Effect of platelet-derived growth factor on voltage-operated calcium channels in rabbit isolated ear artery cells

Imatinib relaxes the pulmonary venous bed of guinea pigs

Background

Recently, the IMPRES study revealed that systemic imatinib improves exercise capacity in patients with advanced pulmonary arterial hypertension. Imatinib blocks the tyrosine kinase activity of the platelet-derived growth factor (PDGF)-receptor (PDGFR), acts antiproliferative and relaxes pulmonary arteries. However so far, the relaxant effects of imatinib on pulmonary veins (PVs) and on the postcapillary resistance are unknown, although pulmonary hypertension (PH) due to left heart disease (LHD) is most common and primarily affects PVs. Next, it is unknown whether activation of PDGFR alters the pulmonary venous tone. Due to the reported adverse effects of systemic imatinib, we evaluated the effects of nebulized imatinib on the postcapillary resistance.

Methods

Precision-cut lung slices (PCLS) were prepared from guinea pigs. PVs were pre-constricted with Endothelin-1 (ET-1) and the imatinib-induced relaxation was studied by videomicroscopy; PDGF-BB-related vascular properties were evaluated as well. The effects of perfused/nebulized imatinib on the postcapillary resistance were studied in cavine isolated perfused lungs (IPL). Intracellular cAMP/cGMP was measured by ELISA in PVs.

Results

In PCLS, imatinib (100 μM) relaxed pre-constricted PVs (126%). In PVs, imatinib increased cAMP, but not cGMP and inhibition of adenyl cyclase or protein kinase A reduced the imatinib-induced relaxation. Further, inhibition of K_ATP-channels, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\mathrm{BK}}_{\mathrm{Ca}}^{2+} $$\end{document}BKCa2+-channels or K_v-channels diminished the imatinib-induced relaxation, whereas inhibition of NO-signaling was without effect. In the IPL, perfusion or nebulization of imatinib reduced the ET-1-induced increase of the postcapillary resistance. In PCLS, PDGF-BB contracted PVs, which was blocked by imatinib and by the PDGFR-β kinase inhibitor SU6668, whereas inhibition of PDGFR-α (ponatinib) had no significant effect. Conversely, PDGFR-β kinase inhibitors (SU6668/DMPQ) relaxed PVs pre-constricted with ET-1 comparable to imatinib, whereas the PDGFR-α kinase inhibitor ponatinib did not.

Conclusions

Imatinib-induced relaxation depends on cAMP and on the activation of K⁺-channels. Perfused or nebulized imatinib significantly reduces the postcapillary resistance in the pre-constricted (ET-1) pulmonary venous bed. Hence, nebulization of imatinib is feasible and might reduce systemic side effects. Conversely, PDGF-BB contracts PVs by activation of PDGFR-β suggesting that imatinib-induced relaxation depends on PDGFR-β-antagonism. Imatinib combines short-term relaxant and long-term antiproliferative effects. Thus, imatinib might be a promising therapy for PH due to LHD.

Enhanced contractile response of the basilar artery to platelet-derived growth factor in subarachnoid hemorrhage

BACKGROUND AND PURPOSE

The level of platelet-derived growth factor (PDGF) in cerebrospinal fluid is elevated in subarachnoid hemorrhage (SAH). Therefore, the contractile effect of PDGF on the basilar artery was examined in SAH.

METHODS

A rabbit double-hemorrhage SAH model was used. In the medial layers of the control basilar artery, PDGF had no effect on contraction up to 1 nmol/L, whereas 3 nmol/L PDGF induced slight contraction. In SAH, PDGF induced an enhanced contraction with an increase in [Ca(2+)](i) at 1 nmol/L and higher concentrations. The levels of [Ca(2+)](i) and tension induced by 1 nmol/L PDGF in SAH were 17% and 20%, respectively, of those obtained with 118 mmol/L K(+) depolarization. The PDGF-induced elevation of [Ca(2+)](i) and contraction seen in SAH were abolished in the absence of extracellular Ca(2+). In alpha-toxin-permeabilized strips of SAH animals, PDGF induced no further development of tension during contraction induced by 300 nmol/L Ca(2+), suggesting no direct effect on myofilament Ca(2+) sensitivity. Genistein at 10 micromol/L completely inhibited the tension induced by 1 nmol/L PDGF. The level of myosin light-chain phosphorylation was significantly increased by 1 nmol/L PDGF.

CONCLUSIONS

These results show that the contractile response to PDGF of the basilar artery was enhanced in SAH. The PDGF-induced contraction depended mostly on tyrosine phosphorylation and Ca(2+)-dependent myosin light-chain phosphorylation. The enhancement of the responsiveness to PDGF may therefore contribute to the development of cerebral vasospasm after SAH.

Signaling pathway underlying stimulation of L-type Ca2+ channels in rabbit portal vein myocytes by recombinant Gbetagamma subunits

In previous studies, we (Callaghan B, Koh SD, and Keef KD, Circ Res 94: 626-633, 2004) have shown that voltage-dependent L-type Ca(2+) channels (Cav) in portal vein myocytes are enhanced when muscarinic M2 receptors are activated with ACh. Current stimulation was coupled to the G protein subunit Gbetagamma along with the downstream mediators phosphatidylinositol-3-kinase (PI3K), protein kinase C (PKC), and c-Src. The present study was designed to determine whether the same second messenger pathway could be identified when exogenous recombinant Gbetagamma subunits are introduced into cells. Smooth muscle myocytes were freshly isolated from rabbit portal vein, and Cav currents were recorded by using the patch-clamp technique. Dialysis of cells with recombinant Gbetagamma (50 nM) significantly increased Cav currents (141%). Nifedipine (1 microM) reduced both control and stimulated currents by approximately 90%. The enhancement of current by Gbetagamma was equivalent to that produced by ACh (142%), whereas the PKC activator phorbol 12,13-dibutyrate (PdBu) gave rise to greater current stimulation (192%). Current stimulation with Gbetagamma, ACh, and PdBu were not associated with changes in the voltage dependence of activation or inactivation. The PI3K inhibitor LY-294002 (20 microM) reduced peak currents by 32% in cells dialyzed with Gbetagamma, whereas the inactive analog LY-303511 resulted in a small but significant reduction in current (12%). The c-Src inhibitor PP2 (1 microM) also significantly reduced currents (34%), whereas the inactive analog PP3 was without effect. These data provide further evidence for the hypothesis that Gbetagamma leads to stimulation of Cav currents in rabbit portal vein myocytes via a signaling pathway that includes PI3K, PKC, and c-Src.

Insensitivity of cardiac delayed-rectifier I(Kr) to tyrosine phosphorylation inhibitors and stimulators

1. The rapidly activating delayed-rectifying K+ current (I(Kr)) in heart cells is an important determinant of repolarisation, and decreases in its density are implicated in acquired and inherited long QT syndromes. The objective of the present study on I(Kr) in guinea-pig ventricular myocytes was to evaluate whether the current is acutely regulated by tyrosine phosphorylation. 2. Myocytes configured for ruptured-patch or perforated-patch voltage-clamp were depolarised with 200-ms steps to 0 mV for measurement of I(Kr) tail amplitude on repolarisations to -40 mV. 3. I(Kr) in both ruptured-patch and perforated-patch myocytes was only moderately (14-20%) decreased by 100 microM concentrations of protein tyrosine kinase (PTK) inhibitors tyrphostin A23, tyrphostin A25, and genistein. However, similar-sized decreases were induced by PTK-inactive analogues tyrphostin A1 and daidzein, suggesting that they were unrelated to inhibition of PTK. 4. Ruptured-patch and perforated-patch myocytes were also treated with promoters of tyrosine phosphorylation, including phosphotyrosyl phosphatase (PTP) inhibitor orthovanadate, exogenous c-Src PTK, and four receptor PTK activators (insulin, insulin-like growth factor-1, epidermal growth factor, and basic fibroblast growth factor). None of these treatments had a significant effect on the amplitude of I(Kr). 5. We conclude that Kr channels in guinea-pig ventricular myocytes are unlikely to be regulated by PTK and PTP.

Integrin Receptor Activation Triggers Converging Regulation of Cav1.2 Calcium Channels by c-Src and Protein Kinase A Pathways

L-type, voltage-gated Ca2+ channels (CaL) play critical roles in brain and muscle cell excitability. Here we show that currents through heterologously expressed neuronal and smooth muscle CaL channel isoforms are acutely potentiated following alpha5beta1 integrin activation. Only the alpha1C pore-forming channel subunit is critical for this process. Truncation and site-directed mutagenesis strategies reveal that regulation of Cav1.2 by alpha5beta1 integrin requires phosphorylation of alpha1C C-terminal residues Ser1901 and Tyr2122. These sites are known to be phosphorylated by protein kinase A (PKA) and c-Src, respectively, and are conserved between rat neuronal (Cav1.2c) and smooth muscle (Cav1.2b) isoforms. Kinase assays are consistent with phosphorylation of these two residues by PKA and c-Src. Following alpha5beta1 integrin activation, native CaL channels in rat arteriolar smooth muscle exhibit potentiation that is completely blocked by combined PKA and Src inhibition. Our results demonstrate that integrin-ECM interactions are a common mechanism for the acute regulation of CaL channels in brain and muscle. These findings are consistent with the growing recognition of the importance of integrin-channel interactions in cellular responses to injury and the acute control of synaptic and blood vessel function.

PDGF stimulates pulmonary vascular smooth muscle cell proliferation by upregulating TRPC6 expression

Capacitative Ca(2+) entry (CCE) through store-operated Ca(2+) (SOC) channels plays an important role in returning Ca(2+) to the sarcoplasmic reticulum (SR) and regulating cytosolic free Ca(2+) concentration ([Ca(2+)](cyt)). A rise in [Ca(2+)](cyt) and sufficient Ca(2+) in the SR are required for pulmonary artery smooth muscle cell (PASMC) proliferation. We tested the hypothesis that platelet-derived growth factor (PDGF)-mediated PASMC growth involves upregulation of c-Jun and TRPC6, a transient receptor potential cation channel. In rat PASMC, PDGF (10 ng/ml for 0.5-48 h) phosphorylated signal transducer and activator of transcription (STAT3), increased mRNA and protein levels of c-Jun, and stimulated cell proliferation. PDGF treatment also upregulated TRPC6 expression and augmented CCE, elicited by passive depletion of Ca(2+) from the SR using cyclopiazonic acid. Furthermore, overexpression of c-Jun stimulated TRPC6 expression and CCE amplitude in PASMC. Downregulation of TRPC6 using an antisense oligonucleotide specifically for human TRPC6 decreased CCE and inhibited PDGF-mediated PASMC proliferation. These results suggest that PDGF-mediated PASMC proliferation is associated with c-Jun/STAT3-induced upregulation of TRPC6 expression. The resultant increase in CCE raises [Ca(2+)](cyt), facilitates return of Ca(2+) to the SR, and enhances PASMC growth.

Genistein inhibits cardiac L-type Ca(2+) channel activity by a tyrosine kinase-independent mechanism

It has been suggested that protein tyrosine kinase (PTK) activity can directly regulate cardiac L-type Ca(2+) channels. This conclusion is based to a large extent on the observation that the PTK inhibitor genistein can inhibit the cardiac L-type Ca(2+) current. The purpose of the present study was to determine whether the ability of genistein to inhibit cardiac L-type Ca(2+) channel activity is due to inhibition of PTK activity. Genistein significantly reduced the magnitude of the L-type Ca(2+) current in guinea pig ventricular myocytes recorded using the whole-cell patch-clamp technique. However, this effect was associated with extracellular, not intracellular, application of the drug. Peroxovanadate (PVN), a potent protein tyrosine phosphatase inhibitor, had no effect on the basal Ca(2+) current. PVN was also ineffective in preventing the inhibitory effect of genistein. Internal perfusion of cells with a pipette solution containing ATPgammaS was used to prevent reversibility of phosphorylation-dependent processes. This treatment did not alter the inhibitory effect of genistein, although it did result in irreversible protein kinase A-dependent regulation of the Ca(2+) current. Bath application of lavendustin A, a PTK inhibitor that is structurally unrelated to genistein, did not affect the Ca(2+) current amplitude. The inhibitory effect of genistein was also associated with a hyperpolarizing shift in the voltage dependence of Ca(2+) channel inactivation. These results are consistent with the conclusion that the cardiac L-type Ca(2+) current is not directly regulated by PTK activity and that the inhibitory effect of genistein is due to direct non-catalytic blockade of the channels.

CNTF inhibits high voltage activated Ca2+ currents in fetal mouse cortical neurones

Neurotrophic factors yield neuroprotection by mechanisms that may be related to their effects as inhibitors of apoptosis as well as their effects on ion channels. The effect of ciliary neurotrophic factor (CNTF) on high-threshold voltage-activated Ca channels in cultured fetal mouse brain cortical neurones was investigated. Addition of CNTF into serum-free growth medium resulted in delayed reduction of the Ca2+ currents. The currents decreased to 50% after 4 h and stabilized at this level during incubation with CNTF for 48 h. Following removal of CNTF the inhibition was completely reversed after 18 h. CNTF reduced the current of all pharmacological subtypes of Ca channels as shown by use of selective blockers of L, N, and P/Q type Ca channels (nifedipine, omega-conotoxin MVIIA, omega-agatoxin IVA). The Ca channel depression was mediated via the CNTF receptor, because enzymatic cleavage of the alpha-subunit glycerophosphatidylinositol anchor of the receptor eliminated the response. The CNTF effect was not elicited through pertussis toxin-sensitive G proteins. Other neurotrophic factors like neurotrophin-3 and insulin-like growth factor-I had no effect on the Ca2+ currents. These results may have important implications for the possible functions of CNTF in the nervous system, such as altered synaptic activity, neuronal excitability and susceptibility to brain ischaemia.

Tyrosine kinases act directly on the alpha1 subunit to modulate Ca(v)2.2 calcium channels

Voltage-operated calcium channels are modulated by tyrosine kinases in different cell types. In this study, I(Ba) was measured by the whole cell voltage-clamp technique in single COS-7 cells overexpressing the Ca(v)2.2 calcium channels encoding N-type currents. Bath application of genistein, a nonselective PTK inhibitor (50-300 microM), concentration-dependently inhibited calcium channel currents, whereas the inactive structural analogue, daidzein, was without effect over the same concentration range. Similarly, PP1, a src family-selective tyrosine kinase inhibitor, inhibited I(Ba) in a concentration-dependent manner (500 nM-5 microM) over a range of test potentials. Expression of the Ca(v)2.2alpha1 (alpha(1B)) subunit alone gave rise to functional channels, and genistein (100 microM) also inhibited currents elicited by the alpha(1B) subunit alone. These results indicate that tyrosine kinase inhibitors are likely to inhibit Ca(v)2.2 calcium channels via an action on the pore-forming alpha(1) subunit and suggest that an endogenous member of the Src family may play a physiological role in modulating these channels.

Lysophosphatidylcholine potentiates vascular contractile responses in rat aorta via activation of tyrosine kinase

We previously reported that while lysophosphatidylcholine (LPC) does not itself produce contraction, it significantly potentiates the contractile responses induced by high-K(+), UK14,304 (a selective alpha(2)-adrenoceptor agonist) and phorbol ester in the endothelium-denuded rat aorta. To further investigate this phenomenon, we examined the effects of genistein and tyrphostin B42 (both tyrosine kinase inhibitors) on the LPC-induced potentiation of the contractile responses to high-K(+) and UK14,304 in the endothelium-denuded rat aorta. Although genistein (3 x 10(-6) M, 10(-5) M) did not affect the high-K(+)-induced contractile response, it selectively inhibited the potentiating effect of LPC on the contraction and it strongly inhibited the LPC-induced augmentation of the associated increases in [Ca(2+)](i). Genistein also attenuated the LPC-induced augmentation effects on both the increase in [Ca(2+)](i) and contractile response induced by the UK14,304. In contrast, daidzein (10(-5) M) did not inhibit the potentiating effect of LPC. Tyrphostin B42 (3 x 10(-5) M) attenuated the potentiating effect of LPC on high K(+)-induced contractions. Western blot analysis showed that LPC increased the tyrosine phosphorylation of a number of proteins, including 42 and 44 kDa proteins and 53 - 64 kDa proteins. These protein phosphorylations were inhibited by genistein. Sodium orthovanadate (10(-4) M), a tyrosine phosphatase inhibitor, also markedly enhanced the high-K(+)-induced contractile responses. This enhancing effect was attenuated by genistein. These results suggest that the LPC-induced augmentation of contractile responses in the rat aorta is due to activation of tyrosine kinase, which in turn regulates Ca(2+) influx.

Stimulation of L-type Ca(2+) channels by increase of intracellular InsP3 in rat retinal pigment epithelial cells

The purpose of this study was to investigate the role of voltage-dependent L-type Ca(2+)channels in intracellular Ca(2+)signaling of the retinal pigment epithelium (RPE). Patch-clamp techniques in conjunction with measurements of the intracellular free Ca(2+)using the Ca(2+)-sensitive fluorescence dye fura-2 were performed using cultured rat RPE cells. Intracellular application of inositol-1,4,5-trisphosphate (InsP3; 10 microM) via the patch-pipette during the whole-cell configuration led to an increase in the intracellular free Ca(2+)([Ca(2+)](i)). This effect could be reduced by the L-type Ca(2+)channel blocker nifedipine (2 microM). At the moment of the maximal rise in [Ca(2+)](i)L-type currents displayed an increase in the current density and shifts in the activation curve and of the steady-state inactivation. Comparable changes of L-type channel activity could be observed by induction of capacitative Ca(2+)entry, a maneuver to release Ca(2+)from intracellular Ca(2+)stores independently from InsP3. The increase in L-type Ca(2+)channel activity and [Ca(2+)](i)by intracellular application of InsP3 or induction of capacitative Ca(2+)entry could be inhibited by blocking tyrosine kinase activity using genistein (5 microM) or tyrphostin 51 (10 microM). It is concluded that L-type Ca(2+)channels are involved in the Ca(2+)/InsP3 second messenger system by generating an influx of extracellular Ca(2+)into the cell. This is enabled by depletion of cytosolic Ca(2+)stores and tyrosine kinase-dependent activation of L-type channels.

Regulation of ion channels by protein tyrosine phosphorylation

Ion channels are regulated by protein phosphorylation and dephosphorylation of serine, threonine, and tyrosine residues. Evidence for the latter process, tyrosine phosphorylation, has increased substantially since this topic was last reviewed. In this review, we present a comprehensive summary and synthesis of the literature regarding the mechanism and function of ion channel regulation by protein tyrosine kinases and phosphatases. Coverage includes the majority of voltage-gated, ligand-gated, and second messenger-gated channels as well as several types of channels that have not yet been cloned, including store-operated Ca2+ channels, nonselective cation channels, and epithelial Na+ and Cl- channels. Additionally, we discuss the critical roles that channel-associated scaffolding proteins may play in localizing protein tyrosine kinases and phosphatases to the vicinity of ion channels.

Phosphoinositide 3-kinase isoforms selectively couple receptors to vascular L-type Ca(2+) channels

Heterodimeric class I phosphoinositide 3-kinase (PI3K) has been shown to be involved in the stimulation of voltage-gated Ca(2+) channels by various mediators. In this study, we bring evidences that vascular L-type Ca(2+) channels can be modulated by both tyrosine kinase-regulated class Ia and G protein-regulated class Ib PI3Ks. Purified recombinant PI3Ks increased the peak Ca(2+) channel current density when applied intracellularly. Furthermore, PI3Kalpha-, beta-, and delta-mediated stimulations of Ca(2+) channel currents were increased by preactivation by a phosphotyrosyl peptide, whereas PI3Kgamma- and beta-mediated effects were increased by Gbetagamma. In freshly isolated and cultured vascular myocytes, angiotensin II and Gbetagamma stimulated L-type Ca(2+) channel current. In contrast, platelet-derived growth factor (PDGF)-BB and the phosphotyrosyl peptide did not stimulate Ca(2+) channel current in freshly isolated cells despite the presence of endogenous PDGF receptors and PI3Kalpha and PI3Kgamma. Interestingly, when endogenous PI3Kbeta expression arose in cultured myocytes, both PDGF and phosphotyrosyl peptide stimulated Ca(2+) channels through PI3Kbeta, as revealed by the inhibitory effect of an anti-PI3Kbeta antibody. These results suggest that endogenous PI3Kbeta but not PI3Kalpha is specifically involved in PDGF receptor-induced stimulation of Ca(2+) channels and that different isoforms of PI3K regulate physiological increases of Ca(2+) influx in vascular myocytes stimulated by vasoconstrictor or growth factor.

Cyclic stretch regulates autocrine IGF-I in vascular smooth muscle cells: implications in vascular hyperplasia

Vascular smooth muscle cells (VSMC) subjected to acute or chronic stretch display enhanced growth rates in vitro and in vivo. Clinical examples of vascular hyperplasia (e.g., systolic hypertension and postinjury restenosis) suggest that local insulin-like growth factor I (IGF-I) expression is enhanced. Therefore, we investigated the role of in vitro cyclic stretch on rat VSMC IGF-I secretion and cellular growth. In serum-free medium, cyclic stretch (1 Hz at 120% resting length for 48 h) stimulated thymidine incorporation approximately 40% above that seen in nonstretched cells. Graded stretch magnitude (100-125% resting length) yielded graded increases in VSMC growth. Exogenous IGF-I increased growth of serum-starved, nonstretched VSMC in a dose-dependent manner, with maximal growth seen with 10(-7) M. IGF-I secretion from stretched cells was 20- to 30-fold greater than from those cells cultured in a static environment. Stretch-induced increases in growth were completely blocked on addition of anti-IGF-I and partially blocked with platelet-derived growth factor (PDGF) antibodies and with a tyrosine kinase inhibitor (tyrphostin-1). Finally, blockade of stretch-activated cation channels with GdCl3 profoundly inhibited stretch-induced growth. We conclude that stretch increases VSMC IGF-I secretion and that such autocrine IGF-I is required for stretch-induced growth. PDGF and stretch-sensitive cation channels are likely additional components of a complex pathway that regulates stretch-induced VSMC seen in systolic hypertension and postinjury restenosis.

Epidermal growth factor: a potent vasoconstrictor in experimental hypertension

We have tested the hypothesis that growth factor signaling pathways are augmented in hypertension, a disease associated with vascular smooth muscle cell growth. Thoracic aorta was dissected from deoxycorticosterone acetate-salt (DOCA-salt) and one kidney, one clip (1K, 1C) hypertensive rats and from sham normotensive rats for use in isolated tissue bath experiments. Systolic blood pressure was significantly higher in DOCA-salt and 1K, 1C than in normotensive sham rats: 192 +/- 7, 185 +/- 10, and 117 +/- 4 mmHg, respectively. Although virtually no contraction to epidermal growth factor (EGF) was observed in endothelium-denuded sham rat aorta [1 +/- 1% phenylephrine (PE) (10 micromol/l)-induced contraction], the maximal EGF-induced contraction was 45 +/- 7% in endothelium-denuded aorta from DOCA-salt hypertensive rats and 39 +/- 7% in aorta from 1K, 1C rats. Although slightly attenuated, a contraction to EGF was still observed in endothelium-intact aortic strips from 28-day DOCA-salt hypertensive rats. We also conducted concentration-response curves to EGF on days 1, 3, 5, 7, 14, and 21 of DOCA-salt therapy. A significant contraction to EGF in aorta from DOCA-salt rats was observed on day 14, when DOCA-salt rats had significantly higher blood pressure than sham rats: 188 +/- 6 and 122 +/- 3 mmHg, respectively. Transforming growth factor-alpha, an agonist of the EGF receptor, contracted DOCA-salt rat aorta (30 +/- 7% PE-induced contraction) but not sham aorta (3 +/- 3%). The EGF receptor tyrosine kinase inhibitor 4,5-dianilinophthalimide (10 micromol/l), the mitogen-activated protein kinase kinase inhibitor PD-098059 (10 micromol/l), and the L-type voltage-gated calcium channel inhibitor diltiazem (1 mol/l), but not the cyclooxygenase inhibitor indomethacin (10 micromol/l), virtually abolished EGF-induced contraction (85, 98, and 99% reduction, respectively). These data support a striking difference in EGF signaling between normotensive and hypertensive animals. Furthermore, they provide evidence that growth factors should be considered vasoconstrictors as well as growth modulators in hypertension.

Role of tyrosine phosphorylation in excitation-contraction coupling in vascular smooth muscle

Increasingly it is recognized that tyrosine phosphorylation plays an important part in the regulation of function in differentiated contractile vascular smooth muscle. Tyrosine kinases and phosphatases are present in large amounts in vascular smooth muscle and have been reported to influence a number of processes crucial to contraction, including ion channel gating, calcium homeostasis and sensitization of the contractile process to [Ca2+]i. This review summarizes current understanding regarding the role of tyrosine phosphorylation in excitation-contraction coupling in blood vessels.

Effect of inhibition of tyrosine phosphatases on voltage-operated calcium channel currents in rabbit isolated ear artery cells

1. The effect of increasing cellular tyrosine phosphorylation by inhibiting endogenous tyrosine phosphatases was examined on voltage-operated calcium channel currents in vascular smooth muscle cells. 2. In single ear artery smooth muscle cells of the rabbit, studied by the whole cell voltage clamp technique, intracellular application of the tyrosine phosphatase inhibitors, sodium orthovanadate (100 microM) and peroxyvanadate (100 microM orthovanadate + 1 mM H2O2) increased voltage-operated calcium channel currents by 56% and 83%, respectively. 3. Bath application of two other membrane permeant tyrosine phosphatase inhibitors, phenylarsine oxide (100 microM) and dephostatin (50 microM) also increased voltage-operated calcium channel currents by 48% and 52%, respectively. 4. The selective tyrosine kinase inhibitor, tyrphostin-23 (100 microM) reduced calcium channel currents by 41%. Pre-incubation with tyrphostin-23 abolished the effects of peroxyvanadate, phenylarsine oxide and dephostatin on calcium channels. 5. Western blot analysis of rabbit ear artery cell lysates showed increased tyrosine phosphorylation of several endogenous proteins following treatment with peroxyvanadate. 6. These results indicate that a number of structurally dissimilar inhibitors of tyrosine phosphatases increase voltage-operated calcium channel currents in arterial smooth muscle cells presumably due to increased tyrosine phosphorylation.

Inhibitory effects of genistein on ATP-sensitive K+ channels in rabbit portal vein smooth muscle

1. Effects on the pinacidil-induced outward current of inhibitors of tyrosine kinases and phosphatases were investigated by use of a patch-clamp method in smooth muscle cells of the rabbit portal vein. 2. A specific tyrosine kinase inhibitor, genistein, inhibited the pinacidil-induced current in a concentration-dependent manner with an IC50 of 5.5 microM. Superfusion of Ca2+-free solution did not affect this inhibitory effect of genistein. At higher concentrations, genistein inhibited the voltage-dependent Ba2+ and K+ currents with IC50 values of > 100 microM and 75 microM respectively. Tyrphostin B46 (30 microM), a tyrosine kinase inhibitor, also inhibited the pinacidil-induced current by 70% of the control. 3. Sodium orthovanadate (100 microM), an inhibitor of tyrosine phosphatase, slightly but significantly enhanced both the pinacidil-induced and delayed rectifier K+ currents. Daidzein (100 microM), an inactive analogue of genistein, did not inhibit these currents. 4. Neither herbimycin A (1 microM), lavendustin A (30 microM), tyrphostin 23 (10 microM), which are also tyrosine kinase inhibitors, nor wortmannin (10 microM), a phosphatidylinositol 3-kinase inhibitor, had an effect on either the pinacidil-induced or delayed rectifier K+ currents. Epidermal growth factor (EGF; 1 microg ml(-1)) did not induce an outward current or enhance the pinacidil-induced current. 5. Pinacidil alone, in the cell-attached configuration, or pinacidil with GDP, in the inside-out configuration, activated a 42 pS channel in the smooth muscle cells of the rabbit portal vein. Genistein (30 microM) reduced the channel's open probability without inducing a change in unitary conductance at any holding potential (-30 to +20 mV). 6. In the inside-out configuration, genistein at 30 microM did not change the mean channel open time, but reduced the burst duration. At 100 microM genistein abolished channel opening. The inhibitory potencies with which 30 and 100 microM genistein acted on the unitary current of the ATP-sensitive K+ channel were similar to those seen in the whole-cell voltage-clamp configuration. 7. Although direct inhibitory actions of genistein on the ATP-sensitive K+ channels are not ruled out, our results suggest that a protein tyrosine kinase may play a role in the regulation of ATP-sensitive K+ channel activity in the rabbit portal vein.

Smooth muscle contractility and protein tyrosine phosphorylation

During the last 5 years several studies have documented an involvement of protein tyrosine kinases (PTKs) in smooth muscle contraction and Ca2+ mobilization. Most of these studies have utilized highly selective inhibitors of PTKs, genistein and tyrphostin and have shown that these inhibitors attenuated smooth muscle contraction induced by growth factors-epidermal growth factor (EGF) and platelet derived growth factor (PDGF) and several vasoactive peptides. It has also been demonstrated that inhibitors of protein tyrosine phosphatases (PTPases) such as vanadate and pervanadate mimic growth factors and vasoactive peptides in causing the contraction of smooth muscle. In this brief review, we have summarized some of the recent observations suggesting a possible link between protein tyrosine phosphorylation pathway and smooth muscle contraction.

Differential effects of lovastatin on mitogen induced calcium influx in human cultured vascular smooth muscle cells

1. In this study the effect of lovastatin, an inhibitor of cholesterol and isoprenoid synthesis, on the rises in intracellular calcium concentration ([Ca2+]i) induced by platelet derived growth factor BB (PDGF-BB), angiotensin II (AII), low density lipoproteins (LDL) and foetal calf serum (FCS) was examined in human cultured vascular smooth muscle cells (VSMC) from saphenous vein. Changes in [Ca2+]i were measured in cell suspensions by the Ca2+ sensitive probe, fura 2. 2. Incubation with lovastatin for 24-26 h markedly reduced the peak rise and sustained phase of [Ca2+]i elevation in response to PDGF-BB but the responses to AII, LDL and FCS were unaffected. Further experiments showed that lovastatin pretreatment inhibited PDGF-BB induced Ca2+ influx but not intracellular Ca2+ release. This inhibition could be overcome by co-incubation with mevalonic acid. 3. Pretreatment of cells with the heterotrimeric G protein inhibitor pertussis toxin for up to 24 h completely abolished AII-induced [Ca2+]i rises but the response to PDGF-BB was unaffected. 4. The tyrosine kinase inhibitor genistein largely abolished PDGF-BB-induced [Ca2+]i elevation but had no significant effect on AII-induced responses. 5. Pre-incubation with lovastatin had no effect on the level of tyrosine phosphorylation of PDGF-beta receptors (as measured by Western blot) in response to the PDGF-BB ligand. 6. PDGF-BB elicits Ca2+ influx via a tyrosine kinase-dependent mechanism distinct from the heterotrimeric G protein coupled pathway utilized by AII. Lovastatin most likely acts by inhibition of isoprenylation (via blockade of isoprenoid synthesis) of an intermediate molecule involved in PDGF-BB-induced Ca2+ influx.

Activation of endogenous c-Src or a related tyrosine kinase by intracellular (pY)EEI peptide increases voltage-operated calcium channel currents in rabbit ear artery cells

The effect of activation of endogenous c-Src tyrosine kinase by (pY)EEI peptide was examined on voltage-operated calcium channel (VOC) currents in arterial smooth muscle cells. In single rabbit ear artery cells intracellular application of (pY)EEI peptide increased calcium channel currents. Inactive, non-phosphorylated YEEI peptide had no effect on currents. Peptide-A, a 21 amino acid inhibitor of c-Src inhibited currents and prevented the effect of (pY)EEI peptide on calcium channel currents. These results indicate that activation of intrinsic c-Src increases VOC and support a role for c-Src in the regulation of VOC in vascular smooth muscle cells.

Platelet-derived growth factor (PDGF): actions and mechanisms in vascular smooth muscle

1. PDGF is a highly hydrophilic cationic glycoprotein (M(r) 28-35kDa) produced by platelets, monocyte/macrophages, endothelial cells and vascular smooth muscle cells under some conditions. 2. Since its original description, PDGF has attracted much attention and it is currently believed to play a role in atherosclerosis and other vascular pathologies. 3. This review describes the vascular biology of PDGF. It particularly focuses on recent findings regarding the intracellular signals activated by PDGF in the context of vascular smooth muscle cell proliferation, migration and, contraction.