Effects of calcium sensitizer OR-1986 on a cardiovascular mortality and myocardial remodelling in hypertensive Dahl/Rapp rats.

Calcium-sensitizing agents have been shown to improve cardiac function in patients suffering from acute decompensated heart failure, however, their long-term effects on cardiac remodeling and cardiovascular mortality are still largely unknown. In the present study we tested the hypothesis whether OR-1896, an active and long-lasting metabolite of calcium sensitizer levosimendan, prevents cardiovascular mortality and hypertension-induced myocardial remodelling in salt-sensitive Dahl/Rapp rats. OR-1896 was given orally to Dahl/Rapp SS rats on high-salt diet (NaCl 7% w/w) for 7 weeks at two different doses (0.5 and 0.05 mg/kg). OR-1896 prevented salt-induced cardiovascular mortality (survival rate 75 % in OR-1896 treated groups vs 38 % in untreated controls, p<0.01), ameliorated cardiac hypertrophy and improved systolic functions of the heart without major influence on systemic blood pressure. OR-1896 also ameliorated salt-induced increase in cardiac ANP mRNA expression and plasma BNP level. Salt-induced cardiac remodelling was associated with 4-fold increase in cardiac p16(INK4a) mRNA expression, a marker of cellular senescence. OR-1896 dose-dependently ameliorated cardiomyocyte senescence. Our findings suggest a therapeutic role for OR-1896 in the prevention of cardiac remodelling in salt-sensitive forms of hypertension. The present study also underscores the importance of cellular senescence in the pathogenesis of salt-induced hypertensive heart disease.

Effects of levosimendan on cardiac remodeling and cardiomyocyte apoptosis in hypertensive Dahl/Rapp rats.

BACKGROUND AND PURPOSE: Progression of heart failure in hypertensive Dahl rats is associated with cardiac remodeling and increased cardiomyocyte apoptosis. This study was conducted to study whether treatment with a novel inotropic vasodilator compound, levosimendan, could prevent hypertension-induced cardiac remodeling and cardiomyocyte apoptosis. EXPERIMENTAL APPROACH: 6-week-old salt-sensitive Dahl/Rapp rats received levosimendan (0.3 mg kg(-1) and 3 mg kg(-1) via drinking fluid) and high salt diet (NaCl 7%) for 7 weeks, Dahl/Rapp rats on low-salt diet served as controls. Blood pressure, cardiac functions by echocardiography, cardiomyocyte apoptosis by TUNEL technique, tissue morphology, myocardial expression of calcium cycling proteins, and markers of neurohumoral activation were determined. KEY RESULTS: Untreated Dahl/Rapp rats on high salt diet developed severe hypertension, cardiac hypertrophy and moderate systolic dysfunction. 38% of Dahl/Rapp rats (9/24) survived the 7-week-follow-up period. Cardiomyocyte apoptosis was increased by 6-fold during high salt diet. Levosimendan improved survival (survival rates in low- and high-dose levosimendan groups 12/12 and 9/12, p<0.001 and p=0.05, respectively), increased cardiac function, and ameliorated cardiac hypertrophy. Levosimendan dose-dependently prevented cardiomyocyte apoptosis. Levosimendan normalized salt-induced increased expression of natriuretic peptide, and decreased urinary noradrenaline excretion. Levosimendan also corrected salt-induced decreases in myocardial SERCA2a protein expression and myocardial SERCA2a/NCX-ratio. CONCLUSIONS AND IMPLICATIONS: Improved survival by the novel inotropic vasodilator levosimendan in hypertensive Dahl/Rapp rats is mediated, at least in part, by amelioration of hypertension-induced cardiac remodeling and cardiomyocyte apoptosis.

Improved survival with simendan after experimental myocardial infarction in rats.

This study compared the effects of simendan, a calcium sensitizer, with those of milrinone and enalapril on survival of rats with healed myocardial infarction. Seven days after ligation-induced myocardial infarction, the rats were randomized to control, milrinone, enalapril, or simendan groups. All compounds were administered via the drinking water for 312 days, at which time there was 80% mortality in the control group--the study's primary endpoint. The infarct sizes were similar across all groups. At endpoint, the mortality rates were: 63% (milrinone), 56% (enalapril) and 53% (simendan); the risk reductions were 25% (P = 0.04 vs. control) and 28% (P = 0.02 vs. control) with enalapril and simendan, respectively. Milrinone had no statistically significant effect on the survival rate. These findings suggest that, like enalapril, simendan improved survival in rats with healed myocardial infarction.

Levosimendan increases diastolic coronary flow in isolated guinea-pig heart by opening ATP-sensitive potassium channels.

Levosimendan, a novel calcium sensitizer developed for the treatment of acute heart failure, is an inodilator that increases coronary flow. Because it was recently shown that levosimendan stimulates potassium current through K(ATP) channels in isolated rat arterial cells, our aim was to assess whether the levosimendan-induced increase in coronary flow is due to the opening of the K(ATP) channels in coronary smooth muscle. The effect of levosimendan on the diastolic coronary flow velocity (DCFV) was measured in the Langendorff perfused spontaneously beating guinea-pig heart in the absence and presence of glibenclamide. Pinacidil was used as a reference compound, and the protein kinase C inhibitor bisindolylmaleimide was used to study the dilatory effect of levosimendan when the K(ATP) channels in smooth muscle are not inhibited by PKC-dependent phosphorylation. Levosimendan (0.01-1 microM) increased DCFV concentration-dependently and was noncompetitively antagonized by 0.1 microM glibenclamide, whereas pinacidil was inhibited competitively by glibenclamide. In the presence of glibenclamide the positive inotropic and chronotropic effects of levosimendan were unaltered. The effect of bisindolylmaleimide and levosimendan on DCFV was additive. The results indicate that levosimendan induced coronary vasodilation through the opening of the K(ATP) channels. Levosimendan and pinacidil probably have different binding sites on the K(ATP) channels. The additive effect of bisindolylmaleimide and levosimendan on the increase of DCFV suggests that the latter binds to the unphosphorylated form of the channel.

Further evidence for the cardiac troponin C mediated calcium sensitization by levosimendan: structure-response and binding analysis with analogs of levosimendan.

Levosimendan, an inodilatory drug discovered using troponin C as a target protein, has a cardiac effect deriving from the calcium sensitization of contractile proteins. The aim of this study was to give further evidence that levosimendan binds to cardiac troponin C and that the binding involves amino acid residues on helixepsilon of the N-terminal domain of this calcium-binding protein. Nine organic molecules, obtained by chemical modification of levosimendan, were tested both for their calcium-dependent binding to troponin C and troponin complex affinity HPLC columns, and for their ability to increase the calcium sensitivity of myofilaments in cardiac skinned fibers. A good correlation between the calcium sensitization and the calcium-dependent binding to troponin complex (r=0.90) and to cardiac troponin C (r=0.91) for the analogs of levosimendan was shown. In addition, the effect of levosimendan on the calcium-induced conformational changes in native and point-mutated cTnC was studied. Cys84-->Ser, Asp87-->Lys and Asp88-->Ala point-mutated cTnC were shown to maintain a high affinity to calcium, but their Ca(2+)titration curves were not influenced by levosimendan as for the native protein. Finally, it was demonstrated that the NMR chemical shifts of the terminal methyl groups of Met47, Met81, and Met85 on calcium-saturated cTnC were changed after addition of levosimendan in water solution at pH 7.4. This effect was not seen when adding an analog of levosimendan, which did not bind to the troponin C affinity HPLC column and did not increase the calcium-induced tension in cardiac skinned fibers.

Calcium sensitivity enhancers.

Patients suffering from acute decompensated heart failure, immediately after infarction, or late in the progression of heart failure, need short-term, positive inotropic support in their therapy. To date, drugs acting through cAMP are used to increase the contractile force of the heart of such patients, although it is well-known that these kind of drugs may trigger arrhythmias and as a result may worsen the long-term prognosis of the patients. Recently, levosimendan, a new drug acting through calcium sensitization of contractile proteins, has shed new light on inotropic therapy, and, importantly, has reduced mortality in acute heart failure patients. For this reason, this review focuses on calcium sensitizers and the mechanisms behind calcium sensitization. The compounds which have been selected for detailed consideration are limited only to positive inotropic compounds that produce calcium sensitization of contractile proteins. In addition, this review elucidates the differences between various calcium sensitizing mechanisms, and mainly focuses on the relaxation of cardiac muscle.

Levosimendan, a calcium sensitizer in cardiac muscle, induces relaxation in coronary smooth muscle through calcium desensitization.

Levosimendan is a pyridazinone-dinitrile derivative belonging to a new class of cardiac inotropic drugs, Ca++ sensitizers. Levosimendan is also a vasodilator both in vitro and in vivo, but its mechanism is not well understood. The cardiac target protein of levosimendan, troponin C, is a Ca++-binding EF-hand protein. This raises the possibility that levosimendan may also interact with smooth muscle EF-hand proteins, such as, calmodulin, the regulatory myosin light chains, or S100 proteins. We investigated the effects of levosimendan on [Ca++]i, and force in porcine coronary arteries, with receptor-mediated (U46619) or KCl stimulation. At high levels of stimulation, levosimendan decreased force without changing or increasing [Ca++]i, measured with the Ca++-sensitive fluorescent probe fura-2 in the intact artery. With lower levels of U46619, levosimendan (1 microM) lowered force by 70% and reduced [Ca++]i by 38%. The relationship between force and [Ca++]i for KCl stimulation are significantly rightward shifted, indicating Ca++ desensitization by levosimendan. In contrast, the phosphodiesterase III inhibitor, milrinone, does not shift the force-Ca++ relations but elicits relaxation via lowering [Ca++]i. There was little change in pHi, indicating that the Ca++ desensitization by levosimendan was not attributable to decreasing pHi. Levosimendan relaxes coronary arteries and lowers [Ca++]i by mechanisms different than milrinone. Our results indicate a lowering of [Ca++]i by levosimendan consistent with opening of potassium channels and a relaxation that is independent of [Ca++]i. Our evidence points to a novel mechanism that might involve the direct effect of levosimendan on the smooth muscle contractile or regulatory proteins themselves.

The role of cAMP- and cGMP-dependent protein kinases in the cardiac actions of the new calcium sensitizer, levosimendan.

OBJECTIVE: The role of phosphodiesterase III inhibition and calcium sensitization in the cardiac actions of levosimendan, (R)-[[4-(1,4,5, 6-tetrahydro-4-methyl-6-oxo-3-pyridazinyl)phenyl]hydrazono]propane dinitrile, was studied. METHODS: Various heart preparations were used to investigate positive inotropy, chromotropy, coronary flow and calcium sensitivity of contractile proteins. The cAMP- and cGMP-dependent protein kinases (PKA and PKG) were inhibited by KT5720 and KT5823, respectively. Furthermore, the synthesis of cAMP was stimulated by forskolin and increased phosphorylation of troponin I was induced by isoprenaline. RESULTS: In Langendorff guinea-pig heart, levosimendan (0.01-1 microM) and milrinone (0.1-10 microM) increased the left ventricular systolic peak pressure almost to the same extent. In the presence of KT5720 (1 microM) milrinone was devoid of positive inotropic activity. In contrast, KT5720 did not antagonize the inotropic effect of levosimendan at < or = 0.03 microM (-up to the EC50 of levosimendan). The effects of levosimendan and milrinone on heart rate and coronary flow were not affected by KT5720. The PKG inhibitor, KT5823 (1 microM), on the other hand, potentiated the levosimendan-induced increase in coronary flow while it had no effect on the increase induced by milrinone. The mechanical parameters were not affected by KT5823. In the papillary muscle, the positive inotropic effect of milrinone but not that of levosimendan was potentiated by forskolin (0.1 microM). In contrast to milrinone, the positive inotropy by levosimendan was decreased by isoprenaline pretreatment (0.1 microM; 3 min). In line with this, the calcium-sensitizing effect of levosimendan was decreased in skinned fibers prepared from isoprenaline-treated hearts. CONCLUSIONS: Our results indicate that the cardiac effects of levosimendan at its therapeutically relevant concentrations were not mediated through PKA or PKG and its positive inotropy is therefore most probably due to the previously reported troponin-C-mediated calcium sensitization of contractile proteins.

Troponin C-mediated calcium sensitization by levosimendan accelerates the proportional development of isometric tension.

The effects of various calcium sensitizers on myosin-actin crossbridge kinetics were evaluated in intact, paced guinea-pig papillary muscle by analysing the velocity of the development of isometric tension (dT/dt) in detail. The effect on association (the whole sequence of events from troponin onward) and dissociation rates of crossbridges was estimated from the rising phase and from the early decay phase of the normalized dT/dt curve. Levosimendan, a calcium sensitizer acting through troponin C, accelerated the proportional association rate and decelerated the dissociation rate of crossbridges. The effect of levosimendan on crossbridge kinetics occurred before the peak twitch tension was achieved. Thus, the compound did not change the actual relaxation phase of twitch tension. Since the effect on the association was more pronounced than on the dissociation of crossbridges, levosimendan shifted the entire twitch tension curve to the left. Based on the dissociation rate analysis levosimendan seems to act preferentially as a calcium sensitizer at low concentrations. At high concentrations the phosphodiesterase III (PDE III) inhibitory properties of levosimendan modulated its effect on the early relaxation processes. In contrast, PDE III inhibition is probably the primary mechanism of action for MCI-154. Pimobendan, and EMD 53998 at low concentrations, whereas their direct effects on crossbridge kinetics contributed to the positive inotropic action at high concentrations. The calcium sensitizing mechanisms of these compounds seemed to be based almost exclusively on the decelerating effect on dissociation of crossbridges.

Cardiac troponin C as a target protein for a novel calcium sensitizing drug, levosimendan.

The role of cardiac troponin C (cTnC) as a target protein for the calcium sensitization by levosimendan, pimobendan, MCI-154 and EMD 53998 was evaluated using purified recombinant human cTnC. For determination of calcium- and magnesium-dependent binding of the compounds to cTnC a new type of cTnC-HPLAC column was used. Furthermore, dansylated cTnC was utilized to study the effect of the calcium sensitizing compounds on calcium-induced conformation of cTnC. Only levosimendan showed calcium-dependent and to a lesser extent magnesium-dependent retention in the cTnC column. The findings indicate that levosimendan binds both to the N-terminal and C-terminal domains of cTnC. In agreement with this, only levosimendan shifted the calcium-induced fluorescence curve of dansylated cTnC to the left. In the control experiments Ca50 and KCa2+ were calculated to be 2.73 microM and 4 x 10(5) M-1, respectively. Levosimendan at 3 microM decreased the value of Ca50 to 1.19 microM. In conclusion, it is suggested that the mechanism of calcium sensitizing effect of levosimendan, unlike that of the other calcium sensitizers, is based on calcium-dependent binding to the N-terminal domain of cTnC. This is proposed to amplify the trigger of contraction induced by cTnC in the cardiac muscle.

Troponin C-mediated calcium sensitization induced by levosimendan does not impair relaxation.

Levosimendan is a novel positive inotropic drug targeted to increase contraction force of the heart through its calcium-dependent binding to troponin C (cTnC). We investigated the calcium-sensitizing effect of levosimendan on contractile proteins as well as its positive inotropic and lusitropic effects in paced guinea pig papillary muscle. We also studied the effect on energy consumption of myosin-actin crossbridges in a myosin ATPase assay. The calcium sensitization induced by levosimendan in fibers skinned with saponin was dependent on the perforation velocity of cell membranes. Levosimendan was almost ineffective in slowly perforated fibers, but was the most potent calcium sensitizer in fibers with rapidly perforated cells. The perforation-dependent calcium sensitization was probably due to changes in phosphorylation state of contractile proteins during the slow dissection of fibers. It is noteworthy that the calcium-sensitizing effect of levosimendan was not affected by acidic pH. Levosimendan at therapeutically relevant (0.3-10 microM) concentrations markedly increased calcium sensitivity both at pH 6.7 and 7.0, being more potent than EMD 53998, pimobendan, and MCI-154. The lack of effect of levosimendan on maximum tension supports the hypothesis that levosimendan increases calcium sensitivity through its action on cTnC. Unlike EMD 53998, levosimendan did not increase myosin ATPase activity, indicating that it did not increase the cycling rate of myosinactin crossbridges. In paced papillary muscles, levosimendan induced positive inotropic effect without changing relaxation time. Thus, levosimendan was devoid of the main negative factors described for calcium sensitizers.

Mechanisms of action of calcium-sensitizing drugs.

This review compares the mechanisms of action of the calcium-sensitizing agents levosimendan, pimobendan, MCI-154, and EMD 53998. By using purified human recombinant troponin-C (cTnC), the role of cTnC as a target protein for these compounds was investigated. Accordingly, the calcium-dependent binding to cTnC in a cTnC-high-performance liquid affinity chromatography (HPLAC) column and the stabilizing effects of the compounds on the calcium-induced conformation of dansylated cTnC were studied. Only levosimendan showed calcium-dependent action on cTnC. Of the studied compounds, levosimendan was the most potent calcium sensitizer in skinned fiber experiments. Furthermore, EMD 53998 and MCI-154, but not levosimendan and pimobendan, increased myosin ATPase activity, indicating that they may enhance the cycling rate of myosin-actin crossbridges. By analyzing the velocity (dT/dt) of isometric tension development in paced papillary muscles, it was shown that levosimendan probably enhances the association rate but decreases the dissociation rate of myosin-actin crossbridges. These effects occurred before the peak twitch tension was achieved. Therefore, levosimendan does not seem to affect the actual relaxation phase. The other calcium sensitizers, however, appear to act mainly by decreasing the dissociation rate of crossbridges. The weak calcium-sensitizing effect of pimobendan may be based on indirectly mediated increase in affinity of cTnC for calcium. MCI-154 might act in a similar way but, like EMD 53998, MCI-154 also acts on myosin-actin crossbridges. We suggest that levosimendan binds in a calcium-dependent manner to the N-terminal domain of cTnC, which magnifies the extent of the contraction produced by cTnC when it is calcium-activated.

Nicotine antagonizes the effects of reserpine on the striatal metabolism of dopamine, 5-hydroxytryptamine and noradrenaline in hypothermic but not in normothermic mice.

Our previous studies indicate that repeated nicotine administration inhibits the release of striatal dopamine in hypothermic mice. To study if similar inhibition occurs in noradrenergic and serotoninergic neurons mice were given (-)-nicotine (3 mg/kg, s.c.) repeatedly at 110, 80, 50, and 20 min before sacrifice. The interactions of nicotine with reserpine were also investigated. Reserpine (5 mg/kg, i.p.) was administered after the second nicotine dose at 60 min before sacrifice. To prevent the effects of nicotine on autonomic ganglia all mice were given hexamethonium (10 mg/kg, i.p.). Experiments were carried out at 20-22 degrees C at which ambient temperature nicotine induced deep hypothermia or at 32-34 degrees C to prevent the drug-induced hypothermia. The changes in striatal metabolism of dopamine, noradrenaline and 5-hydroxytryptamine (5-HT) we Nicotine had temperature dependent effects on the dopamine metabolism which indicates a block of dopaminergic neurons as suggested in our earlier studies. Reserpine per se increased the homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC) contents and decreased the 3-methoxytyramine (3-MT) and dopamine contents at both ambient temperatures. In hypothermic but not in "normothermic", nicotine-treated mice reserpine's effect on dopamine metabolism was almost totally vanished. Nicotine and reserpine per se increased the 3-methoxy-4-hydroxyphenylethylglycol (MOPEG) content and decreased the noradrenaline content at both ambient temperatures. In hypothermic but not in "normothermic" mice nicotine antagonized the reserpine-induced decrease of noradrenaline content. Nicotine tended to decrease the 5-hydroxy-indoleacetic acid (5-HIAA) content in hypothermic mice but increased it in "normothermic" ones.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual effects of nicotine on neuroleptic-induced changes of striatal dopamine metabolism in mice.

The effect of nicotine on the neuroleptic-induced changes in striatal dopamine (DA) metabolism of mice was studied. To investigate the mechanism of action of nicotine, its interactions with apomorphine (APO) and gamma-hydroxybutyric acid (GHBA) were also investigated. Mice were given nicotine, (0.3-3 mg/kg subcutaneously) repeatedly (4 times) at 30 min. intervals. Haloperidol (HAL), (+/-)-sulpiride (SUL), APO or GHBA were administered after the second nicotine dose. Hexamethonium was given to prevent the effects of nicotine on autonomic ganglia. The striatal contents of DA and of its metabolites homovanillic acid (HVA), 3,4-dihydroxyphenylacetic acid (DOPAC) and 3-methoxytyramine (3-MT) were measured. The drug-induced hypothermia in mice was controlled by increasing ambient temperature. At ambient temperature of 32-34 degrees nicotine and HAL increased the striatal DOPAC and HVA contents additively, whereas APO counteracted the effects of nicotine at this ambient temperature. At 20-22 degrees nicotine decreased the 3-MT content which indicates reduced release of DA. In hypothermic mice nicotine inhibited better the HAL- and SUL-induced increases of HVA content than those of DOPAC content suggesting that the neuroleptic-induced increases in DOPAC and HVA contents are mediated partly by different mechanisms. In APO-treated mice both the GHBA- and nicotine-induced decreases of 3-MT content fell further. GHBA did not alter the nicotine-induced decrease of 3-MT content and so this effect of nicotine may be mediated indirectly via GABAergic neurones. Unlike GHBA and APO nicotine decreased 3-MT content only in hypothermic mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in brain monoamine metabolism during withdrawal from chronic oral self-administration of morphine and in response to a morphine challenge in the withdrawn state.

Although p.o. self-administration of morphine is a reliable and convenient means of inducing physical dependence, its effects on brain monoamine metabolism have not been determined. Accordingly, in the present experiment young Wistar rats drank increasing concentrations (0.1-0.5 mg/ml) of morphine in water, or water alone, for 37 days. Half the rats in each group were challenged with morphine (10 mg/kg s.c.) when 27 to 29 hr withdrawn, and half with saline. Rats were sacrificed 2 hr postinjection. Seven brain regions were analyzed for noradrenaline (NA), dopamine (DA), or 5-hydroxytryptamine (5-HT), and their respective metabolites. In all cases in which a comparison could be made with prior work utilizing repeated injections to produce dependence, the p.o. regimen produced the same effects. Thus, the mode of administration does not seem to modify the response of monoaminergic neurons to chronic morphine. In withdrawal, NA turnover increased but DA and 5-HT turnovers decreased. Acute morphine accelerated the turnover of all three monoamines. The NA response was attenuated in some brain regions of withdrawn rats, indicating the development of tolerance to the turnover-enhancing effect of acute morphine in noradrenergic neurons. In contrast, the effect of acute morphine on cerebral 5-HT turnover was not altered, and its effect on cerebral DA turnover was enhanced in withdrawn rats. Our results suggest that there are fundamental differences among the three monoaminergic systems in their capacities for adapting to chronic morphine treatment.

Sequential changes of noradrenaline content of vein grafts in rats: quantitative estimation by high-performance liquid chromatography using electrochemical detection.

Supradiaphragmatic inferior vena cava was transplanted end-to-end into the abdominal aorta of 11-week-old rats of the same inbred strain and same litter using microvascular technique. The grafts were removed 3 days, 4, 8, 16, and 35 weeks postoperatively, and their noradrenaline (NA) content was estimated by high-performance liquid chromatography using electrochemical detection. The amount of NA was significantly lower (P less than .001) in all vein grafts as compared to nontransplanted vena cava. The substantial decrease of NA in the vein grafts throughout the observation period indicates a persistent denervation of the transplant.

Antagonism of the nicotine-induced changes of the striatal dopamine metabolism in mice by mecamylamine and pempidine.

The ability of nicotinic receptor blockers, mecamylamine and pempidine, to antagonize the changes in striatal dopamine (DA) metabolism induced by repeated nicotine administration was studied. The contents of DA and its metabolites 3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were measured. Mice kept at 20-22 degrees C were given nicotine, 3 mg/kg, s.c., four times, at 30 min intervals, and sacrificed 20 min after the last dose. Hexamethonium, 10 mg/kg, i.p., was administered at 30 min before the first nicotine dose in order to prevent the peripheral effects of nicotine. Mecamylamine, 0.6 or 10 mg/kg, i.p., and pempidine, 0.6 or 20 mg/kg, i.p., were given at 60 min before sacrifice. Mecamylamine and pempidine decreased clearly the striatal 3-MT content, which suggests that the nigrostriatal dopaminergic neurons are physiologically controlled by a stimulatory nicotinic mechanism. The repeatedly administered nicotine caused deep hypothermia, and increased the striatal DOPAC content but decreased the 3-MT and HVA contents. The small dose of mecamylamine, which was the only dose found to effectively antagonize the nicotine-induced hypothermia, antagonized the decrease of HVA content. The large but not the small doses of mecamylamine and pempidine antagonized the nicotine-induced increase of DOPAC content but none of the doses studied antagonized the decrease of 3-MT content. Thus it seems that nicotine decreases the 3-MT content by a mechanism distinct from the mechanism mediating the increase of the DOPAC content. The decreased 3-MT content most probably results from desensitization of nicotinic cholinergic receptors (nAChR) and following decrease of cholinergic regulation of nigrostriatal dopaminergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of a novel type of rotating disc electrode and a flow cell with laminar flow pattern for the electrochemical detection of biogenic monoamines and their metabolites after Sephadex gel chromatographic purification and high-performance liquid chromatographic isolation from rat brain.

A novel type of rotating disc electrode and a flow cell with laminar flow pattern were developed and applied to the electrochemical detection of dopamine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, 3-methoxytyramine (3-MT), noradrenaline, 3-methoxy-4-hydroxyphenylethyleneglycol (MOPEG), 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid after HPLC of these compounds. The active surface of the rotating disc working electrode was made from solid paraffin (40% wt/wt) and graphite powder (60% wt/wt). The sensitivity of the detector was proportional to the square root of the angular velocity and was practically independent of the flow rate of the mobile phase. The surface of the working electrode was very large (radius = 12 mm), and so the percentage of oxidation was 24-67% (flow rate = 1.0 ml/min), depending on the compound. Electrical noise between 20 and 40 pA and background current of 20-60 nA were observed. In practice, the sensitivity for the detection of the compounds examined here was 8-16 nA/ng, and so a detection limit of 5 pg/injection could be achieved, when the detector was combined with reversed-phase HPLC. Supernatants obtained from the extracts of the tissue samples (nine brain parts of rat brain were studied) were purified by using Sephadex G-10 gel chromatography. Before this procedure, the proteins of the tissue extracts were precipitated by 0.2 M HClO4, and the excess of HClO4 was precipitated by KOH/HCOOH buffer. Simultaneously, the pH of the extracts was set to 2.4 by the above buffer.(ABSTRACT TRUNCATED AT 250 WORDS)