Identification of protein kinase C isoforms in rat mesenteric small arteries and their possible role in agonist-induced contraction

Mechanism of Ca2+ release and entry during contraction elicited by norepinephrine in rat resistance arteries

The intracellular Ca2+ stores and the mechanisms of Ca2+ entry produced by norepinephrine (NE) were investigated in small mesenteric resistance arteries of the rat. In Ca2+-free medium, NE (10 microM) elicited a transient increase in both intracellular free Ca2+ concentration ([Ca2+]i) and tension that were both drastically reduced by caffeine and only partially reduced by the two sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) blockers thapsigargin and cyclopiazonic acid, despite the presence of SERCA2a and SERCA2b isoforms in the medial smooth muscle layer of the artery. After depletion of intracellular Ca2+ stores with 10 microM NE, addition of exogenous CaCl2 (2.5 mM) produced large and sustained increases in both [Ca2+]i and contraction of the arteries provided that the agonist was continuously present. In these conditions, the responses to CaCl2 were inhibited by the voltage-dependent Ca2+ entry blocker nitrendipine (1 microM), the putative inhibitor of receptor-operated Ca2+ entry SKF-96365 (30 microM), and NiCl2 (1 mM). The inhibition produced by SKF-96365 and NiCl2 was greater than that of nitrendipine. Also, the responses to CaCl2 were greatly reduced or abolished in the presence of the Na+/Ca2+ exchanger inhibitors 1,3-dimethyl-2-thiourea, 3',4'-dichlorobenzamil, MgCl2, and amiloride or after omission of NaCl in the medium. Also, protein kinase C inhibitors, calphostin C and staurosporine, and tyrosine kinase inhibitors, genistein and tyrphostin 23, both reduced the responses to CaCl2. The inhibitory effect of protein kinase C inhibitor and tyrosine kinase were additive. These results suggest that NE releases Ca2+ from intracellular stores that are caffeine sensitive and partially sensitive to SERCA inhibitors. They indicate that in addition to Ca2+ influx via nitrendipine-sensitive and SKF-96365-sensitive channels, Na+/Ca2+ exchanger participates in the CaCl2-induced contraction produced in NE-exposed vessels. The pathway leading to Ca2+ entry probably involves tyrosine kinase and protein kinase C. All the above mechanisms require ongoing receptor stimulation.

Lipid second messengers derived from glycerolipids and sphingolipids, and their role in smooth muscle function

The processes that link activation of an external receptor to the internal mechanisms that elicit a physiological response have been the subject of extensive investigation. It has been established that rather than just being an inert barrier to protect the cell from environmental damage, there are populations of phospholipids located within the plasma membrane that act as a reservoir for signalling molecules and when a receptor binds its appropriate activating ligand a chain of events is initiated which leads to the breakdown of these lipids and the release of second messengers. Such processes are rapid enough for physiological responses to be effected. The purpose of this review is to examine the profile of lipid second messengers derived from glycerophospholipids and sphingolipids. In the former class are included phosphoinositide and phosphatidylcholine and the latter includes sphingomyelin. Hydrolysis of such parent compounds is mediated by phospholipases and the profile of metabolites appears to be agonist specific and modulated by a number of mechanisms including heterotrimeric G-protein subunits, small G-proteins, alterations in intracellular calcium concentration, protein kinase C and tyrosine kinases. The recent interest in sphingolipids, particularly in vascular smooth muscle cells, has been provoked by the observation that ceramide and sphingoid base formation is observed in response to vasoconstrictor hormones.

Signal transduction by protein kinase C in mammalian cells

1. The past two decades have witnessed great advances in our understanding of the role of protein kinase C (PKC) in signal transduction. The Ca(2+)-activated, phospholipid-dependent protein kinase discovered by Nishizuka's group in 1977 is now a family of at least 11 isoforms. Protein kinase C isoforms exist in different proportions in a host of mammalian cells and each isoform has a characteristic subcellular distribution in each cell type. 2. Stimulation of a specific PKC isoform often causes redistribution of the isoform from one subcellular compartment to another compartments where it complexes with and phosphorylates a specific protein substrate. 3. The interaction of a specific PKC isoform with its protein substrate may directly activate a specific function of the cell or may trigger a cascade of protein kinases that ultimately stimulates a specific response in differentiated cells or regulates growth and proliferation in undifferentiated cells.

PKC-dependent signalling mechanisms in differentiated smooth muscle

Protein kinase C (PKC) is now known to play an important physiological role in essentially all cell types. This review will focus on what is known about the kinase in contractile differentiated smooth muscle. Current knowledge on the molecular structure of PKC isoforms will be discussed as they relate to mechanisms of translocation and targeting of the kinase within smooth muscle cells. Studies performed on PKC-dependent signalling pathways in differentiated smooth muscle cells will be discussed with emphasis on studies form our laboratory, especially discussing thin filament linked pathways. Thick filament linked PKC-dependent pathways will be described in more detail elsewhere in this monograph.

Protein kinase C: modulation of vasopressin-induced calcium influx and release in A7r5 vascular smooth muscle cells

This study was guided by the hypothesis that specific isoforms of protein kinase C may participate in modulating increases in intracellular Ca2+ that are induced by stimulation of vascular smooth muscle cells with vasopressin. Immunoblot analysis revealed that A7r5 vascular smooth muscle cells expressed conventional (alpha), novel (delta and epsilon), and atypical (iota/lambda and mu) isoforms of protein kinase C. Stimulation of fura-2-loaded cells with 20 nM vasopressin induced a rapid transient increase in the intracellular concentration of calcium that was followed by a slowly declining component which was above baseline throughout the period of observation. Cell fractionation studies showed that the calcium response was associated with (a) transient translocation of the alpha and delta isoforms of protein kinase C from the cytosolic fraction to the particulate-membrane fraction, (b) sustained translocation of the epsilon isoform, and (c) no translocation of iota/lambda or mu isoforms. Ratiometric and isobestic fluorescence analysis showed that vasopressin-induced Ca2+ influx and release were markedly inhibited in cells that were preincubated with either 1 microM phorbol 12-myristate 13-acetate, or 10 microM 1, 2 dioctanoyl-sn-glycerol, two structurally different activators of protein kinase C. In contrast, vasopressin-induced increases in intracellular Ca2+ were not significantly altered following preincubation with either 1 microM 4alpha-phorbol or 4alpha-phorbol 12,13-didecanoate, analogs of phorbol 12-myristate 13-acetate that do not activate protein kinase C. Moreover, the inhibitory effects of phorbol 12-myristate 13-acetate were prevented by treatment with 1 microM GF109203X, a potent inhibitor of protein kinase C. Taken together, these results show that direct activation of protein kinase C can negatively modulate vasopressin-induced Ca2+ influx and release in cultured vascular smooth muscle cells. They also show that stimulation with vasopressin induces translocation of specific isoforms of protein kinase C, an observation suggesting that one or more of these isoforms may participate in modulation of vasopressin-induced increases in intracellular Ca2+.

Overexpression of protein kinase C alpha enhances lipopolysaccharide-induced nitric oxide formation in vascular smooth muscle cells

Our previous studies showed that lipopolysaccharide (LPS)-induced nitric oxide (NO) synthesis in cardiovascular tissues is attenuated by protein kinase C (PKC) inhibitors. In the current study, we identify a specific PKC isotype involved in the LPS signal transduction pathway that leads to NO formation in rat vascular smooth muscle cells (VSMC). VSMC were transfected with a mammalian expression vector containing a full length PKCalpha cDNA insert, and a stable transfectant overexpressing PKCalpha was obtained as evidenced by increased expression of PKCalpha mRNA and protein. In response to 100 ng/ml LPS stimulation, the PKCalpha transfectants showed a 1.8-fold increase in PKC activity at 30 min and a twofold increase in NO production over 24 hr compared with cells transfected with control plasmids. The LPS-stimulated increase in NO synthesis in PKCalpha transfectants was blocked by the specific PKCalpha inhibitor Gö 6976. After 6 hr LPS treatment, PKCalpha-transfected and control cells showed equivalent increases in mRNA and protein for the inducible NO synthase. NO synthase activity of the cell extracts assayed in the presence of excess substrate and cofactors was not significantly different between PKCalpha-transfected and control cells after LPS stimulation. However, mRNA levels for GTP cyclohydrolase I, a key enzyme in (6R)-tetrahydro-L-biopterin synthesis, and cationic amino acid transporter-2, involved in L-arginine transport, was enhanced in cells overexpressing PKCalpha compared with control cells. These results suggest that PKCalpha plays an important role in LPS-induced NO formation and that a significant portion of this effect may be by means of enhanced substrate availability to the inducible nitric oxide synthase enzyme.

Platelet-derived growth factor-BB, insulin-like growth factor-I, and phorbol ester activate different signaling pathways for stimulation of vascular smooth muscle cell migration

Vascular smooth muscle cell (VSMC) migration is an important process in the development of vascular occlusive disease. To investigate mitogen regulation of VSMC migration, a cell-layer-scrape assay was used to measure migration 20 h after stimulation of VSMC with platelet-derived growth factor-BB (PDGF-BB), insulin-like growth factor I (IGF-I), or phorbol 12-myristate 13-acetate (PMA). The contributions of cell proliferation were eliminated by treatment of VSMC with hydroxyurea, which suppressed DNA synthesis.PDGF-BB stimulated VSMC migration 2.5-fold, while PMA and IGF-I stimulated migration 1.7- and 1.5-fold, respectively. The importance of protein kinase C (PKC), ERK, and phosphoinositide-3' kinase (PI3 kinase) in mitogen-stimulated migration was investigated, using specific inhibitors of these signaling molecules. PDGF-BB-stimulated migration was inhibited by the general PKC inhibitor RO 31-8220 (40%), the MEK inhibitor PD98059 (31%), and the PI3 kinase inhibitor wortmannin (22%) but not by PMA-induced downregulation of conventional and novel PKC isoforms. IGF-I-stimulated migration was inhibited by RO 31-8220 (34%) and wortmannin (37%) but was much less affected by PD98059 (19%) or PKC downregulation (10%). PMA-stimulated migration was inhibited by RO 31-8220 (53%), PD98059 (50%), wortmannin (45%), and PKC downregulation (47%). Western analysis confirmed that ERK was strongly activated by PDGF-BB and PMA but not by IGF-I. To examine potential in vivo negative regulators of VSMC migration, we analyzed the ability of heparin, an analogue of heparan sulfate, and TGFbeta to attenuate mitogen-stimulated migration. Heparin but not TGFbeta inhibited VSMC migration stimulated by all three mitogens. Delayed-addition experiments showed that RO 31-8220 retained substantial inhibitory activity even if added 3 h after PMA or IGF-I stimulation and 5 h after PDGF-BB addition, suggesting that sustained PKC activation is important for migration. The MEK inhibitor retained some effectiveness for 5 h after PDGF-BB stimulation but only 1 h after PMA addition. Western analysis showed ERK activation was transient after PMA treatment but sustained for 6 h after PDGF-BB treatment. Heparin strongly inhibited migration even if added 5-7 h after mitogen stimulation, suggesting that heparin may inhibit both short- and long-term signals necessary for migration. The present studies indicate that PMA and IGF-I activate a limited number of second messengers resulting in moderate stimulation of migration; in contrast PDGF-BB stimulates multiple signaling pathways resulting in strong stimulation of migration and lessened sensitivity to inhibitory signals.

Rapid activation of the novel serine/threonine protein kinase, protein kinase D by phorbol esters, angiotensin II and PDGF-BB in vascular smooth muscle cells

Protein kinase D (PKD) is a novel serine/threonine kinase structurally distinct from all protein kinase C (PKC) isoforms but which like classic and novel PKCs is activated by phorbol esters and diacylglycerol. This study investigated the regulation of PKD in vascular smooth muscle cells (VSMC) by physiological regulators of VSMC function and growth factors. Treatment of rabbit aortic VSMC with phorbol ester, angiotensin II and PDGF-BB all stimulated PKD activity in a time- and concentration-dependent manner in VSMC. The effect of angiotensin II was particularly rapid and potent (maximum stimulation within 1 min and at 0.5 nM). In contrast, the maximum effect of PDGF-BB was obtained after 5 min. Other factors, including basic FGF, IGF-I, IGF-II, endothelin-1 and endothelin-2, had no effect on PKD activity in VSMC. These results show for the first time that PKD activity is regulated in VSMC, and is activated by the vasoconstrictor angiotensin II. PKD may be an important mediator for the biological function(s) of one or more PKC isoforms in VSMC and/or may represent a component of a novel PKC-independent signalling pathway in VSMC.

Protein kinase C alterations in aortic vascular smooth muscle cells from rats with cirrhosis

BACKGROUND/AIMS

Alterations in signal transduction in vascular smooth muscle cells may contribute to vascular hyporeactivity in cirrhosis. Protein kinase C plays a role in vascular cell contraction by modifying contractile proteins and intracellular [Ca2+] homeostasis. The aim of this study was to examine the vascular reactivity and expression of protein kinase Calpha in aortae from rats with cirrhosis.

METHODS

The contractile response to phorbol ester, a protein kinase C activator, was evaluated in endothelium-denuded aortic rings from normal and cirrhotic rats. Protein kinase Calpha expression was determined by Western blot analysis.

RESULTS

Maximal contraction was significantly less marked in cirrhotic (1.24+/-0.24 g) than in control (3.43+/-0.27 g) aortae. Phorbol myristate-acetate-induced contraction was dependent on extracellular [Ca2+] concentrations, as shown by a reduction in maximal contraction when control and cirrhotic aortic rings were exposed to a Ca2+-free medium. Increasing the intracellular [Ca2+], by incubation with a Ca2+ ionophore, significantly increased the maximal contraction induced by phorbol myristate-acetate in cirrhotic but not in control rat aortae. Protein kinase Calpha expression was significantly lower in aortae in cirrhotic than in control rats.

CONCLUSION

These results confirm alterations in protein kinase C in aortae from cirrhotic rats.

Intracellular translocation of PKC isoforms in canine pulmonary artery smooth muscle cells by ANG II

Our goals were to identify the isoforms of protein kinase C (PKC) present in primary cultures of canine pulmonary artery smooth muscle cells (PASMCs) and to determine whether angiotensin II (ANG II) triggers translocation of specific PKC isoforms to discreet intracellular locations. Isoform-specific antibodies and Western blot analysis were utilized to identify the isoforms of PKC in PASMCs. Indirect immunofluorescence and confocal microscopy were used to examine the subcellular distribution of PKC isoforms. Inositol phosphate production was used to assess phospholipase C activation, and fura 2 was utilized to monitor intracellular Ca2+ concentration in response to ANG II. Six isoforms (alpha, delta, epsilon, zeta, iota/lambda, and mu) of PKC were identified by Western blot analysis. Immunolocalization of 5 isoforms (alpha, delta, zeta, iota/lambda, and mu) revealed a unique pattern of staining for each individual isoform. ANG II caused translocation of PKC-alpha from the cytosol to the nuclear envelope and of PKC-delta to the myofilaments. In contrast, cytosolic PKC-zeta did not translocate, but nuclear PKC-zeta was upregulated. Translocation of PKC-alpha and PKC-delta and upregulation of PKC-zeta in response to ANG II were blocked by the ANG II type 1-receptor antagonist losartan. In addition, ANG II stimulated inositol phosphate production and intracellular Ca2+ concentration oscillations, which were blocked by losartan. Thus activation of ANG II type 1 receptors triggers the phosphoinositide signaling cascade, resulting in translocation or upregulation of specific PKC isoforms at discreet intracellular sites. The alpha and zeta isoforms may act to regulate nuclear events, whereas PKC-delta may be involved in modulating contraction via actions on the myofilaments.

Protein kinase C mediates increase of Ca2+ sensitivity for contraction by cholinoceptor partial agonist in ileal longitudinal muscle of guinea pig

1. Experiments were designed to study the roles of protein kinase C in carbachol- and pilocarpine-induced contraction and the increase in cytosolic Ca2+ concentration ([Ca2+]i) in guinea pig ileal longitudinal muscle. 2. The protein kinase C inhibitors, GF 109203X (10 microM), calphostin C (10 microM) and H-7 (10 microM), reduced the maximum of the concentration response curve produced by pilocarpine more effectively than that produced by carbachol. 3. The slopes of the regression lines between [Ca2+]i and tension development for pilocarpine and carbachol in tissues treated with GF 109203X were significantly gentler than those for untreated tissues. 4. The protein kinase C alpha- and beta 1 selective inhibitor Goe 6976 (1 microM) decreased both [Ca2+]i and contraction, but did not affect the slopes of the regression lines for pilocarpine and carbachol. 5. These results suggest that protein kinase C (both n- and/or a-type) plays an important role in the increase of Ca2+ sensitivity of the contractile element, and that pilocarpine mainly activates the protein kinase C-dependent pathways for contractile mechanisms in guinea pig ileal longitudinal muscle.

Farnesol inhibits L-type Ca2+ channels in vascular smooth muscle cells

Earlier experiments with animal and human arteries have shown that farnesol, a natural 15-carbon (C15) isoprenoid, is an inhibitor of vasoconstriction (Roullet, J.-B., Xue, H., Chapman, J., McDougal, P., Roullet, C. M., and McCarron, D. A. (1996) J. Clin. Invest. 97, 2384-2390). We report here that farnesol reduced KCl- and norepinephrine-dependent cytosolic Ca2+ transients in fura-2-loaded intact arteries. An effect on Ca2+ signaling was also observed in cultured aortic smooth muscle cells (A10 cells). In these cells, farnesol reduced KCl-induced [Ca2+]i transients and mimicked the inhibitory effect of Ca2+-free medium on the [Ca2+]i response to both 12,13-phorbol myristate acetate, a protein kinase C activator, and thapsigargin, a specific endoplasmic reticulum ATPase inhibitor. Perforated patch-clamp experiments further showed in two vascular smooth muscle cell lines (A10 and A7r5), a reversible, dose-dependent inhibitory effect of farnesol on L-type Ca2+ currents (IC50 = 2.2 microM). Shorter (C10, geraniol) and longer (C20, geranylgeraniol) isoprenols were inactive. L-type Ca2+ channel blockade also occurred under tight (gigaohm) seal configuration using cell-attached, single-channel analysis, thus suggesting a possible action of farnesol from within the intracellular space. We finally demonstrated that farnesol did not affect Ca2+-sensitive pathways implicated in smooth muscle contraction, as tested with alpha-toxin permeabilized arteries. Altogether, our results indicate that farnesol is an inhibitor of vascular smooth muscle Ca2+ signaling with plasma membrane Ca2+ channel blocker properties. The data have implications for the endogenous and pharmacological regulation of vascular tone by farnesol or farnesol analogues.

Role of protein kinase C in calcium sensitization during muscarinic stimulation in airway smooth muscle

Muscarinic receptor stimulation increases Ca2+ sensitivity, i.e., the amount of force produced at a constant submaximal cytosolic Ca2+ concentration ([Ca2+]i), in permeabilized smooth muscle preparations. It is controversial whether this increase in Ca2+ sensitivity is in part mediated by protein kinase C (PKC). With the use of a beta-escin permeabilized canine tracheal smooth muscle (CTSM) preparation, the effect of four putative PKC inhibitors [calphostin C, chelerythrine chloride, a pseudosubstrate inhibitor for PKC [PKC peptide-(19-31)], and staurosporine] on Ca2+ sensitization induced by acetylcholine (ACh) plus GTP was determined. Preincubation with each of the inhibitors did not affect subsequent Ca2+ sensitization induced by muscarinic receptor stimulation in the presence of a constant submaximal [Ca2+]i, neither did any of these compounds reverse the increase in Ca2+ sensitivity induced by ACh plus GTP. Administration of a 1,2-diacylglycerol analog, 1-oleoyl-2-acetyl-sn-glycerol, did not induce Ca2+ sensitization at a constant submaximal [Ca2+]i. Thus we found no evidence that PKC mediates increases in Ca2+ sensitivity produced by muscarinic receptor stimulation in permeabilized CTSM.

Identification of the protein kinase C isoenzymes in human lung and airways smooth muscle at the protein and mRNA level

The protein kinase C (PKC) isoenzymes expressed by human peripheral lung and tracheal smooth muscle resected from individuals undergoing heart-lung transplantation were identified at the protein and mRNA level. Western immunoblot analyses of human lung identified multiple PKC isoenzymes that were differentially distributed between the soluble and particulate fraction. Thus, PKC alpha, PKC betaII, PKC epsilon, and PKC zeta were recovered predominantly in the soluble fraction whereas the eta isoform was membrane-associated together with trace amounts of PKC alpha and PKC epsilon. PKC beta1-like immunoreactivity was occasionally seen although the intensity of the band was uniformly weak. Immunoreactive bands corresponding to PKCs gamma, delta, or theta were never detected. Reverse transcription-polymerase chain reaction (RT-PCR) of RNA extracted from human lung using oligonucleotide primer pairs that recognise unique sequences in each of the PKC genes amplified cDNA fragments that corresponded to the predicted sizes of PKC alpha, PKC betaI, PKC betaII, PKC epsilon, PKC zeta, and PKC eta (consistent with the expression of PKC isoenzyme protein) and, in addition, mRNA for PKC delta; PCR fragments of the expected size for the supposedly muscle-specific isoform, PKC theta, or the atypical isoenzyme, PKC lambda, were never obtained. The complement and distribution of PKC isoforms in human trachealis were similar, but not identical, to human lung. Thus, immunoreactive bands corresponding to the alpha, betaI, betaII, epsilon, and zeta isoenzymes of PKC were routinely labelled in the cytosolic fraction. In the particulate material PKC alpha, PKC epsilon, PKC alpha, PKC eta, and PKC mu were detected by immunoblotting. With the exception of PKC zeta, RT-PCR analyses confirmed the expression of the PKC isoforms detected at the protein level and, in addition, identified mRNA for PKC delta. Collectively, these data clearly demonstrate the expression of multiple PKC isoenzymes in human lung and tracheal smooth muscle, suggesting that they subserve diverse multifunctional roles in these tissues.

Possible role of atypical protein kinase C activated by arachidonic acid in Ca2+ sensitization of rabbit smooth muscle

1. Diacylglycerol (DAG; 10 microM), an activator of conventional and novel protein kinases C (cPKCs and nPKCs), induced Ca2+ sensitization of force in isolated intact and alpha-toxin-permeabilized femoral artery (FA) and portal vein (PV), and increased the phosphorylation of myosin light chain (MLC20) at the same peptides phosphorylated by myosin light chain kinase. 2. Ca2+ sensitization by DAG was specifically inhibited by a pseudosubstrate peptide inhibitor of cPKCs (PKC alpha(22-30) peptide; 50 microM). Similarly, GF 109203X (600 nM), an inhibitor of cPKCs and nPKCs, completely abolished Ca2+ sensitization by phorbol 12,13-dibutyrate (PDBu; 1 microM). In contrast, Ca2+ sensitization induced by the alpha1-adrenergic agonist phenylephrine (100 microM) was not inhibited by these inhibitors of cPKCs and nPKCs. 3. A pseudosubstrate peptide inhibitor of the atypical PKCs (aPKCs) PKC zeta(116-124) (50 microM) significantly (about 50%) inhibited the Ca2+ sensitization of force and MLC20 phosphorylation induced by 100 microM phenylephrine and by 300 microM arachidonic acid, but not that by DAG (10 microM) or PDBu (1 microM). 4. A phospholipase A2 (PLA2) inhibitor, ONO-RS-082 (10 microM), abolished the release of arachidonic acid and partially (by 40%) inhibited the Ca2+ sensitization induced by phenylephrine in FA smooth muscle. This effect was not additive to the inhibition observed with the aPKC inhibitor peptide, suggesting that arachidonic acid and aPKCs exert their effects via the same pathway, probably through activation of aPKC(s) by arachidonic acid. 5. Western blot analysis with antibodies to aPKCs revealed aPKCs zeta, lambda (or iota) and an unidentified 64 kDa protein. The distribution (cytosolic and particulate) of these proteins was not affected by PDBu (1 microM). 6. Our results are consistent with a significant role for atypical (or related) PKCs through a PLA2-arachidonic acid-aPKC pathway in agonist-induced Ca2+ sensitization, in parallel with a similar, but minor role of the DAG-cPKC cascade. The inability of the combination of the two (aPKC and cPKC) inhibitors to completely eliminate Ca2+ sensitization also suggests the presence of a third, still unidentified, pathway of this mechanism.

Inositol phosphate responses in portal veins from portal hypertensive rats: receptor- and nonreceptor-mediated responses

BACKGROUND/AIMS

Venous hyporesponsiveness in portal hypertension has been reported previously by us. The present study was undertaken to investigate possible changes of phosphoinositide signal transduction pathway in the portal veins from portal hypertensive rats

METHODS

Portal hypertension was induced by partial portal vein ligation. Fourteen days after surgery, portal veins were removed for measurement of [3H]inositol phosphate responses to both receptor- and nonreceptor-mediated stimuli.

RESULTS

Basal [3H]inositol phosphate formation was similar between the two groups. Both phenylephrine and angiotensin II stimulated [3H]inositol phosphate formation in portal veins, but the responses were attenuated in the portal hypertensive group. In contrast, the [3H]inositol phosphate formation by nonreceptor-mediated stimuli (GTP gamma S, NaF/AlCl3, and phospholipase C) was similar between the two groups.

CONCLUSION

Our results showed that the receptor-mediated [3H]inositol phosphate formation was attenuated, while the non-receptor-mediated formation was unaltered, in the portal vein from portal hypertensive rats.

MCI-826 is a potent and selective antagonist of peptide leukotrienes (p-LTs) and has characteristics distinctive from those of FPL 55712

Antagonistic effects of a newly synthesized compound, (E)-2,2-diethyl-3'-[2-[2-(4-isopropyl)thiazolyl]ethenyl]succinanilic+ ++ acid sodium salt (MCI-826) on the contraction of the isolated guinea pig trachea and human bronchus induced by various agonists including peptide leukotrienes (p-LTs), histamine, acetylcholine (ACh), prostaglandin (PG) D2 and others were investigated and compared with the effects of a p-LT antagonist, FPL 55712, in some experiments. MCI-826 potently antagonized LTD4- and LTE4-induced contractions at extremely low concentrations in the isolated guinea pig trachea with pA2 values of 8.3 and 8.9, respectively, on a molar basis. These values indicated that MCI-826 is over 100 times stronger than FPL 55712. Similarly, MCI-826 at 10(-8) g/ml (2.4 x 10(-8) M) markedly antagonized LTD4-induced contractions of the isolated human bronchus. Although FPL 55712 fairly inhibited the 10(-9) g/ml LTC4-induced contraction of the isolated guinea pig trachea, MCI-826 had little effect on the contraction at high concentrations like 3 x 10(-6) g/ml (7.1 x 10(-6) M). MCI-826 modestly affected the other agonist-induced contractions and the resting tonus of the isolated guinea pig trachea at 10(-6) g/ml (2.4 x 10(-6) M) or higher concentrations, but FPL 55712 caused fair inhibition of some of those contractions and gradually lowered the resting tonus with time. These results indicate that MCI-826 is a highly potent and selective antagonist of LTD4 and LTE4 and can be a useful tool for biological and pharmacological experiments on p-LTs.