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PubMed1999

Concerted regulation of wild-type p53 nuclear accumulation and activation by S100B and calcium-dependent protein kinase C.

The calcium ionophore ionomycin cooperates with the S100B protein to rescue a p53-dependent G(1) checkpoint control in S100B-expressing mouse embryo fibroblasts and rat embryo fibroblasts (REF cells) which express the temperature-sensitive p53Val135 mutant (C. Scotto, J. C. Deloulme, D. Rousseau, E. Chambaz, and J. Baudier, Mol. Cell. Biol. 18:4272-4281, 1998). We investigated in this study the contributions of S100B and calcium-dependent PKC (cPKC) signalling pathways to the activation of wild-type p53. We first confirmed that S100B expression in mouse embryo fibroblasts enhanced specific nuclear accumulation of wild-type p53. We next demonstrated that wild-type p53 nuclear translocation and accumulation is dependent on cPKC activity. Mutation of the five putative cPKC phosphorylation sites on murine p53 into alanine or aspartic residues had no significant effect on p53 nuclear localization, suggesting that the cPKC effect on p53 nuclear translocation is indirect. A concerted regulation by S100B and cPKC of wild-type p53 nuclear translocation and activation was confirmed with REF cells expressing S100B (S100B-REF cells) overexpressing the temperature-sensitive p53Val135 mutant. Stimulation of S100B-REF cells with the PKC activator phorbol ester phorbol myristate acetate (PMA) promoted specific nuclear translocation of the wild-type p53Val135 species in cells positioned in early G(1) phase of the cell cycle. PMA also substituted for ionomycin in the mediating of p53-dependent G(1) arrest at the nonpermissive temperature (37.5 degrees C). PMA-dependent growth arrest was linked to the cell apoptosis response to UV irradiation. In contrast, growth arrest mediated by a temperature shift to 32 degrees C protected S100B-REF cells from apoptosis. Our results suggest a model in which calcium signalling, linked with cPKC activation, cooperates with S100B to promote wild-type p53 nuclear translocation in early G(1) phase and activation of a p53-dependent G(1) checkpoint control.

Animals↗
PubMed2006

Role of cyclic GMP and calcineurin in homologous and heterologous desensitization of natriuretic peptide receptor-A.

The natriuretic peptide receptor-A (NPR-A) mediates natriuretic, hypotensive, and antihypertrophic effects of natriuretic peptides through the production of cGMP. In pathological conditions such as heart failure, these effects are attenuated by homologous and heterologous desensitization mechanisms resulting in the dephosphorylation of the cytosolic portion of the receptor. In contrast with natriuretic peptide-induced desensitization, pressor hormone-induced desensitization is dependent on protein kinase C (PKC) stimulation and (or) cytosolic calcium elevation. Mechanisms by which PKC and Ca(2+) promote NPR-A desensitization are not known. The role of cGMP and of the cytosolic Ca(2+) pathways in NPR-A desensitization were therefore studied. In contrast with the activation of NPR-A by its agonist, activation of soluble guanylyl cyclases of LLC-PK1 cells by sodium nitroprusside also leads to a production of cGMP but without altering NPR-A activation. Consequently, cGMP elevation per se does not appear to mediate homologous desensitization of NPR-A. In addition, cytosolic calcium increase is required only for the heterologous desensitization pathway since the calcium chelator BAPTA-AM blocks only PMA or ionomycin-induced desensitization. Calcineurin inhibitors block the NPR-A guanylyl cyclase heterologous desensitization induced by ionomycin, suggesting an essential role for this Ca(2+)-stimulated phosphatase in NPR-A desensitization. In summary, the present report demonstrates that neither cGMP nor Ca(2+) cytosolic elevation cause NPR-A homologous desensitization. Our results also indicate for the first time a role for calcineurin in NPR-A heterologous desensitization.

Animals↗
PubMed1989

Characterization of ion movements and their relationship to muscarinic receptor binding and excitation-contraction coupling in guinea pig ileal longitudinal smooth muscle.

The relationship between ion movements (sodium uptake and potassium release) and agonist-induced contractile responses or muscarinic receptor binding was investigated in the guinea pig ileal longitudinal muscle (GPLM). Sodium uptake and potassium release were agonist-dependent, concentration-dependent, and stereoselective, with the following rank order of maximum ion movement: muscarinic agonists greater than histamine greater than substance P = serotonin. Potassium depolarization did not initiate sodium uptake or potassium release. Sodium uptake was rapid and monophasic, preceding potassium release which was biphasic in nature. Full muscarinic agonists produced equal maximal increases in sodium uptake, while maximal potassium release varied for all muscarinic agonists and in addition differed from sodium uptake in the following ways: time course, stereoselectivity, sensitivity to calcium antagonists, modulation by the guanylyl nucleotide derivative, 5'-guanylylimidodiphosphate (Gpp(NH)p), and inhibition by muscarinic receptor blockade with benzilylcholine mustard. The calcium ionophores A23187 and ionomycin (SQ23377) did not produce any sodium uptake; A23187 but not ionomycin produced potassium release comparable to that evoked by muscarinic agonists. Ion movement in response to combinations of agonists were not additive. Muscarinic agonist binding as measured by competition for [3H]quinuclidinyl benzilate ([3H]QNB) binding, was best described by multiple sites and was regulated by Gpp(NH)p. Excellent correlations were observed between the dissociation constants for binding and sodium uptake, potassium release, and contraction. The best correlations were those between the pharmacologic responses and the high affinity binding site in the absence, and the low affinity site in the presence, of Gpp(NH)p, respectively. Furthermore, the potencies of muscarinic agonists to evoke ion movements and to inhibit [3H]QNB binding were similar, and from one to two orders of magnitude less than those for contraction. It is suggested that contraction and potassium release were mediated by the high affinity, and sodium uptake by the low and average affinity muscarinic agonist binding sites, respectively. These findings suggest an agonist-activated receptor-effector coupling model in GPLM that leads to the activation of sodium uptake, potassium release, and subsequently, contraction.

Animals↗
PubMed2003

Inhibition of interferon-gamma expression by osmotic shrinkage of peripheral blood lymphocytes.

A hypertonic environment, as it prevails in renal medulla or in hyperosmolar states such as hyperglycemia of diabetes mellitus, has been shown to impair the immune response, thus facilitating the development of infection. The present experiments were performed to test whether hypertonicity influences activation of T lymphocytes. To this end, peripheral blood lymphocytes (PBL) of cytomegalovirus (CMV)-positive donors were stimulated by human leukocyte antigen (HLA)-A2-restricted CMV epitope NLVPMVATV to produce interferon (IFN)-gamma at varying extracellular osmolarity. As a result, increasing extracellular osmolarity during exposure to the CMV antigen indeed decreased IFN-gamma formation. Addition of NaCl was more effective than urea. A 50% inhibition was observed at 350 mosM by addition of NaCl. The combined application of the Ca(2+) ionophore ionomycin (1 microg/ml) and the phorbol ester phorbol 12-myristate 13-acetate (PMA; 5 microg/ml) stimulated IFN-gamma production, an effect again reversed by hyperosmolarity. Moreover, hyperosmolarity abrogated the stimulating effect of ionomycin (1 microg/ml) and PMA (5 microg/ml) on the transcription factors activator protein (AP)-1, nuclear factor of activated T cells (NFAT), and NF-kappaB but not Sp1. In conclusion, osmotic cell shrinkage blunts the stimulatory action of antigen exposure on IFN-gamma production, an effect explained at least partially by suppression of transcription factor activation.

Apoptosis↗
PubMed2002

CaM kinase II-dependent activation of tyrosine kinases and ERK1/2 in vascular smooth muscle.

In vascular smooth muscle (VSM) and many other cells, G protein receptor-coupled activation of mitogen-activated protein kinases has been linked, in part, to increases in free intracellular Ca(2+). Previously, we demonstrated that ionomycin-, angiotensin II-, and thrombin-induced activation of extracellular signal-regulated kinase (ERK)1/2 in VSM cells was attenuated by pretreatment with KN-93, a selective inhibitor of the multifunctional Ca(2+)/calmodulin-dependent protein kinase (CaM kinase II). In the present study, we show that the Ca(2+)-dependent pathway leading to activation of ERK1/2 is preceded by nonreceptor proline-rich tyrosine kinase (PYK2) activation and epidermal growth factor (EGF) receptor tyrosine phosphorylation and is attenuated by inhibitors of src family kinases or the EGF receptor tyrosine kinase. Furthermore, we demonstrate that pretreatment with KN-93 or a CaM kinase II inhibitor peptide inhibits Ca(2+)-dependent PYK2 activation and EGF receptor tyrosine phosphorylation in response to ionomycin, ATP, and platelet-derived growth factor but has no effect on phorbol 12,13-dibutyrate- or EGF-induced responses. The results implicate CaM kinase II as an intermediate in the Ca(2+)/calmodulin-dependent activation of PYK2.

Animals↗
PubMed2005

Intermediate-conductance Ca2+-activated K+ channel is expressed in C2C12 myoblasts and is downregulated during myogenesis.

We report here the expression in C2C12 myoblasts of the intermediate-conductance Ca2+-activated K+ (IK(Ca)) channel. The IK(Ca) current, recorded under perforated-patch configuration, had a transient time course when activated by ionomycin (0.5 microM; peak current density 26.2 +/- 3.7 pA/pF; n = 10), but ionomycin (0.5 microM) + 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one (100 microM) evoked a stable outward current (28.4 +/- 8.2 pA/pF; n = 11). The current was fully inhibited by charybdotoxin (200 nM), clotrimazole (2 microM), and 5-nitro-2-(3-phenylpropylamino)benzoic acid (300 microM), but not by tetraethylammonium (1 mM) or D-tubocurarine (300 microM). Congruent with the IK(Ca) channel, elevation of intracellular Ca2+ in inside-out patches resulted in the activation of a voltage-insensitive K+ channel with weak inward rectification, a unitary conductance of 38 +/- 6 pS (at negative voltages), and an IC50 for Ca2+ of 530 nM. The IK(Ca) channel was activated metabotropically by external application of ATP (100 microM), an intracellular Ca2+ mobilizer. Under current-clamp conditions, ATP application resulted in a membrane hyperpolarization of approximately 35 mV. The IK(Ca) current downregulated during myogenesis, ceasing to be detectable 4 days after the myoblasts were placed in differentiating medium. Downregulation was prevented by the myogenic suppressor agent basic FGF (bFGF). We also found that block of the IK(Ca) channel by charybdotoxin did not inhibit bFGF-sustained myoblast proliferation. These observations show that in C2C12 myoblasts the IK(Ca) channel expression correlates inversely with differentiation, yet it does not appear to have a role in myoblast proliferation.

Adenosine Triphosphate↗
PubMed1993

Ezrin-calpain I interactions in gastric parietal cells.

Gastric ezrin, a membrane-cytoskeletal linker with sequence homology to talin and erythrocyte band 4.1, has been associated with the remodeling of parietal cell apical membrane that occurs with adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase stimulation. Here we examine the interrelationship between parietal cell ezrin and Ca(2+)-dependent protease activity. Addition of Ca2+ to sonicated gastric gland preparations rendered a relatively selective proteolysis of the 80-kDa ezrin, accompanied by the appearance of a 55-kDa breakdown product. Ca(2+)-dependent proteolysis of ezrin was blocked by E64, a cysteine protease inhibitor, or calpastatin, indicating calpain as the responsible protease. Degradation of ezrin in intact gastric glands was achieved by varying extracellular [Ca2+] and [ionomycin]. Ezrin degradation in situ was rapid and relatively selective, although Ca(2+)-dependent degradation of some spectrin-like bands was also observed. The effect of activated calpain I on parietal cell function was assessed by probing the secretory response to histamine stimulation using [14C]aminopyrine uptake, along with parallel measurements of calpain activity, over a wide range of ionomycin. Activation of calpain, as evidenced by loss of parietal cell ezrin, was correlated with decreased AP uptake by stimulated gastric glands, supporting a role for ezrin in the oxyntic secretory process. The calpain-ezrin interaction established here, and the similarities of calpain with talin and erythrocyte band 4.1, suggest a common feature to this family of ezrin/band 4.1/talin proteins that have been implicated in membrane-cytoskeletal association.

Animals↗
PubMed1993

AVP-induced activation of MAP kinase in vascular smooth muscle cells is mediated through protein kinase C.

Arginine vasopressin (AVP) has been shown to stimulate tyrosine phosphorylation and activation of p42 mitogen-activated protein (MAP) kinase (p42MAPK) in vascular smooth muscle cells (VSMC). In VSMC, AVP increases free intracellular Ca2+ concentration ([Ca2+]i) and activates protein kinase C (PKC) through activation of phospholipase C. The contribution of PKC and [Ca2+]i in p42MAPK regulation was therefore determined. Activation of PKC by phorbol 12-myristate 13-acetate (PMA) stimulated tyrosine phosphorylation and activation of p42MAPK to the same extent as AVP. Inhibition of PKC by staurosporine or downregulation of PKC by PMA pretreatment abolished AVP-induced stimulation of p42MAPK. When [Ca2+]i was elevated to the same level as with AVP, using either ionomycin (0.1 microM) or thapsigargin (0.1 microM), MAP kinase was only partially activated. Elevation of [Ca2+]i to supraphysiological levels by 1 microM ionomycin stimulated MAP kinase activity to the same extent as AVP. This effect was blocked by downregulation of PKC. The intracellular Ca2+ chelator BAPTA [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid] blocked AVP-induced [Ca2+]i increase but did not affect AVP stimulation of p42MAPK. Thus AVP-induced activation of p42MAPK requires only the activation of PKC but not an increase in [Ca2+]i.

Animals↗
PubMed1995

Cholinergic activation of phospholipase D in lacrimal gland acini is independent of protein kinase C and calcium.

To determine if rat lacrimal gland acini contain phospholipase D (PLD) activity, we took advantage of PLD's unique ability, in the presence of ethanol, to catalyze a transphosphatidylation reaction to produce phosphatidylethanol (PEth). Lacrimal gland acini were labeled for 3 h with [14C]stearic acid, preincubated for 20 min in the presence of 2% ethanol, and incubated for 20 min with or without agonists. Total cellular lipids were then extracted and analyzed by thin-layer chromatography, and the radioactivity was determined by liquid scintillation counting. Carbachol (1 mM), a cholinergic agonist, stimulated the production of both [14C]PEth and [14C]phosphatidic acid ([14C]PA) twofold. This effect was completely blocked by the muscarinic antagonist atropine (10 microM). [14C]PEth accumulation was also stimulated twofold by the active phorbol esters, 4 beta-phorbol 12-myristate 13-acetate and 4 beta-phorbol 12,13-dibutyrate at 1 microM. Ionomycin (1 microM), a Ca2+ ionophore, also stimulated the production of [14C]PEth twofold. In contrast to carbachol, neither phorbol esters nor ionomycin stimulated [14C]PA production. Neither [14C]PEth nor [14C]PA production was altered by epinephrine (1 mM), a nonselective adrenergic agonist, or phenylephrine (0.1 and 1 mM), a specific alpha 1-adrenergic agonist. We concluded that PLD activity, modulated by muscarinic receptors, protein kinase C, and Ca2+, but not by adrenergic receptors, is present in rat lacrimal gland acini. We also concluded that cholinergic activation of PLD appears to be independent of PKC and Ca2+.

Animals↗
PubMed1993

Role of cytosolic calcium in regulation of cytoskeletal gene expression by insulin.

Insulin and calcium ionophores rapidly stimulated transcription of the cytoskeletal beta- and gamma-actin genes in serum-deprived rat H4-II-E hepatoma cells. The calcium ionophore A23187 (1 microM) stimulated transcription of the beta-actin gene by 7.3-, 5.4-, and 2.6-fold and the gamma-actin gene by 5.9-, 5.6-, and 2.6-fold at 15, 30, and 60 min, respectively. Ionomycin (1 microM) similarly increased beta- and gamma-actin transcription. Insulin stimulated beta-actin transcription 11.4-fold and gamma-actin 8.4-fold at 30 min. alpha-Tubulin transcription was induced by both insulin and calcium ionophores but to a lesser degree. The effects of A23187 or ionomycin together with insulin for 30 min were no greater than those of insulin alone. Insulin alone, however, did not significantly increase measurable intracellular calcium concentrations in fura-2-loaded cells. When cytosolic calcium was chelated using quin2 acetoxymethyl ester, the ability of A23187 to increase beta- and gamma-actin transcription was completely abolished, whereas insulin's ability to stimulate actin transcription was only partially inhibited. This suggests that the regulation of gene transcription by insulin may include calcium-dependent pathways but strongly implies that calcium-independent pathways are also utilized.

Actins↗
PubMed2004

Inhibition of ACh-stimulated exocytosis by NSAIDs in guinea pig antral mucous cells: autocrine regulation of mucin secretion by PGE2.

The effects of indomethacin (IDM) and aspirin (ASA) on ACh (10 microM) -stimulated exocytotic events were studied in guinea pig antral mucous cells by using video optical microscopy. IDM or ASA, which inhibits cyclooxygenase (COX), decreased the frequency of ACh-stimulated exocytotic events by 30% or 60%, respectively. The extent of inhibition induced by ASA (60%) decreased by 30% when IDM or arachidonic acid (AA, the substrate of COX) was added. IDM, unlike ASA, appears to induce the accumulation of AA, which enhances the frequency of ACh-stimulated exocytotic events in ASA-treated cells. ONO-8713 (100 microM; an inhibitor of the EP1-EP4 prostaglandin receptors) and N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide, HCl (H-89, 20 microM; an inhibitor of PKA) also decreased the frequency of ACh-stimulated exocytotic events by 60%. However, the supplementation of PGE(2) (1 microM) prevented the IDM-induced decrease in the frequency of ACh-stimulated exocytotic events. SC-560 (an inhibitor of COX-1) decreased the frequency of ACh-stimulated exocytotic events by 30%, but NS-398 (an inhibitor of COX-2) did not. Moreover, IDM decreased the frequency of exocytotic events stimulated by ionomycin, suggesting that COX-1 activity is stimulated by an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)). ACh and ionomycin increased PGE(2) release in antral mucosal cells. In conclusion, in ACh-stimulated antral mucous cells, an increase in [Ca(2+)](i) activates Ca(2+)-regulated exocytotic events and PGE(2) release mediated by COX-1. The released PGE(2) induces the accumulation of cAMP, which enhances the Ca(2+)-regulated exocytosis. The autocrine mechanism mediated by PGE(2) maintains the high-level mucin release from antral mucous cells during ACh stimulation.

Acetylcholine↗
PubMed2002

Cholinergic inhibition of electrogenic sodium absorption in the guinea pig distal colon.

Submucosal cholinergic and noncholinergic neurons in intestines have been shown to be involved in regulating epithelial transport functions, particularly stimulating Cl(-) secretion. This study investigates the role of submucosal cholinergic neurons in regulating electrogenic Na(+) absorption in distal colon. Amiloride-sensitive short-circuit current (I(sc)) and (22)Na(+) flux were measured in mucosal and mucosal-submucosal preparations mounted in Ussing chambers. In the mucosal preparation, carbachol (CCh) added to the serosal side inhibited amiloride-sensitive I(sc) and amiloride-sensitive (22)Na(+) absorption. The inhibitory effect of CCh was observed at approximately 0.1 microM, and maximum inhibition of approximately 70% was attained at approximately 30 microM (IC(50) = approximately 1 microM). CCh-induced inhibition of amiloride-sensitive I(sc) was almost totally abolished by 10 microM atropine. Treatment of the tissue with ionomycin markedly reduced amiloride-sensitive I(sc), but a subsequent addition of CCh further decreased it. Also, CCh still had an inhibitory effect, although significantly attenuated, after the tissue had been incubated with a low-Ca(2+) solution containing ionomycin and BAPTA-AM. Applying electrical field stimulation to submucosal neurons in the mucosal-submucosal preparation resulted in inhibition of amiloride-sensitive I(sc), approximately 33% of this inhibition being atropine sensitive. Physostigmine inhibited amiloride-sensitive I(sc), this effect being abolished by atropine. In conclusion, submucosal cholinergic and noncholinergic neurons were involved in inhibiting electrogenic Na(+) absorption in colon. This inhibition by cholinergic neurons was mediated by muscarinic receptor activation.

Acetylcholine↗
PubMed2005

[Ca2+]i-independent contractile force generation by rat hepatic stellate cells in response to endothelin-1.

The contractile force generated by hepatic stellate cells in response to endothelin-1 contributes to sinusoidal blood flow regulation and hepatic fibrosis. This study's aim was to directly test the widely held view that changes in cytosolic Ca2+ concentration ([Ca2+]i) mediate stellate cell force generation. Contractile force generation by primary cultures of rat hepatic stellate cells grown in three-dimensional collagen gels was directly and quantitatively measured using a force transducer. Stellate cell [Ca2+]i, myosin activation, and migration were quantified using standard techniques. [Ca2+]i was modulated using ionomycin, BAPTA, KCl, and removal of extracellular Ca2+. Removal of extracellular Ca2+ did not alter endothelin-1-stimulated force development or [Ca2+]i. Ionomycin, a Ca2+ ionophore, triggered an increase in [Ca2+]i that was three times greater than that stimulated by endothelin-1, but only induced 16% of the force and 38% of the myosin regulatory light chain (MLC) phosphorylation induced by endothelin-1. Physiological increases in [Ca2+]i induced by hyperkalemia had no effect on contractile force. Loading BAPTA, a Ca2+ chelator, in stellate cells completely blocked endothelin-1-induced increases in [Ca2+]i but had no effect on endothelin-1-stimulated force generation or MLC phosphorylation. In contrast, Y-27632, a selective rho-associated kinase inhibitor, inhibited endothelin-1-stimulated force generation by at least 70% and MLC phosphorylation by at least 80%. Taken together, these observations indicate that changes in [Ca2+]i are neither necessary nor sufficient for contractile force generation by rat stellate cells. Our results challenge the current model of contractile regulation in hepatic stellate cells and have important implications for our understanding of hepatic pathophysiology.

Animals↗
PubMed1985

Relation between free cytosolic calcium and amylase release by pancreatic acini.

Pancreatic acini were loaded with the Ca-selective fluorescent indicator quin-2 by incubation with its acetyoxymethyl ester. Loading acini with 844 +/- 133 microM quin-2 altered neither their ultrastructure nor their viability. The rate of amylase release from quin-2-loaded acini in response to the secretagogue carbachol, however, was significantly smaller than that of control acini. Studies in which acini were loaded with both quin-2 and a similar Ca-chelating compound, BAPTA, indicated that this reduced amylase release was related to the Ca buffering properties of quin-2. The concentration of free intracellular Ca calculated from the fluorescence of quin-2 was 90 +/- 18 nM. Stimulation by carbachol of acini suspended in media containing 1.25 mM Ca caused a rapid, transient enhancement of this value. After stimulation amylase release, the onset of the rise in free cytosolic Ca levels was observed in 1.1 +/- 0.1 s following the addition of agonist, and peak Ca levels (545 +/- 112 nM) were obtained within 5.3 +/- 0.3 s. For concentrations of carbachol less than or equal to 10(-6) M, a stoichiometric relation was found between stimulated amylase release and the peak concentration of free cytosolic Ca achieved. At higher concentrations of carbachol, however, the peak free cytosolic Ca remained constant while amylase release declined. The latency of the rise in intracellular Ca following stimulation of acini suspended in Ca-free media was not different from that observed for acini suspended in normal media, but the rise time was significantly prolonged. In the presence of extracellular Ca, the intracellular level of Ca remained elevated 2.8-fold above basal levels for at least 15 min following stimulation with 10(-6) M carbachol, whereas it had returned to near resting levels by 15 min when either 3 X 10(-7) or 3 X 10(-5) M carbachol was the stimulus. The Ca ionophore ionomycin (10-6 M) induced changes in the level of free cytosolic Ca similar to those caused by 10(-6) M carbachol. Ionomycin, however, stimulated only approximately one-third as much amylase release. These data suggest that factors in addition to changes in free cytosolic Ca may be important in regulating enzyme secretion by pancreatic acinar cells.

Aminoquinolines↗
PubMed1991

Kinetics and regulation of a polarized Na(+)-H+ exchanger from Caco-2 cells, a human intestinal cell line.

The kinetics and regulation of Na(+)-H+ exchange were studied using BCECF to measure pHi in Caco-2 cells grown on membrane filters. Na(+)-H+ exchange was defined as a Na(+)-dependent H+ efflux in response to an acid load imposed by an NH4Cl prepulse in the absence of added CO2. Na(+)-H+ exchange was present exclusively on the basolateral membrane, had a Kt (Na+) of 21 +/- 2 mM, and an ID50 for amiloride dependent on medium [Na+] with an apparent Ki for amiloride of 3 microM. Na(+)-H+ exchange rates had a greater than first-order dependence on intracellular [H+], suggesting the presence of an internal proton modifier site. Results also suggest that Na(+)-H+ exchange is kinetically inactivated at resting pHi (7.35 +/- 0.02), since neither removal of Na+ nor addition of amiloride affected resting pHi, although monensin alkalinized cells to pHi 7.6. To evaluate regulation of Na(+)-H+ exchange, cells were exposed to either forskolin, 1,9-dideoxyforskolin (a noncyclase-activating forskolin derivative), 8-BrcAMP, E. coli STa toxin, ionomycin, phorbol dibutyrate, or cellular shrinkage in hypertonic medium. Only forskolin and 1,9-dideoxyforskolin caused a significant change (inhibition) in Na(+)-H+ exchange rate. Experiments performed with the Ussing chamber-voltage clamp technique verified that forskolin, 8-BrcAMP, E. coli STa toxin, ionomycin, and phorbol dibutyrate increased transepithelial Isc, verifying that all the regulatory pathways tested were functional and responsive to agonists. Results suggested that the Isc was due to Cl- secretion, since no net transcellular Na+ or Cl- flux was detected in basal conditions, and the Isc response to forskolin was abolished by omission of serosal Cl-. Because forskolin, but not 1,9-dideoxyforskolin, increased both cellular cAMP and Isc, the inhibition of Na(+)-H+ exchange by forskolin derivatives was mediated by a mechanism not involving activation of adenylyl cyclase. In conclusion, Caco-2 cells use a basolateral Na(+)-H+ exchanger to regulate pHi, but this exchanger is not affected by cell shrinkage or second messenger pathways that regulate Na(+)-H+ exchangers in other cell systems.

Amiloride↗
PubMed1994

Mechanism of cholinergic stimulation of glucose oxidation in isolated gastric glands.

The mechanisms of cholinergic activation of carbohydrate metabolism were investigated in isolated rabbit gastric glands. Carbachol stimulated the rate of glucose oxidation in a dose-dependent fashion with a half-maximal effect occurring at approximately 9 microM. Atropine and omeprazole, but not cimetidine, completely blocked the stimulation induced by carbachol. Direct activation of the H(+)-K(+)-adenosinetriphosphatase by NH+4 caused a significant stimulation of glucose oxidation that was totally abolished by oligomycin and by the mitochondrial uncouplers dinitrophenol and carbonyl cyanide p-trifluoromethoxyphenylhydrazone. These latter agents did not abolish the stimulating effect of carbachol on glucose oxidation. Ionomycin increased the rate of glucose oxidation in a dose-dependent manner, and this effect was not blocked by oligomycin. The metabolic effect of ionomycin was reduced but not abolished by omeprazole. 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester eliminated the carbachol-induced stimulation of glucose oxidation and partially inhibited the effect of NH+4. The mitochondrial enzymes pyruvate dehydrogenase and oxoglutarate dehydrogenase were activated by physiological concentrations of calcium in the isolated mitochondria. This effect was blocked by incubation with ruthenium red.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2003

Mechanism of glutamate stimulation of CO production in cerebral microvessels.

Dilation of piglet pial arterioles to glutamate involves carbon monoxide (CO) produced from heme by heme oxygenase-2 (HO-2). Piglet cerebral microvessels and endothelial and smooth muscle cells grown on microcarrier beads were used to address the hypothesis that glutamate increases endothelial CO production by increasing HO-2 catalytic activity. CO was measured by gas chromatography/mass spectrometry. Glutamate increased CO production from endogenous heme by cerebral microvessels, endothelial cells, and smooth muscle cells. Glutamate increased the conversion of exogenous heme to CO. Protein tyrosine kinase inhibition blocked glutamate stimulation of CO production. Inhibition of protein tyrosine phosphatases stimulated CO production. Conversely, neither phorbol myristate acetate nor H-7 changed glutamate stimulation of CO production. The mechanism of HO-2 stimulation by glutamate appears to be independent of cytosolic Ca, because stimulation of CO production by glutamate was the same in Careplete medium, Ca-free medium with ionomycin, and Careplete medium with ionomycin. Therefore, glutamate appears to increase HO-2 catalytic activity in cerebral microvessels via a tyrosine kinase mediated pathway.

Animals↗
PubMed1997

Dual role of cGMP in modulation of macromolecule permeability of aortic endothelial cells.

The effect of guanosine 3',5'-cyclic monophosphate (cGMP) on cytosolic Ca2+ dynamics and associated alterations in macromolecule permeability was investigated in cultured monolayers of aortic endothelial cells. Addition of the membrane-permeable cGMP analogue 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP, 5 x 10(-4)M) or activators of the soluble (3-morpholinosydnonimine, 10(-5) M) or the particulate guanylyl cyclase (atrial natriuretic peptide, 10(-7) M) to unstimulated monolayers led to a decrease in permeability (8-BreGMP: 62 +/- 8% of control) without affecting low basal cytosolic Ca2+ concentration ([Ca2+]i, 87 +/- 8 nM). In contrast, under conditions of elevated [Ca2+]i (503 +/- 95 nM) and increased permeability (155 +/- 7% of control) induced by 10(-6) M ionomycin, 8-BrcGMP, 3-morpholinosydnonimine, or atrial natriuretic peptide provoked a further increase in permeability (8-BrcGMP: 255 +/- 27%). These agents failed to increase permeability when added before or after the ionomycin-triggered transitory rise in [Ca2+]i. The increase in permeability in response to 8-BrcGMP was due to a secondary further rise in [Ca2+]i (758 +/- 87 nM), which was abolished in the absence of extracellular Ca2+, indicating influx of exogenous Ca2+ as the cause. Changes in [Ca2+]i and permeability were inhibited, in the presence of the Rp diastereomer of 8-(4-chlorophenylthio)guanosine 3',5'-cyclic monophosphothioate (2 x 10(-5) M), an inhibitor of the cGMP-dependent protein kinase. These findings show that, depending on [Ca2+]i, cGMP can play opposite roles in endothelial permeability in one and the same cell preparation.

Animals↗