochondrial inhibitors that simultaneously block [Ca²⁺]_i signals. Together, the data reviewed here indicate that Ca²⁺-mobilizing stimulus generates mitochondrial ROS, which, in turn, facilitate [Ca²⁺]_i signals, a new aspect in the biology of mitochondria. Finally, the potential implications for biological modeling are discussed. mitochondria; calcium     

Enhanced response to caffeine and 4-chloro-m-cresol in malignant hyperthermia-susceptible muscle is related in part to chronically elevated resting [Ca2+]i

Malignant hyperthermia (MH) is a potentially fatal pharmacogenetic syndrome caused by exposure to halogenated volatile anesthetics and/or depolarizing muscle relaxants. We have measured intracellular Ca²⁺ concentration ([Ca²⁺]_i) using double-barreled, Ca²⁺-selective microelectrodes in myoballs prepared from skeletal muscle of MH-susceptible (MHS) and MH-nonsusceptible (MHN) swine. Resting [Ca²⁺]_i was approximately twofold in MHS compared with MHN quiescent myoballs (232 ± 35 vs. 112 ± 11 nM). Treatment of myoballs with caffeine or 4-chloro-m-cresol (4-CmC) produced an elevation in [Ca²⁺]_i in both groups; however, the concentration required to cause a rise in [Ca²⁺]_i elevation was four times lower in MHS than in MHN skeletal muscle cells. Incubation of MHS cells with the fast-complexing Ca²⁺ buffer BAPTA reduced [Ca²⁺]_i, raised the concentration of caffeine and 4-CmC required to cause an elevation of [Ca²⁺]_i, and reduced the amount of Ca²⁺ release associated with exposure to any given concentration of caffeine or 4-CmC to MHN levels. These results suggest that the differences in the response of MHS skeletal myoballs to caffeine and 4-CmC may be mediated at least in part by the chronic high resting [Ca²⁺]_i levels in these cells. calcium homeostasis; 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid     

Suppression of cAMP by phosphoinositol/Ca2+ pathway in the cardiac kappa -opioid receptor

To determine whetherthe phosphoinositol/Ca²⁺ pathwayinteracts with the adenylate cyclase/adenosine 3',5'-cyclicmonophosphate (cAMP) pathway in the cardiac -receptor, the effectsof U-50488, a specific -receptor agonist, on the intracellularCa²⁺ concentration([Ca²⁺]_i)and forskolin-induced accumulation of cAMP in rat ventricular myocyteswere determined after interference of thephosphoinositol/Ca²⁺ pathway.U-50488 suppressed the forskolin-induced accumulation of cAMP andelevated[Ca²⁺]_i,which were blocked by norbinaltorphimine, a specific -receptor antagonist, and pertussis toxin. The effects of U-50488 werequalitatively similar to those of A-23187, aCa²⁺ ionophore, but opposite tothose of1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-acetoxymethyl ester (AM), a[Ca²⁺]_ichelator. Abolition of U-50488-induced elevation of[Ca²⁺]_iby BAPTA-AM also abolished the effect of U-50488 on forskolin-induced accumulation of cAMP. Inhibition of the phospholipase C by specific inhibitors, U-73122 and neomycin, abolished the effects of U-50488 onboth[Ca²⁺]_iand forskolin-induced accumulation of cAMP. The results showed for thefirst time that -receptor stimulation may suppress cAMP accumulationvia activation of thephosphoinositol/Ca²⁺ pathway inthe rat heart.

     

Regulation of cytoplasmic calcium levels by two nitric oxide receptors

We examined the effects of dissolved nitric oxide (NO) gas oncytoplasmic calcium levels ([Ca²⁺]_i) in C6glioma cells under anoxic conditions. The maximum elevation (27 ± 3 nM) of [Ca²⁺]_i was reached at 10 µM NO. Asecond application of NO was ineffective if the first was >0.5 µM.The NO donor diethylamine/NO mimicked the effects of NO. Acute exposureof the cells to low calcium levels was without effect on the NO-evokedresponse. Thapsigargin (TG) increased [Ca²⁺]_iand was less effective if cells were pretreated with NO. Hemoglobin inhibited the effects of NO at a molar ratio of 10:1. 8-Bromo-cGMP waswithout effect on the NO-evoked response. If cells were pretreated withTG or exposed chronically to nominal amounts of calcium, NO decreased[Ca²⁺]_i. The results suggest that C6 gliomacells have two receptors for NO. One receptor (NO_A)elevates [Ca²⁺]_i and resides on theendoplasmic reticulum (ER). The other receptor (NO_B)decreases [Ca²⁺]_i and resides on theplasmalemma or the ER. The latter receptor dominates when the level ofcalcium within intracellular stores is diminished.

     

Cellular Responses to Mechanical Stress: Invited Review: Effects of flow on vascular endothelial intracellular calcium signaling of rat aortas ex vivo

To study the effects of flow on in situendothelial intracellular calcium concentration([Ca²⁺]_i) signaling, rat aortic rings wereloaded with fura 2, mounted on a tissue flow chamber, and divided intocontrol and flow-pretreated groups. The latter was perfused with bufferat a shear stress of 50 dyns/cm² for 1 h. Endothelial[Ca²⁺]_i responses to ACh or shear stresseswere determined by ratio image analysis. Moreover, ACh-induced[Ca²⁺]_i elevation responses were measured ina calcium-free buffer, or in the presence of SKF-96365, to elucidatethe role of calcium influx in the flow effects. Our results showed that1) ACh increased endothelial[Ca²⁺]_i in a dose-dependent manner, and theseresponses were incremented by flow-pretreatment; 2) thedifferences in ACh-induced [Ca²⁺]_i elevationbetween control and flow-pretreated groups were abolished by SKF-96365or by Ca²⁺-free buffer; and 3) in the presenceof 10⁵ M ATP, shear stress induced dose-dependent[Ca²⁺]_i elevation responses that were notaltered by flow-pretreatment. In conclusion, flow-pretreatment augmentsthe ACh-induced endothelial calcium influx in rat aortas ex vivo.

     

Modulation of intracellular Ca2+ release and capacitative Ca2+ entry by CaMKII inhibitors in bovine vascular endothelial cells

The effects of inhibitors of CaMKII on intracellular Ca²⁺ signaling were examined in single calf pulmonary artery endothelial (CPAE) cells using indo-1 microfluorometry to measure cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i). The three CaMKII inhibitors, KN-93, KN-62, and autocamtide-2-related inhibitory peptide (AIP), all reduced the plateau phase of the [Ca²⁺]_i transient evoked by stimulation with extracellular ATP. Exposure to KN-93 or AIP alone in the presence of 2 mM extracellular Ca²⁺ resulted in a dose-dependent increase of [Ca²⁺]_i consisting of a rapid and transient Ca²⁺ spike followed by a small sustained plateau phase of elevated [Ca²⁺]_i. Exposure to KN-93 in the absence of extracellular Ca²⁺ caused a transient rise of [Ca²⁺]_i, suggesting that exposure to CaMKII inhibitors directly triggered release of Ca²⁺ from intracellular endoplasmic reticulum (ER) Ca²⁺ stores. Repetitive stimulation with KN-93 and ATP, respectively, revealed that both components released Ca²⁺ largely from the same store. Pretreatment of CPAE cells with the membrane-permeable inositol 1,4,5-trisphosphate (IP₃) receptor blocker 2-aminoethoxydiphenyl borate caused a significant inhibition of the KN-93-induced Ca²⁺ response, suggesting that exposure to KN-93 affects Ca²⁺ release from an IP₃-sensitive store. Depletion of Ca²⁺ stores by exposure to ATP or to the ER Ca²⁺ pump inhibitor thapsigargin triggered robust capacitative Ca²⁺ entry (CCE) signals in CPAE cells that could be blocked effectively with KN-93. The data suggest that in CPAE cells, CaMKII modulates Ca²⁺ handling at different levels. The use of CaMKII inhibitors revealed that in CPAE cells, the most profound effects of CaMKII are inhibition of release of Ca²⁺ from intracellular stores and activation of CCE. Ca²⁺/calmodulin-dependent kinase II; calcium regulation; capacitative calcium entry     

Dual effects of n-alcohols on fluid secretion from guinea pig pancreatic ducts

Ethanol strongly augments secretin-stimulated, but not acetylcholine (ACh)-stimulated, fluid secretion from pancreatic duct cells. To understand its mechanism of action, we examined the effect of short-chain n-alcohols on fluid secretion and intracellular Ca²⁺ concentration ([Ca²⁺]_i) in guinea pig pancreatic ducts. Fluid secretion was measured by monitoring the luminal volume of isolated interlobular ducts. [Ca²⁺]_i was estimated using fura-2 microfluorometry. Methanol and ethanol at 0.3–10 mM concentrations significantly augmented fluid secretion and induced a transient elevation of [Ca²⁺]_i in secretin- or dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP)-stimulated ducts. However, they failed to affect fluid secretion and [Ca²⁺]_i in unstimulated and ACh-stimulated ducts. In contrast, propanol and butanol at 0.3–10 mM concentrations significantly reduced fluid secretion and decreased [Ca²⁺]_i in unstimulated ducts and in ducts stimulated with secretin, DBcAMP, or ACh. Both stimulatory and inhibitory effects of n-alcohols completely disappeared after their removal from the perfusate. Propanol and butanol inhibited the plateau phase, but not the initial peak, of [Ca²⁺]_i response to ACh as well as the [Ca²⁺]_i elevation induced by thapsigargin, suggesting that they inhibit Ca²⁺ influx. Removal of extracellular Ca²⁺ reduced [Ca²⁺]_i in duct cells and completely abolished secretin-stimulated fluid secretion. In conclusion, there is a distinct cutoff point between ethanol (C2) and propanol (C3) in their effects on fluid secretion and [Ca²⁺]_i in duct cells. Short-chain n-alcohols appear to affect pancreatic ductal fluid secretion by activating or inhibiting the plasma membrane Ca²⁺ channel. intracellular calcium; acetylcholine     

PKA induces Ca2+ release and enhances ciliary beat frequency in a Ca2+-dependent and -independent manner

The intent of this work was to evaluate the role of cAMP inregulation of ciliary activity in frog mucociliary epithelium and toexamine the possibility of cross talk between the cAMP- andCa²⁺-dependent pathways in thatregulation. Forskolin and dibutyryl cAMP induced strong transientintracellular Ca²⁺ concentration([Ca²⁺]_i)elevation and strong ciliary beat frequency enhancement with prolongedstabilization at an elevated plateau. The response was not affected byreduction of extracellular Ca²⁺concentration. The elevation in[Ca²⁺]_iwas canceled by pretreatment with1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM, thapsigargin, and a phospholipase C inhibitor, U-73122. Underthose experimental conditions, forskolin raised the beat frequency to amoderately elevated plateau, whereas the initial strong rise infrequency was completely abolished. All effects were canceled by H-89,a selective protein kinase A (PKA) inhibitor. The results suggest adual role for PKA in ciliary regulation. PKA releasesCa²⁺ from intracellular stores,strongly activating ciliary beating, and, concurrently, producesmoderate prolonged enhancement of the beat frequency by aCa²⁺-independent mechanism.

     

Regulation of cAMP dynamics by Ca2+ and G protein-coupled receptors in the pancreatic -cell: a computational approach

In this report we describe a mathematical model for the regulation of cAMP dynamics in pancreatic β-cells. Incretin hormones such as glucagon-like peptide 1 (GLP-1) increase cAMP and augment insulin secretion in pancreatic β-cells. Imaging experiments performed in MIN6 insulinoma cells expressing a genetically encoded cAMP biosensor and loaded with fura-2, a calcium indicator, showed that cAMP oscillations are differentially regulated by periodic changes in membrane potential and GLP-1. We modeled the interplay of intracellular calcium (Ca²⁺) and its interaction with calmodulin, G protein-coupled receptor activation, adenylyl cyclases (AC), and phosphodiesterases (PDE). Simulations with the model demonstrate that cAMP oscillations are coupled to cytoplasmic Ca²⁺ oscillations in the β-cell. Slow Ca²⁺ oscillations (<1 min^–1) produce low-frequency cAMP oscillations, and faster Ca²⁺ oscillations (>3–4 min^–1) entrain high-frequency, low-amplitude cAMP oscillations. The model predicts that GLP-1 receptor agonists induce cAMP oscillations in phase with cytoplasmic Ca²⁺ oscillations. In contrast, observed antiphasic Ca²⁺ and cAMP oscillations can be simulated following combined glucose and tetraethylammonium-induced changes in membrane potential. The model provides additional evidence for a pivotal role for Ca²⁺-dependent AC and PDE activation in coupling of Ca²⁺ and cAMP signals. Our results reveal important differences in the effects of glucose/TEA and GLP-1 on cAMP dynamics in MIN6 β-cells. adenylyl cyclase; calcium ion; glucagon-like peptide 1; modeling; oscillations     

Regulation of exocytosis by purinergic receptors in pancreatic duct epithelial cells

In epithelial cells, several intracellular signals regulate the secretion of large molecules such as mucin via exocytosis and the transport of ions through channels and transporters. Using carbon fiber amperometry, we previously reported that exocytosis of secretory granules in dog pancreatic duct epithelial cells (PDEC) can be stimulated by pharmacological activation of cAMP-dependent protein kinase (PKA) or protein kinase C (PKC), as well as by an increase of intracellular free Ca²⁺ concentration ([Ca²⁺]_i). In this study, we examined whether exocytosis in these cells is modulated by activation of endogenous P2Y receptors, which increase cAMP and [Ca²⁺]_i. Low concentrations of ATP (<10 µM) induced intracellular Ca²⁺ oscillation but no significant exocytosis. In contrast, 100 µM ATP induced a sustained [Ca²⁺]_i rise and increased the exocytosis rate sevenfold. The contribution of Ca²⁺ or cAMP pathways to exocytosis was tested by using the Ca²⁺ chelator BAPTA or the PKA inhibitors H-89 or Rp-8-bromoadenosine 3',5'-cyclic monophosphorothioate. Removal of [Ca²⁺]_i rise or inhibition of PKA each partially reduced exocytosis; when combined, they abolished exocytosis. In conclusion, ATP at concentrations >10 µM stimulates exocytosis from PDEC through both Ca²⁺ and cAMP pathways. secretion; amperometry; photometry; calcium, adenosine 3',5'-cyclic monophosphate     

Effect of intracellular pH on depolarization-evoked calcium influx in human sperm

Human sperm are endowed with putative voltage-dependent calcium channels (VDCC) that produce measurable increases in intracellular calcium concentration ([Ca²⁺]_i) in response to membrane depolarization with potassium. These channels are blocked by nickel, inactivate in 1–2 min in calcium-deprived medium, and are remarkably stimulated by NH₄Cl, suggesting a role for intracellular pH (pH_i). In a previous work, we showed that calcium permeability through these channels increases approximately onefold during in vitro "capacitation," a calcium-dependent process that sperm require to fertilize eggs. In this work, we have determined the pH_i dependence of sperm VDCC. Simultaneous depolarization and pH_i alkalinization with NH₄Cl induced an [Ca²⁺]_i increase that depended on the amount of NH₄Cl added. VDCC stimulation as a function of pH_i showed a sigmoid curve in the 6.6–7.2 pH_i range, with a half-maximum stimulation at pH 7.00. At higher pH_i (7.3), a further stimulation occurred. Calcium release from internal stores did not contribute to the stimulating effect of pH_i because the [Ca²⁺]_i increase induced by progesterone, which opens a calcium permeability pathway that does not involve gating of VDCC, was unaffected by ammonium. The ratio of pH_i-stimulated-to-nonstimulated calcium influx was nearly constant at different test depolarization values. Likewise, depolarization-induced calcium influx in pH_i-stimulated and nonstimulated cells was equally blocked by nickel. In our capacitating conditions pH_i increased 0.11 pH units, suggesting that the calcium influx stimulation observed during sperm capacitation might be partially caused by pH_i alkalinization. Additionally, a calcium permeability pathway triggered exclusively by pH_i alkalinization was detected. mammalian sperm; capacitation; intracellular calcium     

Taste responses to divalent cations in the frog glossopharyngeal nerve: competitive inhibition of the Mg2+ response by Ca2+

In the frog glossopharyngeal nerve, single water fibers respondto low CaCl₂ (1–2 mM) and relatively high MgCl₂ (100 mM).In the present study, it was found that stimulation by a mixtureof low CaCl₂ and relatively high MgCl₂ led to a small response.This suggests that the Ca⁺ response is inhibited by the presenceof Mg²⁺ and the Mg²⁺ response is inhibited by the presence ofCa²⁺. Hence, it is suggested that there are different receptorsites for divalent cations in single water fibers of the frogglossopharyngeal nerve, a calcium receptor site (X_Ca) responsiblefor the Ca²⁺ response and a magnesium receptor site (X_Mg) responsiblefor the Mg²⁺ response. It has been reported that Mg²⁺ inhibitsthe Ca²⁺ response by competing with Ca²⁺ for X_Ca (Kitada andShimada, 1980). In the present study, the inhibition of theMg²⁺ response by Ca²⁺ was examined quantitatively under theassumption that the magnitude of the neural response is proportionalto the amount of MgX_Mg complex minus a constant (the thresholdconcentration of the MgX_Mg complex). The results obtained indicatethat Ca²⁺ competes with Mg²⁺ for X^Mg. The apparent dissociationconstants for MgX_Mg complex and CaX_Mg complex, which were obtainedfrom the present study, were 8.0 x 10^–2 M and 7.2 x 10^–4M, respectively. Thus, competition between Ca⁺ and Mg²⁺ forthe distinct receptor sites involved in taste reception wasdemonstrated by the results described in this paper. Since thedivalent cations do not always bring about activation of tastereceptors, the responses to salts in the frog glossopharyngealnerve cannot be explained in terms of changes in the surfacepotential outside the taste cells. The present results suggestthat there exist multiple specific receptor sites for cationsinvolved in salt taste responses, and only the binding of eachseparate cation to its appropriate receptor sites leads to activationof the receptor and the initiation of impulses in sensory nerveendings.     

Sperm extract and inositol 1,4,5-triphosphate induce cytosolic calcium rise in the central cell of Torenia fournieri

Microinjection of soluble sperm extract and Calcium Green-1 10 kDa-dextran conjugate (CG-1) into the mature central cell of Torenia fournieri induced a significant rise in cytosolic free calcium concentration ([Ca²⁺]_i). The rise reached a maximum at 20 min after injection and then steadily declined. Nevertheless, a relatively high level of [Ca²⁺]_i was maintained even 40 min after injection. Microinjection of sperm extract of maize into Torenia central cells, however, did not trigger any increase in [Ca²⁺]_i, suggesting the possibility of distinct triggers in different species. We also injected caged inositol 1,4,5-triphosphate (InsP₃) and caged cyclic ADP-ribose (cADPR) into Torenia central cells to compare the pattern of Ca²⁺ rise induced by the sperm extract. The results showed that [Ca²⁺]_i elevation triggered by the release of InsP₃ after photolysis appears much faster than that induced by sperm extract. The increase in [Ca²⁺]_i reached a maximum at 70-80 s and dropped to the resting level within 300 s after photolysis. Microinjection of cADPR, however, did not induce any changes in [Ca²⁺]_i. The results indicate that sperm extract might contain factors triggering the release of Ca²⁺ in the central cell.     

Tunicamycin increases intracellular calcium levels in bovine aortic endothelial cells

Tunicamycin is anucleoside antibiotic that inhibits protein glycosylation andpalmitoylation. The therapeutic use of tunicamycin is limited inanimals because of its toxic effects, particularly in cerebralvasculature. Tunicamycin decreases palmitoylation of the endothelialisoform of nitric oxide synthase, stimulates nitric oxide synthesis,and increases the concentration of intracellular calcium([Ca²⁺]_i)in bovine aortic endothelial cells (B. J. Buckley and A. R. Whorton.FASEB J. 11: A110, 1997). In the present study,we investigated the mechanism by which tunicamycin alters[Ca²⁺]_iusing the Ca²⁺-sensitive dye fura2. We found that tunicamycin increased[Ca²⁺]_iwithout increasing levels of inositol phosphates. When cells wereincubated in the absence of extracellularCa²⁺,[Ca²⁺]_irapidly rose in response to tunicamycin, although a full response wasnot achieved. The pool of intracellularCa²⁺ mobilized by tunicamycinoverlapped with that mobilized by thapsigargin. Extracellular nickelblocked a full response to tunicamycin when cells were incubated in thepresence of extracellular Ca²⁺.The effects of tunicamycin on[Ca²⁺]_iwere partially reversed by washing out the drug, and the remainder ofthe response was inhibited by removing extracellularCa²⁺. These results indicate thattunicamycin mobilizes Ca²⁺ fromintracellular stores in a manner independent of phospholipase Cactivation and increases the influx ofCa²⁺ across the plasma membrane.

     

Thrombin-, bradykinin-, and arachidonic acid-induced Ca2+ signaling in Ehrlich ascites tumor cells

Stimulation ofsingle Ehrlich ascites tumor cells with agonists (bradykinin, thrombin)and with arachidonic acid (AA) induces increases in the freeintracellular Ca²⁺ concentration([Ca²⁺]_i)in the presence and absence of extracellularCa²⁺, measured using theCa²⁺-sensitive probe fura 2. Sequential stimulation with two agonists elicits sequential increasesin[Ca²⁺]_i,unlike addition of the same agonist twice. Bradykinin and thrombin haveadditive effects on[Ca²⁺]_iin Ca²⁺-free medium. Thephosphoinositidase C inhibitor U-73122 inhibits the agonist-inducedincreases in[Ca²⁺]_i,whereas ryanodine has no effect. Pretreatment of cells in Ca²⁺-free medium with thapsigarginabolishes the bradykinin-induced increase in[Ca²⁺]_ibut not the response to thrombin. The AA-induced response is notinhibited by U-73122 and cannot be mimicked by the inactive structuralanalog trifluoromethylarachidonyl ketone. Pretreatment of the cellswith 50 µM AA (but not with 10 µM AA) abolishes the agonist-inducedincrease in[Ca²⁺]_i.Thus bradykinin, thrombin, and AA induce increases in[Ca²⁺]_iin Ehrlich cells due to Ca²⁺ entryand release from intracellular stores. Thrombin causes release ofCa²⁺ from an intracellular storethat is insensitive to bradykinin and is not depleted by thapsigarginbut is depleted by AA.