Unexpected drug effects on autonomic function during myocardial ischaemia

Regulation of the Cardiovascular System by Histamine

Histamine mediates a wide range of cellular responses, including allergic and inflammatory reactions, gastric acid secretion, and neurotransmission in the central nervous system. Histamine also exerts a series of actions upon the cardiovascular system but may not normally play a significant role in regulating cardiovascular function. During tissue injury, inflammation, and allergic responses, mast cells (or non-mast cells) within the tissues can release large amounts of histamine that leads to noticeable cardiovascular effects. Owing to intensive research during several decades, the distribution, function, and pathophysiological role of cardiovascular H₁- and H₂-receptors has become recognized adequately. Besides the recognized H₁- and H₂-receptor-mediated cardiovascular responses, novel roles of H₃- and H₄-receptors in cardiovascular physiology and pathophysiology have been identified over the last decade. In this review, we describe recent advances in our understanding of cardiovascular function and dysfunction mediated by histamine receptors, including H₃- and H₄-receptors, their potential mechanisms of action, and their pathological significance.

Blockade of spinal nerves attenuates myocardial apoptosis in acute myocardial ischaemia/infarction in rats

BACKGROUND AND OBJECTIVES

Evidence showed involvement of neural activity in acute myocardial ischaemia but the knowledge about the role in the pathology is still limited. The objective of this study was to examine whether low cervical and upper thoracic epidural anaesthesia could reduce apoptosis of the myocardium induced by 3-h and 6-h permanent coronary artery occlusion (CAO) in rats.

METHODS

The left anterior descending branch of coronary artery was ligated in anaesthetized Sprague-Dawley rats with and without precondition of epidural anaesthesia at the level of upper thoracic segments. Apoptosis of the myocardium insulted by acute myocardial ischaemia and infarction was assessed using terminal deoxynucleotidyl transferase-mediated dUTP nick endlabelling and caspase assay.

RESULTS

The apoptotic rates of the myocytes were 16.90 +/- 1.38 and 19.75 +/- 1.47% in the groups of CAO 3 h and CAO 6 h, respectively, whereas in the epidural groups, 10.25 +/- 1.07 and 13.05 +/- 1.19% of the apoptotic rates were detected at 3 and 6 h of CAO, significantly lower than that in the time-matched CAO animals without epidural anaesthesia (P < 0.01). Lower caspase-3 activity was observed in epidural anaesthesia group, 1.41 +/- 0.11 folds of the sham (versus 1.86 +/- 0.14 folds in CAO-3 h group) and 2.0 +/- 0.18 folds (versus 2.87 +/- 0.20 folds in CAO-6 h group) at 3 and 6 h of CAO, respectively, which were statistically significant (P < 0.05).

CONCLUSION

The findings may indicate blockade of spinal nerves could reduce apoptosis of myocardium in acute myocardial ischaemia and infarction.

Cardiac mast cell-derived renin promotes local angiotensin formation, norepinephrine release, and arrhythmias in ischemia/reperfusion

Having identified renin in cardiac mast cells, we assessed whether its release leads to cardiac dysfunction. In Langendorff-perfused guinea pig hearts, mast cell degranulation with compound 48/80 released Ang I-forming activity. This activity was blocked by the selective renin inhibitor BILA2157, indicating that renin was responsible for Ang I formation. Local generation of cardiac Ang II from mast cell-derived renin also elicited norepinephrine release from isolated sympathetic nerve terminals. This action was mediated by Ang II-type 1 (AT1) receptors. In 2 models of ischemia/reperfusion using Langendorff-perfused guinea pig and mouse hearts, a significant coronary spillover of renin and norepinephrine was observed. In both models, this was accompanied by ventricular fibrillation. Mast cell stabilization with cromolyn or lodoxamide markedly reduced active renin overflow and attenuated both norepinephrine release and arrhythmias. Similar cardioprotection was observed in guinea pig hearts treated with BILA2157 or the AT1 receptor antagonist EXP3174. Renin overflow and arrhythmias in ischemia/reperfusion were much less prominent in hearts of mast cell-deficient mice than in control hearts. Thus, mast cell-derived renin is pivotal for activating a cardiac renin-angiotensin system leading to excessive norepinephrine release in ischemia/reperfusion. Mast cell-derived renin may be a useful therapeutic target for hyperadrenergic dysfunctions, such as arrhythmias, sudden cardiac death, myocardial ischemia, and congestive heart failure.

beta 2-Adrenergic stimulation is involved in the contractile dysfunction of the stunned heart

Endogenous catecholamines released during myocardial ischemia have been considered both to aggravate cell injury and exacerbate arrhythmias and to exert a protective action on the post-ischemic contractile function. The present work was addressed to look for evidence to explain this controversy. The effects of cardiac catecholamine depletion and of alpha- and beta-adrenoceptor (AR) blockade on the post-ischemic contractile dysfunction, as well as its possible relationship with cardiac oxidative stress, were studied in isolated and perfused rat hearts submitted to 20 min of ischemia and 30 min of reperfusion (stunning). Catecholamine depletion improves the contractile recovery in the stunned heart. This mechanical effect was associated with decreased levels of lipid peroxidation. A similar enhancement of the contractile function during reperfusion was detected after the simultaneous blockade of alpha 1- and beta-ARs with prazosin plus propranolol. To ascertain which specific AR pathway was involved in the effects of catecholamines on the stunned heart, selective AR blockers, prazosin (alpha 1-blocker), atenolol (beta 1-blocker), ICI 118,551 (beta 2-blocker) and selective inhibitors of Gi-PI3K pathway (pertussis toxin and wortmannin) were alternatively combined. The results indicate that catecholamines released during ischemia exert a dual action on the contractile behavior of the stunned heart: a deleterious effect, related to the activation of the beta 2-AR-Gi-PI3K-pathway, which was counteracted by a beneficial effect, triggered by the stimulation of alpha 1-AR. Neither the depression nor the enhancement of the post-ischemic contractile recovery were related with the increase in ROS formation induced by endogenous catecholamines.

Increased severity of reperfusion arrhythmias in mouse hearts lacking histamine H3-receptors

We had previously reported that activation of histamine H(3)-receptors (H(3)R) on cardiac adrenergic nerve terminals decreases norepinephrine (NE) overflow from ischemic hearts and alleviates reperfusion arrhythmias. Thus, we used transgenic mice lacking H(3)R (H(3)R(-/-)) to investigate whether ischemic arrhythmias might be more severe in H(3)R(-/-) hearts than in hearts with intact H(3)R (H(3)R(+/+)). We report a greater incidence and longer duration of ventricular fibrillation (VF) in H(3)R(-/-) hearts subjected to ischemia. VF duration was linearly correlated with NE overflow, suggesting a possible cause-effect relationship between magnitude of NE release and severity of reperfusion arrhythmias. Thus, our findings strengthen a protective antiarrhythmic role of H(3)R in myocardial ischemia. Since malignant tachyarrhythmias cause sudden death in ischemic heart disease, attenuation of NE release by selective H(3)R agonists may represent a new approach in the prevention and treatment of ischemic arrhythmias.

Ectonucleotidase in sympathetic nerve endings modulates ATP and norepinephrine exocytosis in myocardial ischemia

We recently reported that ATP, coreleased with norepinephrine (NE) from cardiac sympathetic nerves, increases NE exocytosis via a positive feedback mechanism. A neuronal ectonucleotidase (E-NTPDase) metabolizes the released ATP, decreasing NE exocytosis. Excessive NE release in myocardial ischemia exacerbates cardiac dysfunction. Thus, we studied whether the ATP-mediated autocrine amplification of NE release is operative in ischemia and, if so, whether it can be modulated by E-NTPDase and its recombinant equivalent, solCD39. Isolated, guinea pig hearts underwent 10- or 20-min ischemic episodes, wherein NE was released by exocytosis and reversal of the NE transporter, respectively. Furthermore, to restrict the role of E-NTPDase to transmitter ATP, sympathetic nerve endings were isolated (cardiac synaptosomes) and subjected to increasing periods of ischemia. Availability of released ATP at the nerve terminals was either increased via E-NTPDase inhibition or diminished by enhancing ATP hydrolysis with solCD39. P2X receptor blockade with PPADS was used to attenuate the effects of released ATP. We found that, in short-term ischemia (but, as anticipated, not in protracted ischemia, where NE release is carrier-mediated), ATP exocytosis was linearly correlated with that of NE. This indicates that by limiting the availability of ATP at sympathetic terminals, E-NTPDase effectively attenuates NE exocytosis in myocardial ischemia. Our findings suggest a key role for neuronal E-NTPDase in the control of adrenergic function in the ischemic heart. Because excessive NE release is an established cause of dysfunction in ischemic heart disease, solCD39 may offer a novel therapeutic approach to myocardial ischemia and its consequences.

Norepinephrine release from the ischemic heart is greatly enhanced in mice lacking histamine H3 receptors

We previously reported that histamine H(3) receptors (H(3)Rs) are present in cardiac sympathetic nerve endings (cSNE) of animals and humans, where they attenuate norepinephrine (NE) release in normal and hyperadrenergic states, such as myocardial ischemia. The recent creation of a transgenic line of mice lacking H(3)R provided us with the opportunity to assess the relevance of H(3)R in the ischemic heart. We isolated SNE from hearts of wild-type (H(3)R(+/+)) and knockout (H(3)R(-/-)) mice and found that basal NE release from H(3)R(-/-) cSNE was approximately 60% greater than that from H(3)R(+/+) cSNE. NE exocytosis evoked by K(+)-induced depolarization of cSNE from H(3)R(+/+) mice was attenuated by activation of either H(3)R or adenosine A(1) receptors (A(1)R). In contrast, NE release from cSNE of H(3)R(-/-) was unaffected by H(3)R agonists, but it was still attenuated by A(1)R activation. When isolated mouse hearts were subjected to ischemia for 20 min, NE overflow into the coronaries was 2-fold greater in the H(3)R(-/-) hearts than in those from H(3)R(+/+) mice. Furthermore, whereas stimulation of H(3)R or A(1)R reduced ischemic NE overflow from H(3)R(+/+) hearts by 50%, only A(1)R, but not H(3)R activation, reduced NE release in H(3)R(-/-). Our data demonstrate that NE release from cSNE can be modulated by various heteroinhibitory receptors (e.g., H(3)R and A(1)R) and that H(3)Rs are particularly important in modulating NE release in myocardial ischemia. Inasmuch as excessive NE release is clinically recognized as a major cause of arrhythmic cardiac dysfunction, our findings reveal a significant cardioprotective role of H(3)R on cSNE.

Carrier-mediated norepinephrine release and reperfusion arrhythmias induced by protracted ischemia in isolated perfused guinea pig hearts: effect of presynaptic modulation by alpha(2)-adrenoceptor in mild hypothermic ischemia

Yohimbine, an alpha(2)-adrenoceptor antagonist, has been reported to protect hypoxic myocardium and inhibit carrier-mediated norepinephrine (NE) release and reperfusion arrhythmias (ventricular fibrillation; VF) in normothermic ischemia. In heart surgery, mild hypothermic (tepid) cardioplegia has been reported to reduce metabolic demand and permit immediate recovery of cardiac function. Therefore, we determined the effect of yohimbine on NE release and reperfusion arrhythmias in isolated perfused guinea pig hearts of tepid temperature (32 degrees C) ischemia model. Stepwise increase of global ischemia period (20, 40, and 60 min) induced a progressive increase of NE release and duration of VF. Neuronal uptake 1 inhibitor desipramine (100 nM) and Na(+)-H(+) exchanger inhibitor 5-N-ethyl-N-isopropyl-amiloride (10 microM) decreased NE and VF in 60-min hypothermic ischemia. This indicated that NE release induced by protracted tepid ischemia was due to carrier-mediated release. Yohimbine (1 microM) markedly reduced NE release and VF (p < 0.01 versus control) and 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine [UK 14,304 (UK); 10 microM], an alpha(2)-adrenoceptor agonist, increased NE release and VF (p < 0.01 versus control). Yohimbine (1 microM) prevented the potentiated effect of UK (10 microM) in hypothermia (p < 0.01 versus UK). Our findings indicate that presynaptic reduction of carrier-mediated NE release seems to be one of the most important factors controlling reperfusion arrhythmias, and alpha(2)-adrenoceptor blockade by yohimbine (1 microM) in tepid ischemia may contribute to effective myocardial protection in terms of NE release and reperfusion arrhythmia.

Ischemia promotes renin activation and angiotensin formation in sympathetic nerve terminals isolated from the human heart: contribution to carrier-mediated norepinephrine release

We recently reported that in the ischemic human heart, locally formed angiotensin II activates angiotensin II type 1 (AT(1)) receptors on sympathetic nerve terminals, promoting reversal of the norepinephrine transporter in an outward direction (i.e., carrier-mediated norepinephrine release). The purpose of this study was to assess whether cardiac sympathetic nerve endings contribute to local angiotensin II formation, in addition to being a target of angiotensin II. To this end, we isolated sympathetic nerve endings (cardiac synaptosomes) from surgical specimens of human right atrium and incubated them in ischemic conditions (95% N(2,) sodium dithionite, and no glucose for 70 min). These synaptosomes released large amounts of endogenous norepinephrine via a carrier-mediated mechanism, as evidenced by the inhibitory effect of desipramine on this process. Norepinephrine release was further enhanced by preincubation of synaptosomes with angiotensinogen and was prevented by two renin inhibitors, pepstatin-A and BILA 2157BS, as well as by the angiotensin-converting enzyme inhibitor enalaprilat and the AT(1) receptor antagonist EXP 3174 [2-N-butyl-4-chloro-1-[2'-(1H-tetrazol-5-yl)biphenyl-4-yl] methyl]imidazole-5-carboxylic acid]. Western blot analysis revealed the presence of renin in cardiac sympathetic nerve terminals; renin abundance increased ~3-fold during ischemia. Thus, renin is rapidly activated during ischemia in cardiac sympathetic nerve terminals, and this process eventually culminates in angiotensin II formation, stimulation of AT(1) receptors, and carrier-mediated norepinephrine release. Our findings uncover a novel autocrine/paracrine mechanism whereby angiotensin II, formed at adrenergic nerve endings in myocardial ischemia, elicits carrier-mediated norepinephrine release by activating adjacent AT(1) receptors.

EctoNucleotidase in cardiac sympathetic nerve endings modulates ATP-mediated feedback of norepinephrine release

ATP, coreleased with norepinephrine, affects adrenergic transmission by acting on purinoceptors at sympathetic nerve endings. Ectonucleotidases terminate the actions of ATP. Previously, we had preliminary evidence for ectonucleotidase activity in cardiac sympathetic nerve terminals. Therefore, we investigated whether this ectonucleotidase might influence norepinephrine release in the heart. Sympathetic nerve endings isolated from guinea pig heart (cardiac synaptosomes) were rich in Ca(2+)-dependent ectonucleotidase activity, as measured by metabolism of exogenously added radiolabeled ATP or ADP. By its inhibitor profile, ectonucleotidase resembled ectonucleoside triphosphate diphosphohydrolase 1 (E-NTPDase1). Exogenous ATP elicited concentration-dependent norepinephrine release from cardiac synaptosomes (EC(50) 0.96 microM). This release was antagonized by the P2X receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (10 microM) and potentiated by the P2Y receptor antagonist 2'-deoxy-N(6)-methyladenosine-3',5'-diphosphate (MRS 2179) (30 nM). Norepinephrine release promoted by ATP was also potentiated by the nucleotidase inhibitor 6-N,N-diethyl-beta-gamma-dibromomethylene-D-adenosine-5'-triphosphate (ARL67156) (30 microM) and blocked by a recombinant, soluble form of human E-NTPDase1 (solCD39). In contrast, ARL67156 had no effect on norepinephrine release induced by the nonhydrolyzable analog, alpha, beta-methyleneadenosine-5'-triphosphate (alpha,beta-MeATP). Depolarization of cardiac synaptosomes with K(+) elicited release of endogenous norepinephrine. This was attenuated by PPADS and solCD39 and potentiated by MRS 2179 and ARL67156. Importantly, our results demonstrate that facilitation of ATP-induced norepinephrine release from cardiac sympathetic nerves is a composite of two autocrine components: positive, mediated by P2X receptors, and negative, mediated by P2Y receptors. Modulation of norepinephrine release by coreleased ATP is terminated by endogenous as well as exogenous ectonucleotidase. We propose that ectonucleotidase control of norepinephrine release should provide cardiac protection in hyperadrenergic states such as myocardial ischemia.

Oral administration of Lactobacillus induces cardioprotection

OBJECTIVES

Our goal was to investigate the potential of oral administration of Lactobacillus to induce cardioprotection against ischemia-reperfusion injury in rat hearts, and to determine whether this protection is associated with myocardial catalase activation and heat shock protein 70 expression.

DESIGN

Lactobacillus was administered orally to rats on a daily basis, from 1 to 21 days before global ischemia. Global ischemia was produced by full-stop flow in isolated Langendorff-perfused rat hearts for 20 minutes and was followed by reperfusion. Similarly, saline was administered to control animals.

RESULTS

Lactobacillus significantly reduced reperfusion tachyarrhythmia and improved functional recovery of the ischemic rat hearts. These cardioprotective effects were associated with reduction of norepinephrine release at the first minute of reperfusion, activation of myocardial catalase, and overexpression of 70 kd heat stress protein at ischemia and reperfusion (p < 0.05).

CONCLUSIONS

Oral administration of Lactobacillus produces marked cardioprotection against ischemia-reperfusion injury. This effect is attributed to activation of the cellular defense system, manifested by activation of the antioxidant pathway, and by expression of protective proteins. Norepinephrine is involved in this process. The results of this study suggest that Lactobacillus, which is generally considered safe, could serve as a basis for the development of a new agent for preventive therapy of various ischemic heart syndromes.

Coupling of histamine H3 receptors to neuronal Na+/H+ exchange: a novel protective mechanism in myocardial ischemia

In myocardial ischemia, adrenergic nerves release excessive amounts of norepinephrine (NE), causing dysfunction and arrhythmias. With anoxia and the concomitant ATP depletion, vesicular storage of NE is impaired, resulting in accumulation of free NE in the axoplasm of sympathetic nerves. Intraneuronal acidosis activates the Na(+)/H(+) exchanger (NHE), leading to increased Na(+) entry in the nerve terminals. These conditions favor availability of the NE transporter to the axoplasmic side of the membrane, causing massive carrier-mediated efflux of free NE. Neuronal NHE activation is pivotal in this process; NHE inhibitors attenuate carrier-mediated NE release. We previously reported that activation of histamine H(3) receptors (H(3)R) on cardiac sympathetic nerves also reduces carrier-mediated NE release and alleviates arrhythmias. Thus, H(3)R activation may be negatively coupled to NHE. We tested this hypothesis in individual human SKNMC neuroblastoma cells stably transfected with H(3)R cDNA, loaded with the intracellular pH (pH(i)) indicator BCECF. These cells possess amiloride-sensitive NHE. NHE activity was measured as the rate of Na(+)-dependent pH(i) recovery in response to an acute acid pulse (NH(4)Cl). We found that the selective H(3)R-agonist imetit markedly diminished NHE activity, and so did the amiloride derivative EIPA. The selective H(3)R antagonist thioperamide abolished the imetit-induced NHE attenuation. Thus, our results provide a link between H(3)R and NHE, which may limit the excessive release of NE during protracted myocardial ischemia. Our previous and present findings uncover a novel mechanism of cardioprotection: NHE inhibition in cardiac adrenergic neurons as a means to prevent ischemic arrhythmias associated with carrier-mediated NE release.

Therapeutic potential of H(3)-receptor agonists in myocardial infarction

Sympathetic over-activity accompanied by excessive noradrenaline (NA) release within the heart is a recognised cause of dysfunction in myocardial ischaemia. Myocardial infarction is often accompanied by arrhythmias with high morbidity and mortality. Indeed, NA enhances intracellular Ca(2+) by increasing its influx through voltage-dependent channels, mobilising it from intracellular stores and favouring its inward transport by Na(+)/Ca(2+) exchange. Ca(2+) overload eventually results in dysrhythmia and uncoordinated myocyte contraction. Moreover, NA increases metabolic demand. In concert with other contributing factors, this will aggravate the primary ischaemia and initiate a vicious cycle that can culminate in myocardial damage and heart failure. Therefore, reduction of NA release from cardiac sympathetic nerves is an important protective measure. Adrenergic nerves possess inhibitory receptors, such as alpha(2)-adrenoceptors, adenosine A(1)-receptors and histamine H(3)-receptors (H(3)R). In myocardial infarction, NA is released by both exocytotic (Ca(2+)-dependent) and carrier-mediated (Na(+)/H(+) exchange-dependent) mechanisms, associated with short-term and protracted ischaemia, respectively. Unlike alpha(2)-adrenoceptor agonists that reduce NA exocytosis, but enhance carrier-mediated NA release, H(3)R agonists inhibit both exocytotic and carrier-mediated NA release. Moreover, unlike adenosine A(1)-receptor agonists, H(3)R agonists do not depress sinoatrial and atrioventricular nodes, nor cause bronchoconstriction. Therefore, stimulation of H(3)R on cardiac sympathetic nerve endings is an important new way to protect the heart from the consequences of ischaemia and infarction. Although H(3)R agonists alleviate reperfusion arrhythmias in isolated hearts by reducing NA release, this protective action needs to be demonstrated in classical in vivo models of occlusion/reperfusion. Regardless, H(3)R agonists offer the promise of a novel strategy in the treatment of myocardial ischaemia and infarction.

Modulation of adrenergic receptors during left ventricular hypertrophy development and after regression by captopril

The objective of this study was to analyze adrenergic receptors during cardiac hypertrophy development, after establishment of cardiac hypertrophy and after regression of cardiac hypertrophy by an angiotensin-converting enzyme inhibitor. Left ventricular hypertrophy (LVH) was induced by abdominal aortic stenosis. After surgery, plasma norepinephrine concentrations (PNE) and left ventricular adrenergic receptors from rat hearts subjected to aortic stenosis were assessed during cardiac hypertrophy development (at 3, 7, 15, and 30 days of aortic stenosis), once cardiac hypertrophy had been established (7 and 14 weeks after the stenosis) and after regression of cardiac hypertrophy by an antihypertensive dose (200 mg/kg/day) of captopril. The presence of LVH was observed from day 7 after stenosis. PNE had significantly increased after 15 days but returned to control values 30 days after surgery. The density of alpha1-adrenoceptors was found to decrease with development of hypertrophy. Once hypertrophy had been established, 7 weeks from stenosis, PNE was not different from control; however, the density of alpha1-adrenoceptors continued to diminish, whereas PNE and the density of beta-adrenoceptors were no different from control values. Fourteen weeks after stenosis, a significant decrease in PNE was recorded, and no change in alpha1- but an increase in beta-adrenoceptors was observed. LVH was reversed by treatment with captopril; PNE was similar in control and stenosed treated animals. The density of alpha1-adrenoceptors was decreased when compared with control animals, and no change in the density of beta-adrenoceptors was observed with treatment. In conclusion, a decrease of alpha1-adrenoceptors was associated with LVH development and earlier stages of established cardiac hypertrophy. Later stages of established cardiac hypertrophy were characterized by no change in alpha1- and an increase in beta-adrenoceptors. Treatment with captopril induced LVH regression and decreased the number of alpha1-adrenoceptors without any change in beta-adrenoceptors.

Catechol-O-methyltransferase activity in CHO cells expressing norepinephrine transporter

1. We examined the existence of catecholamine metabolizing enzymes (catechol-O-methyltransferase, COMT, and monoamine oxidase, MAO) in CHO cells transfected with norepinephrine (NE) transporter (NET) cDNA. 2. NET activity was studied by incubating cells with [3H]-NE (0. 5 microCi ml-1, 20 min) in a Na+ containing medium. Incubation with [3H]-NE lead to [3H] accumulation at 47797+/-4864 d.p.m. per well. Specific inhibitors of NET abolished this uptake. 3. During post-uptake incubation, [3H] leaked rapidly from cells and the extracellular phase comprised 89% of total radioactivity within 40 min. Both [3H] retention and [3H] 'leakage' were largely unaffected by inhibitors for MAO. In contrast, COMT inhibitors, U-0521 and Ro 41-0960, dose-dependently increased intracellular [3H]-NE retention with a maximal increase of 4.5 fold. The EC50 for Ro 41-0960 was 139-times lower than that of U-0521. U-0521 largely inhibited [3H] 'leakage' and doubled the apparent Vmax for [3H]-NE uptake. 4. Addition of U-0521 during uptake incubation increased intracellular NE content by 8 fold. Normetanephrine, the COMT-dependent metabolite of NE, was formed in large quantities during post-uptake incubation. U-0521 significantly inhibited the formation of NMN with an equal preservation of intracellular NE. 5. CHO cells expressing NET possess COMT activity, which is responsible for the metabolism of NE to form lipophilic metabolite normetanephrine. The apparent 'properties' of the NET function expressed in CHO cells changed, after inhibition of COMT, in such a way closer to that described in the native neuronal preparations.

Protection of neuronal uptake-1 inhibitors in ischemic and anoxic hearts by norepinephrine-dependent and -independent mechanisms

Cardiac ischemia and anoxia induce massive norepinephrine (NE) release, which is mediated by a reverse operation of uptake-1 and can be suppressed by uptake-1 inhibitors. We studied effects of uptake-1 inhibitors on incidence of ventricular fibrillation (VF%) and myocardial contracture in perfused rat hearts under ischemic or anoxic conditions. NE release occurred in hearts during ischemia or anoxia and was largely inhibited by desipramine, imipramine, and cocaine. The generation of inositol 1,4,5-trisphosphate (InsP3) during reperfusion also was abolished by desipramine. During anoxia/reoxygenation, VF (93 and 71%, respectively) and myocardial contracture occurred and were significantly inhibited by desipramine and by NE depletion. Regional ischemia and reperfusion induced high VF% (86 and 100%, respectively), which was reduced or abolished by desipramine and imipramine at 0.03 and 0.3 microM. During the ischemic phase, cocaine was similarly antiarrhythmic, as was a combination of timolol and prazosin, but NE depletion was not. In NE-depleted hearts, cocaine or the combination of timolol and prazosin showed limited effect on VF%, whereas both desipramine and imipramine abolished VF. In anesthetized rats in vivo, ischemic VF% was reduced by desipramine (30 vs. 92%; p < 0.01). In conclusion, uptake-1 inhibitors protect hearts against ischemia/reperfusion- and anoxia/reoxygenation-induced arrhythmias, partly because of the inhibition of locally mediated NE release. Other actions of desipramine and imipramine may contribute to the overall efficacy.

Activation of histamine H3-receptors inhibits carrier-mediated norepinephrine release during protracted myocardial ischemia. Comparison with adenosine A1-receptors and alpha2-adrenoceptors

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 pH(i) 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 approximately 80% by desipramine (10 nmol/L) and 70% by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA, 10 micromol/L), inhibitors of norepinephrine uptake and NHE, respectively. The H3-receptor agonist imetit (0.1 micromol/L) decreased carrier-mediated norepinephrine release by approximately 50%. This effect was blocked by the H3-receptor antagonist thioperamide (0.3 micromol/L), indicating that H-receptor activation inhibits carrier-mediated norepinephrine release. At lower concentrations, imetit (10 nmol/L) or EIPA (3 micromol/L) did not inhibit carrier-mediated norepinephrine release. However, a 25% inhibition occurred with imetit (10 nmol/L) and EIPA (3 micromol/L) combined. This synergism suggests an association between H-receptors and NHE. Conceivably, activation of H-receptors may lead to inhibition of NHE. In fact, alpha2-adrenoceptor activation, which is known to stimulate NHE, enhanced norepinephrine release, whereas alpha2-adrenoceptor blockade attenuated it. Furthermore, activation of adenosine A1-receptors markedly attenuated norepinephrine release, whereas their inhibition potentiated it. Because norepinephrine directly correlated with the severity of reperfusion arrhythmia and imetit reduced the incidence of ventricular fibrillation by 50%, our findings with H-receptor agonists may further the development of novel pharmacological means to reduce reperfusion arrhythmias in the clinical setting.