Increase by lysophosphatidylcholines of smooth muscle Ca2+ sensitivity in alpha-toxin-permeabilized small mesenteric artery from the rat

Downregulation of L-type Ca2+ channel in rat mesenteric arteries leads to loss of smooth muscle contractile phenotype and inward hypertrophic remodeling

L-type Ca2+ channels (LTCCs) are important for vascular smooth muscle cell (VSMC) contraction, as well as VSMC differentiation, as indicated by loss of LTCCs during VSMC dedifferentiation. However, it is not clear whether loss of LTCCs is a primary event underlying phenotypic modulation or whether loss of LTCCs has significance for vascular structure. We used small interference RNA (siRNA) transfection in vivo to investigate the role of LTCCs in VSMC phenotypic expression and structure of rat mesenteric arteries. siRNA reduced LTCC mRNA and protein expression in rat mesenteric arteries 3 days after siRNA transfection to 12.7 ± 0.7% and 47.3 ± 13%, respectively: this was associated with an increased resting intracellular Ca2+ concentration ([Ca2+]i). Despite the high [Ca2+]i, the contractility was reduced (tension development to norepinephrine was 3.5 ± 0.2 N/m and 0.8 ± 0.2 N/m for sham-transfected and downregulated arteries respectively; P < 0.05). Expression of contractile phenotype marker genes was reduced in arteries downregulated for LTCCs. Phenotypic changes were associated with a 45% increase in number of VSMCs and a consequent increase of media thickness and media area. Ten days after siRNA transfection arterial structure was again normalized. The contractile responses of LTCC-siRNA transfected arteries were elevated in comparison with matched controls 10 days after transfection. The study provides strong evidence for causal relationships between LTCC expression and VSMC contractile phenotype, as well as novel data addressing the complex relationship between VSMC contractility, phenotype, and vascular structure. These findings are relevant for understanding diseases, associated with phenotype changes of VSMC and vascular remodeling, such as atherosclerosis and hypertension.

Lysophosphatidylcholine contents in plasma LDL in patients with type 2 diabetes mellitus: relation with lipoprotein-associated phospholipase A2 and effects of simvastatin treatment

Increased lipoprotein-associated PLA(2) (Lp-PLA(2)) predicts the future development of cardiovascular diseases. Although lysophosphatidylcholine (lyso-PC) produced by Lp-PLA(2) may contribute to its proatherogenic activity, the relation between Lp-PLA(2) and lyso-PC content in LDL remains unclarified. We determined the correlation between lyso-PC content in LDL and serum concentrations of Lp-PLA(2), chemokines, oxidative and inflammatory markers and microvascular complications in 32 patients with type 2 diabetes mellitus free of macroangiopathy. We also investigated the effect of simvastatin treatment on Lp-PLA(2) and lyso-PC content in 26 hypercholesterolemic patients with type 2 diabetes mellitus. 1-palmitoyl lyso-PC was measured using electrospray ionization-liquid chromatography/mass spectrometry and Lp-PLA(2) by ELISA. Lyso-PC content in LDL was significantly higher in diabetic patients than in control healthy subjects. Lyso-PC content correlated significantly with Lp-PLA(2) levels (r=0.56, p<0.0001), and was significantly higher in patients with preproliferative or proliferative retinopathy and those with nephropathy than the control. Simvastatin treatment reduced serum Lp-PLA(2) and lyso-PC content in LDL. Our findings suggest that Lp-PLA(2) has the proatherogenic activity by contributing to the production of lyso-PC in circulating LDL.

Atherogenic role of lysophosphatidylcholine in low-density lipoprotein modified by phospholipase A2 and in diabetic patients: protection by nitric oxide donor

The aim of our study was to investigate the atherogenic role of lysophosphatidylcholine (lyso-PC) in low-density lipoprotein (LDL) under diabetic environment. Expression of monocyte chemoattractant protein-1 (MCP-1) mRNA and nuclear factor-kappa B (NF-kappaB)-DNA binding activity were determined in human umbilical vein endothelial cells (HUVEC) incubated with native or glycoxidized LDL, LDL modified by phospholipase A2 (PLA2) and LDL isolated from diabetic patients. Lyso-PC contents in LDL were measured using electrospray ionization-liquid chromatography/mass spectrometry (ESI-LC/MS). Lyso-PC contents were higher in glycoxidized LDL and PLA2-treated LDL compared with native LDL. Glycoxidized LDL and enrichment of lyso-PC by PLA2 treatment resulted in upregulation of MCP-1 mRNA expression through increased NF-kappaB activity in HUVEC. Moreover, LDL isolated from diabetics contained more lyso-PC than that from nondiabetic subjects, and induced higher MCP-1 mRNA expression and NF-kappaB activity in HUVEC. In both in vitro and human studies, palmitoyl- and stearoyl-lyso-PC contents correlated with MCP-1 expression and NF-kappaB activity. Preincubation with 4-ethyl-2-hydroxyimino-5-nitro-3-hexenamide, a NO donor, abrogated increased expression of MCP-1 mRNA and high NF-kappaB activity induced by PLA2-treated LDL and by LDL isolated from diabetic patients. Our results suggest that lyso-PC contents in LDL play an important role in atherogenesis under diabetic condition, which could be prevented by increased availability of vascular NO.

Tyrosine kinases regulate intracellular calcium during alpha(2)-adrenergic contraction in rat aorta

We have demonstrated enhanced contractile sensitivity to the alpha(2)-adrenoreceptor (alpha(2)-AR) agonist UK-14304 in arteries from rats made hypertensive with chronic nitric oxide synthase (NOS) inhibition (LHR) compared with arteries from normotensive rats (NR); additionally, this contraction requires Ca(2+) entry. We hypothesized that tyrosine kinases augment alpha(2)-AR contraction in LHR arteries by increasing Ca(2+). The tyrosine kinase inhibitor tyrphostin 23 significantly attenuated UK-14304 contraction of denuded thoracic aortic rings from NR and LHR. However, tyrphostin 23 did not alter UK-14304 contraction in ionomycin-permeabilized aorta, which indicates that tyrosine kinases regulate intracellular Ca(2+) concentration. The Src family inhibitor PP1 and the epidermal growth factor receptor kinase inhibitor AG-1478 did not alter alpha(2)-AR contraction, whereas the mitogen-activated protein kinase extracellular signal-regulated kinase kinase inhibitor PD-98059 attenuated the contraction. Contraction to CaCl(2) in ionomycin-permeabilized LHR rings was greater than in NR rings. UK-14304 augmented CaCl(2) contraction in ionomycin-permeabilized rings from both groups but to a greater extent in LHR aorta. Together, these data suggest that alpha(2)-AR stimulates contraction via two pathways. One, which is enhanced with NOS inhibition hypertension, activates Ca(2+) sensitivity and is independent of tyrosine kinases. The other is tyrosine kinase dependent and regulates intracellular Ca(2+) concentration.

Acute and chronic NOS inhibition enhances alpha(2)- adrenoreceptor-stimulated RhoA and Rho kinase in rat aorta

We demonstrated that arteries from rats made hypertensive with chronic nitric oxide (NO) synthase (NOS) inhibition (N(omega)-nitro-L-arginine in drinking water, LHR) have enhanced contractile sensitivity to alpha(2)-adrenergic receptors (alpha(2)-AR) agonist UK-14304 compared with arteries from normotensive rats (NR). NO may regulate vascular tone in part through suppression of RhoA and Rho kinase (ROK). We hypothesized that enhanced RhoA and ROK activity augments alpha(2)-AR contraction in LHR aortic rings. Y-27632 eliminated UK-14304 contraction in LHR and NR aortic rings. The order of increasing sensitivity to Y-27632 was the following: endothelium-intact NR, LHR, and endothelium-denuded NR. UK-14304 stimulated RhoA translocation to the membrane fraction in LHR and denuded NR but not in intact NR aorta. Basally, more RhoA was present in the membrane fraction in denuded NR than in intact NR or LHR aorta. Relaxation to S-nitroso-N-acetyl-penicillamine and Y-27632 in denuded ionomycin-permeabilized rings was greater in NR than in LHR. Together these studies indicate alpha(2)-AR contraction depends on ROK activity more in NR than LHR aorta. Additionally, endogenous NO may regulate RhoA activation, whereas chronic NOS inhibition appears to cause RhoA desensitization.

Role of protein kinase C in myogenic calcium-contraction coupling of rat cannulated mesenteric small arteries

1. The present study was designed to determine the role of protein kinase C (PKC) in the myogenic response of small arteries. In particular, we tested whether inhibition of PKC reverses the previously found pressure-induced elevation of contractile element calcium sensitivity. 2. Rat mesenteric small arteries were cannulated and pressurized. The internal diameter was continuously monitored with a video camera and intracellular calcium levels were measured by means of fura-2. Myogenic responses were observed when the pressure was raised stepwise from 20 to 60 and then to 100 mmHg in physiological saline solution and during application of phenylephrine (0.1 or 1 micromol/L) or potassium (36 mmol/L). 3. The PKC inhibitors H-7 (20 micromol/L), staurosporine (100 nmol/L) and calphostin C (10 nmol/L) all completely abolished the myogenic response. Whereas staurosporine caused an ongoing reduction in intracellular calcium, pressure-induced calcium transients were not affected by either H-7 or calphostin C. In particular, the slope of the wall tension-calcium relationship remained similar in the presence of both H-7 and calphostin C, despite an upward shift of this relationship to higher calcium levels in the case of calphostin C. 4. These results show that activity of PKC isoform(s) is essential for myogenic calcium-contraction coupling.

Role of lysophosphatidylcholine in the desensitization of beta-adrenergic receptors by Ca(2+) sensitization in tracheal smooth muscle

Lysophosphatidylcholine (Lyso-PC) is generally considered to promote tissue inflammation. To determine the involvement of exogenous Lyso-PC in the beta-adrenergic desensitization by phospholipase A2, we examined the inhibitory effects of isoproterenol (ISO) on tension and intracellular Ca(2+) concentration by methacholine (MCh) after continuous exposure to Lyso-PC in guinea-pig tracheal smooth muscle, using isometric tension recordings and fura-2 signal (F340/F380 ratio). Pre- exposure to 10 microM Lyso-PC markedly reduced subsequent inhibition by 0.3 microM ISO against 1 microM MCh-induced contraction in a time-dependent manner. In contrast, values of percent F340/F380 ratio for MCh with ISO were not affected after exposure to Lyso-PC. In the presence of Y-27632, a selective rho-kinase inhibitor, a reduction in subsequent relaxation by ISO after exposure to Lyso-PC was inhibited in a concentration-dependent manner. Preincubation with cholera toxin also inhibited reduced responsiveness to ISO by Lyso-PC. Pre-exposure to Lyso-PC did not attenuate subsequent relaxation by agents that bypass beta-adrenergic receptors. These results indicate that continuous exposure to Lyso-PC may cause homologous desensitization of beta-adrenergic receptors via an augmentation in sensitivity to Ca(2+) by rho, a small G protein, in airway smooth muscle, and that activation of the stimulatory G protein of adenylyl cyclase, G(s), may prevent this phenomenon.

Marked dissociation between intracellular Ca2+ level and contraction on exposure of rat aorta to lysophosphatidylcholine

We investigated the relationship between tension development and the cytosolic free Ca2+ level ([Ca2+]i) on exposure of the endothelium-denuded isolated rat aorta to palmitoyl-L-alpha-lysophosphatidylcholine. Lysophosphatidylcholine concentration-dependently induced a gradual increase in [Ca2+]i. Application of 10(-4) M lysophosphatidylcholine induced a large and sustained tonic increase in [Ca2+]i (the peak [Ca2+]i was 125.2 +/- 11.5% of the 80 mM K+-induced response) but only a small contraction (4.0 +/- 1.4% of the 80 mM K+ induced contraction). The sustained increase in [Ca2+]i was attenuated when extracellular Ca2+ was removed but it was unaffected by verapamil or 1-(5-isoquinolinesulphonyl)-2-methylpiperazine dihydrochloride (H-7). Digitonin also produced a gradual increase in [Ca2+]i but with a pronounced contraction. Triton X-100 (0.1%) produced a marked elevation in [Ca2+]i with no detectable contraction. Triton X-100, however, caused a rapid leakage of fura PE-3. Treatment with 10(-4) M lysophosphatidylcholine for 1 or 2 h did not affect the contractile response induced by 80 mM K+ and this treatment did not release lactate dehydrogenase from the rat aorta. Treatment with lysophosphatidylcholine did not affect either the cyclic AMP level or the cyclic GMP level in endothelium-denuded aortic tissues. These results show that in the rat aorta lysophosphatidylcholine produces a large increase in [Ca2+]i (possibly in a non-contractile compartment) which does not induce contraction. Thus, the increase in [Ca2+]i induced by lysophosphatidylcholine (i) requires external Ca2+ but is not due to an increased Ca2+ influx through voltage-dependent L-type Ca2+ channels, (ii) is not primarily due to protein kinase C activation and (iii) is probably not due to a detergent action (like those of digitonin and triton X-100). The relative lack of a contractile response to lysophosphatidylcholine is not due to formation of cyclic AMP or cyclic GMP.

Lysophosphatidylcholine potentiates vascular contractile responses by enhancing vasoconstrictor-induced increase in cytosolic free Ca2+ in rat aorta

We investigated the effects of palmitoyl-L-alpha-lysophosphatidylcholine on the contractile responses of the endothelium-denuded rat aorta to high K+, noradrenaline, UK14,304 (5-bromo-6-[2-imidazolin-2-ylamino]-quinoxaline) (a selective alpha2 adrenoceptor agonist) and phorbol 12-myristate 13-acetate (PMA). Lysophosphatidylcholine at concentrations from 10(-6) M to 10(-4) M did not contract aortic strips. However, lysophosphatidylcholine strongly potentiated the UK14,304-induced contraction. High K+ - and PMA-induced contractions were also potentiated. In contrast, the noradrenaline-induced contraction was only slightly potentiated by 10(-5) M lysophosphatidylcholine. In fura PE-3-loaded aortic strips, lysophosphatidylcholine (10(-5) M) markedly augmented the increase in both cytosolic free Ca2+ ([Ca2+]i) and contractile tension induced by UK14,304, high K+ and PMA. Nicardipine (10(-7) M) and 10(-6) M Ro-31-8220 (¿1-[3-(amidinothio)propyl-1H-indoyl-3-yl]-3-(1-methyl-1H-++ +indoyl-3-yl)-maleimide-methane sulfate) strongly inhibited the increase in [Ca2+]i and contractile tension induced by UK14,304 and in the presence of these inhibitors, the enhancing effects of lysophosphatidylcholine were attenuated. However, the enhancing effect on high K+ -induced contraction was not affected by Ro-31-8220. These results suggest that lysophosphatidylcholine may cause an augmentation of the increase in [Ca2+]i induced by UK14,304 which response is depend on protein kinase C activation and in this way potentiate contractile responses in the rat aorta. Protein kinase C independent mechanisms may also be involved in the enhancing effect of lysophosphatidylcholine on smooth muscle contraction.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Involvement of protein kinase C in the supersensitivity to 5-HT caused by oxidized low-density lipoproteins

The effect of native (n-LDL) and oxidized (ox-LDL) low-density lipoproteins and lysophosphatidylcholines (LPCs) on: (1) vasodilator responses induced by acetylcholine (ACh) in intact rabbit aorta segments, and (2) vasoconstrictor responses to serotonin (5-HT), and potassium (K+) in endothelium denuded segments was investigated. In intact vessels, 100 microg/ml ox-LDL did not modify ACh-induced relaxation, while it was diminished by 300 microg/ml ox-LDL and abolished by 50 microM LPCs. In contrast, this relaxation was unaltered by n-LDL (100 or 300 microg/ml). In deendothelialized arteries, 100 and 300 microg/ml n-LDL as well as 50 microM LPCs did not modify the contractions induced by 5-HT or K+, while 100 or 300 microg/ml ox-LDL increased the 5-HT-induced contraction, without altering those induced by 75 mM K+. Incubation with 100 or 300 microg/ml ox-LDL increased the contractile response to the protein kinase C (PKC) activator phorbol 12,13-dibutyrate (PDB) (0.1-1 microM) in a concentration-dependent manner, which was blocked by staurosporine (0.1 microM), and unaltered by (50 microM) calphostin C or (50 microM) chelerythrine, the three are PKC inhibitors. Preincubation with 0.05 microM PDB increased the contraction elicited by 5-HT, while staurosporine decreased the PDB-induced contraction, and prevented the 5-HT response increase caused by 300 microg/ml ox-LDL. These results suggest that only ox-LDL reduces endothelium-dependent relaxation and elicits PKC activation, and that this activation mediates, at least in part, the vasoconstrictor response to 5-HT.

Involvement of tyrosine phosphorylation in endothelin-1-induced calcium-sensitization in rat small mesenteric arteries

1. We have studied the effect of endothelin-1 stimulation on protein tyrosine phosphorylation levels in intact small mesenteric arteries of the rat and investigated the effects of tyrosine kinase inhibition on the contractile response to this agonist. 2. Endothelin-1 stimulated a rapid (20 s), sustained (up to 20 min) and concentration-dependent (1-100 nM) increase in protein tyrosine phosphorylation levels which coincided temporally with the contractile response in intact and alpha-toxin permeabilized small artery preparations. Tyrosine phosphorylation was increased in four main clusters of proteins of apparent molecular mass 28-33, 56-61, 75-85 and 105-115 kDa. Endothelin-1-induced protein tyrosine phosphorylation was independent of extracellular calcium, antagonized by the tyrosine kinase inhibitor tyrphostin A23 but not by the inactive tyrphostin A1. 3. In intact small arteries tyrphostin A23 inhibited the force developed to endothelin-1 at all concentrations studied; at higher concentrations (10 and 100 nM) the profile of contraction was altered from a sustained to a transient response. Tyrphostin A1 inhibited the contractile response to endothelin-1 at all concentrations except 100 nM; the profile of the response was not altered. Neither tyrphostin affected the transient phasic contraction induced by endothelin-1 (100 nM) in the absence of extracellular calcium. 4. In rat alpha-toxin permeabilized mesenteric arteries endothelin-1 caused a concentration-dependent increase in force in the presence of 10 microM GTP and low (pCa 6.7) constant calcium, demonstrating increased sensitivity of the contractile apparatus to calcium. Tyrphostin A23 inhibited this response by approximately 50%, tyrphostin A1 did not affect endothelin-1-induced calcium sensitization of force. 5. We conclude that increased tyrosine phosphorylation is important in the contractile response induced by endothelin-1 in intact small mesenteric arteries. Furthermore our data implicate activation of this signalling pathway in the tonic phase of contraction possibly through modulation of the sensitivity of the contractile apparatus to calcium.

Calphostin C-sensitive enhancements of force by lysophosphatidylinositol and diacylglycerols in mesenteric arteries from the rat

1. A pharmacological characterization was made of the effects of lysophosphatidyl-inositol (lysoPI) and -ethanolamine (lysoPE) on the Ca(2+)-sensitivity of contraction in alpha-toxin permeabilized rat mesenteric arteries. The effect of GTP gamma S (G-protein activator), diacylglycerols (DAGs, dioctanoyl glycerol (diC8) and 1-stearoyl-2-arachidonoyl-sn-glycerol) and phorbol myristate acetate (PMA, protein kinase C (PKC) activator) on Ca(2+)-sensitivity was also assessed. 2. LysoPI increased the Ca(2+)-sensitivity, demonstrated by both an increase in tension induced by 1 microM [Ca2+]free and an increase in the Ca(2+)-sensitivity of Ca2+ concentration-tension curves. LysoPE did not enhance force or Ca(2+)-sensitivity. 3. GTP gamma S enhanced force at constant Ca2+, increased the Ca(2+)-sensitivity, and increased force under Ca(2+)-free conditions. PMA also increased force at constant Ca2+ and increased Ca(2+)-sensitivity, but caused no force development under Ca(2+)-free conditions. 4. DAGs, both diC8 and the more physiological relevant DAG, 1-stearoyl-2-arachidonoyl-sn-glycerol, enhanced force at constant Ca2+ and increased the Ca(2+)-sensitivity. DiC8, in contrast to 1-stearoyl-2-arachidonoyl-sn-glycerol, caused force development under Ca(2+)-free conditions and substantially enhanced force at maximal Ca(2+)-induced contraction. GDP-beta-S abolished the increased Ca(2+)-sensitization induced by noradrenaline, but not that by DAGs. 5. The PKC inhibitor calphostin C completely abolished Ca(2+)-sensitization induced by all of the Ca(2+)-sensitizing agents. 6. These results show that lysoPI can increase the Ca(2+)-sensitivity of smooth muscle contraction, and the Ca(2+)-sensitization induced by DAGs was not completely G-protein mediated, because it was not inhibited by GDP-beta-S. A central role for PKC in regulation of Ca(2+)-sensitization in rat mesenteric small arteries was indicated by the abolishment of Ca(2+)-sensitization by calphostin C.