Effects of cicletanine on whole-cell currents of single smooth muscle cells from the guinea-pig portal vein

Phenytoin inhibits contractions of rat gastrointestinal and portal vein smooth muscle by inhibiting calcium entry

BACKGROUND

Phenytoin is widely used as a second-line treatment for status epilepticus. Besides its well-known cardiac pro-arrhythmogenicity, side effects on other organ systems have received less attention.

METHODS

This study investigates the effects of phenytoin on gastrointestinal tissue function using an in vitro model of smooth muscle preparations from rats by combining registrations of pharmacological effects on mechanical contractions, electric field potentials, and dynamic intravital fluorescence microscopy.

KEY RESULTS

When added to the bathing solution at a concentration of 30 μM, phenytoin reduced the frequency of spontaneous activity significantly in antrum and portal vein preparations to 72.2 ± 36.5% (p = 0.022) and 80.7 ± 24.4% (p = 0.037) of control values, respectively. At a concentration of 100 μM, the height of spontaneous contractions declined to 9.8 ± 19.6% (p = 0.005) (antrum), 15.7 ± 28.2% (p = 0.004) (portal vein), and 31.8 ± 31.3% (p = 0.005) (colon) in comparison to the control conditions before the application of phenytoin. Depolarization triggered increases in calcium dependent fluorescence signals were reduced by 52.8 ± 39.1% (p = 0.012) The inhibition of spontaneous activity caused by phenytoin was reduced in the presence of the L-type calcium channel agonist BAY K8644(-).

CONCLUSIONS & INFERENCES

Phenytoin exerts strong inhibitory effects on the spontaneous and stimulated contractile activity of smooth muscles from both the upper and lower gastrointestinal tract. The mechanism underlying this effect is not related to the sodium channel blocking activity of phenytoin, but is rather caused by an inhibition of calcium entry through voltage dependent L-type calcium channels. The results of this study should raise vigilance to gastrointestinal complications in patients treated with phenytoin.

Cicletanine stimulates eNOS phosphorylation and NO production via Akt and MAP kinase/Erk signaling in sinusoidal endothelial cells

The function of the endothelial isoform of nitric oxide synthase (eNOS) and production of nitric oxide (NO) is altered in a number of disease states. Pharmacological approaches to enhancing NO synthesis and thus perhaps endothelial function could have substantial benefits in patients. We analyzed the effect of cicletanine, a synthetic pyridine with potent vasodilatory characteristics, on eNOS function and NO production in normal (liver) and injured rat sinusoidal endothelial cells, and we studied the effect of cicletanine-induced NO on stellate cell contraction and portal pressure in an in vivo model of liver injury. Sinusoidal endothelial cells were isolated from normal and injured rat livers. After exposure to cicletanine, eNOS phosphorylation, NO synthesis, and the signaling pathway regulating eNOS activation were measured. Cicletanine led to an increase in eNOS (Ser¹¹⁷⁷) phosphorylation, cytochrome c reductase activity, L-arginine conversion to L-citrulline, as well as NO production. The mechanism of the effect of cicletanine appeared to be via the protein kinase B (Akt) and MAP kinase/Erk signaling pathways. Additionally, cicletanine improved NO synthesis in injured sinusoidal endothelial cells. NO production induced by cicletanine in sinusoidal endothelial cells increased protein kinase G (PKG) activity as well as relaxation of stellate cells. Finally, administration of cicletanine to mice with portal hypertension induced by bile duct ligation led to reduction of portal pressure. The data indicate that cicletanine might improve eNOS activity in injured sinusoidal endothelial cells and likely activates hepatic stellate cell NO/PKG signaling. It raises the possibility that cicletanine could improve intrahepatic vascular function in portal hypertensive patients.

Characterization of ion currents of murine CD117(pos) stem cells in vitro and their modulation under AT2 R stimulation

AIM

Hematopoietic stem cells, especially CD117(pos) cells, have been found to possess a regenerative potential in various tissues, in particular cardiac muscle. However, the characterization of the relevant ion currents of stem cells prior to implantation lacks documentation. Activation of angiotensin II type 2 receptor (AT2 R) can lead to further cell differentiation and receptor auto-expression and might thus influence electrophysiological properties of CD117(pos) stem cells. This study was designed to functionally characterize membrane currents of CD117(pos) cells under normal and AT2 R-stimulated conditions.

METHODS

CD117(pos) murine bone marrow stem cells were isolated with MACS technique and stimulated for the AT2 R with angiotensin II and losartan for 3-5 days prior to patch-clamp measurements. RT-PCR was used to determine channel expression. Endothelial properties were analysed with immunocytochemistry and acLDL uptake assay.

RESULTS

A well-expressed inward rectifying current (IKir ) was identified in cultured CD117(pos) cells. Furthermore, a ZD 7288 (HCN channel blocker)-sensitive current component was isolated. Voltage-dependent potassium currents and chloride currents were less expressed. A small fraction of cells demonstrated voltage- and time-dependent inward currents. In AT2 R-stimulated cells inward rectifying the hyperpolarization-induced inward currents were slightly attenuated on the translational level but showed increased mRNA expression. Cultured CD117(pos) cells express CD31 and VEGFR-2 and significantly increased the uptake of acLDL.

CONCLUSIONS

CD117(pos) cells do not have properties of action potential-generating cells and moderately change their excitability during AT2 R stimulation. Electrophysiological and molecular properties of control and AT2 R-stimulated cells point to a differentiation to vascular endothelial cells. This could increase beneficial vascularization in injured tissues.

Cicletanine stimulates nitric oxide release and scavenges superoxide in endothelial cells

Cicletanine ((+/-)3-(4-chlorophenyl)-1,3-dihydro-7-hydroxy-6-methylfuro-[3,4-c] pyridine) 3-(4-chlorophenyl)-1,3-dihydro-7-hydroxy-6-methylfuro-[3,4-c] pyridine) is a novel antihypertensive vasodilator with an incompletely understood mechanism of action. In the studies described here, the release of nitric oxide and superoxide (O2-) stimulated by cicletanine was measured simultaneously in the endothelium of isolated rat aortic rings. Highly sensitive electrochemical nitric oxide and O2- microsensors were placed near the surface of endothelial cells and the kinetics of nitric oxide and O2- release were monitored in situ. The response times for nitric oxide and O2- microsensors were 100 micros and 50 micros, respectively, and detection limit was 10(-9) M. Cicletanine stimulated nitric oxide release in aorta endothelium at (micromolar) therapeutic concentrations that were consistent with the concentrations of the compound to induce endothelium-dependent vasorelaxation in isolated rat aorta. The peak concentration of nitric oxide was 160+/-8 nM. This concentration was about 70% and was 60% lower as compared with the nitric oxide peak concentration observed after stimulation with receptor-independent agonist (calcium ionophore A23187) and receptor-dependent agonist (acetylcholine), respectively. However, after administration of cicletanine, only a small concentration of O2- was recorded (peak 3.1+/-0.2 nM) contrary to a large concentration (27+/-1.35 nM) observed after stimulation with A23187). Cicletanine not only stimulated nitric oxide release but also was a potent scavenger of O2- at nanomolar level. Both of these effects may contribute to potent vasorelaxation properties of cicletanine and its long-term therapeutic actions, resulting in cardiovascular tissue protection.

Cicletanine: new insights into its pharmacological actions

Cicletanine ((+/-)3-(4-chlorophenyl)-1,3-dihydro-7-hydroxy-6-methylfuro-[3,4-c ] pyridine) 3-(4-chlorophenyl)-1,3-dihydro-7-hydroxy-6-methylfuro-[3,4-c] pyridine) is a novel antihypertensive agent that has been shown to possess vasorelaxant, natriuretic, and diuretic properties in preclinical and clinical studies. The mechanism(s) by which cicletanine induces these biological effects has not been definitely established, although it appears to differ from that of other classes of antihypertensive drugs. The salidiuretic activity appears to be the result of an action of the sulfoconjugated metabolite of cicletanine, which inhibits the apical Na+-dependent Cl-/HCO3- anion exchanger in the distal convoluted tubule. The mechanism of the vasodilating effect of cicletanine seems to be complex; it may include stimulation of vascular prostaglandin synthesis, inhibition of the low Km cyclic GMP phosphodiesterases, and blockade of Ca2+ channels either directly or indirectly through a K+-channel opening effect. The drug has also been shown to interact with alpha-adrenergic, vascular histamine, and muscarinic receptors. We have also reviewed the other vascular effects of the drug, such as stimulation of nitric oxide synthesis and inhibition of both myosin light chain kinase and protein kinase C. Cicletanine protects cardiovascular and renal systems against the injuries induced by hypertension, in addition to its lowering of arterial pressure. Similarly to the vasorelaxant action of cicletanine, the various properties of the drug likely contribute to its protective effect against injury in hypertension.

Effects of the antihypertensive agent, cicletanine, on noradrenaline release and vasoconstriction in perfused mesenteric artery of SHR

1. The mechanism by which cicletanine (CIC) exerts its antihypertensive effects has not been fully elucidated. The present study was undertaken to examine the effects of in vivo and in vitro treatment with CIC on the pressor response and noradrenaline (NA) overflow during periarterial nerve stimulation (PNS) in perfused mesenteric arterial beds isolated from spontaneously hypertensive rats (SHR). 2. CIC at a dose of 50 mg kg(-1) day(-1) was administered orally to both SHR and normotensive Wistar-Kyoto rats (WKY) from the 6th to 10th week of age. At the 10th week, the isolated mesenteric arterial bed was perfused with Krebs-Henseleit buffer and changes in perfusion pressure and NA overflow during PNS were measured. 3. Chronic treatment with CIC suppressed the age-related elevation of systemic blood pressure in SHR but not in WKY. 4. The PNS (20 Hz)-induced mesenteric vasoconstrictor response and NA overflow were greater in SHR than in WKY. In the vasculature of SHR chronic treatment with CIC resulted in a significant attenuation of the vasoconstriction and the NA overflow during PNS, whereas it did not alter vasoconstrictor responses to bolus injections of KCl and phenylephrine. 5. Treatment with 30 microM CIC in vitro diminished the PNS-induced vasoconstriction and NA overflow but not the NA- and KCl-induced vasoconstriction in the vasculature of untreated SHR. 6. In the vasculature of SHR PNS-induced NA overflow was attenuated by prostaglandin E2 (0.05 microM), whereas it was augmented by the cyclo-oxygenase inhibitor diclofenac-Na (30 microM). In the presence of diclofenac, in vitro treatment with CIC did not attenuate the NA overflow during PNS. 7. The results suggest that the antihypertensive effect of CIC in SHR is partially due to the presynaptic inhibition of NA release during sympathetic nerve activation. Transjunctional inhibition of NA release by prostaglandins may contribute to the inhibitory action of CIC on NA release in the vasculature of SHR.

Cicletanine-induced decreases in cytosolic Ca2+ level and contraction in vascular smooth muscle

The mechanism by which cicletanine (3-(4-chlorophenyl)-1,3-dihydro-7-hydroxy-6-methylfuro-[3,4-c]pyri dine) induces vasodilatation was examined in isolated vascular smooth muscle. Cicletanine inhibited the contraction induced by high K+, norepinephrine (NE) and prostaglandin F2alpha in a concentration-dependent manner in rat aorta. High K+ (15.8-72.7 mM) elicited elevation of cytosolic Ca2+ level ([Ca2+]i) and contraction in a concentration-dependent manner. Cicletanine (300 microM) inhibited the high K+-induced contractions without changing the [Ca2+]i/tension relationship. NE (3-300 nM) elicited greater contractions than high K+ at a given [Ca2+]i, suggesting that NE increased Ca2+ sensitivity of the contractile elements. Cicletanine inhibited the NE-induced contractions without changing the slope of the [Ca2+]i/tension relationship. Cicletanine inhibited the transient increases in both [Ca2+]i and muscle tension elicited by NE but not the transient increase in [Ca2+]i elicited by caffeine in Ca2+-free solution. Cicletanine did not inhibit contraction induced by Ca2+ in the permeabilized rabbit mesenteric artery with alpha-toxin. These results suggest that cicletanine inhibits vascular smooth muscle contraction by multiple mechanisms: 1) inhibition of Ca2+ influx via voltage-dependent Ca2+ channel and 2) inhibition of Ca2+ release mediated by the alpha-adrenoceptors, but not by caffeine.

Modulation by low sodium intake of glomerular response to cicletanine and atrial natriuretic factor

1. The aim of the study was to investigate whether cicletanine (CIC), as a potential inhibitor of cyclic GMP phosphodiesterase, is able to restore glomerular response to atrial natriuretic factor (ANF) in rats under conditions of diet deprived of sodium. We examined the effects of CIC on glomerular filtration rate (GFR), natriuresis and nephrogenous cyclic GMP excretion in response to ANF and the effects of both agents on intracapillary volume and cyclic GMP accumulation in isolated glomeruli of rats on normal and low sodium diets. 2. CIC (0.25 mg min-1 kg-1 BW) of ANF (0.5 microgram min-1 BW) alone, given in pharmacological doses, increased Cin significantly in normal sodium rats, whereas the effect of each agent was blunted in low sodium diet rats. Pretreatment with CIC restored the increase in C(in) in response to ANF infusion in low sodium diet rats. In rats on either diet, there were no differences in the extent of diuresis and natriuresis induced by CIC or ANF alone. In contrast to FENa, combined effects of both agents on V and UNa V in rats on normal and low sodium diets were observed. 3. In normal sodium diet rats, CIC 10(-4) M or ANF 10(-6) M alone inhibited angiotensin II 10(-6) M (AII)-induced decrease in intracapillary volume reflected by the glomerular [3H]-inulin space (GIS). In contrast, CIC or ANF alone did not inhibit AII-induced decrease in GIS in low sodium diet rats. Both agents given together inhibited AII-induced decrease in GIS in low sodium diet rats. 4. CIC both alone and in combination with ANF increased nephrogenous cyclic GMP excretion and cyclic GMP accumulation in isolated glomeruli of rats on normal and low sodium diets. In rats on either diet, CIC abolished the difference in ANF-stimulated increase in nephrogenous cyclic GMP excretion and cyclic GMP accumulation in glomeruli. 5. These results suggest that CIC and ANF alone induce relaxation of glomeruli and a resultant increase in glomerular filtration rate in normal sodium diet rats; in contrast, these effects are blunted in the low sodium diet rats. CIC restores glomerular response to ANF in low sodium diet rats, apparently involving inhibition of the cyclic GMP phosphodiesterase.

Inhibitory effects of cicletanine on smooth muscle in comparison to those of nifedipine and sodium nitroprusside

The inhibitory effect of cicletanine was studied, in comparison to the effects of nifedipine and sodium nitroprusside (SNP), in various types of smooth muscle: portal vein and iliac artery of rabbit; gastric fundus and antrum of rabbit and guinea pig; guinea pig taenia coli and uterus. In all types of tissue the nifedipine-sensitive component (LCA, L-type calcium channel dependent activation) was inhibited by cicletanine (threshold concentration 10(-6) mol/l to 10(-5) mol/l). The nifedipine to resistant component (NLCA) was in some tissues preferentially inhibited by SNP (gastric fundus) and in other tissues preferentially by cicletanine (portal vein), with graded intermediate forms (iliac artery). Consequently, the inhibitory effect of cicletanine on NLCA is different in mechanism to that of SNP. Only papaverine suppressed all types of activation.