Eicosanoid-induced Ca2+ release and sustained contraction in Ca(2+)-free media are mediated by different signal transduction pathways in rat aorta

Mechanism of thromboxane receptor-induced vasoconstriction in human saphenous vein

Saphenous vein (SV) is one of the most widely used graft material in patients undergoing coronary artery bypass graft surgery (CABG). Thromboxane A2 (TXA2) is implicated in graft failure by inducing vasoconstriction and platelet aggregation. The aim of this study is to investigate the mechanism involved in TXA2-induced vasoconstriction in human SV. The role of different inhibitors and blockers on U46619 (TXA2-mimetic)-induced vasoconstriction is investigated by using an isolated organ bath system. Relaxation responses to several mediators are evaluated in SV pre-contracted with U46619 and compared with those pre-contracted with phenylephrine. Our results demonstrate that U46619-induced contraction is completely blocked by myosin light chain kinase inhibitor ML-9 or TP receptor antagonist BAY u3405. Furthermore, U46619-induced contraction is partially inhibited by phospholipase C inhibitor U73122, protein kinase C inhibitor calphostin C, Rho-kinase inhibitor Y-27632, L-type calcium channel blocker nifedipine, store-operated channel inhibitor SKF96365 or removal of extracellular calcium. Relaxation responses to NO donor (sodium nitroprusside), guanylate cyclase (GC) stimulator (riociguat), phosphodiesterase (PDE) inhibitors (sildenafil, IBMX), adenylate cyclase (AC) activator (forskolin) and acetylcholine (ACh) are markedly reduced when U46619 is used as a pre-contraction agent. Our results demonstrate that influx of extracellular Ca2+ (through L-type calcium channels and store-operated calcium channels) and intracellular Ca2+ release together with Ca2+ sensitization (through Rho-kinase activation) are necessary components for TXA2-induced vasoconstriction in SV. Moreover, more pronounced decrease in vasorelaxation induced by several mediators (SNP, riociguat, sildenafil, IBMX, forskolin, and ACh) in the presence of U46619 when compared with phenylephrine suggests that there is a crosstalk between the TP receptor signaling pathway and PDE, AC, GC enzymes. We believe that the investigation of mechanism of the TXA2-induced vasoconstriction in SV will provide additional information for the prevention of SV graft failure.

Involvement of phosphatidylcholine-specific phospholipase C in thromboxane A₂ receptor-mediated extracellular Ca²⁺ influx in rat aorta

An involvement of signal transduction other than phosphatidylinositol turnover in thromboxane A(2) receptor (TP receptor)-mediated vascular contraction was investigated in rat aorta. The contraction induced by U46619, a TP receptor agonist, at low concentrations (≤ 30 nM) was partially inhibited by verapamil, an inhibitor of voltage-dependent Ca(2+) channels (VDCC), and was further diminished in Ca(2+)-free solution. Twenty nanomolar of U46619 induced contraction and elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)), which were consisted of two phases; slowly developing first phase followed by quickly rising second phase. The second phase was inhibited by verapamil, and all the [Ca(2+)](i) response was abolished in Ca(2+)-free solution. The contraction and [Ca(2+)](i) elevation induced by 20 nM U46619 were not inhibited by U73122, an inhibitor of phosphatidylinositol-specific phospholipase C, or GF109203X, a protein kinase C inhibitor, but were abolished by D609, an inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC). However, D609 had no effect on those induced by 1 μM phenylephrine. The U46619-induced responses were also partially inhibited by cation channel blockers, 2-APB and LOE908. The inhibition by LOE908 was abolished in the presence of verapamil, suggesting that LOE908-sensitive cation channels lead to the activation of VDCC by depolarizing plasma membrane. In contrast, 2-APB further diminished the U46619-induced [Ca(2+)](i) elevation in the presence of verapamil. In conclusion, TP receptor stimulation is suggested to be coupled with PC-PLC. Diacylglycerol produced by PC-PLC seems to activate two types of cation channels independently of PKC, which in turn leads to VDCC-dependent and independent Ca(2+) influx, thereby eliciting contraction.

Mechanisms of the prostaglandin F2alpha-induced rise in [Ca2+]i in rat intrapulmonary arteries

The mechanisms by which prostaglandin F(2alpha) (PGF(2alpha)) increases intracellular Ca2+ concentration [Ca2+]i in vascular smooth muscle remain unclear. We examined the role of store-, receptor- and voltage-operated Ca2+ influx pathways in rat intrapulmonary arteries (IPA) loaded with Fura PE-3. Low concentrations (0.01-1 microM) of PGF(2alpha) caused a transient followed by a plateau rise in [Ca2+]i. Both responses became maximal at 0.1 microM PGF(2alpha). At higher concentrations of PGF(2alpha), a further slower rise in [Ca2+]i was superimposed on the plateau. The [Ca2+]i response to 0.1 microM PGF(2alpha) was mimicked by the FP receptor agonist fluprostenol, whilst the effect of 10 microM PGF(2alpha) was mimicked by the TP receptor agonist U-46619. The plateau rise in [Ca2+]i in response to 0.1 microM PGF(2alpha) was insensitive to diltiazem, and was abolished in Ca2+-free physiological salt solution, and by pretreatment with La3+, 2-APB, thapsigargin or U-73122. The rises in [Ca2+]i in response to 10 microM PGF(2alpha) and 0.01 microM U-46619 were partially inhibited by diltiazem. The diltiazem-resistant components of both of these responses were inhibited by 2-APB and La3+ to an extent which was significantly less than that seen for the response to 0.1 microM PGF(2alpha), and were also much less sensitive to U-73122. The U-46619 response was also relatively insensitive to thapsigargin. When Ca2+ was replaced with Sr2+, the sustained increase in the Fura PE-3 signal to 0.1 microM PGF(2alpha) was abolished, whereas 10 microM PGF(2alpha) and 0.05 microM U-46619 still caused substantial increases. These results suggest that low concentrations of PGF(2alpha) act via FP receptors to cause IP3-dependent Ca2+ release and store operated Ca2+ entry (SOCE). U-46619 and 10-100 microM PGF(2alpha) cause a TP receptor-mediated Ca2+ influx involving both L-type Ca2+ channels and a receptor operated pathway, which differs from SOCE in its susceptibility to La3+, 2-APB and thapsigargin, does not require phospholipase C activation, and is Sr2+ permeable.

Hypoxia induces hypersensitivity and hyperreactivity to thromboxane receptor agonist in neonatal pulmonary arterial myocytes

PPHN, caused by perinatal hypoxia or inflammation, is characterized by an increased thromboxane-prostacyclin ratio and pulmonary vasoconstriction. We examined effects of hypoxia on myocyte thromboxane responsiveness. Myocytes from 3rd-6th generation pulmonary arteries of newborn piglets were grown to confluence and synchronized in contractile phenotype by serum deprivation. On the final 3 days of culture, myocytes were exposed to 10% O2 for 3 days; control myocytes from normoxic piglets were cultured in 21% O2. PPHN was induced in newborn piglets by 3-day hypoxic exposure (Fi(O2) 0.10); pulmonary arterial myocytes from these animals were maintained in normoxia. Ca2+ mobilization to thromboxane mimetic U-46619 and ATP was quantified using fura-2 AM. Three-day hypoxic exposure in vitro results in increased basal [Ca2+]i, faster and heightened peak Ca2+ response, and decreased U-46619 EC50. These functional changes persist in myocytes exposed to hypoxia in vivo but cultured in 21% O2. Blockade of Ca2+ entry and store refilling do not alter peak U-46619 Ca2+ responses in hypoxic or normoxic myocytes. Blockade of ryanodine-sensitive or IP3-gated intracellular Ca2+ channels inhibits hypoxic augmentation of peak U-46619 response. Ca2+ response to ryanodine alone is undetectable; ATP-induced Ca2+ mobilization is unaltered by hypoxia, suggesting no independent increase in ryanodine-sensitive or IP3-linked intracellular Ca2+ pool mobilization. We conclude hypoxia has a priming effect on neonatal pulmonary arterial myocytes, resulting in increased resting Ca2+, thromboxane hypersensitivity, and hyperreactivity. We postulate that hypoxia increases agonist-induced TP-R-linked IP3 pathway activation. Myocyte thromboxane hyperresponsiveness persists in culture after removal from the initiating hypoxic stimulus, suggesting altered gene expression.

Cyclic AMP affects the haemocyte responses of larval Galleria mellonella to selected antigens

Signal transduction of the innate immediate responses of insect haemocytes to foreign matter is rarely considered. Herein using a combination of adenylate cyclase inhibitors and activators and phosphodiesterase inhibitors we determined that cyclic adenosine monophosphate (cAMP) at high levels normally impairs non-self response. Haemocyte contact with glass and bacteria lowered cAMP in vitro. Inactive phosphodiesterases, including type 4, impaired haemocyte reactions in vitro. Using the drugs in vivo to modulate adenylate cyclase and phosphodiesterases altered the total and types of haemocytes. Adenylate cyclase inhibitors and etazolate (a type 4 phosphodiesterase inhibitor) alone produced changes in the haemograms similar to those caused by Bacillus subtilis. Sequential injections of an enzyme modulator followed by B. subtilis impaired bacterial removal due (1) in the case of enzyme inhibitors, to the removal of haemocytes prior to bacterial challenge and (2) in the case of forskolin and IBMX to the shut-down of the haemocytes. Activating adenylate cyclase or inhibiting phosphodiesterase impaired bacterial removal when co-injecting the compounds and bacteria.

Pressor responses to platelet-activating factor and thromboxane are mediated by Rho-kinase

Platelet-activating factor (PAF) contracts smooth muscle of airways and vessels primarily via release of thromboxane. Contraction of smooth muscle is thought to be mediated either by calcium and inositol trisphosphate (IP(3))-dependent activation of the myosin light chain kinase or, alternatively, via the recently discovered Rho-kinase pathway. Here we investigated the contribution of these two pathways to PAF and thromboxane receptor-mediated broncho- and vasoconstriction in two different rat models: the isolated perfused lung (IPL) and precision-cut lung slices. Inhibition of the IP(3) receptor (1-10 microM xestospongin C) or inhibition of phosphatidylinositol-specific PLC (30 microM L-108) did not affect bronchoconstriction but attenuated the sustained vasoconstriction by PAF. Inhibition of myosin light chain kinase (35 microM ML-7) or of calmodulin kinase kinase (26 microM STO609), which regulates the phosphorylation of the myosin light chain, had only a small effect on PAF- or thromboxane-induced pressor responses. Similarly, calmidazolium (10 microM), which inhibits calmodulin-dependent proteins, only weakly reduced the airway responses. In contrast, Y-27632 (10 microM), a Rho-kinase inhibitor, attenuated the thromboxane release triggered by PAF and provided partial or complete inhibition against PAF- and thromboxane-induced pressor responses, respectively. Together, our data indicate that PAF- and thus thromboxane receptor-mediated smooth muscle contraction depends largely on the Rho-kinase pathway.

Role of Cl− currents in rat aortic smooth muscle activation by prostaglandin F2α

The aim of this study was to determine the role of Cl(-) channel activation in prostaglandin F(2 alpha)-stimulated aortic contraction and in membrane depolarization during stimulation with prostaglandin F(2 alpha) in an aortic smooth muscle cell line (A7r5). The Cl(-) channel antagonists 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), indanyloxyacetic acid-94 (IAA-94) and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) were found to decrease (P<0.05) the maximum tension generated by rat thoracic aortic segments during stimulation with prostaglandin F(2 alpha) and to shift the concentration-response relationship to the right. In the presence of Nifedipine and Cesium, rat aorta-derived A7r5 smooth muscle cells demonstrated outwardly rectifying voltage-dependent currents that were inhibited by NPPB, IAA-94 and DIDS. Both inward and outward currents were enhanced (P<0.05) following addition of prostaglandin F(2 alpha) (4 micromol/l, final concentration) to the bath solution and this increase was completely inhibited by NPPB. In the absence of Cesium, the addition of prostaglandin F(2 alpha) (4 micromol/l) to the extracellular bath solution either depolarized or hyperpolarized the cell membrane depending on the equilibrium potential for Cl(-) ions. Our results indicate that altered Cl(-) conductance is an important mechanism mediating membrane depolarization and contraction of aortic smooth muscle cells during stimulation with prostaglandin F(2 alpha). Given the significant role that prostaglandin F(2 alpha) and its biologically active isomers, the F(2) isoprostanes, play in the control of vascular tone during hypoxic and oxidative stress in the systemic circulation, alterations in Cl(-) channel function and expression may represent an important mechanism in the pathogenesis of abnormal blood flow regulation in disease states.

Variations in cell signaling pathways for different vasoconstrictor agonists in renal circulation of the rat

BACKGROUND

Major cell signaling pathways involved in agonist-induced vasoconstriction are recognized to be Ca2+ mobilization via inositol-1,4,5 triphosphate (IP3), Ca2+ influx through l-type channels, activation of protein kinase C (PKC), and of Rho-associated kinase (ROK). However, their contribution for renal vasoconstriction induced by different agonists is not well characterized.

METHODS

Increasing doses of angiotensin II (Ang II), norepinephrine, and arginine vasopressin (AVP) were infused into the left renal artery of anesthetized rats to reduce renal blood flow from a threshold value to about 50%. Rightward shift of the dose-response curves due to coinfusion of inhibitors served to assess contribution of different pathways: trimethoxybenzoate (TMB-8) against Ca2+ mobilization, nifedipine against Ca2+ influx, staurosporine and Ro-318220 against PKC, and Y-27632 and HA-1077 against ROK. Effects of inhibitors were also determined for renal response to a single dose of U-46619, a thromboxane A2 agonist. Composite response to U-46619 consisting of a fast and slow component did not permit determination of dose-response curves.

RESULTS

Inhibition of ROK by Y-27632 or HA-1077 had the largest effect on renal responses to agonists. They shifted dose-response curves of Ang II, norepinephrine, and AVP to sevenfold and higher values. Staurosporine, nifedipine, and TMB-8 had variable effect on agonist responses. They attenuated effects of Ang II and norepinephrine in an additive manner, and each of them increased effective dose values about fourfold. TMB-8 did not attenuate response to AVP and U-46619. Staurosporine and nifedipine diminished effects of AVP in a nonadditive manner, and attenuated additively the fast component of U-46619 response.

CONCLUSION

In contrast to other cell signaling pathways, ROK plays a common role for all vasoconstrictor agonistsis in renal circulation.

Essential role of rho kinase in the Ca2+ sensitization of prostaglandin F(2alpha)-induced contraction of rabbit aortae

Inhibition of dephosphorylation of the 20 kDa myosin light chain (MLC(20)) is an important mechanism for the Ca(2+)-induced sensitization of vascular smooth muscle contraction. We investigated whether this mechanism operates in prostaglandin F(2alpha) (PGF(2alpha))-induced contraction of rabbit aortic smooth muscle and, if so, whether protein kinase C (PKC) or rho-associated kinase (rho kinase) contribute to the inhibition of dephosphorylation. In normal medium, PGF(2alpha) (10 microM) increased the phosphorylation of MLC(20) and developed tension. The rho-kinase inhibitors fasudil and hydroxyfasudil inhibited these changes, despite having no effect on a phorbol-ester-induced MLC(20) phosphorylation. After treatment with verapamil or chelation of external Ca(2+) with EGTA, PGF(2alpha) increased the MLC(20) phosphorylation and tension without an increase in [Ca(2+)](i), all of which were sensitive to fasudil and hydroxyfasudil. ML-9, a MLC kinase inhibitor, quickly reversed the KCl-induced MLC(20) phosphorylation and contraction to the resting level. However, fractions of PGF(2alpha)-induced contraction and MLC(20) phosphorylation were resistant to ML-9 but were sensitive to fasudil. Ro31-8220 (10 microM), a PKC inhibitor, did not affect the phosphorylation of MLC(20) and the tension caused by PGF(2alpha), thus excluding the possibility of the involvement of PKC in the PGF(2alpha)-induced MLC(20) phosphorylation. PGF(2alpha) increased phosphorylation at Thr654 of the myosin binding subunit (MBS) of myosin phosphatase, which is a target of rho kinase, and fasudil decreased the phosphorylation. These data suggest that the PGF(2alpha)-induced contraction is accompanied by the inhibition of MLC(20) dephosphorylation through rho kinase-induced MBS phosphorylation, leading to Ca(2+) sensitization of contraction. An actin-associated mechanism may also be involved in the PGF(2alpha)-induced sensitization.

Endothelin and prostaglandin H(2)/thromboxane A(2) enhance myogenic constriction in hypertension by increasing Ca(2+) sensitivity of arteriolar smooth muscle

The myogenic response of skeletal muscle arterioles is enhanced in hypertension because of the release of endothelin (ET) and prostaglandin H(2) (PGH(2))/thromboxane A(2) (TxA(2)) from the endothelium. We hypothesized that ET and PGH(2)/TxA(2) modulate Ca(2+) signaling in arteriolar smooth muscle and thereby enhance myogenic constriction. Thus, simultaneous changes in intracellular Ca(2+) concentration in smooth muscle ([Ca(2+)](i)), measured by fura 2 microfluorometry (expressed as Ca(2+) fluorescence ratio [R(Ca)]), and diameter were obtained as a function of intraluminal pressure (P(i)) in isolated cannulated gracilis muscle arterioles (diameter approximately 120 micrometer) of normotensive Wistar rats (WR) and spontaneously hypertensive rats (SHR). In the absence of extracellular Ca(2+), increases in P(i) from 20 to 160 mm Hg increased the passive diameter of arterioles without changes in R(Ca). In the presence of extracellular Ca(2+) and endothelium, increases in P(i) elicited similar increases in R(Ca) (30+/-7% for control and 33+/-8% for SHR at 160 mm Hg) but a significantly (P<0.05) greater constriction of SHR arterioles compared with WR arterioles (at 160 mm Hg, 55+/-4% versus 38+/-2%, respectively, of passive diameter). In the absence of the endothelium, P(i)-induced changes in the R(Ca) and diameter of SHR and WR arterioles did not differ significantly. Also, a step increase in P(i) (from 80 to 140 mm Hg) elicited a similar increase in R(Ca) but greater constrictions in SHR versus WR arterioles. In the presence of the TxA(2) receptor inhibitor SQ29,548 and the ET(A) receptor inhibitor BQ123, there was no difference between responses of SHR and WR arterioles. In WR arterioles, increasing concentrations of KCl elicited a significant increase in R(Ca) (38+/-7% at 80 mmol/L) and completely constricted the arterioles. In contrast, constrictions to ET (52+/-7% at 3x10(-12) mol/L) and the TxA(2) agonist U46619 (40+/-8% at 3x10(-9) mol/L) were not accompanied by increases in R(Ca) at submaximal concentrations. Collectively, these findings suggest that in hypertension, endothelium-derived ET and PGH(2)/TxA(2) increase the Ca(2+) sensitivity of the contractile apparatus of arteriolar smooth muscle; thus, the similar increases in [Ca(2+)](i) in response to the elevation of intraluminal pressure elicit greater myogenic constriction.

Vascular reactivity to nifedipine and Ca(2+) in vitro: the role of preactivation, wall tension and geometry

The purpose of the study was to investigate the influence of preactivation, wall tension and geometry on the reactivity of porcine coronary arteries to nifedipine and extracellular Ca(2+) in vitro. Porcine large coronary arteries were mounted as ring and cylindrical preparations and studied by wire- and balloon-based techniques. The sensitivity and maximal responses to nifedipine were more pronounced in 25 mM K(+) compared to 10 microM prostaglandin F(2alpha)-contracted preparations. Vascular sensitivity to nifedipine and Ca(2+) was enhanced under isometric compared to isobaric conditions. Under isometric conditions in the presence of 25 mM K(+), coronary rings were more sensitive to nifedipine, but less sensitive to Ca(2+) compared to cylindrical segments. In cylindrical segments, circumferential and axial tension increases augmented the extracellular Ca(2+)-dependent spontaneous resting tone and the sensitivity to extracellular Ca(2+). Coronary rings showed no resting tone at various resting tensions. These results suggest that preactivation, wall tension and vessel geometry are important determinants of Ca(2+)-influxes via nifedipine-sensitive voltage-gated Ca(2+) channels. Furthermore, axial wall tension appears to be a modulator of nifedipine-insensitive transmembrane Ca(2+)-influx that may play a role for the tone and reactivity in large coronary arteries.

Halothane decreases calcium sensitivity of rat aortic smooth muscle

PURPOSE

To examine the effect of halothane on the cytosolic Ca2+ concentration ([Ca2+]i)-tension relationship of rat aortic smooth muscle.

METHODS

Rat aortic rings without endothelia were loaded with the fluorescent Ca2+ indicator, Fura PE3-AM, and then mounted in organ baths. The changes in isometric tension and [Ca2+]i were measured simultaneously. In one series ionomycin (10 nM-3 microM) was added to normal Krebs' solution cumulatively in the absence and presence of halothane (1.5%, 3%). In the other series, CaCl2 (0.3-3 mM) was added to Ca2+-free Krebs' solution including high KCl (50 mM), phenylephrine (100 nM) or prostaglandin F2alpha (PGF2alpha, 1-3 microM) in the absence and presence of halothane (1.5%, 3%). The linear part of [Ca2+]i-tension relationship was analyzed by a linear regression.

RESULTS

Halothane, 1.5%, had no effect on the normal [Ca2+]i-tension relationship obtained with the calcium ionophore, ionomycin (10 nM-3 microM), but halothane 3% decreased the slope of the relationship (0.239 +/- 0.037 for control and 0.110 +/- 0.010 for halothane 3%, P < 0.05). Halothane, 1.5% and 3%, did not change the [Ca2+]i-tension relationship obtained with CaCl2 (0.3-3 mM) in the presence of high KCl (50 mM) or phenylephrine (100 nM). In contrast, halothane, 3%, inhibited the intercept of [Ca2+]i-tension relationship obtained with CaCl2 (0.3-3 mM) in the presence of prostaglandin F2alpha (PGF2alpha, 1-3 microM) (45.708 +/- 4.233 for control and 26.997 +/- 2.522 for halothane 3%, P < 0.01).

CONCLUSION

Halothane decreases the Ca2+ sensitivity and that in the presence of PGF2.

ANG II- and TxA(2)-induced mesenteric vasoconstriction in rats is mediated by separate cell signaling pathways

Studies in vitro have demonstrated that vasoconstrictor agents increase intracellular Ca(2+) and activate protein kinase C (PKC) to elevate vascular tone. The aim of the present study was to determine the importance of these signaling pathways for angiotensin II (ANG II) and thromboxane A(2) (TxA(2)) in regulating mesenteric blood flow (MBF) in vivo. In anesthetized rats increasing doses of ANG II or the TxA(2) agonist U-46619 were administered into the superior mesenteric artery to reduce MBF. Intra-arterial infusion of inhibitors served to examine the contribution of different pathways: 8-(diethylamino)octyl 3,4,5-trimethoxybenoate hydrochloride (TMB-8) to inhibit intracellular Ca(2+) release, nifedipine to block transmembrane Ca(2+) influx through the L-type Ca(2+) channel, and staurosporine to inhibit PKC. Each of the inhibitors attenuated ANG II-induced reductions in MBF, and all dose-response curves were shifted to the right to an approximately threefold higher ANG II dose. Combinations of the inhibitors revealed that their effects were additive; together they abolished the vasoconstrictor action of ANG II completely. In contrast, the dose-response curve for U-46619 was not affected by any of the inhibitors infused either separately or together. The results demonstrate that a rise in intracellular Ca(2+) and activation of PKC are major mediators of the vasoconstrictor effect of ANG II in mesenteric circulation, but they play a subordinate role, if any, for the effects of TxA(2). Because TxA(2) plays a major role only under pathological conditions, the uncontrolled vasoconstriction appears to be associated with the recruitment of novel signal transduction pathways.

Effects of cell-permeant calcium chelators on contractility in monkey basilar artery

Vasospasm after traumatic or aneurysmal subarachnoid hemorrhage is associated with smooth muscle contraction, a process that results in part from increased intracellular calcium in smooth muscle cells. These experiments tested the hypothesis that chelation of intracellular calcium with the cell-permeant calcium chelator, 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetracetic acid acetoxymethyl ester (BAPTA-AM), decreases smooth muscle contraction in response to agents that cause contraction by increasing intracellular calcium. Effects of BAPTA-AM on vasoconstriction induced by KCl, prostaglandin F2alpha (PGF2alpha), caffeine, and erythrocyte hemolysate were tested on monkey basilar artery under isometric tension. BAPTA-AM, 30 and 100 micromol/L, caused a significant decrease in resting tension in rings with and without endothelium (30 micromol/L; 8+/-6% [n.s.] and 14+/-5%, 100 micromol/L; 19+/-3% and 32+/-6%,p < 0.05, paired t test). Contractions to caffeine were significantly decreased by 30 micromol/L BAPTA-AM and were abolished at 100 micromol/L in rings with and without endothelium (p < 0.05). BAPTA-AM, 100 micromol/L, competitively inhibited contractions to PGF2alpha. BAPTA-AM, 100 micromol/L, significantly decreased the maximum contractions to KCI in rings with and without endothelium (p < 0.05). There were no significant effects of BAPTA-AM on contractions induced by hemolysate in rings with endothelium but in rings without endothelium, BAPTA-AM, 100 micromol/L, significantly inhibited contractions. In rings with endothelium contractions to hemolysate could be significantly reduced by BAPTA-AM plus indomethacin or indomethacin alone, suggesting that hemolysate releases an eicosanoid from the endothelium by a pathway that is not inhibited by BAPTA. These results suggest that the ability of BAPTA-AM to inhibit smooth muscle contractions will depend on the agonists mediating the contraction. In response to erythrocyte hemolysate, loading of endothelial cells with BAPTA-AM increases the release of a vasoconstricting eicosanoid from these cells that counteracts the decreased contraction caused by loading of smooth muscle cells with BAPTA-AM.

Studies on the endothelin-1-induced contraction of rat granulation tissue pouch mediated by myofibroblasts

A granulation tissue pouch, mostly composed of myofibroblasts, was prepared by injecting rats subcutaneously with croton oil. The contraction of the granulation tissue pouch caused by endothelin-1 (ET-1) and the effects of the ET receptor antagonists, BQ123 and BQ788 on it were thus examined. ET-1 produced contractions in a dose-dependent manner. Pretreatment with BQ123, an ETA receptor selective antagonist, shifted the dose-response curve to the right, whereas pretreatment with BQ788, an ETB receptor selective antagonist, showed little effect. IRL1620, an ETB receptor selective agonist, did not cause any contraction in the granulation tissue pouch. The existence of both ETA and ETB receptors in the granulation tissue pouch and in cultured myofibroblasts was demonstrated by RT-PCR. Intracellular Ca2+ mobilization in Fura-2/acetyl-methoxy ester loaded cultured myofibroblasts isolated from the granulation tissue was also examined. ET-1 produced a transient increase in [Ca2+]i followed by a sustained elevation of [Ca2+]i, whereas IRL1620 caused only a transient peak. These results suggest that the ET-1 induced contraction of granulation tissue is mainly mediated through the mobilization of Ca2+ from the extracellular space caused by stimulation with ETA receptor.

Stimulated prostaglandin E2 release from rat skin, in vitro

The excitatory effect of bradykinin (BK) and of low pH on nociceptors appears to partly depend on secondary release of prostaglandins from the surrounding tissue. Rat skin, in vitro, is introduced as a novel model to measure basal and stimulated release of PGE2 and, in future, other substances relevant to nociception, such as neuropeptides. Flaps of hairy skin (n=57) from the rat saphenous region of the hindpaw were subcutaneously excised and fixed on acrylic rods, the corium side exposed. The preparations were equilibrated in carbogen gassed "synthetic interstitial fluid" (SIF) for 30 minutes. The skin flaps were then immersed for 5 minutes each in 9 consecutive glass tubes, which were mounted in a shaking bath at 32 degrees C. Each tube was filled with 5 ml of gassed SIF, the third tube contained inflammatory mediator(s) dissolved in SIF or solutions of low pH. After passage of the skin flap, the eluates were deep frozen (-70 degrees C) and the PGE2 content measured, off-line, using an enzyme immuno-assay. As stimulants, BK at 10(-5) M (n=9) and 10(-6) M (n=4) and BK in equimolar combination with histamine (HA) and serotonin (5-HT; 10(-5) M: n=8, 10(-6) M: n=6, 10(-7) M: n=6) dose-dependently increased PGE2 release. Considering the total amount of PGE2 secreted the combination of inflammatory mediators caused a significantly greater release of PGE2 at 10(-5) and 10(-6) M (p<0.01, Kruskal-Wallis test) than BK stimulation alone. Racemic flurbiprofen caused a profound depression of basal and stimulated release. Solutions of high proton concentration are known to stimulate and sensitize nociceptors. However, phosphate buffered SIF at pH 6.1 and 6.4 caused a substantial and significant decrease of the PGE2 release, probably due to low-pH block of phospholipases. Thus, algogenic potency of mediators does not necessarily match their pro-inflammatory action.

Mechanisms of hydrogen peroxide-induced contraction of rat aorta

It has been suggested that reactive oxygen species may be involved in the regulation of vascular tone. However, the underlying mechanisms remain to be elucidated. The present studies were designed to investigate the contractile effects of hydrogen peroxide (H2O2), one of the reactive oxygen species, on isolated ring segments of rat aorta with and without endothelium. H2O2 induced an endothelium-independent contraction in isolated rat aorta ring segments in a concentration-dependent manner at concentrations from 5 x 10(-6) to 5 x 10(-3) M. H2O2-induced contractions of denuded rat aorta rings were stronger than those on intact rat aorta segments. The contractile effects of H2O2 were inhibited completely by 1200 u/ml catalase. The presence of 1.0 microM Fe2+ or 10 microM proadifen, a cytochrome P450 monooxygenase inhibitor, potentiated the contractile effect of H2O2 on isolated rat aorta segments. 1 mM deferoxamine (a Fe2+ chelator) or 100 microM dimethyl sulfoxide (a hydroxyl radical scavenger) significantly attenuated the vessel contractions induced by hydrogen peroxide plus Fe2+ or hydrogen peroxide itself. Removal of extracellular Ca2+ ([Ca2+]0), addition of 5 microM verapamil, administration of a protein kinase C inhibitor (staurosporine), treatment with an inhibitor of protein tyrosine phosphorylation (genistein) or employment of 5.0 microM indomethacin resulted in a significant attenuation of the contractile responses of the vessels to H2O2. Pharmacological antagonists (e.g. a muscarinic acetylcholine receptor antagonist (atropine), an antagonist of histamine H1 receptors (diphenhydramine), an antagonist of histamine H2 receptors (cimetidine), an alpha-adrenoceptor antagonist (phentolamine), a beta-adrenoceptor antagonist (propranolol) and an antagonist of serotonin receptor (methysergide)) did not inhibit or attenuate the contractions induced by H2O2. Exposure of primary aortic smooth muscle cells to H2O2 (5 x 10(-6) to 5 x 10(-3) M) produced significant rises of intracellular Ca2+ ([Ca2+]i) within 20 s. Employment of 1.0 microM Fe2+ markedly enhanced the increment in [Ca2+]i in the smooth muscle cells. 10 microM proadifen treatment failed to alter the hydrogen peroxide-induced increment in [Ca2+]i of the smooth muscle cells. However, the presence of 5 microM indomethacin significantly attenuated the rise in [Ca2+]i in smooth muscle cells. The present results suggest that H2O2 can induce contractions of rat aorta segments, at pathophysiological concentrations, which are Ca2+-dependent. Hydroxyl radicals (.OH), cyclooxygenase products, protein kinase C and products of protein tyrosine phosphorylation appear to play some role in hydrogen peroxide-induced contractions. Metabolites catalyzed by cytochrome P450-dependent enzymes (upon treatment with hydrogen peroxide) appear to exert a vasodilator effect on rat aorta segments. Lastly, some unidentified mediators, produced by a cytochrome P450 inhibitor (proadifen), during hydrogen peroxide treatment, appear to play some role in contraction of vascular smooth muscle of rat aorta segments in vitro.

Mechanisms of prostaglandin F2 alpha and histamine-induced contractions in human chorionic vasculature

We investigated the signaling pathways modulating histamine- and prostaglandin F2 alpha (PGF2 alpha)-induced contractions of human chorionic vasculature. Neomycin, a phospholipase C (PLC) inhibitor, attenuated PGF2 alpha and histamine contractile responses 40 and 60%, respectively. AIF4-, a G protein stimulant, induced a strong contraction alone but blocked histamine- and PGF2 alpha-induced contractions. Staurosporine (100 nM), a protein kinase C (PKC) inhibitor, attenuated the PGF2 alpha-dependent contractions by 50% but did not affect the histamine response. However, higher nonspecific inhibitory concentrations of staurosporine (1-2 microM) abolished histamine and PGF2 alpha contractile responses, presumably by inhibiting other protein kinases. Although, the PKC phorbol 12-myristate 13-acetate (PMA) did not affect basal tension or PGF2 alpha-dependent contractions, the histamine response was attenuated by 30%. Sodium nitroprusside (SNP), a guanylyl cyclase stimulant, strongly attenuated histamine- and PGF2 alpha-induced contractions. Tension increases were similarly attenuated by forskolin and isobutylmethylxanthine (IBMX), which increase intracellular cyclic AMP. In vessel rings prelabeled with [3H]myoinositol, PGF2 alpha and histamine increased [3H]inositol phosphate (IP) production 400 and 100%, respectively, indicating that PLC is stimulated by both agonists. Neomycin inhibited histamine- and PGF2 alpha-induced increases in [3H]IP production 60 and 40%, respectively. Staurosporine (0.1-1 microM) and PMA did not affect histamine- or PGF2 alpha-stimulated IP production. AIF4-alone increased IP production but blocked histamine- and PGF(2 alpha)-dependent IP increases. These observations suggest that at least part of the contractile responses due to PGF2 alpha and histamine are associated with stimulation of PLC through an AIF4(-)-sensitive G protein. The role of PKC is variable, because PGF2 alpha but not histamine tension responses were attenuated by PKC inhibition. In addition, therapeutic agents that increase cyclic AMP and cyclic GMP attenuated histamine- and PGF2 alpha-induced contractions in human chorionic vasculature, although histamine responses were relatively more sensitive to these agents.

Inhibitory effect of phorbol 12,13-dibutyrate on norepinephrine-induced contraction in rabbit iris dilator muscle

The hypothesis that the increase in Ca2+ sensitivity on norepinephrine-induced contraction of smooth muscles and also the decrease of the norepinephrine-induced sustained level of intracellular Ca2+ concentration are produced by the activation of protein kinase C was tested. Phorbol 12,13-dibutyrate (PDB; 10(-6) M) relaxed the norepinephrine-induced sustained contraction in a concentration-dependent manner. On pretreatment with PDB a transient contraction was produced by the application of norepinephrine, but the sustained contraction was significantly reduced. The sustained elevations of intracellular Ca2+ concentration ([Ca2+]i) and the contraction induced by norepinephrine in fura-2-loaded preparations were decreased by the application of PDB. These inhibitory effects were antagonized by potent protein kinase inhibitors, 2-(1-(3-dimethylaminopropyl)-indol-3-yl)-3-(-indol-3-yl)-maleimide (GF 109203X) (10 (-6) M) and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) (10 (-6) M), but were not affected by a protein kinase A/G inhibitor, N-(2-cinnamylaminoethyl)-5-isoquinolinesulfonamide (H-88) (10(-6) M). The slope of the regression line for norepinephrine for [Ca2+]i and tension was significantly steeper than those obtained with high K+. Also, on pretreatment with PDB the Ca2+ sensitivity of the K(+)-induced contraction was decreased, but the Ca2+ sensitivity of norepinephrine-induced contraction tended to be increased. These observations indicate that PDB induces a decrease of [Ca2+]i on Ca2+ mobility and an increase of Ca2+ sensitivity on contraction of smooth muscle through the activation of protein kinase C.

Characterization of endothelin receptor subtypes mediating Ca2+ mobilization and contractile response in rabbit iris dilator muscle

1. We investigated the characteristics of endothelin (ET)-induced contraction and changes in intracellular Ca2+ concentration ([Ca2+]i) using the fura-2-loaded and non-loaded rabbit iris dilator. ET-1 and ET-2 (3-100 nM) and ET-3 (30-100 nM) caused contraction in a concentration-dependent fashion. 2. The selective ETB-receptor agonists, IRL1620 and sarafotoxin S6c produced only a small contraction or no contraction at a concentration of 1 microM. The rank order of potencies for the contraction (pD2 value) was ET-1 = ET-2 > ET-3 >> sarafotoxin S6c = IRL1620. 3. The contractile response to ET-3 was antagonized by pretreatment with BQ-123 (10 nM), a selective ETA receptor antagonist. The contractile responses to ET-1 and ET-2 were antagonized by pretreatment with BQ-123 (10 microM), but not at a concentration of 10 nM. 4. ETs increased [Ca2+]i and sustained muscle contraction. ET-1 (100 nM), ET-2 (100 nM), and ET-3 (1 microM) induced an elevation of [Ca2+]i consisting of two components: first a rapid and transient elevation to reach a peak, followed by a second, sustained elevation; a sustained contraction was produced without a transient contraction. The ETB receptor-selective agonist, IRL1620 (1 microM) and sarafotoxin S6c (1 microM) also induced a rapid and transient elevation of [Ca2+]i to reach a peak and a sustained elevation, together with only a small contraction or no contraction. 5. ET-1 (100 nM) induced a transient increase in [Ca2+]i in a Ca(2+)-free, 2 mM EGTA-containing physiological saline solution (Ca(2+)-free PSS), and a small sustained contraction which was significantly different from that induced by ET-1 (100 nM) in normal PSS. The ET-1-induced increase in [Ca2+]i and sustained contraction were not affected by the voltage-dependent Ca2+ channel blocker, nicardipine (10 microM). The ET-1-induced transient increase in [Ca2+]i was significantly reduced by the sarcoplasmic reticulum (SR) Ca(2+)-ATPase inhibitor, cyclopiazonic acid (30 microM); however, the ET-1-induced sustained contraction was not affected by this agent. 6. The selective ETA receptor antagonist, BQ-123 (100 nM) reduced the ET-3 (100 nM)-induced contraction, but did not affect the transient increase or elevation of the second phase of [Ca2+]i. However, this antagonist at 1 microM did not affect the ET-1 (100 nM)- and ET-2 (100 nM)-induced elevation of [Ca2+]i and contractile response, or the IRL1620-induced elevation of [Ca2+]i. 7. The selective ETB receptor antagonist, BQ-788 (1 microM) reduced the transient increase in [Ca2+]i induced by ET-1 (30 nM), ET-2 (30 nM), ET-3 (100 nM) and IRL1620 (1 microM), but did not affect the sustained elevation of [Ca2+]i and contractile responses produced by ET-1, ET-2 and ET-3. 8. Pretreatment with IRL1620 (1 microM) reduced the increase in [Ca2+]i induced by IRL1620 (1 microM) and sarafotoxin S6c (1 microM), as well as the ET-1 (100 nM)-, ET-2 (100 nM)- and ET-3 (1 microM)-induced elevation of [Ca2+]i, whereas in the presence of IRL1620, ET-1-, ET-2- and ET-3-induced contractions were unaltered. 9. These results suggest that ETA and ETB receptor subtypes exist in the rabbit iris dilator muscle, and that the ETA receptor is divided into: (1) BQ-123-sensitive ETA subtypes activated by ET-1, ET-2 and ET-3, and (2) BQ-123-insensitive ETA subtypes activated by ET-1 and ET-2, which cause the sustained increase of [Ca2+]i and contraction; in contrast, ETB receptor subtypes are activated by ET-1, ET-2, ET-3, IRL1620 and sarafotoxin S6c and cause the transient and sustained increase in [Ca2+]i which is not able to contract the smooth muscle.

Role of calcium in thromboxane B2-mediated injury to rabbit gastric mucosal cells

A sustained increase in cytosolic Ca2+ can damage gastric mucosal cells. The present study has examined the role of Ca2+ in thromboxane B2 (TXB2)-mediated damage of rabbit isolated gastric mucosal cells. Cells were isolated from rabbit oxyntic mucosa by collagenase-EDTA digestion. Cell metabolic activity and cell damage were estimated by alamar blue dye absorbance and trypan blue uptake, respectively. Cellular Ca2+ was monitored by indo-1 dye fluorescence. Addition of TXB2 (10(-6) and 10(-8) M) to the cell suspension resulted in a decrease in metabolic activity, and this effect was reduced when Ca2+ was removed from the incubation medium. TXB2 addition to the incubation medium resulted in an increase in cytosolic Ca2+ and incubation of cells with the intracellular Ca2+ chelator, BAPTA-AM (20 microM), reduced cell injury in response to TXB2. Incubation of cells with the Ca2+ ionophore A23187 (1-25 microM) resulted in a dose-dependent increase in trypan blue uptake and a reduction in cell metabolism. Cell injury in response to A23187 were exacerbated by addition of TXB2 (10(-8) M) to the cell suspension. TXB2 treatment reduced cellular content of reduced glutathione (GSH), while exogenous GSH addition (10 mM) reduced TXB2-mediated cell injury. These data demonstrate that TXB2 can directly injure gastric mucosal cells. Gastric mucosal cellular damage in response to TXB2 is mediated in part by a disruption of Ca2+ homeostasis as well as a reduction in cellular GSH content.

Variable potency of nitrergic-nitrovasodilator relaxations of the mouse anococcygeus against different forms of induced tone

1. U46619 (thromboxane A2 receptors; 0.002-1 microM), carbachol (muscarinic M3 receptors; 0.1-100 microM), cyclopiazonic acid (CPA; Ca(2+)-ATPase inhibitor; 0.1-30 microM) and K+ (5-100 mM) produced concentration-dependent contractions of the mouse isolated anococcygeus muscle. Equi-effective, submaximal concentrations of each agent were used in further experiments (40 nM U46619; 5 microM carbachol; 5 microM CPA; 70 mM K+). 2. Nifedipine (1 microM) totally abolished contractile responses to K+; those to U46619, carbachol and CPA were reduced by only 20-30% in the presence of nifedipine, but were greatly reduced (> 90%) by a combination of nifedipine and SKF 96365 (0.1-40 microM). 3. In Ca(2+)-free medium, contractions to K+ and CPA were abolished. Small residual responses remained to both carbachol and U46619; those to carbachol were transient, could not be repeated in the continued absence of Ca2+ and were prevented by pre-incubation with CPA, but unaffected by SKF 96365; those to U46619 were sustained, could be repeated in the absence of Ca2+, and were resistant to CPA and SKF 96365. 4. Tone induced by all four agents could be relaxed by sodium nitroprusside (SNP), but with a clear order of potency. SNP (pIC40) was most effective against U46619 (7.92), less so against carbachol (6.80) and CPA (6.68), and least potent against K+ (5.94). A similar order of potency was observed with 8Br-cyclic GMP (50 microM) and nitrergic field stimulation (1-20 Hz). 5. The relaxant potency of SNP was similar in normal Krebs solution and in high K+ (70 mM) Krebs containing 1 microM nifedipine. 6. Inclusion of SNP (0.01-1 microM) or 8Br-cyclic GMP (50 microM) in the Ca2+-free medium inhibited the transient residual response to carbachol. Inclusion of similar concentrations of SNP or 8Br-cyclic GMP,during Ca2+ re-loading, increased the subsequent residual contraction to carbachol in Ca2+-free medium.7. At higher concentrations, SNP (0.1-10 microM) produced a partial relaxation of the sustained contraction to U46619 in Ca2+-free medium.8. Thus, the relaxant potency of the nitrergic stimuli was dependent on the agent and mechanism used to induce tone in the preparation. Examination of the contractile/relaxant interactions suggests that altered Ca2+ sequestration and inhibition of contractile protein function may underlie nitrergic relaxations of this tissue.

Nitrergic stimulation does not inhibit carbachol-induced inositol phosphate generation in the rat anococcygeus

Carbachol (50 microM) produced a rapid, transient increase in inositol-1,4,5-trisphosphate (IP3) levels in the rat anococcygeus; the peak increase observed at 10 s (3-fold above controls) was greatly reduced in the presence of atropine (100 nM), but was unaffected by nitrergic stimulation (10 Hz), sodium nitroprusside (10 microM) or 8-Br-cyclic GMP (200 microM). Following loading of muscles with [3H]myo-inositol, subsequent exposure to carbachol for 30 min resulted in a 6-fold increase in the accumulation of [3H]inositol-1-monophosphate; again, this action of carbachol was greatly attenuated by atropine, but unaffected by nitrergic stimulation, sodium nitroprusside or 8-Br-cyclic GMP. It is concluded that inhibition of agonist-induced generation of inositol phosphates cannot explain the ability of nitrergic activation to relax (by 54-62%) carbachol-induced tone in this tissue.