Thapsigargin, a Ca(2+)-ATPase inhibitor, relaxes rat aorta via nitric oxide formation

Betulinic Acid Increases eNOS Phosphorylation and NO Synthesis via the Calcium-Signaling Pathway

Betulinic acid (BA) is a naturally occurring pentacyclic triterpene that attenuates vascular diseases and atherosclerosis, but the mechanism by which it stimulates endothelial nitric oxide synthase (eNOS) is unclear. eNOS is the key regulatory enzyme in the vascular endothelium. This study examined the intracellular pathways underlying the effects of BA on eNOS activity and endothelial nitric oxide (NO) production in endothelial cells. BA treatment induced both eNOS phosphorylation at Ser1177 and NO production. It also increased the level of intracellular Ca(2+) and phosphorylation of Ca(2+)/calmodulin-dependent kinase IIα (CaMKIIα) and Ca(2+)/calmodulin-dependent protein kinase kinase β (CaMKKβ). Inhibition of the L-type Ca(2+) channel (LTCC) and the ryanodine receptor (RyR) abolished BA-induced intracellular levels of Ca(2+) and eNOS phosphorylation. Treatment with W7 (a CaM antagonist), KN-93 (a selective inhibitor of CaMKII), and STO 609 (a selective inhibitor of CaMKK) suppressed eNOS phosphorylation and NO production. Moreover, AMP-activated protein kinase (AMPK) was induced by BA, and BA-induced eNOS phosphorylation was inhibited by compound C, an AMPK inhibitor. Taken together, these results indicate that BA activates eNOS phosphorylation and NO synthesis via the Ca(2+)/CaMKII and Ca(2+)/CaMKK/AMPK pathways. These findings provide further insight into the eNOS signaling pathways involved in the antiatherosclerosis effects of BA.

Endothelium-dependent contractions to acetylcholine, ATP and the calcium ionophore A 23187 in aortas from spontaneously hypertensive and normotensive rats

The present study was designed to determine whether or not an increase in endothelial intracellular concentration of calcium ([Ca2+]i) evokes endothelium-dependent contractions in the aorta from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Acetylcholine, adenosine triphosphate (ATP) and the calcium ionophore, A 23187, produced endothelium-dependent relaxations in isolated aortic rings of both WKY and SHR. These relaxations in response to the three agonists were significantly smaller in the SHR when compared with the WKY. Endothelium-dependent contractions to acetylcholine, ATP and A 23187 were observed only in the aorta isolated from the SHR. In the presence of NG-nitro-L-arginine, an NO synthase inhibitor, the endothelium-dependent contractions in response to acetylcholine, ATP and A 23187 were potentiated significantly in the aorta SHR and were unmasked in that of WKY. However, the contractions were still significantly greater in SHR than in WKY. These contractions were abolished by indomethacin and valeryl salicylate (two cyclo-oxygenase inhibitors) as well as by S 18886 (a TP-receptor antagonist), indicating that the endothelium-dependent contraction produced by the three agonists share the same characteristics. The results of the present study indicate that the release/generation of endothelium-derived contracting factor, requires an increase in endothelial [Ca2+]i.

Endothelium-dependent noradrenergic hyperresponsiveness induced by thapsigargin in human saphenous veins: role of thromboxane and calcium

To further investigate the mechanisms which regulate sympathetic vascular tone, we studied the effects of the sarcoplasmic reticulum Ca(2+)-ATPase inhibitor, thapsigargin, on the vasoconstriction induced by transmural nerve stimulation and noradrenaline in superfused human saphenous vein rings. The contractions induced by both transmural nerve stimulation and noradrenaline were potentiated by thapsigargin in endothelium-intact, but not in endothelium-denuded vessels. This potentiation was unaffected by the non-selective endothelin ET(A/B) receptor antagonist, Ro 47-0203 (4-tert-Butyyl-N-[6-(2-hydroxy-ethoxy)-5-(2-methoxy-phenoxy)-2,2'-bipyrimidin-4yl]benzene sulfonamide), or by the nitric oxide (NO) synthase inhibitor, L-NNA (N(omega)-nitro-L-arginine), but was inhibited by the thromboxane A(2) receptor antagonist, Bay u3405 (3(R)-[[(4-flurophenyl) sulphonyl]amino-1,2,3,4-tetrahydro-9H-carbazole-9-propanoic acid]) or by the thromboxane A(2) synthase inhibitor, UK 38485 (3-(1H-imidazol-1-yl-methyl)-2-methyl-1H-indole-1-propanoic acid). Moreover, the thapsigargin-induced noradrenergic hyperresponsiveness, as well as that produced by subthreshold concentrations of the thromboxane A(2) mimetic, U 46619, were blocked by the Ca(2+) channel antagonist, verapamil. In conclusion, our results indicate that thapsigargin enhances the contractions produced by sympathetic nerve stimulation in human saphenous vein rings through the endothelial release of thromboxane A(2) that potentiates the vasoconstriction induced by the noradrenergic mediator with a verapamil-sensitive mechanism.

Intracellular alkalinization augments capacitative Ca2+ entry in vascular smooth muscle cells

Agonist-induced Ca2+ influx of vascular smooth muscle cells is thought to be triggered by depletion of intracellular Ca2+ stores. This study investigated the effects of intracellular alkalinization on capacitative Ca2+ entry in A7r5 rat aortic smooth muscle cells. Intracellular alkalinization was induced by NH(4)Cl. Transplasmalemmal Ca2+ influx due to Ca2+ store depletion induced by thapsigargin, which was abolished by pretreatment of the cells with SKF-96365 but not affected by that with verapamil, was significantly increased by pretreatment with NH(4)Cl. Neither 5-hydroxytryptamine-induced inositol monophosphate accumulation nor intracellular Ca2+ release from its stores was affected by NH(4)Cl. These results suggest that intracellular alkalinization acts on the process(es) after depletion of Ca2+ stores and facilitates capacitative Ca2+ entry in vascular smooth muscle cells.

Involvement of Ca2+/calmodulin-dependent protein kinase II in endothelial NO production and endothelium-dependent relaxation

Nitric oxide (NO) is synthesized from l-arginine by the Ca(2+)/calmodulin-sensitive endothelial NO synthase (NOS) isoform (eNOS). The present study assesses the role of Ca(2+)/calmodulin-dependent protein kinase II (CaMK II) in endothelium-dependent relaxation and NO synthesis. The effects of three CaMK II inhibitors were investigated in endothelium-intact aortic rings of normotensive rats. NO synthesis was assessed by a NO sensor and chemiluminescence in culture medium of cultured porcine aortic endothelial cells stimulated with the Ca(2+) ionophore A23187 and thapsigargin. Rat aortic endothelial NOS activity was measured by the conversion of l-[(3)H]arginine to l-[(3)H]citrulline. Three CaMK II inhibitors, polypeptide 281-302, KN-93, and lavendustin C, attenuated the endothelium-dependent relaxation of endothelium-intact rat aortic rings in response to acetylcholine, A23187, and thapsigargin. None of the CaMK II inhibitors affected the relaxation induced by NO donors. In a porcine aortic endothelial cell line, KN-93 decreased NO synthesis and caused a rightward shift of the concentration-response curves to A23187 and thapsigargin. In rat aortic endothelial cells, KN-93 significantly decreased bradykinin-induced eNOS activity. These results suggest that CaMK II was involved in NO synthesis as a result of Ca(2+)-dependent activation of eNOS.

EDHF contributes to strain-related differences in pulmonary arterial relaxation in rats

Pulmonary arteries from the Madison (M) strain relax more in response to acetylcholine (ACh) than those from the Hilltop (H) strain of Sprague-Dawley rats. We hypothesized that differences in endothelial nitric oxide (NO) synthase (eNOS) expression and function, metabolism of ACh by cholinesterases, release of prostacyclin, or endothelium-derived hyperpolarizing factor(s) (EDHF) from the endothelium would explain the differences in the relaxation response to ACh in isolated pulmonary arteries. eNOS mRNA and protein levels as well as the NO-dependent relaxation responses to thapsigargin in phenylephrine (10(-6) M)-precontracted pulmonary arteries from the M and H strains were identical. The greater relaxation response to ACh in M compared with H rats was also observed with carbachol, a cholinesterase-resistant analog of ACh, a response that was not modified by pretreatment with meclofenamate (10(-5) M). N(omega)-nitro-L-arginine (10(-4) M) completely abolished carbachol-induced relaxation in H rat pulmonary arteries but not in M rat pulmonary arteries. Precontraction with KCl (20 mM) blunted the relaxation response to carbachol in M rat pulmonary arteries and eliminated differences between the M and H rat pulmonary arteries. NO-independent relaxation present in the M rat pulmonary arteries was significantly reduced by 17-octadecynoic acid (2 microM) and was completely abolished by charybdotoxin plus apamin (100 nM each). These findings suggest that EDHF, but not NO, contributes to the strain-related differences in pulmonary artery reactivity. Also, EDHF may be a metabolite of cytochrome P-450 that activates Ca(2+)-dependent K(+) channels.

Thapsigargin, a Ca(2+)-ATPase inhibitor, relaxes guinea pig tracheal smooth muscle by producing epithelium-dependent relaxing factors

A non-phorbol ester-type tumor promoter, thapsigargin has been reported to deplete Ca(2+) stores in endothelial cells by inhibiting Ca(2+)-ATPase, which in turn increases intracellular Ca(2+) by mobilization of extracellular Ca(2+), leading to activation of constitutive nitric oxide synthase (cNOS) and resultant generation of nitric oxide (NO). In the present study, to evaluate the role of Ca(2+) in the release of epithelium-dependent relaxing factor (EpDRF), we determined the effect of thapsigargin (10(-6) M) on the contraction evoked by exogenous Ca(2+) or acetylcholine (10(-5) M) in epithelium-denuded or epithelium-intact smooth muscle from guinea pig trachea. The following results were obtained: (1) In epithelium-denuded smooth muscle, the contraction evoked by exogenous Ca(2+) in Ca(2+)-free solution or by acetylcholine (10(-5) M) in Ca(2+)-containing solution did not change within 20 min after thapsigargin application, but the contraction evoked by exogenous Ca(2+) increased markedly after 120 min, indicating that thapsigargin had no effect on smooth muscle itself within 20 min of application. The following experiments were performed within 20 min of thapsigargin application. (2) In epithelium-intact smooth muscle, thapsigargin significantly suppressed the contraction evoked by acetylcholine, suggesting that thapsigargin stimulate the epithelium to produce EpDRF. N(G)-nitro-L-arginine methylester (L-NAME) partly, but significantly, attenuated this inhibitory effect of thapsigargin. (3) In epithelium-denuded smooth muscle, atropine (10(-6) M) and L-NAME (10(-5) M) did not change the contraction evoked by exogenous Ca(2+) after application of thapsigargin, suggesting that thapsigargin did not stimulate acetylcholine and NO release from nerve terminals. These results suggest that thapsigargin (10(-6) M) may stimulate EpDRF, including NO and other factor(s) by Ca(2+)-dependent mechanisms.

Possible involvement of Ca2+ entry and its pharmacological characteristics responsible for endothelium-dependent, NO-mediated relaxation induced by thapsigargin in guinea-pig aorta

Thapsigargin, a specific inhibitor of Ca(2+)-pump Ca(2+)-ATPase in the sarcoplasmic/endoplasmic reticulum (SR/ER), produces an endothelium-dependent vascular relaxation. In the present study, pharmacological features of thapsigargin-induced endothelium-dependent relaxation were functionally characterized in the isolated guinea-pig aorta especially focusing on the Ca2+ mobilization mechanisms in endothelial cells. Thapsigargin-induced endothelium-dependent vascular relaxation was markedly suppressed by N(G)-nitro-L-arginine (L-NNA) and calmidazolium, suggesting that the vascular relaxation to thapsigargin is largely attributable to endothelium-derived nitric oxide (NO) produced as a result of the activation of Ca2+, calmodulin-dependent NO synthase (NOS). Removal of Ca2+ from the external solution abolished the endothelium-dependent relaxation of guinea-pig aorta in response to thapsigargin. Thapsigargin-induced endothelium-dependent relaxation was inhibited more strongly compared with the endothelium-independent relaxation to an NO donor, SIN-1 (3-(4-morpholinyl)-sydnonimine), when the artery preparation was preconstricted with a high concentration (80 mM) of KCl instead of agonistic stimulation. Endothelium-dependent relaxation induced by thapsigargin was not affected by diltiazem, a blocker of L-type voltage-gated Ca2+ channels. SK&F96365 (1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1 H-imidazole) and Ni2+, both of which block capacitative Ca(2+) entry, did not show any appreciable inhibitory effects on the endothelium-dependent relaxation to thapsigargin. These findings suggest that in guinea-pig aorta, endothelium-dependent NO-mediated relaxation induced by thapsigargin is preceded by the increase in the cytosolic free Ca2+ concentrations ([Ca2+]cyt) following the depletion of stored Ca2+ in thapsigargin-sensitive store sites in endothelial cells. Although the increase in [Ca2+]cyt responsible for the activation of endothelium NOS leading to thapsigargin-induced vascular relaxation may be ascribed to the capacitative Ca2+ entry from extracellular space, the Ca2+ entry mechanism stimulated with thapsigargin is deficient in sensitivity to SK&F96365 and Ni2+ in the endothelium of guinea-pig aorta.

Hormonally controlled chloride movement across Drosophila tubules is via ion channels in stellate cells

Anion conductance across the Drosophila melanogaster Malpighian (renal) tubule was investigated by a combination of physiological and transgenic techniques. Patch-clamp recordings identified clusters of 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive "maxi-chloride" channels in a small domain of the apical membrane. Fluid secretion assays demonstrated sensitivity to the chloride channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid, diphenylamine-2-carboxylate, anthracene-9-carboxylic acid, and niflumic acid. Electrophysiological analysis showed that the calcium-mediated increase in anion conductance was blocked by the same agents. Vibrating probe analysis revealed a small number of current density hot spots, coincident with "stellate" cells, that were abolished by low-chloride saline or the same chloride channel blockers. GAL-4-targeted expression of an aequorin transgene revealed that the neurohormone leucokinin elicits a rapid increase in intracellular calcium levels in stellate cells that precedes the fastest demonstrable physiological effect. Taken together, these data show that leucokinins act on stellate cells through intracellular calcium to increase transcellular chloride conductance through channels. As electrogenic cation conductance is confined to principal cells, the two pathways are spatially segregated in this tissue.

Thapsigargin inhibits the response to acetylcholine and substance P but does not interfere with the responses to endothelium-independent agents

We investigated the influence of the Ca(2+)-ATPase inhibitor thapsigargin (TG) on the vasorelaxant response to different endothelium-dependent and endothelium-independent relaxing agents in an isolated thoracic aorta preparation of the rabbit, precontracted by norepinephrine (NE). Pretreatment with 100 microM L-arginine methyl ester (L-NAME) an inhibitor of nitric oxide (NO) synthesis, completely prevented acetylcholine (ACh)-induced relaxation; the inactive stereoisomer D-NAME did not modify the effect of ACh. The exposure of the preparations to 1 microM TG induced a slowly developing slight increase in the basal tension during 30-min contact. The same concentration of TG also slightly reduced the response to the subsequent administration of NE. The antagonist effect of TG on the ACh response was concentration dependent in the range between 0.1 and 10 microM. A 30-min pretreatment with 1 microM TG appeared to be sufficient to induce a consistent antagonism of the ACh (0.01-10 microM) concentration-relaxant effect curve, since an increase to 60 min did not produce a further significant increment in the degree of the antagonist effect. The concentration-dependent relaxation induced by substance P (SP 0.1-3 nM) was also significantly antagonized by 1 microM TG. The effect of the calcium ionophore A23187 (0.01-1 microM) was reduced by the Ca(2+)-ATPase inhibitor only at the higher concentrations tested (0.3-1 microM). However, a 30-min contact time with 1 microM TG was completely ineffective in antagonizing the concentration-relaxant response curves to the two nitrovasodilators sodium nitroprusside (SNP 0.1-100 microM) and nitroglycerin (NTG 1-300 nM) and to the cyclic GMP analogue 8-Bromo-cyclic GMP (3-100 microM). The effects of the beta-adrenoceptor agonist isoprenaline (ISO 0.1-10 microM) and of the direct adenylate cyclase activator forskolin (FK 0.01-10 microM) were also completely unaffected by 1 microM TG. These results demonstrate that TG affects the response to agents that induce an endothelium-dependent relaxation through receptor-dependent calcium mobilization. However, they do not support the hypothesis that sarcoplasmic pump activity is essential for the development of a vasorelaxant response to endothelium-independent agents.

Inhibition by SK&F96365 of NO-mediated relaxation induced by Ca2(+) -ATPase inhibitors in rat thoracic aorta

1. We investigated the effect of SK&F96365, a putative inhibitor of receptor-operated Ca2+ entry, on the endothelium-dependent, NO-mediated relaxation and cyclic GMP formation induced by Ca2(+)-ATPase inhibitors in rat thoracic aorta. 2. SK&F96365 inhibited cyclopiazonic acid or thapsigargin-induced relaxation and cyclic GMP formation mediated by a constitutive NO synthase, which is known to be activated by the Ca2+ that enters into the endothelial cells via plasma membrane Ca2+ channels subsequent to depletion of stored Ca2+ by Ca2(+)-ATPase inhibitors. 3. SK&F96365 also inhibited relaxation and cyclic GMP formation induced by acetylcholine, without affecting those induced by nitroprusside and A23187. 4. Ni2+ attenuated relaxation and cyclic GMP formation induced by cyclopiazonic acid and acetylcholine. 5. In contrast, the voltage-dependent Ca2+ channel blocker, nifedipine, did not affect the relaxation caused by Ca2(+)-ATPase inhibitors. 6. These results suggest that endothelium-dependent, NO-mediated relaxation of the arteries induced by Ca2(+)-ATPase inhibitors is triggered by the Ca2+ that enters into endothelial cells via receptor-operated channels (SK&F96365-sensitive channels) subsequent to depletion of stored Ca2+ as a result of inhibition of the Ca2(+)-ATPase (Ca2+ pump) of the stores.

NMDA-mediated activation of the NO/cGMP pathway: characteristics and regulation in cultured neocortical neurones

The linkage of the N-methyl-D-aspartate (NMDA) subtype of L-glutamate receptor to the nitric oxide (NO)/3, 5'-cyclic guanosine monophosphate (cGMP) intracellular signalling system was investigated in murine neocortical cultures by examining the effects of NMDA antagonists, NO synthase inhibitors, and drugs targeting second messenger systems on NMDA-stimulated synthesis of cGMP. NMDA-stimulated synthesis of cGMP was time- and concentration-dependent, and inhibited by competitive (LY 274614, 100 mu M) and non-competitive NMDA antagonists (MK-801 30 mu M, 7-chlorokynurenate 100 mu M, and ifenprodil 100 mu M). NO synthase inhibitors (NG-nitro-L-arginine, KN-62, diphenyleneiodonium) and LY 83583, an inhibitor of guanylate cyclase, all inhibited NMDA-stimulated cGMP synthesis in a concentration-dependent manner, demonstrating its dependence on the two enzymes. Phorbol 12-myristyl 13-acetate (0.1 mu M), arachidonic acid (1 mu M), and thapsigargin (10 mu M) produced approximately 50% inhibition of NMDA-induced cGMP synthesis. These observations demonstrate that all domains of the NMDA receptor-complex and of NO synthase are active in neocortical neuronal cultures, and that the essential NO/cGMP signalling system has complex interactions with other second messengers.

Different mechanisms can activate Ca2+ entrance via cation currents in endothelial cells

Effects of Endothelin-1 (ET-1) and cyclopiazonic acid (CPA) on non-specific cation channels in cultured bovine pulmonary artery endothelial cells (BPAECs) were investigated using the patch-clamp technique. In a bath solution containing Ca2+ as a permeant cation, 10 nM ET-1 increased inward and outward currents and this current reversed at -10 mV instead of -60 mV. Under similar conditions, 10 microM CPA, an inhibitor of Ca2+ pumps in the sarcoplasmic reticulum, also increased both currents which now reversed near -10 mV. An inorganic Ca2+ influx blocker, La3+ at 50 microM completely blocked ET-1 and CPA-evoked currents restoring the reversal potential to -60 mV. ET-1 and CPA evoked currents were partially blocked by 50 microM SK&F 96365 (a putative inhibitor of receptor-mediated Ca2+ entry). ET-1 and CPA increased Ca2+ influx by activation of the Ca(2+)-permeable non-specific cation channels, which are gated by the depletion of intracellular Ca2+ stores in endothelial cells. These results, together with a previous study demonstrating that this Ca2+ entrance pathway can be opened directly by one vasodilator (LP-805) reveal that different mechanisms exist to activate Ca2+ entrance into endothelial cells. All may allow sustained release of endothelium-derived relaxing factor (EDRF).