Activation of Histamine H3-Receptors Inhibits Carrier-Mediated Norepinephrine Release During Protracted Myocardial Ischemia
Comparison With Adenosine A1-Receptors and α2-Adrenoceptors
Abstract We previously showed that prejunctional histamine H3-receptors downregulate norepinephrine exocytosis, which is markedly enhanced in early myocardial ischemia. In the present study, we investigated whether H3-receptors modulate nonexocytotic norepinephrine release during protracted myocardial ischemia. In this setting, decreased pHi in sympathetic nerve endings sequentially leads to a compensatory activation of the Na+-H+ antiporter (NHE), accumulation of intracellular Na+, reversal of the neuronal uptake of norepinephrine, and thus carrier-mediated release of norepinephrine. Accordingly, norepinephrine overflow from isolated guinea pig hearts undergoing 20-minute global ischemia and 45-minute reperfusion was attenuated ≈80% by desipramine (10 nmol/L) and 70% by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA, 10 μmol/L), inhibitors of norepinephrine uptake and NHE, respectively. The H3-receptor agonist imetit (0.1 μmol/L) decreased carrier-mediated norepinephrine release by ≈50%. This effect was blocked by the H3-receptor antagonist thioperamide (0.3 μmol/L), indicating that H3-receptor activation inhibits carrier-mediated norepinephrine release. At lower concentrations, imetit (10 nmol/L) or EIPA (3 μmol/L) did not inhibit carrier-mediated norepinephrine release. However, a 25% inhibition occurred with imetit (10 nmol/L) and EIPA (3 μmol/L) combined. This synergism suggests an association between H3-receptors and NHE. Conceivably, activation of H3-receptors may lead to inhibition of NHE. In fact, α2-adrenoceptor activation, which is known to stimulate NHE, enhanced norepinephrine release, whereas α2-adrenoceptor blockade attenuated it. Furthermore, activation of adenosine A1-receptors markedly attenuated norepinephrine release, whereas their inhibition potentiated it. Because norepinephrine release directly correlated with the severity of reperfusion arrhythmia and imetit reduced the incidence of ventricular fibrillation by 50%, our findings with H3-receptor agonists may further the development of novel pharmacological means to reduce reperfusion arrhythmias in the clinical setting.
- norepinephrine release
- histamine H3-receptors
- adenosine A1-receptors
We have recently reported the presence of histamine H3-receptors on sympathetic nerve terminals in the heart.1 We found that their activation by an endogenous ligand, most likely histamine, leads to the attenuation of norepinephrine exocytosis in pathophysiological conditions associated with enhanced adrenergic activity, such as acute myocardial ischemia.2
Protracted myocardial ischemia causes an even greater release of norepinephrine from sympathetic nerves. In this case, excessive norepinephrine release is “carrier mediated” rather than exocytotic.3 4 It is caused by a reversal of the carrier responsible for the reuptake of norepinephrine by nerve endings.3 4 5 Exaggerated norepinephrine release increases oxygen demand by stimulating heart rate and contractility and decreases oxygen supply by constricting the coronary vessels.6 This accelerates the progression of cell damage in the ischemic area and border zones and potentiates the arrhythmogenicity of norepinephrine.6 7 Therefore, limiting norepinephrine release and its dysfunctional consequences is of vital importance.
The purpose of the present study was to assess the potential of H3-receptor activation as a novel way to attenuate norepinephrine release in protracted myocardial ischemia. In this context, we compared the effectiveness of the H3-receptor with that of other prejunctional modulatory receptors, namely, the α2-adrenoceptor and the adenosine A1-receptor.
Materials and Methods
Male Hartley guinea pigs (Hilltop, Pa) weighing 250 to 300 g were killed by cervical dislocation under light anesthesia with CO2 vapor. After midline thoracotomy, the heart was rapidly excised and perfused retrogradely at a constant pressure of 40 cm H2O in a Langendorff apparatus with Ringer’s solution at 37°C saturated with 100% O2 (pH 7.5).8 The composition of the Ringer’s solution was (mmol/L) NaCl 154.0, KCl 5.61, NaHCO3 5.55, CaCl2 2.16, and dextrose 5.95 (pH 7.5). As previously described,2 hearts were first perfused with oxygenated Ringer’s solution for 30 to 45 minutes until heart rate, left ventricular contractile force, and coronary flow reached a steady state. Heart rate was determined from surface electrograms recorded from the right atrium and the left ventricle. Isometric left ventricular contractile force was measured with a force-displacement transducer (model FT03, Grass) and continuously recorded on a polygraph.
After a 30-minute preischemic stabilization period, normothermic global ischemia was induced by complete interruption of coronary perfusion for 20 minutes. This was followed by a 45-minute reperfusion period. The coronary effluent was collected into tubes. In the preischemic period, tubes were replaced every 5 minutes. In the first 10 minutes of reperfusion, tubes were replaced every 2 minutes and every 5 minutes during the last 35 minutes. The volume of effluent collected for each period was measured and subsequently analyzed for norepinephrine and histamine content. All drugs were added to the perfusion solution. Hearts were perfused with a given drug or drug combination for the entire duration of the experiment, beginning 30 minutes before global ischemia. Hearts were weighed at the end of the experiment.
In the absence of drugs (ie, in control conditions), preischemic left ventricular contractile force and coronary flow were 3.3±0.2 g and 4.9±0.3 mL/min (n=10), respectively. After global ischemia, when cardiac standstill occurred, contractile force slowly recovered during reperfusion and reached a maximum of 72±3% of preischemic control. Coronary flow increased by 44±7% over preischemic values during the first 2 minutes of reperfusion; it then slowly decreased to a level 17±4% below preischemic values, which was reached by 45 minutes of reperfusion.
Norepinephrine and Histamine Assays
Norepinephrine and histamine were assayed in the coronary perfusate by high-performance liquid chromatography coupled to electrochemical detection and by a commercial enzyme immunoassay kit (Immunotech International), respectively, as previously described.2 Values are expressed in picomoles per gram of wet heart weight. Briefly, perchloric acid and EDTA were added to samples to achieve final concentrations of 0.01N and 0.025%, respectively. After a short period of storage (<2 weeks) at −70°C, the samples were thawed. The norepinephrine present in the effluent was adsorbed on acid-washed alumina adjusted at pH 8.6 with Tris/2% EDTA buffer and then extracted into 150 μL of 0.1N perchloric acid. These final sample aliquots were kept frozen until injected onto a 3-μm ODS reverse-phase column (3.2×100 mm, Bioanalytical Systems, Inc) with an applied potential of 0.65 V. The mobile phase consisted of monochloroacetic acid (75 mmol/L), Na2EDTA (0.5 mmol/L), sodium octylsulfate (0.5 mmol/L), and acetonitrile (1.5%) at pH 3.0. The flow rate was 1.0 mL/min. Dihydroxybenzylamine was added to each sample as an internal standard before alumina extraction and used for recovery calculation. The recovery of norepinephrine was 77%, and the detection limit was ≈0.2 pmol. The histamine present in the coronary effluent was assayed with the use of a commercial enzyme immunoassay kit (Immunotech International Co). The recovery of histamine was ≈100%, and the detection limit was ≈0.02 pmol.
Values are expressed as mean±SEM. Comparison of more than two groups was performed by ANOVA, with the Bonferroni t test used for post hoc analysis. A value of P<.05 was considered statistically significant. The Yates’ corrected χ2 test was used to analyze the difference in the incidence of arrhythmias.
The selective H3-receptor agonist imetit and the selective adenosine A1-receptor antagonist N-0861 were gifts of Prof C.R. Ganellin, Department of Chemistry, University College, London, UK, and of Whitby Research, Inc, respectively. 5-(N-Ethyl-N-isopropyl)-amiloride (EIPA), N6-cyclopentyladenosine (CPA), idazoxan hydrochloride, (−)norepinephrine bitartrate, rauwolscine hydrochloride, and UK 14,304 were purchased from Research Biochemicals International. Yohimbine hydrochloride and desipramine hydrochloride were purchased from Sigma Chemical Co. Thioperamide maleate was purchased from Cookson Chemicals. EIPA, desipramine, thioperamide, and UK 14,304 were dissolved in 99.8% dimethyl sulfoxide, and further dilutions were made with perfusion buffer. At the concentration used (ie, 0.1%), dimethyl sulfoxide had no effect on any preparation in these studies. N-0861 was dissolved in 95% ethanol. Other drugs were dissolved in water.
Norepinephrine Overflow From Isolated Hearts Subjected to Ischemia and Reperfusion
The cumulative norepinephrine overflow during the 45-minute reperfusion of isolated guinea pig hearts subjected to global ischemia for 20 minutes rose from an undetectable level to 798±60 pmol/g (mean±SEM, n=5). Most of the norepinephrine was released in the first 10 minutes of reperfusion (Fig 1⇓, inset). During reperfusion, there was also a marked increase in histamine overflow. In the first 2 minutes, histamine overflow into the coronary effluent increased fourfold (5.1±1.0 pmol/g per minute) over the preischemic level (1.2±0.2 pmol/g per minute) and then declined to approximately twofold (2.4±0.4 pmol/g per minute) in the following 4 minutes (n=5).
Ischemia/Reperfusion, Norepinephrine Overflow, and Its Pharmacological Modulation: Effects of Norepinephrine Neuronal Reuptake Blockers and Na+-H+ Exchange Inhibitors
Desipramine (10 nmol/L), a blocker of norepinephrine neuronal reuptake, suppressed norepinephrine overflow during reperfusion after 20-minute global ischemia in isolated guinea pig hearts (Fig 1⇑). The Na+-H+ exchanger inhibitor EIPA (10 μmol/L) also markedly decreased norepinephrine overflow caused by ischemia/reperfusion (Fig 1⇑). In the absence of drugs, the incidence of ventricular fibrillation during reperfusion was 100%. In contrast, no fibrillation occurred in hearts perfused with desipramine or EIPA.
Ischemia/Reperfusion, Norepinephrine Overflow, and Its Pharmacological Modulation: Effects of Histamine H3-Receptor Agonists
The selective histamine H3-receptor agonist imetit (0.1 μmol/L) markedly decreased (≈50%) norepinephrine overflow during reperfusion after 20-minute global ischemia in isolated guinea pig hearts (Fig 2A⇓). The selective histamine H3-receptor antagonist thioperamide (0.3 μmol/L) did not by itself affect norepinephrine overflow during reperfusion; however, thioperamide prevented the inhibitory effect of imetit (Fig 2A⇓). Since both EIPA and imetit attenuated norepinephrine overflow during reperfusion, we tested whether H3-receptor stimulation might be coupled to inhibition of the Na+-H+ exchanger. For this, we evaluated the effects of subthreshold concentrations of EIPA and imetit added in combination. As shown in Fig 2B⇓, neither imetit at 10 nmol/L nor EIPA at 3 μmol/L affected norepinephrine overflow. In combination, however, both compounds significantly decreased norepinephrine overflow (by 25%) during reperfusion.
Ischemia/Reperfusion, Norepinephrine Overflow, and Its Pharmacological Modulation: Effects of α2-Adrenoceptor Agonists and Antagonists
The α2-adrenoceptor agonist UK 14,304 (10 μmol/L) significantly potentiated (by 30%) norepinephrine overflow during reperfusion after 20-minute global ischemia in isolated guinea pig hearts (Fig 3A⇓). The α2-adrenoceptor antagonist yohimbine (1 μmol/L) prevented the potentiating effect of UK 14,304. Furthermore, yohimbine by itself markedly decreased norepinephrine overflow (by 50%) and so did two other α2-adrenoceptor antagonists: rauwolscine (1 μmol/L), which is structurally similar to yohimbine, and idazoxan (10 μmol/L), which is unrelated to yohimbine (Fig 3A⇓); the inhibition with rauwolscine and idazoxan was 30% and 40%, respectively. Shown in Fig 3B⇓ is the finding that subthreshold concentrations of yohimbine (30 nmol/L) and EIPA (3 μmol/L), which by themselves did not influence norepinephrine overflow, significantly attenuated norepinephrine overflow (by 35%) when added in combination.
Ischemia/Reperfusion, Norepinephrine Overflow, and Its Pharmacological Modulation: Effects of Adenosine A1-Receptor Activation and Blockade
The selective adenosine A1-receptor agonist CPA (0.1 μmol/L) markedly decreased norepinephrine overflow (by 75%) during reperfusion after 20-minute global ischemia in isolated guinea pig hearts (Fig 4A⇓). The selective adenosine A1-receptor antagonist N-0861 (5 μmol/L) prevented the inhibitory effect of CPA. Furthermore, N-0861 by itself enhanced norepinephrine overflow by 40%. Shown in Fig 4B⇓ is the finding that subthreshold concentrations of CPA (3 nmol/L) and EIPA (3 μmol/L), which by themselves did not influence norepinephrine overflow, significantly attenuated it (by 35%) when added in combination.
Effects of H3-Receptor, α2-Adrenoceptor, and A1-Receptor Ligands on Reperfusion Arrhythmias
After 20-minute global ischemia, ventricular fibrillation invariably occurred in all control hearts at the beginning of reperfusion and persisted for 3.05±0.74 minutes (n=14). The effects of H3-receptor, α2-adrenoceptor, and A1-receptor ligands on the incidence of ventricular fibrillation are presented in Fig 5⇓. The selective histamine H3-receptor agonist imetit (0.1 μmol/L) decreased the incidence of ventricular fibrillation by 50%. This effect was prevented by the selective H3-receptor antagonist thioperamide (0.3 μmol/L, Fig 5A⇓). As shown in Fig 5B⇓ and 5C⇓, the α2-adrenoceptor antagonist yohimbine (1 μmol/L) and the selective adenosine A1-receptor agonist CPA (0.1 μmol/L) each prevented the occurrence of ventricular fibrillation. The effect of yohimbine was counteracted by the concomitant administration of the α2-adrenoceptor agonist UK 14,304 (10 μmol/L, Fig 5B⇓), whereas the effect of CPA was abolished by the selective adenosine A1-receptor antagonist N-0861 (5 μmol/L).
Shown in Fig 6⇓ is the relationship between norepinephrine overflow and duration of ventricular fibrillation in 51 hearts subjected to 20-minute global ischemia followed by 45-minute reperfusion either in the absence or in the presence of various drugs. It is evident that the arrhythmia lasted progressively longer as the norepinephrine overflow increased. Moreover, agents that increased norepinephrine release by ≈50%, either via α2-adrenoceptor stimulation or adenosine A1-receptor blockade, more than doubled the duration of ventricular fibrillation. In contrast, agents that stimulate histamine H3- and adenosine A1-receptors or block α2-adrenoceptors, norepinephrine neuronal uptake (uptake1), and Na+-H+ exchanger all decreased norepinephrine release and shortened or abolished ventricular fibrillation.
Our data clearly indicate that histamine H3-receptor activation attenuates carrier-mediated norepinephrine release caused by protracted myocardial ischemia and suggest that this effect may be associated with an inhibition of the Na+-H+ exchanger in sympathetic nerve endings. Analogous to histamine H3-receptor activation, α2-adrenoceptor blockade and adenosine A1-receptor stimulation also attenuated carrier-mediated norepinephrine release, most likely by inhibiting the Na+-H+ exchanger. Whatever the pharmacological agent used to positively or negatively modulate norepinephrine release, we consistently found that as the norepinephrine overflow increased or decreased from control levels, reperfusion arrhythmias became more or less severe, respectively, thus implicating norepinephrine release as a major cause of reperfusion arrhythmias.
We found that subjecting isolated guinea pig hearts to global ischemia for 20 minutes greatly augmented norepinephrine overflow during reperfusion. Depending on the length of the ischemic episode, different mechanisms are likely to mediate the enhancement in norepinephrine release. Relatively short ischemic periods (ie, ≈10 minutes) are thought to augment norepinephrine exocytosis from adrenergic nerves.3 With longer-lasting ischemia (ie, ≈20 minutes), metabolically deprived ion pumps fail and H+ accumulates in sympathetic nerve endings, leading to a compensatory activation of the Na+-H+ antiporter and intracellular Na+ accumulation.3 4 This, coupled with a decreased vesicular storage of norepinephrine and its consequent accumulation in the axoplasm, causes a reversal of uptake1, such that norepinephrine is actively carried out of the sympathetic nerve terminal (carrier-mediated norepinephrine release).3 4 5 With more protracted ischemia (ie, >30 minutes), cell lysis occurs, and norepinephrine leaks out of the neurons.3
Having previously demonstrated that histamine H3-receptors attenuate the enhancement in norepinephrine exocytosis, which occurs after 10-minute global ischemia,2 we have now assessed whether H3-receptor activation modifies the marked increase in norepinephrine overflow that follows 20-minute global ischemia and is accompanied by severe reperfusion arrhythmias. That a reversal of the neuronal norepinephrine uptake was the cause of this release was verified by the finding that the norepinephrine transport inhibitor desipramine blocked the increase in overflow (see Fig 1⇑). If enhanced exocytosis had been the mechanism involved in the increased release of norepinephrine in these studies, then desipramine would have potentiated norepinephrine overflow.2 Moreover, that an activation of the Na+-H+ exchanger created the conditions that favored a reversal of the neuronal uptake was demonstrated by our finding that the antiporter inhibitor EIPA blocked the increase in norepinephrine overflow (see Fig 1⇑). Notably, desipramine and EIPA each prevented the occurrence of reperfusion arrhythmias. The fact that a marked attenuation of norepinephrine release coincided with the absence of arrhythmias implicates norepinephrine as a major cause of reperfusion arrhythmias. Indeed, we found that the severity of reperfusion arrhythmias was directly associated with an increase in norepinephrine overflow (see Fig 6⇑).
The selective histamine-H3 receptor agonist imetit9 markedly attenuated the increase in norepinephrine release that occurred during reperfusion after 20-minute global ischemia and that was due to the reversal of uptake1. Because the selective H3-receptor antagonist thioperamide10 prevented the effects of imetit, our findings indicate that activation of histamine H3-receptors inhibits carrier-mediated norepinephrine release. These modulatory receptors are most likely located on sympathetic nerve endings, since their presence can be directly demonstrated in isolated adrenergic terminals (ie, cardiac synaptosomes).1 As to the mechanism of the H3-receptor–mediated inhibition of norepinephrine release in the setting of protracted myocardial ischemia, our data suggest an association between H3-receptor activation and Na+-H+ antiport inhibition. Indeed, we found that subthreshold concentrations of imetit and EIPA acted synergistically to significantly reduce norepinephrine release when added in combination.
We also found that activation of adenosine A1-receptors markedly attenuated norepinephrine release, whereas their inhibition potentiated it. Furthermore, subthreshold concentrations of the adenosine A1-receptor agonist CPA and the antiporter inhibitor EIPA markedly inhibited norepinephrine overflow when added in combination (see Fig 4B⇑). Thus, it is possible that the adenosine A1-receptor–induced inhibition of carrier-mediated norepinephrine release involves an antagonism of the Na+-H+ exchanger, similar to what we postulate for the histamine H3-receptor.
The hypothesis that the H3-receptor–mediated inhibition of carrier-mediated norepinephrine release involves a decrease in the Na+-H+ exchanger activity is further supported by our findings with selective ligands for α2-adrenoceptors. Indeed, α2-receptor activation, which has been previously associated with a stimulation of the Na+-H+ exchanger,11 enhanced norepinephrine release. This action was inhibited by α2-adrenoceptor blockade (see Fig 3A⇑). Moreover, α2-adrenoceptor blockade by itself inhibited the norepinephrine overflow caused by ischemia/reperfusion, an indication that norepinephrine released in this condition activates prejunctional α2-adrenoceptors and thus the antiporter. Indeed, subthreshold concentrations of the α2-adrenoceptor blocker yohimbine and the antiporter inhibitor EIPA markedly inhibited norepinephrine overflow when added in combination (see Fig 3B⇑).
To date, very little is known regarding the transductional mechanisms involved in the effects mediated by histamine H3-receptors. In a gastric epithelial tumor cell line, activation of H3-receptors was tied to the inhibition of phosphoinositide metabolism.12 It is conceivable that during prolonged myocardial ischemia, H3-receptor activation may also lead to the inhibition of phosphoinositide turnover in sympathetic nerve endings, which would result in a decrease in protein kinase C activity and inhibition of the Na+-H+ antiporter. Inasmuch as selective adenosine A1-agonists reportedly inhibit phosphoinositide turnover in neural tissue,13 adenosine A1-receptors could also attenuate carrier-mediated norepinephrine release by inhibiting the antiporter. Conversely, the stimulation of norepinephrine release by α2-adrenoceptor activation, and its inhibition by α2-adrenoceptor blockade, could be explained by the reported α2-adrenoceptor–induced stimulation of the antiporter.11
During reperfusion, there was a marked increase in the overflow of histamine into the coronary effluent. Accordingly, we would have assumed H3-receptors to be activated by the endogenous ligand, similar to what we had previously observed in hearts subjected to 10-minute global ischemia.2 However, the H3-receptor antagonist thioperamide did not by itself potentiate norepinephrine overflow after 20-minute global ischemia (see Fig 2A⇑), suggesting a lack of H3-receptor activation. The finding that thioperamide potentiated exocytotic,2 but not carrier-mediated, norepinephrine overflow could be interpreted as an indication that two subsets of H3-receptors, with different affinities and perhaps transductional mechanisms, may be involved in the modulation of exocytotic and carrier-mediated norepinephrine release. The first subset could be coupled to N-type Ca2+ channels via a pertussis toxin–sensitive Gi/Go protein,14 whereas the other could be coupled to the Na+-H+ exchanger via a G protein, phospholipase C, and phosphoinositide turnover.
Accordingly, we hypothesize that high-affinity H3-receptors inhibit the enhanced norepinephrine exocytosis after 10-minute global ischemia. After 20-minute global ischemia, these receptors become desensitized as a result of the more prolonged histamine release. Thus, only nonactivated low-affinity H3-receptors would remain available for stimulation by selective H3-agonists. No potentiation of carrier-mediated norepinephrine release occurred with selective H3-receptor antagonists, because according to this hypothesis, low-affinity H3-receptors would not be occupied by endogenous histamine. Nevertheless, imetit, which has a 60-fold higher affinity for the H3-receptor than does histamine,9 is still able to bind to the low-affinity H3-receptors and inhibit norepinephrine release. The existence of two H3-receptor subtypes has indeed been proposed in other tissues.15 16 Whether these receptor subtypes exist in cardiac adrenergic nerve terminals and whether they may operate via distinct signal transduction mechanisms remain to be determined.
H3-receptor activation markedly reduced the incidence of ventricular fibrillation during reperfusion and greatly shortened its duration in the remaining cases, demonstrating that these modulatory receptors mitigate the dysfunctional consequences of prolonged myocardial ischemia. Moreover, α2-adrenoceptor blockade and adenosine A1-receptor activation prevented reperfusion arrhythmias. Because all of these antiarrhythmic effects coincided with a marked reduction in norepinephrine overflow, our findings highlight the importance of nonexocytotic norepinephrine release17 in the generation of reperfusion arrhythmias.
Activation of adenosine A1-receptors with CPA caused a greater inhibition of norepinephrine release than did activation of H3-receptors with imetit (compare Figs 2A⇑ and 4A⇑). Also, the effect of CPA in combination with EIPA was greater than that of imetit in combination with EIPA (compare Figs 2B⇑ and 4B⇑). This could be interpreted to suggest that adenosine plays a more important role than histamine in negatively modulating carrier-mediated norepinephrine release in protracted ischemia/reperfusion, possibly because high-affinity H3-receptors have become desensitized in these conditions. Nevertheless, the availability of low-affinity H3-receptors would still enable the use of potent H3-agonists for the prevention of reperfusion arrhythmias.
Indeed, although H3- and adenosine A1-receptor stimulation, as well as α2-adrenoceptor blockade, reduced carrier-mediated norepinephrine release, H3-receptor stimulation may be more advantageous than adenosine A1-receptor activation or α2-adrenoceptor blockade. Unlike adenosine A1-receptor stimulation,18 H3-receptor activation has no negative chronotropic and dromotropic effects. Furthermore, H3-receptors modulate both exocytotic2 and carrier-mediated norepinephrine release associated with acute and protracted myocardial ischemia, respectively. In contrast, α2-adrenoceptor blockade inhibits carrier-mediated norepinephrine release but enhances norepinephrine exocytosis.2
Thus, we have found a very strong correlation between the amount of norepinephrine release and the severity of reperfusion arrhythmias. We have previously reported the presence of H3-receptors on sympathetic nerve endings in the human heart.1 Furthermore, others have demonstrated carrier-mediated norepinephrine release in ischemic human heart specimens.5 Collectively, these findings may further the development of novel pharmacological means to reduce reperfusion arrhythmias in the clinical setting.
In conclusion, histamine H3-receptor activation attenuates both exocytotic1 2 14 and carrier-mediated norepinephrine release associated with acute and protracted myocardial ischemia, respectively. The mechanisms of action are likely to differ. Inhibition of exocytosis probably involves a decreased Ca2+ entry via N-type channels,14 whereas inhibition of carrier-mediated norepinephrine release probably involves an antagonism of the Na+-H+ exchanger.
This study was supported by National Institutes of Health grants HL-34215 and HL-46403. Dr Imamura was a Fellow of the American Heart Association, New York City Affiliate, Inc.
Presented as preliminary data at the Experimental Biology ’95 meeting in Atlanta, Ga, and published in abstract form (FASEB J. 1995;9:A9).
- Received September 13, 1995.
- Accepted December 5, 1995.
- © 1996 American Heart Association, Inc.
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