Rho kinase mediates serum-induced contraction in fibroblast fibers independent of myosin LC20 phosphorylation

Fibroblasts form fibers when grown inculture medium containing native type 1 collagen. The contractileforces generated can be precisely quantified and used to analyze thesignal transduction pathways regulating fibroblast contraction. Calfserum (30%) induces a sustained contraction that is accompanied by atransient increase in intracellular calcium([Ca²⁺]_i). W-7, a calmodulin inhibitor,KN-62, an inhibitor of calcium/calmodulin-dependent protein kinase, andML-7, a myosin light-chain kinase inhibitor, had no effects on eitherthe contraction or the [Ca²⁺]_i responses.Neither genistein, a tyrosine kinase inhibitor, nor calphostin C, aprotein kinase C inhibitor, had major effects on force or[Ca²⁺]_i. In contrast, the Rho kinaseinhibitors(R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide (Y-27632) and HA1077 depressed the contraction in a dose-dependent manner without affecting the [Ca²⁺]_iresponse. Stress fiber formation was also suppressed by Y-27632. Surprisingly, calf serum, Y-27632, and calf serum plus Y-27632 did notalter mono- or diphosphorylation of the myosin regulatory light chain(MRLC) compared with control untreated fibers. These results suggestthat the sustained contraction of NIH 3T3 fibroblast fibers induced bycalf serum is mediated by Rho kinase but is independent of a sustainedincrease in [Ca²⁺]_i, calcium/calmodulin- orprotein kinase C-dependent pathways, or increases in MRLC phosphorylation.

     

L-type voltage-dependent Ca2+ channels in cerebral microvascular endothelial cells and ET-1 biosynthesis

We investigatedthe role of intracellular calcium concentration([Ca²⁺]_i) in endothelin-1 (ET-1) production,the effects of potential vasospastic agents on[Ca²⁺]_i, and the presence of L-typevoltage-dependent Ca²⁺ channels in cerebral microvascularendothelial cells. Primary cultures of endothelial cells isolated frompiglet cerebral microvessels were used. Confluent cells were exposed toeither the thromboxane receptor agonist U-46619 (1 µM),5-hydroxytryptamine (5-HT; 0.1 mM), or lysophosphatidic acid (LPA; 1 µM) alone or after pretreatment with the Ca²⁺-chelatingagent EDTA (100 mM), the L-type Ca²⁺ channel blockerverapamil (10 µM), or the antagonist of receptor-operated Ca²⁺ channel SKF-96365 HCl (10 µM) for 15 min. ET-1production increased from 1.2 (control) to 8.2 (U-46619), 4.9 (5-HT),or 3.9 (LPA) fmol/µg protein, respectively. Such elevated ET-1biosynthesis was attenuated by verapamil, EDTA, or SKF-96365 HCl. Toinvestigate the presence of L-type voltage-dependent Ca²⁺channels in endothelial cells, the [Ca²⁺]_isignal was determined fluorometrically by using fura 2-AM. Superfusionof confluent endothelial cells with U-46619, 5-HT, or LPA significantlyincreased [Ca²⁺]_i. Pretreatment ofendothelial cells with high K⁺ (60 mM) or nifedipine (4 µM) diminished increases in [Ca²⁺]_i inducedby the vasoactive agents. These results indicate that 1)elevated [Ca²⁺]_i signals are involved in ET-1biosynthesis induced by specific spasmogenic agents, 2) theincreases in [Ca²⁺]_i induced by thevasoactive agents tested involve receptor as well as L-typevoltage-dependent Ca²⁺ channels, and 3) primarycultures of cerebral microvascular endothelial cells express L-typevoltage-dependent Ca²⁺ channels.

     

Acidosis antagonizes intracellular calcium response to kappa -opioid receptor stimulation in the rat heart

To study the effects of -opioid receptor stimulation onintracellular Ca²⁺ concentration([Ca²⁺]_i)homeostasis during extracellular acidosis, we determined the effects of-opioid receptor stimulation on[Ca²⁺]_iresponses during extracellular acidosis in isolated single ratventricular myocytes, by a spectrofluorometric method. U-50488H (10-30 µM), a selective -opioid receptor agonist, dosedependently decreased the electrically induced[Ca²⁺]_itransient, which results from the influx ofCa²⁺ and the subsequentmobilization of Ca²⁺ from thesarcoplasmic reticulum (SR). U-50488H (30 µM) also increased theresting[Ca²⁺]_iand inhibited the[Ca²⁺]_itransient induced by caffeine, which mobilizesCa²⁺ from the SR, indicating thatthe effects of the -opioid receptor agonist involved mobilization ofCa²⁺ from its intracellular poolinto the cytoplasm. The Ca²⁺responses to 30 µM U-50488H were abolished by 5 µMnor-binaltorphimine, a selective -opioid receptorantagonist, indicating that the event was mediated by the -opioidreceptor. The effects of the agonist on[Ca²⁺]_iand the electrically induced[Ca²⁺]_itransient were significantly attenuated when the extracellular pH(pH_e) was loweredto 6.8, which itself reduced intracellular pH(pH_i) and increased[Ca²⁺]_i.The inhibitory effects of U-50488H were restored during extracellular acidosis in the presence of 10 µM ethylisopropyl amiloride, a potentNa⁺/H⁺exchange blocker, or 0.2 mM Ni²⁺,a putativeNa⁺/Ca²⁺exchange blocker. The observations indicate that acidosismay antagonize the effects of -opioid receptor stimulation viaNa⁺/H⁺andNa⁺/Ca²⁺exchanges. When glucose at 50 mM, known to activate theNa⁺/H⁺exchange, was added, both the resting[Ca²⁺]_iand pH_i increased. Interestingly,the effects of U-50488H on [Ca²⁺]_iand the electrically induced[Ca²⁺]_itransient during superfusion with glucose were significantly attenuated; this mimicked the responses during extracellular acidosis. When a high-Ca²⁺ (3 mM) solutionwas superfused, the resting[Ca²⁺]_iincreased; the increase was abolished by 0.2 mMNi²⁺, but thepH_i remained unchanged. Like theresponses to superfusion with high-concentration glucose andextracellular acidosis, the responses of the[Ca²⁺]_iand electrically induced[Ca²⁺]_itransients to 30 µM U-50488H were also significantly attenuated. Results from the present study demonstrated for the first time thatextracellular acidosis antagonizes the effects of -opioid receptorstimulation on the mobilization ofCa²⁺ from SR. Activation of bothNa⁺/H⁺andNa⁺/Ca²⁺exchanges, leading to an elevation of[Ca²⁺]_i,may be responsible for the antagonistic action of extracellular acidosis against -opioid receptor stimulation.

     

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.

     

Ca(2+) mobilization through dorsal root ganglion Ca(2+)-sensing receptor stably expressed in HEK293 cells

The rat dorsal root ganglion (DRG) Ca²⁺-sensing receptor (CaR) was stably expressed in-frame as an enhanced green fluorescent protein (EGFP) fusion protein in human embryonic kidney (HEK)293 cells, and is functionally linked to changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i). RT-PCR analysis indicated the presence of the message for the DRG CaR cDNA. Western blot analysis of membrane proteins showed a doublet of 168–175 and 185 kDa, consistent with immature and mature forms of the CaR.EGFP fusion protein, respectively. Increasing extracellular [Ca²⁺] ([Ca²⁺]_e) from 0.5 to 1 mM resulted in increases in [Ca²⁺]_i levels, which were blocked by 30 µM 2-aminoethyldiphenyl borate. [Ca²⁺]_e-response studies indicate a Ca²⁺ sensitivity with an EC₅₀ of 1.75 ± 0.10 mM. NPS R-467 and Gd³⁺ activated the CaR. When [Ca²⁺]_e was successively raised from 0.25 to 4 mM, peak [Ca²⁺]_i, attained with 0.5 mM, was reduced by 50%. Similar reductions were observed with repeated applications of 10 mM Ca²⁺, 1 and 10 µM NPS R-467, or 50 and 100 µM Gd³⁺, indicating desensitization of the response. Furthermore, Ca²⁺ mobilization increased phosphorylated protein kinase C (PKC) levels in the cells. However, the PKC activator, phorbol myristate acetate did not inhibit CaR-mediated Ca²⁺ signaling. Rather, a spectrum of PKC inhibitors partially reduced peak responses to Ca_e²⁺. Treatment of cells with 100 nM PMA for 24 h, to downregulate PKC, reduced [Ca²⁺]_i transients by 49.9 ± 5.2% (at 1 mM Ca²⁺) and 40.5 ± 6.5% (at 2 mM Ca²⁺), compared with controls. The findings suggest involvement of PKC in the pathway for Ca²⁺ mobilization following CaR activation. desensitization; protein kinase C     

Rho activation in excitatory agonist-stimulated vascular smooth muscle

Small GTPase Rho and its downstream effector, Rho kinase, havebeen implicated in agonist-stimulated Ca²⁺ sensitization of20-kDa myosin light chain (MLC₂₀) phosphorylation andcontraction in smooth muscle. In the present study we demonstrated forthe first time that excitatory receptor agonists induce increases inamounts of an active GTP-bound form of RhoA, GTP-RhoA, in rabbit aorticsmooth muscle. Using a pull-down assay with a recombinant RhoA-bindingprotein, Rhotekin, we found that a thromboxane A₂ mimetic,U-46619, which induced a sustained contractile response, induced asustained rise in the amount of GTP-RhoA in a dose-dependent mannerwith an EC₅₀ value similar to that for the contractile response. U-46619-induced RhoA activation was thromboxaneA₂ receptor-mediated and reversible. Other agonistsincluding norepinephrine, serotonin, histamine, and endothelin-1 (ET-1)also stimulated RhoA, albeit to lesser extents than U-46619. Incontrast, ANG II and phorbol 12,13-dibutyrate failed to increaseGTP-RhoA. The tyrosine kinase inhibitor genistein substantiallyinhibited RhoA activation by these agonists, except for ET-1. Thusexcitatory agonists induce Rho activation in an agonist-specificmanner, which is thought to contribute to stimulation ofMLC₂₀ phosphorylation Ca²⁺ sensitivity.

     

EGF inhibits muscarinic receptor-mediated calcium signaling in a human salivary cell line

The effects of epidermal growth factor(EGF) on intracellular calcium ([Ca²⁺]_i)responses to the muscarinic agonist carbachol were studied in a humansalivary cell line (HSY). Carbachol (10⁴ M)-stimulated[Ca²⁺]_i mobilization was inhibited by 40%after 48-h treatment with 5 × 10¹⁰ M EGF. EGF alsoreduced carbachol-induced [Ca²⁺]_i inCa²⁺-free medium and Ca²⁺ influx followingrepletion of extracellular Ca²⁺. UnderCa²⁺-free conditions, thapsigargin, an inhibitor ofCa²⁺ uptake to internal stores, induced similar[Ca²⁺]_i signals in control and EGF-treatedcells, indicating that internal Ca²⁺ stores were unaffectedby EGF; however, in cells exposed to thapsigargin, Ca²⁺influx following Ca²⁺ repletion was reduced by EGF.Muscarinic receptor density, assessed by binding of the muscarinicreceptor antagonistL-[benzilic-4,4'-³HCN]quinuclidinyl benzilate([³H]QNB), was decreased by 20% after EGF treatment.Inhibition of the carbachol response by EGF was not altered by phorbolester-induced downregulation of protein kinase C (PKC) but was enhancedupon PKC activation by a diacylglycerol analog. Phosphorylation of mitogen-activated protein kinase (MAP kinase) and inhibition of thecarbachol response by EGF were both blocked by the MAP kinase pathwayinhibitor PD-98059. The results suggest that EGF decreases carbachol-induced Ca²⁺ release from internal stores andalso exerts a direct inhibitory action on Ca²⁺ influx. Adecline in muscarinic receptor density may contribute to EGF inhibitionof carbachol responsiveness. The inhibitory effect of EGF is mediatedby the MAP kinase pathway and is potentiated by a distinct modulatorycascade involving activation of PKC. EGF may play a physiological rolein regulating muscarinic receptor-stimulated salivary secretion.

     

Mechanical signals and mechanosensitive modulation of intracellular [Ca(2+)] in smooth muscle

We testedthe hypothesis that strain is the primary mechanical signal in themechanosensitive modulation of intracellular Ca²⁺concentration ([Ca²⁺]_i) in airway smoothmuscle. We found that [Ca²⁺]_i wassignificantly correlated with muscle length during isotonic shorteningagainst 20% isometric force (F_iso). When the isotonic loadwas changed to 50% F_iso, data points from the 20 and 50% F_iso experiments overlapped in thelength-[Ca²⁺]_i relationship. Similarly, datapoints from the 80% F_iso experiments clustered near thosefrom the 50% F_iso experiments. Therefore, despite 2.5- and4-fold differences in external load, [Ca²⁺]_idid not deviate much from the length-[Ca²⁺]_irelation that fitted the 20% F_iso data. Maximal inhibition of sarcoplasmic reticular (SR) Ca²⁺ uptake by 10 µMcyclopiazonic acid (CPA) did not significantly change[Ca²⁺]_i in carbachol-induced isometriccontractions and isotonic shortening. CPA also did not significantlychange myosin light-chain phosphorylation or force redevelopment whencarbachol-activated muscle strips were quickly released from optimallength (L_o) to 0.5 L_o. These results are consistent with thehypothesis and suggest that SR Ca²⁺ uptake is not theunderlying mechanism.

     

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     

Increase in cytosolic Ca2+ induced by elevation of extracellular Ca2+ in skeletal myogenic cells

Cytoplasmic Ca²⁺concentration ([Ca²⁺]_i) variation is akey event in myoblast differentiation, but the mechanism by which itoccurs is still debated. Here we show that increases of extracellular Ca²⁺ concentration ([Ca²⁺]_o)produced membrane hyperpolarization and a concentration-dependent increase of [Ca²⁺]_i due to Ca²⁺influx across the plasma membrane. Responses were not related toinositol phosphate turnover and Ca²⁺-sensing receptor.[Ca²⁺]_o-induced[Ca²⁺]_i increase was inhibited byCa²⁺ channel inhibitors and appeared to be modulated byseveral kinase activities. [Ca²⁺]_i increasewas potentiated by depletion of intracellular Ca²⁺ storesand depressed by inactivation of the Na⁺/Ca²⁺exchanger. The response to arginine vasopressin (AVP), which inducesinositol 1,4,5-trisphosphate-dependent[Ca²⁺]_i increase in L6-C5 cells, was notmodified by high [Ca²⁺]_o. On the contrary,AVP potentiated the [Ca²⁺]_i increase in thepresence of elevated [Ca²⁺]_o. Other clones ofthe L6 line as well as the rhabdomyosarcoma RD cell line and thesatellite cell-derived C2-C12 line expressed similar responses to high[Ca²⁺]_o, and the amplitude of the responseswas correlated with the myogenic potential of the cells.

     

Sphingosylphosphorylcholine stimulates mitogen-activated protein kinase via a Ca2+-dependent pathway

In cultured porcine aortic smooth muscle cells,sphingosylphosphorylcholine (SPC), ATP, or bradykinin (BK) induced arapid dose-dependent increase in the cytosolicCa²⁺ concentration([Ca²⁺]_i)and also stimulated inositol 1,4,5-trisphosphate(IP₃) generation. Pretreatmentof cells with pertussis toxin blocked the SPC-induced IP₃ generation and[Ca²⁺]_iincrease but had no effect on the action of ATP or BK. In addition, SPCstimulated the mitogen-activated protein kinase (MAPK) and increasedDNA synthesis, whereas neither ATP nor BK produced such effects. Boththe SPC-induced MAPK activation and DNA synthesis were pertussis toxinsensitive. SPC-induced MAPK activation was blocked by treatment ofcells with the phospholipase C inhibitor, U-73122, or the intracellularCa²⁺-ATPase inhibitor,thapsigargin, but not by removal of extracellular Ca²⁺. Lysophosphatidic acidinduced cellular responses similar to SPC in a pertussistoxin-sensitive manner in terms of[Ca²⁺]_iincrease, IP₃ generation, MAPKactivation, and DNA synthesis. Platelet-derived growth factor (PDGF)also induced a[Ca²⁺]_iincrease, MAPK activation, and DNA synthesis in the same cells; however, the PDGF-induced MAPK activation was not sensitive to pertussis toxin and changes in[Ca²⁺]_i.SPC-induced MAPK activation was inhibited by pretreatment of cells withstaurosporine, W-7, or calmidazolium. Our results suggest that, inporcine aortic smooth muscle cells, MAPK is not activated by theincrease in[Ca²⁺]_iunless a pertussis toxin-sensitive G protein is simultaneously stimulated, indicating the role ofCa²⁺ in pertussis toxin-sensitiveG protein-mediated MAPK activation.

     

Different kinases regulate activation of voltage-dependent calcium channels by depolarization in GH3 cells

The L-type Ca²⁺ channel is the primary voltage-dependent Ca²⁺-influx pathway in many excitable and secretory cells, and direct phosphorylation by different kinases is one of the mechanisms involved in the regulation of its activity. The aim of this study was to evaluate the participation of Ser/Thr kinases and tyrosine kinases (TKs) in depolarization-induced Ca²⁺ influx in the endocrine somatomammotrope cell line GH3. Intracellular Ca²⁺ concentration ([Ca²⁺]_i) was measured using a spectrofluorometric method with fura 2-AM, and 12.5 mM KCl (K⁺) was used as a depolarization stimulus. K⁺ induced an abrupt spike (peak) in [Ca²⁺]_i that was abolished in the presence of nifedipine, showing that K⁺ enhances [Ca²⁺]_i, preferably activating L-type Ca²⁺ channels. H89, a selective PKA inhibitor, significantly reduced depolarization-induced Ca²⁺ mobilization in a concentration-related manner when it was applied before or after K⁺, and okadaic acid, an inhibitor of Ser/Thr phosphatases, which has been shown to regulate PKA-stimulated L-type Ca²⁺ channels, increased K⁺-induced Ca²⁺ entry. When PKC was activated by PMA, the K⁺-evoked peak in [Ca²⁺]_i, as well as the plateau phase, was significantly reduced, and chelerythrine (a PKC inhibitor) potentiated the K⁺-induced increase in [Ca²⁺]_i, indicating an inhibitory role of PKC in voltage-dependent Ca²⁺ channel (VDCC) activity. Genistein, a TK inhibitor, reduced the K⁺-evoked increase in [Ca²⁺]_i, but, unexpectedly, the tyrosine phosphatase inhibitor orthovanadate reduced not only basal Ca²⁺ levels but, also, Ca²⁺ influx during the plateau phase. Both results suggest that different TKs may act differentially on VDCC activation. Activation of receptor TKs with epidermal growth factor (EGF) or vascular endothelial growth factor potentiated K⁺-induced Ca²⁺ influx, and AG-1478 (an EGF receptor inhibitor) decreased it. However, inhibition of the non-receptor TK pp60 c-Src enhanced K⁺-induced Ca²⁺ influx. The present study strongly demonstrates that a complex equilibrium among different kinases and phosphatases regulates VDCC activity in the pituitary cell line GH3: PKA and receptor TKs, such as vascular endothelial growth factor receptor and EGF receptor, enhance depolarization-induced Ca²⁺ influx, whereas PKC and c-Src have an inhibitory effect. These kinases modulate membrane depolarization and may therefore participate in the regulation of a plethora of intracellular processes, such as hormone secretion, gene expression, protein synthesis, and cell proliferation, in pituitary cells. phosphatases; protein kinase A; protein kinase C; epidermal growth factor     

Role of glycolytically generated ATP for CaMKII-mediated regulation of intracellular Ca2+ signaling in bovine vascular endothelial cells

The role of glycolytically generated ATP in Ca²⁺/calmodulin-dependent kinase II (CaMKII)-mediated regulation of intracellular Ca²⁺ signaling was examined in cultured calf pulmonary artery endothelial (CPAE) cells. Exposure of cells (extracellular Ca²⁺ concentration = 2 mM) to glycolytic inhibitors 2-deoxy-D-glucose (2-DG), pyruvate (pyr) + -hydroxybutyrate (-HB), or iodoacetic acid (IAA) caused an increase of intracellular Ca²⁺ concentration ([Ca²⁺]_i). CaMKII inhibitors (KN-93, W-7) triggered a similar increase of [Ca²⁺]_i. The rise of [Ca²⁺]_i was characterized by a transient spike followed by a small sustained plateau of elevated [Ca²⁺]_i. In the absence of extracellular Ca²⁺ 2-DG caused an increase in [Ca²⁺]_i, suggesting that inhibition of glycolysis directly triggered release of Ca²⁺ from intracellular endoplasmic reticulum (ER) Ca²⁺ stores. The inositol-1,4,5-trisphosphate receptor (IP₃R) inhibitor 2-aminoethoxydiphenyl borate abolished the KN-93- and 2-DG-induced Ca²⁺ response. Ca²⁺ release was initiated in peripheral cytoplasmic processes from which activation propagated as a [Ca²⁺]_i wave toward the central region of the cell. Focal application of 2-DG resulted in spatially confined elevations of [Ca²⁺]_i. Propagating [Ca²⁺]_i waves were preceded by [Ca²⁺]_i oscillations and small, highly localized elevations of [Ca²⁺]_i (Ca²⁺ puffs). Inhibition of glycolysis with 2-DG reduced the KN-93-induced Ca²⁺ response, and vice versa during inhibition of CaMKII 2-DG-induced Ca²⁺ release was attenuated. Similar results were obtained with pyr + -HB and W-7. Furthermore, 2-DG and IAA caused a rapid increase of intracellular Mg²⁺ concentration, indicating a concomitant drop of cellular ATP levels. In conclusion, CaMKII exerts a profound inhibition of ER Ca²⁺ release in CPAE cells, which is mediated by glycolytically generated ATP, possibly through ATP-dependent phosphorylation of the IP₃R. Ca²⁺/calmodulin-dependent kinase II; glycolysis; calcium regulation     

Ca2+-dependent inhibition of NHE3 requires PKC alpha which binds to E3KARP to decrease surface NHE3 containing plasma membrane complexes

The intestinal brush border (BB) Na⁺/H⁺ exchanger isoform 3 (NHE3) is acutely inhibited by elevation in the concentration of free intracellular Ca²⁺ ([Ca²⁺]_i) by the cholinergic agonist carbachol and Ca²⁺ ionophores in a protein kinase C (PKC)-dependent manner. We previously showed that elevating [Ca²⁺]_i with ionomycin rapidly inhibited NHE3 activity and decreased the amount of NHE3 on the plasma membrane in a manner that depended on the presence of the PDZ domain-containing protein E3KARP (NHE3 kinase A regulatory protein, also called NHERF2). The current studies were performed in PS120 fibroblasts (NHE-null cell line) stably transfected with NHE3 and E3KARP to probe the mechanism of PKC involvement in Ca²⁺ regulation of NHE3. Pretreatment with the general PKC inhibitor, GF109203X prevented ionomycin inhibition of NHE3 without altering basal NHE3 activity. Similarly, the Ca²⁺-mediated inhibition of NHE3 activity was blocked after pretreatment with the conventional PKC inhibitor Gö-6976 and a specific PKC pseudosubstrate-derived inhibitor peptide. [Ca²⁺]_i elevation caused translocation of PKC from cytosol to membrane. PKC bound to the PDZ1 domain of GST-E3KARP in vitro in a Ca²⁺-dependent manner. PKC and E3KARP coimmunoprecipitated from cell lysates; this occurred to a lesser extent at basal [Ca²⁺]_i and was increased with ionomycin exposure. Biotinylation studies demonstrated that [Ca²⁺]_i elevation induced oligomerization of NHE3 in total lysates and decreased the amount of plasma membrane NHE3. Treatment with PKC inhibitors did not affect the oligomerization of NHE3 but did prevent the decrease in surface amount of NHE3. These results suggest that PKC is not necessary for the Ca²⁺-dependent formation of the NHE3 plasma membrane complex, although it is necessary for decreasing the membrane amounts of NHE3, probably by stimulating NHE3 endocytosis. Na absorption; PDZ domains; signal complex     

Hypoxic vasodilation in porcine coronary artery is preferentially inhibited by organ culture

Hypoxia (95% N₂-5%CO₂) elicits an endothelium-independent relaxation(45-80%) in freshly dissected porcine coronary arteries. Pairedartery rings cultured at 37°C in sterile DMEM (pH ~7.4) for 24 h contracted normally to KCl or 1 µM U-46619. However, relaxation inresponse to hypoxia was sharply attenuated compared with control (fresharteries or those stored at 4°C for 24 h). Hypoxicvasorelaxation in organ cultured vessels was reduced at both high andlow stimulation, indicating that both Ca²⁺-independent andCa²⁺-dependent components are altered. In contrast,relaxation to G-kinase (sodium nitroprusside) or A-kinase (forskolinand isoproterenol) activation was not significantly affected by organculture. Additionally, there was no difference in relaxation afterwashout of the stimulus, indicating that the inhibition is specific toacute hypoxia-induced relaxation. Simultaneous force and intracellularcalcium concentration ([Ca²⁺]_i) measurementsindicate the reduction in [Ca²⁺]_i concomitantwith hypoxia at low stimulus levels in these tissue is abolished byculture. Our results indicate that organ culture at 37°C specificallyattenuates hypoxic relaxation in vascular smooth muscle by alteringdynamics of [Ca²⁺]_i handling and decreasing aCa²⁺-independent component of relaxation. Thus organculture can be a novel tool for investigating the mechanisms ofhypoxia-induced vasodilation.

     

Ca(2+) regulates fluid shear-induced cytoskeletal reorganization and gene expression in osteoblasts

Osteoblasts subjected to fluid shearincrease the expression of the early response gene, c-fos, andthe inducible isoform of cyclooxygenase, COX-2, two proteins linked tothe anabolic response of bone to mechanical stimulation, in vivo. Theseincreases in gene expression are dependent on shear-induced actinstress fiber formation. Here, we demonstrate that MC3T3-E1osteoblast-like cells respond to shear with a rapid increase inintracellular Ca²⁺ concentration([Ca²⁺]_i) that wepostulate is important to subsequent cellular responses to shear. Totest this hypothesis, MC3T3-E1 cells were grown on glass slides coatedwith fibronectin and subjected to laminar fluid flow (12 dyn/cm²). Before application of shear, cells were treatedwith two Ca²⁺ channel inhibitors or various blockers ofintracellular Ca²⁺ release for 0.5-1 h. Althoughgadolinium, a mechanosensitive channel blocker, significantly reducedthe [Ca²⁺]_i response, neithergadolinium nor nifedipine, an L-type channel Ca²⁺ channelblocker, were able to block shear-induced stress fiber formation andincrease in c-fos and COX-2 in MC3T3-E1 cells. However, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraaceticacid-AM, an intracellular Ca²⁺ chelator, or thapsigargin,which empties intracellular Ca²⁺ stores, completelyinhibited stress fiber formation and c-fos/COX-2 production in shearedosteoblasts. Neomycin or U-73122 inhibition of phospholipase C, whichmediates D-myo-inositol 1,4,5-trisphosphate (IP₃)-induced intracellular Ca²⁺ release, alsocompletely suppressed actin reorganization and c-fos/COX-2 production.Pretreatment of MC3T3-E1 cells with U-73343, the inactive isoform ofU-73122, did not inhibit these shear-induced responses. These resultssuggest that IP₃-mediated intracellular Ca²⁺release is required for modulating flow-induced responses in MC3T3-E1 cells.

     

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.

     

Involvement of JAK2 and Src kinase tyrosine phosphorylation in human growth hormone-stimulated increases in cytosolic free Ca2+ and insulin secretion

We previously reported that human growth hormone (hGH) increases cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) and proliferation in pancreatic -cells (Sjöholm Å, Zhang Q, Welsh N, Hansson A, Larsson O, Tally M, and Berggren PO. J Biol Chem 275: 21033–21040, 2000) and that the hGH-induced rise in [Ca²⁺]_i involves Ca²⁺-induced Ca²⁺ release facilitated by tyrosine phosphorylation of ryanodine receptors (Zhang Q, Kohler M, Yang SN, Zhang F, Larsson O, and Berggren PO. Mol Endocrinol 18: 1658–1669, 2004). Here we investigated the tyrosine kinases that convey the hGH-induced rise in [Ca²⁺]_i and insulin release in BRIN-BD11 -cells. hGH caused tyrosine phosphorylation of Janus kinase (JAK)2 and c-Src, events inhibited by the JAK2 inhibitor AG490 or the Src kinase inhibitor PP2. Although hGH-stimulated rises in [Ca²⁺]_i and insulin secretion were completely abolished by AG490 and JAK2 inhibitor II, the inhibitors had no effect on insulin secretion stimulated by a high K⁺ concentration. Similarly, Src kinase inhibitor-1 and PP2, but not its inactive analog PP3, suppressed [Ca²⁺]_i elevation and completely abolished insulin secretion stimulated by hGH but did not affect responses to K⁺. Ovine prolactin increased [Ca²⁺]_i and insulin secretion to a similar extent as hGH, effects prevented by the JAK2 and Src kinase inhibitors. In contrast, bovine GH evoked a rise in [Ca²⁺]_i but did not stimulate insulin secretion. Neither JAK2 nor Src kinase inhibitors influenced the effect of bovine GH on [Ca²⁺]_i. Our study indicates that hGH stimulates rise in [Ca²⁺]_i and insulin secretion mainly through activation of the prolactin receptor and JAK2 and Src kinases in rat insulin-secreting cells. c-Src; growth hormone receptor; prolactin receptor; Ca²⁺-induced Ca²⁺ release     

Regulation of smooth muscle calcium sensitivity: KCl as a calcium-sensitizing stimulus

KCl has long been used as a convenient stimulus to bypass G protein-coupled receptors (GPCR) and activate smooth muscle by a highly reproducible and relatively "simple" mechanism involving activation of voltage-operated Ca²⁺ channels that leads to increases in cytosolic free Ca²⁺ ([Ca²⁺]_i), Ca²⁺-calmodulin-dependent myosin light chain (MLC) kinase activation, MLC phosphorylation and contraction. This KCl-induced stimulus-response coupling mechanism is a standard tool-set used in comparative studies to explore more complex mechanisms generated by activation of GPCRs. One area where this approach has been especially productive is in studies designed to understand Ca²⁺ sensitization, the relationship between [Ca²⁺]_i and force produced by GPCR agonists. Studies done in the late 1980s demonstrated that a unique relationship between stimulus-induced [Ca²⁺]_i and force does not exist: for a given increase in [Ca²⁺]_i, GPCR activation can produce greater force than KCl, and relaxant agents can produce the opposite effect to cause Ca²⁺ desensitization. Such changes in Ca²⁺ sensitivity are now known to involve multiple cell signaling strategies, including translocation of proteins from cytosol to plasma membrane, and activation of enzymes, including RhoA kinase and protein kinase C. However, recent studies show that KCl can also cause Ca²⁺ sensitization involving translocation and activation of RhoA kinase. Rather than complicating the Ca²⁺ sensitivity story, this surprising finding is already providing novel insights into mechanisms regulating Ca²⁺ sensitivity of smooth muscle contraction. KCl as a "simple" stimulus promises to remain a standard tool for smooth muscle cell physiologists, whose focus is to understand mechanisms regulating Ca²⁺ sensitivity. K⁺ depolarization; cell signaling; signal transduction; contraction     

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