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CARDIOVASCULAR
Institut für experimentelle und klinische Pharmakologie und Toxikologie, Universitätsklinikum Schleswig-Holstein, Campus Lübeck, Germany (C.B., A.D.); Medizinische Klinik II, Universitätsklinikum Schleswig-Holstein, Campus Lübeck, Germany (T.K., I.S., F.S.); Institut für Pharmakologie, Klinikum der Universität zu Köln, Köln, Germany (E.S.); and Herzzentrum Segeberger Kliniken GmbH, Bad Segeberg, Germany (G.R.)
Received September 10, 2003; accepted December 5, 2003.
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Abstract |
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; Kurz et al., 1995; Imamura et al., 1996; Hatta et al., 1999; Levi and Smith, 2000; Oka et al., 2002; Sesti et al., 2003). In contrast, during prolonged periods of reduced coronary flow (low flow ischemia), exocytotic NE release decreases, resulting in a uniform suppression of NE overflow from the heart (Du and Riemersma, 1991). Because ischemia in vivo is neither absolute nor homogeneous, heterogeneity of NE release has substantial clinical implication, because it forms the substrate for malignant ventricular arrhythmias (Schömig et al., 1991). So far, the underlying metabolic alterations that affect sympathetic neurotransmission have been studied intensively in the setting of stopflow ischemia; however, less is known concerning the modulation of transmitter release during coronary low flow.
It is well known that adenosine rapidly accumulates in the interstitial space in response to metabolic distress (Fredholm et al., 2001; Mubagwa and Flameng, 2001). Pharmacological blockade of presynaptic adenosine A1-receptors attenuates suppression of exocytotic NE release during low flow ischemia as well as reperfusion, whereas presynaptic 
2-adrenoceptors lose their autoinhibitory potential (Du and Riemersma, 1991; Burgdorf et al., 2001). In parallel with adenosine accumulation, breakdown of intracellular ATP triggers the opening of ATP-sensitive potassium (KATP) channels in ischemic myocardium (Yokoshiki et al., 1998; Grover and Garlid, 2000). Because KATP channels are present at noradrenergic neurons (Dunn-Meynell et al., 1997, 1998), it is conceivable that axonal hyperpolarization in response to KATP channel opening modulates NE release. Indeed, pharmacological activation of KATP channels has recently been shown to reduce the exocytotic release of NE in nonischemic atrial tissue of guinea pigs (Oe et al., 1999). However, NE overflow from the heart is not only determined by the release itself but also by the elimination of NE from the synaptic cleft. Recently, molecular identification of the extraneuronal monoamine transporter (EMT, uptake2) has been reported (Gründemann et al., 1998; Gründemann and Schömig, 2000). The characteristics distinguishing EMT from NET (uptake1) include the ability to transport NE in a Na+- and Cl--independent manner, inhibition by corticosterone, as well as dependence on the membrane potential as the driving force (Kekuda et al., 1998; Wu et al., 1998). In vitro studies using clonal cancer cells of human renal tubules have documented that cell membrane hyperpolarization in response to KATP channel opening enhances cellular catecholamine uptake via EMT (Schömig et al., 1992). From the latter findings, it is tempting to conclude that in addition to neuronal KATP channels, extraneuronal KATP channels may modulate sympathetic neurotransmission during ischemia via EMT. However, previous findings from isolated myocytes and intact rat hearts suggested that the EMT, at least in myocardial tissue, operates independently from KATP channels, because neither KATP channel opening nor blockade influenced cellular catecholamine uptake by EMT (Bryan-Lluka and Vuocolo, 1993; Obst et al., 1996).
We therefore sought to evaluate the role of neuronal KATP channels and of EMT on NE overflow during coronary low flow ischemia.
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Materials and Methods |
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Heart Preparation. Seven-week-old male Wistar rats (180–250 g body weight; Charles River, Sulzfeld, Germany) were anesthetized with 150 mg kg-1 thiopental sodium intraperitoneally. After medial laparotomy and injection of 500 IU kg-1 heparin sodium into the inferior vena cava, the thorax was opened and the heart rapidly removed and weighed. Isolated hearts were mounted in a Langendorff apparatus and were perfused via the ascending aorta with 95% O2, 5% CO2 saturated Krebs-Henseleit solution (37°C, pH 7.4).
NE Release. Two 13 x 8 mm concavely shaped metal paddles were attached to the epicardium in opposite positions for electrical field stimulation (effective voltage 5 V, stimulation frequency 6 Hz, pulse width 2 ms). During the experiment, hearts were stimulated twice (1 min each). For determination of endogenous NE release, effluent was collected immediately before, during, and for 2 min after electrical stimulation. The exocytotic nature of stimulation-induced NE release has been documented previously (Seyfarth et al., 1993).
Each heart was equilibrated for 25 min at a constant flow of 8 ml min-1. Unless otherwise stated, desipramine (0.1 µmol l-1) was given to all hearts from the 11th min to prevent nonexocytotic NE release via the neuronal NET (Schömig, 1990). Subsequent to equilibration, hearts were subjected to low coronary flow (0.4 ml min-1, i.e., 95% flow reduction). The first stimulation (baseline NE release) was carried out in the 20th min of equilibration. The second stimulation was performed in four different groups of hearts either in the final minute of equilibration or 5, 30, or 120 min after the onset of low flow (Fig. 1). Additionally, in a separate group with 120 min of low flow ischemia, the second stimulation was induced after 5 min of reperfusion at normal flow of 8 ml min-1. In hearts with continuous perfusion at normal flow, electrical stimulations were evoked at corresponding times to maintain comparability to low flow ischemic experiments (Fig. 1). To exclude nonexocytotic NE release, additional 5, 30, and 120 min of low flow experiments were performed without electrical stimulation, and washout of NE was assessed within the first 5 min of reperfusion at normal flow.
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Pharmacological interventions on stimulation-induced NE overflow during low and normal coronary flow were carried out in separate groups of hearts. Addition of drugs [glibenclamide (KATP channel blockade), corticosterone (EMT blockade), and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; adenosine A1-receptor blockade)] was started 2 min before flow reduction and was continued throughout the remaining experiment. Otherwise, the experimental protocol was identical to that described above. In hearts without flow reduction, pharmacological interventions [glibenclamide, corticosterone, DPCPX, cromakalim (KATP channel opening), diazoxide (KATP channel opening), 2-chloro-N6-cyclopentyladenosine (CCPA; adenosine A1-receptor activation)] were performed for 30 min after equilibration. The second stimulation was carried out in the final minute of drug infusion (Fig. 1). In three additional groups, cromakalim was given during the final 15 min of corticosterone or glibenclamide infusion and CCPA during the final 15 min of glibenclamide infusion, respectively.
Endogenous NE was determined in the coronary venous effluent by high-performance liquid chromatography and electrochemical detection as described previously in detail (Schömig et al., 1987).
Modulation of EMT. Isolated hearts were perfused throughout the experiment at a flow of 8 ml min-1. After 25 min of equilibration, hearts were labeled for 1.5 min with the EMT substrate N-[methyl-3H]4-phenylpyridinium ([3H]MPP+) at a final concentration of 1.25 µmol l-1 (Fig. 1) (Russ et al., 1992). Thereafter, each heart was perfused for further 18.5 min. Fractions of coronary venous effluent were collected at 30 s intervals after the onset of labeling for determination of radioactivity. As previously reported, during steady-state conditions 90% of total outward transport of intracellular [3H]MPP+ occurs via EMT in reversal of its normal transport direction (Russ et al., 1992). Therefore, pharmacological interventions targeting EMT activity (cromakalim and corticosterone) were started 10 min after the beginning of [3H]MPP+ loading and were continued until the end of the experiment (Fig. 1).
Desipramine (0.1 µmol l-1) was given to all hearts from the 11th min of equilibration to inhibit neuronal uptake of [3H]MPP+ (Javitch et al., 1985).
[3H]MPP+ was determined in each sample corresponding to 30 s of perfusion. For determination of the final myocardial [3H]MPP+ content, each heart was hydrolyzed in 6 mol l-1 potassium hydroxide for 18 h at 95°C. After neutralization with 1 mol l-1 perchloric acid, a clear supernatant was obtained by 30 min of centrifugation (3000g). One milliliter of the myocardial extract or venous effluent was transferred to a scintillation vial containing 10 ml of scintillation fluid (Quicksafe A; Zinsser Analytic, Frankfurt, Germany). The radioactivity of the samples was measured in a scintillation beta counter (1409 liquid scintillation counter; PerkinElmer Wallac, Gaithersburg, MD).
Drugs. Desipramine, corticosterone, and CCPA were purchased from Sigma-Aldrich (Taufkirchen, Germany). Glibenclamide, cromakalim, diazoxide, and DPCPX were obtained from Tocris Cookson Inc. (Bristol, UK). [3H]MPP+ (specific activity 80 Ci mmol-1) was obtained from American Radiolabeled Chemicals (St. Louis, MO). Heparin sodium was purchased as Liquemin N 25000 from Roche Diagnostics (Grenzach-Wyhlen, Germany) and thiopental sodium as Trapanal 0.5 g from Byk Gulden (Konstanz, Germany). The composition of the Krebs-Henseleit solution was 142 mmol l-1 Na+, 4 mmol l-1 K+, 1.85 mmol l-1 Ca2+, 1.1 mmol l-1 Mg2+, 135 mmol l-1 Cl-, 0.22 mmol l-1 H2PO4-, 16.7 mmol l-1 HCO3-, 11 mmol l-1 glucose, and 0.027 mmol l-1 EDTA (salts from Merck, Darmstadt, Germany).
Calculations and Statistics. Overflow of endogenous NE was determined as the total NE content in the coronary effluent during and for 2 min after each stimulation and was expressed in picomoles per gram of heart weight. Absolute values of NE overflow are presented under Results in parentheses as follows: NE overflow induced by the second stimulation (intervention) versus stimulation-induced NE overflow at baseline. Normalized data were expressed as NE overflow in percentage of baseline. [3H]MPP+ in each 30 s coronary venous fraction was normalized to the total myocardial [3H]MPP+ content present in the tissue when the sample was collected. The results are expressed as fractional [3H]MPP+ overflow.
All data are given as means ± S.E.M. of n experiments. Inner group comparison (NE overflow induced by the second stimulation versus stimulation-induced NE overflow at baseline) was assayed by two-tailed paired Student's t test. Comparisons between groups (normalized NE overflow, fractional [3H]MPP+ overflow) were assayed by one-way analysis of variance followed by Bonferroni's post hoc test when three or more groups were compared. Two-tailed unpaired Student's t test was performed when two groups were compared. A value of p < 0.05 was considered statistically significant.
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Effects of Low and Normal Coronary Flow on Stimulation-Induced NE Overflow. Directly before flow reduction, stimulation-induced NE overflow did not differ from baseline (294 ± 42 versus 292 ± 40 pmol g-1, n = 8). In contrast, NE overflow decreased by 37 ± 4 and 56 ± 4% within 5 and 30 min of low flow (5 min: 218 ± 15 versus 349 ± 28 pmol g-1, p < 0.01, n = 6; 30 min: 117 ± 19 versus 260 ± 30 pmol g-1, p < 0.01, n = 8) (Fig. 2). Inhibition of NE overflow remained constant for up to 120 min (115 ± 15 versus 264 ± 28 pmol g-1, p < 0.01, n = 8). Five minutes of reperfusion at normal flow after 120-min ischemia restored NE overflow (263 ± 26 versus 297 ± 25 pmol g-1, n = 8), suggesting a functional rather than structural alteration of sympathetic neurotransmission during low flow (Fig. 2). Corresponding periods of 5, 30, 120, and 125 min of normal perfusion did not affect NE overflow compared with baseline (5 min: 330 ± 24 versus 342 ± 30 pmol g-1, n = 4; 30 min: 235 ± 13 versus 238 ± 12 pmol g-1, n = 8; 120 min: 249 ± 17 versus 264 ± 24 pmol g-1, n = 8; 125 min: 364 ± 37 versus 378 ± 44 pmol g-1, n = 4) (Fig. 2).
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In a separate set of experiments, NE overflow was determined in the absence of NET blockade by desipramine. In this setting, the absolute amounts of stimulation-induced NE overflow at baseline were 3-fold lower than those without NET blockade. Within 30 min of low flow, NE overflow decreased to 58 ± 6% of baseline overflow (53 ± 7 versus 91 ± 2 pmol g-1, p < 0.01, n = 4), whereas the respective controls with 30 min of normal flow revealed no effect on NE overflow (87 ± 5 versus 94 ± 6 pmol g-1, 93 ± 4%, n = 8). The inhibition of NE overflow at 30 min low flow did not differ significantly from the inhibition that was observed in the presence of NET blockade.
Role of KATP Channels, EMT, and Adenosine A1-Receptors on Stimulation-Induced NE Overflow at Low and Normal Coronary Flow. NE overflow induced by the second stimulation (i.e., during pharmacological intervention) was normalized to the intraindividual NE overflow at baseline. Within the separate experimental groups, baseline NE overflow did not differ significantly and was comparable with that of untreated hearts subjected to either low flow or normal flow. Normalized data of untreated hearts served as control.
None of the drugs induced a detectable NE overflow before stimulation. Blockade of KATP channels with glibenclamide (30 µmol l-1) had no effect on NE overflow at 5 min of low flow (n = 8); however, glibenclamide increased NE overflow 1.9- and 1.5-fold at 30 and 120 min (each group n = 4) (Fig. 3). EMT blockade with corticosterone (30 µmol l-1) elicited a 1.6-, 2.4-, and 2-fold increase of NE overflow after 5 min (n = 8), 30 min (n = 6), and 120 min (n = 4) of low flow. Likewise, adenosine A1-receptor blockade with DPCPX (1 µmol l-1) significantly enhanced NE overflow after 30 min of low flow (n = 6) (Fig. 3). To assess whether the effect of EMT blockade on NE overflow was only operative in the presence of NET blockade by desipramine, additional experiments were performed without desipramine. After 30 min of low flow, corticosterone (30 µmol l-1) markedly attenuated the suppression of NE overflow (86 ± 7% of baseline NE overflow, n = 8, p < 0.05 versus 58 ± 6% at 30 min of low flow without desipramine and corticosterone).
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At normal coronary flow, neither KATP channel blockade with glibenclamide (30 µmol l-1, n = 6) nor EMT blockade with corticosterone (30 µmol l-1, n = 4) had an effect on NE overflow (Fig. 4). Likewise, adenosine A1-receptor blockade with DPCPX (1 µmol l-1, n = 6) did not affect NE overflow (108 ± 8% of baseline NE overflow). In contrast, when KATP channels were opened with cromakalim (100 µmol l-1, n = 4) NE overflow was slightly but significantly suppressed (Fig. 4). Interestingly, diazoxide (100 µmol l-1, n = 6), a structurally different KATP channel opener, did not reduce NE overflow (102 ± 4% of baseline NE overflow). The inhibitory effect of cromakalim remained evident in the presence of EMT blockade by corticosterone (30 µmol l-1, n = 6). However, the suppressive effect of cromakalim was not found when KATP channels were blocked with glibenclamide (30 µmol l-1, n = 6) (Fig. 4). Because it is well known that adenosine A1-receptors activate KATP channels (Fredholm et al., 2001; Mubagwa and Flameng, 2001), we also determined whether the inhibitory effect of CCPA (adenosine A1-receptor agonist) on NE overflow can be antagonized by KATP channel blockade with glibenclamide (30 µmol l-1). In hearts treated with CCPA (0.1 µmol l-1, n = 4) exclusively, NE overflow decreased by 40 ± 6%; however, the suppressive effect of CCPA was still observed in the presence of glibenclamide (n = 4), suggesting that presynaptic adenosine A1-receptors modulate NE release independently of KATP channels (Fig. 5).
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Role of KATP Channels and EMT on [3H]MPP+ Overflow. The modulatory effects of cromakalim (100 µmol l-1) and corticosterone (30 µmol l-1) on EMT activity was assessed at normal coronary flow by means of [3H]MPP+ overflow. The drugs were added to the perfusion buffer during steady-state conditions 8.5 min after termination of [3H]MPP+ loading. Control hearts did not receive drugs (each group n = 4).
During the loading period, a major fraction of [3H]MPP+ was not retained in the heart causing high fractional overflow (Fig. 6). Within 1.5 min after termination of labeling, [3H]MPP+ overflow decreased to 
8.5% in control-, cromakalim-, and corticosterone-treated hearts. In control hearts, fractional [3H]MPP+ overflow stabilized at a level of 6%. In contrast, [3H]MPP+ overflow decreased 2.5-fold 2 min after addition of corticosterone, an effect that persisted until the end of the experiment. KATP channel opening with cromakalim had no modulatory effect on EMT activity, as indicated by an unaffected [3H]MPP+ overflow throughout the experiment (Fig. 6).
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Discussion |
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) that activation of presynaptic adenosine A1-receptors contributes to the suppression of exocytotic NE release during low flow ischemia. The results of the current study suggest that flow reduction impairs sympathetic neurotransmission as a result of metabolic alterations (KATP channel opening, adenosine A1-receptor activation) that may affect both exocytotic catecholamine secretion and extraneuronal catecholamine elimination.
We could demonstrate that activation of EMT contributes importantly to the suppression of NE overflow. This effect occurs specifically in the condition of low flow ischemia because in isolated rat hearts subjected to either normoxic perfusion, hypoxic perfusion, or stopflow ischemia, pharmacological blockade of EMT had no effect on NE overflow (Schömig et al., 1984; Carlsson et al., 1986). However, our findings indicate that the EMT is selectively activated during moderate ischemia because corticosterone did not influence NE overflow in nonischemic conditions. Notably, previous studies using nonmyocardial cells and incubated segments from guinea pig trachealis muscle have documented that cell membrane hyperpolarization stimulates the EMT (Schömig et al., 1992; Bryan-Lluka and Vuocolo, 1993; Kekuda et al., 1998; Wu et al., 1998). KATP channels couple the membrane potential to the metabolic state of the cells (Yokoshiki et al., 1998; Grover and Garlid, 2000). KATP channels are closed during physiological conditions but open during the course of ischemia and thereby evoke hyperpolarization. Therefore, it was suggested that endogenous KATP channel opening during low flow ischemia activates the EMT, resulting in a suppression of NE overflow. Hence, to assess this hypothesis, we have determined whether cromakalim (KATP channel opener) would influence EMT activity. EMT activity was measured by means of fractional [3H]MPP+ overflow. EMT transports the neurotoxin [3H]MPP+ more efficiently than [3H]NE as is evident from higher rate constants for inwardly and outwardly directed specific transport (Russ et al., 1992). As expected, corticosterone decreased [3H]MPP+ overflow, indicating functional suppression of EMT, whereas cromakalim did not affect [3H]MPP+ overflow. However, the lack of effect of cromakalim on EMT activity is in accordance with previous findings from quiescent myocytes and isolated beating rat hearts, that neither membrane hyperpolarization nor depolarization is effective to modulate EMT in myocardial tissue (Bryan-Lluka and Vuocolo, 1993; Obst et al., 1996). Together with the observation that corticosterone increased NE overflow at 5 min of low flow ischemia before KATP channels are opened because glibenclamide had no effect (Fig. 3), our results suggest that activation of EMT during low flow is not induced by KATP channel opening. This contention is further substantiated by the finding that neither cromakalim nor glibenclamide influenced postsynaptic isoprenaline uptake in intact rat hearts (Bryan-Lluka and Vuocolo, 1993). However, the underlying endogenous stimulus for enhanced EMT activity during myocardial low flow ischemia remains unclear.
Previous investigations have focused primarily on the effects of KATP channel modulation on neurotransmitter release in the brain. Because ATP-sensitive K+ conductance is present in noradrenergic neurons (Dunn-Meynell et al., 1997, 1998), the possibility arises that KATP channel opening in response to ischemia influences NE release from the heart. In fact, we found that glibenclamide substantially increased NE overflow after 30 and 120 min of myocardial low flow ischemia, indicating endogenous activation of KATP channels. The lack of effect of glibenclamide at preserved coronary flow is in accordance with the hypothesis that KATP channels are not activated during nonischemic conditions (Yokoshiki et al., 1998; Grover and Garlid, 2000). In contrast, when KATP channels were pharmacologically opened with cromakalim, NE overflow was suppressed. Together with the finding that cromakalim had no effect on [3H]MPP+ overflow, the latter observations indicate that opening of KATP channels impairs catecholamine release directly at presynaptic nerve endings. The ultrastructure of the KATP channel is unique among ion channels because it is a complex of two proteins: the sulfonylurea receptor (SUR) and a smaller protein Kir6.2 (Aguilar-Bryan et al., 1995; Inagaki et al., 1995). It was concluded from previous investigations that the therapeutically important KATP channel openers (e.g., cromakalim, pinacidil, and nicorandil) act specifically on the SUR2 muscle isoforms but, except for diazoxide, remain ineffective on the SUR1 neuronal/pancreatic isoform (Inagaki et al., 1995; Babenko et al., 1998; D'hahan et al., 1999). In the present study, we found that diazoxide had no effect on NE overflow. Functional studies using isolated guinea pig atria revealed that KATP channel activation with diazoxide decreases the heart rate response to sympathetic nerve stimulation (Mohan and Paterson, 2000). Notably, Oe et al. (1999) have documented an inhibitory effect of cromakalim on stimulation-induced NE release from nonischemic guinea pig atrium that was antagonized by glibenclamide, whereas pinacidil dose dependently increased NE release. Our results are consistent with recent findings stating that nicorandil and cromakalim inhibit nonexocytotic NE release in ischemic myocardium from dog and rabbit (Miura et al., 2001; Remme et al., 2001) and provide evidence that KATP channels are present at presynaptic nerve endings, modulating sympathetic neurotransmission during myocardial ischemia. However, from the latter findings it is evident that both the experimental model and animal species have an important influence on the response of sympathetic neurotransmission to distinct pharmacologic KATP channel openers.
It is well established that noradrenergic nerve terminals are equipped with inhibitory adenosine A1-receptors and histamine H3-receptors (Boehm and Kubista, 2002). Because postsynaptic adenosine A1-receptors are coupled to KATP channels (Fredholm et al., 2001; Mubagwa and Flameng, 2001), it is conceivable that the inhibitory effect on sympathetic neurotransmission would also involve KATP channels present at the peripheral nerve ending. In the present study, however, glibenclamide did not antagonize the suppressive effect of CCPA (adenosine A1-receptor agonist), suggesting that adenosine A1-receptors and KATP channels provide independent targets to intervene on adrenergic neurotransmission. Accordingly, in atrial tissue from guinea pig, KATP channel blockade with glibenclamide was unable to interact with the inhibitory effect of exogenous adenosine on NE release (Oe et al., 1999). In hypoxic myocardium of humans and guinea pigs, a rise in endogenous histamine release is likely to inhibit noradrenergic overactivity through an activation of inhibitory presynaptic histamine H3-receptors of sympathetic nerve terminals (Levi and Smith, 2000). In the rat heart, however, such a prejunctional effect of histamine could not be demonstrated (Mazenot et al., 1999). This has been attributed to an insensitivity of the rat to histamine (McLeod et al., 1994; Mazenot et al., 1999). Therefore, in the current study, we did not determine whether histamine H3-receptor activation/blockade modulates sympathetic neurotransmission during low flow ischemia.
Several limitations apply to the present study. First, most experiments were performed in the presence of desipramine to prevent nonexocytotic NE release (Schömig, 1990). This does not correspond to myocardial low flow ischemia in vivo. However, in additional experiments without NET blockade, suppression of NE overflow during low flow also emerged as a significant effect that did not differ from the inhibition observed in the presence of NET blockade. Moreover, in this setting, corticosterone markedly attenuated the suppression of NE overflow during low flow, suggesting that the EMT was activated independently from NET activity. Second, only single concentrations of the various drugs were used. Concentrations were taken from previous published animal studies (Schömig et al., 1984; Carlsson et al., 1986; Oe et al., 1999; Burgdorf et al., 2001). Third, we have no data of myocardial function and infarction parameters and we can only speculate about the functional implications of sympathetic impairment during low flow ischemia. It is apparent that sympathetic dysfunction during low flow ischemia has substantial clinical implications because it may worsen the inotropic impairment of hibernating myocardium and aggravate subendocardial ischemia as neuronal activation influences transmural flow. Furthermore, heterogeneity of NE release may predispose to malignant ventricular arrhythmias (Schömig et al., 1991). On the other hand, it is conceivable that suppression of NE release during low flow ischemia is of potential benefit, because it protects the heart from excessive sympathetic drive and postpones irreversible myocardial damage.
In conclusion, prolonged periods of myocardial low flow ischemia result in a suppression of NE overflow from the heart. In addition to adenosine A1-receptor stimulation, endogenous activation of KATP channels and of EMT mediates suppression of NE overflow during low coronary flow. KATP channels modulate sympathetic neurotransmission primarily at the presynaptic nerve ending, most likely through inhibition of NE release. EMT influences sympathetic neurotransmission independently from KATP channels at the postsynaptic membrane by increasing NE elimination from the synaptic cleft.
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Footnotes |
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
ABBREVIATIONS: NE, norepinephrine; NET, neuronal monoamine transporter; KATP channel, ATP-sensitive potassium channel; EMT, extraneuronal monoamine transporter; DPCPX, 8-cyclopentyl-1,3-dipropylxanthine; CCPA, 2-chloro-N6-cyclopentyladenosine; [3H]MPP+, N-[methyl-3 phenylpyridinium; SUR, sulfonylurea receptor.
Address correspondence to: Christof Burgdorf, Institut für experimentelle und klinische Pharmakologie und Toxikologie, Universitätsklinikum Schleswig-Holstein, Campus Lübeck, Ratzeburger Allee 160, 23538 Lübeck, Germany. E-mail: chbrg{at}compuserve.de
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) and normal (
) coronary flow on stimulation-induced NE overflow. NE overflow was intraindividually normalized to the stimulation-induced baseline NE overflow. Results are expressed as means ± S.E.M. (n = 4–8 each group). **, p < 0.01 versus normal flow; 
, glibenclamide, 30 µmol l-1) and EMT blockade (
, corticosterone, 30 µmol l-1) on stimulation-induced NE overflow after 5, 30, and 120 min of low flow. B, effect of adenosine A1-receptor blockade with DPCPX (1 µmol l-1)on stimulation-induced NE overflow after 30 min of low flow. NE overflow was intraindividually normalized to the stimulation-induced baseline NE overflow. Results are expressed as means ± S.E.M. (n = 4–8 each group). **, p < 0.01 versus respective low flow period without pharmacological intervention (
