Neuropeptide Y is a vasoconstrictor of human coronary arteries

Neuropeptide Y (NPY) is a 36-amino-acid polypeptide which coexists with catecholamines in many adrenergic and noradrenergic neurons. It has been demonstrated to exert pressor effects in the perfused guinea pig heart and to constrict large cerebral and coronary blood vessels in animal studies. To determine if NPY might be a human coronary vasoconstrictor, the authors studied its effect on postmortem human coronary arteries. Proximal epicardial coronary rings were studied in a superfusion apparatus in Krebs-Ringer bicarbonate buffer (37 degrees C, pH 7.4) presaturated with 95% O2-5% CO2. Concentration-response curves were obtained using NPY in 0.1% bovine serum albumin in buffer and the responses were compared to those obtained in the presence of alpha 1, beta, and cyclooxygenase antagonists. A dose-related constrictor effect was obtained with NPY, which was significantly more potent than noradrenaline, constriction often being seen at 10(-12) M concentration. A vasorelaxant effect was seen in nonatherosclerotic vessels at higher concentrations. The vasoconstriction produced by noradrenaline was potentiated by subthreshold concentrations of NPY. The vasoconstrictor effect of NPY was not inhibited by prazosin (10(-6) M), and the vasodilatory effect was not inhibited by propranolol (10(-5) M). Indomethacin (3 X 10(-6) M) did not alter either vasoconstriction or vasorelaxation. The authors conclude that NPY is a potent constrictor of the human coronary artery at concentrations that may be achievable in vivo; it may thus be a contributor to sympathetic enhancement of coronary artery tone.     

Interaction of neuropeptide Y and the sympathetic nervous system in vascular control in man

BACKGROUND. There is increasing evidence that neuropeptide Y (NPY) contributes to the autonomic control of the circulation. NPY coexists with noradrenaline in perivascular nerve terminals, may be released during sympathetic stimulation, and is a potent constrictor of the human coronary circulation and other vascular beds. In vitro studies show that NPY can act either directly on vascular smooth muscle or indirectly by modulation of the presynaptic release or the postsynaptic actions of noradrenaline. It is unclear to what extent these mechanisms operate in vivo. METHODS AND RESULTS. The effect on forearm blood flow of intra-arterial NPY was studied in six volunteers during coinfusion of noradrenaline and during reflex sympathetic stimulation induced by lower-body negative pressure. NPY alone induced a dose-dependent reduction of forearm blood flow in all subjects studied, to a maximum of 49 +/- 6.1%. The reduction of flow during infusion of noradrenaline alone was 42 +/- 8%. The response to noradrenaline was unaffected by coinfusion of a threshold constrictor dose of NPY (50 pmol/min). Furthermore, the reflex sympathetic vasoconstrictor response to 20 cm H2O of lower-body negative pressure was similar in both the infused and control arms during the infusion of 50 pmol/min NPY. The response to noradrenaline was abolished by alpha-blockade with phentolamine, but the flow reduction induced by NPY was unaffected by alpha-blockade. CONCLUSIONS. NPY is a potent constrictor of human forearm resistance vessels and has a direct effect independent of alpha-receptors. NPY has no detectable modulating effect in vivo on the action of endogenous or infused noradrenaline.     

Responses of the rabbit epicardial coronary artery to acetylcholine and adrenoceptor agonists.

STUDY OBJECTIVE--The aim was to identify the role of the endothelium in mediating the responses to acetylcholine in the rabbit coronary artery, and to determine whether alpha or beta adrenergic stimulation may cause relaxation via endothelial receptors in the coronary arteries of this species. DESIGN--Responses to acetylcholine and adrenoceptor agonists were compared in isolated ring preparations with and without endothelium. The adrenoceptor agonists were examined in the presence of phentolamine or propranolol to block alpha and beta adrenoceptors, respectively. EXPERIMENTAL MATERIAL--30 New Zealand white rabbits (2.3-3.4 kg) were killed by an overdose of barbiturate and exsanguination, and the left epicardial coronary artery was dissected free. Ring preparations were suspended in organ baths under isometric tension and, where required, the tone of the preparations was raised by KC1. MEASUREMENTS AND MAIN RESULTS--Concentrations of acetylcholine up to 10(-6) mol.litre-1 produced dose dependent relaxation of the preparations with endothelium intact, but no relaxation in preparations denuded of endothelium. At higher concentrations, a marked vasoconstrictor response was seen in all preparations regardless of the presence of endothelium. At basal tone, acetylcholine produced vasoconstriction which reached a maximum of 1.0 (SEM 0.14)g tension in preparations with endothelium and 1.74(0.27) g tension in those without endothelium (p less than 0.05). In coronary arteries pretreated with 50 mumol.litre-1 phenoxybenzamine to block alpha adrenoceptors, noradrenaline, isoprenaline, and salbutamol produced dose dependent relaxation of the preparations; this was unaffected by the absence of endothelium. In vessels not pretreated with phenoxybenzamine, propranolol inhibited the relaxation to noradrenaline and isoprenaline but again there was no difference between vessels with and without endothelium. CONCLUSIONS--In the rabbit isolated epicardial coronary artery, acetylcholine produces an endothelium dependent relaxant response over a limited concentration range; a vasoconstrictor response via smooth muscle dominates at higher concentrations. beta Adrenoceptors mediating relaxation are present on the smooth muscle, but there was no evidence for either alpha or beta adrenoceptor mediated responses via the endothelium. Important differences with coronary arteries from other species are discussed.     

Enhanced neuropeptide Y immunoreactivity and vasoconstriction in mesenteric small arteries from spontaneously hypertensive rats

Enhanced sympathetic nerve activity is thought to play a role in the pathogenesis of hypertension. The purpose of the present study was to investigate the mechanisms underlying the enhanced vasocontractile response to perivascular stimulation of mesenteric arteries isolated from female spontaneously hypertensive rats (SHR). Innervation of mesenteric small arteries was evaluated by immunohistochemistry and confocal microscopy while functional studies were conducted in a microvascular myograph. The distribution of nerve terminals immunoreactive for tyrosine hydroxylase (TH) and neuropeptide Y (NPY) was similar in mesenteric small arteries from Wistar-Kyoto (WKY) and SHR rats. However, immunointensity of TH or NPY immunoreactivities were much higher in small arteries from SHR compared to WKY. Expressed as percentage of contractions elicited by 124 mM K(+), concentration-response curves for noradrenaline (NA) and NPY were shifted leftward in SHR compared with WKY rats. The combination of noradrenaline (1 microM) and NPY (10 nM) contracted mesenteric arteries from WKY and SHR to higher levels than compared to either contractile agent added alone. The NPY Y(1) receptor antagonist, BIBP 3226, inhibited these contractions with 87 +/- 0.7 and 80 +/- 1.3% (p < 0.05, n = 6) in arteries from WKY and SHR rats, respectively. In arteries incubated with the alpha(1)-adrenoceptor antagonist, prazosin, and preactivated with vasopressin, electrical field stimulation evoked contractions which were more pronounced in mesenteric arteries from SHR compared to WKY rats. BIBP 3226 partially inhibited these contractions. In vasopressin-activated arteries BIBP 3226 caused rightward shifts of the concentration-response curves for NPY in mesenteric arteries from SHR rats, but in addition it also abolished the maximal NPY contraction in arteries from WKY rats. In the presence of BIBP 3226, low concentrations (1 pM to 10 nM) of NPY caused relaxations in arteries from WKY, but not in segments from SHR rats. Mechanical removal of the endothelium abolished NPY relaxation in arteries from WKY. In arteries activated with vasopressin and exposed to either forskolin or sodium nitroprusside, the addition of NPY evoked contractions which were more pronounced in arteries from SHR compared to WKY arteries. The present study suggests that enhanced NPY content and vasoconstriction to NPY in arteries from hypertensive rats can contribute to the enhanced sympathetic nerve activity and vascular resistance in female hypertensive rats. Endothelial cell dysfunction as well as alterations in smooth muscle response to NPY seem to contribute to the enhanced vasoconstriction in arteries from hypertensive animals.     

Age related attenuation of the endothelium dependent relaxation to noradrenaline in isolated pig coronary arteries.

STUDY OBJECTIVE--The aim was to investigate the relationship between vascular aging and endothelial regulation of the vascular tone by various agonists in isolated pig coronary arteries. EXPERIMENTAL MATERIALS AND DESIGN--Coronary artery rings from young (4-6 month old) and aged (3-4 years old) Yorkshire pigs were studied under isometric tension by organ chamber experiments. Cumulative concentration-response curves were obtained for vasorelaxing agents including noradrenaline (with beta blocker), substance P, bradykinin, and glyceryl trinitrate in rings precontracted with prostaglandin F2 alpha (PGF2 alpha). The degree of atherosclerosis of the coronary arteries was examined by light microscopy after the pharmacological experiments. MEASUREMENTS AND MAIN RESULTS--In the presence of propranolol (3 x 10(-6)M), noradrenaline (10(-8)-10(-5)M) caused endothelium dependent relaxation. The maximum relaxation was significantly greater in the young [45.4(SEM 2.5)% of the magnitude of precontraction induced by PGF2 alpha] than in the aged group [26.9(3.0)%] (p less than 0.001). However, magnitudes of relaxation to substance P, bradykinin, and glyceryl trinitrate were not significantly different between the young and the aged groups. The alpha 2 antagonist yohimbine (3 x 10(-6)M) inhibited the endothelium dependent relaxation to noradrenaline, but the alpha 1 antagonist prazosin (10(-6)M) failed to inhibit the response. Gossypol (3 x 10(-5)M) and methylene blue (10(-5)M), both inhibitors of endothelium dependent relaxation, abolished alpha 2 adrenoceptor mediated relaxation, but a cyclo-oxygenase inhibitor indomethacin (10(-5)M) did not affect the response. Histologically almost all young pig coronary arteries were free of atherosclerotic changes, whereas aged arteries had fatty streaks with slightly narrowed lumen in 22 of 38 rings (58%). The remaining aged pig coronary rings were free from atherosclerosis. Comparison of the endothelium dependent relaxation to noradrenaline between the rings with fatty streaks and those without lesions in the aged pigs suggested that the attenuated response was due both to vascular aging and to fatty streak development. CONCLUSIONS--The endothelium dependent relaxation to noradrenaline via the alpha 2 adrenoceptor was attenuated by vascular aging and also by fatty streak formation in isolated pig coronary arteries. Thus vascular aging may affect the sympathetic regulation of the coronary arterial tone by the attenuation of endothelium dependent relaxation to catecholamines via alpha 2 adrenoceptors.     

Characterization of Neuropeptide Y Receptors Mediating Contraction, Potentiation and Inhibition of Relaxation

Nilsson T, You J, Sun X, Hedner T, Edvinsson L. Characterization of neuropeptide Y receptors mediating contraction potentiation and inhibition of relaxation.

In addition to its direct vasoconstrictive effect, neuropeptide Y (NPY) potentiates noradrenaline- (NA) induced contraction and inhibits acetylcholine- (ACh) induced relaxation: The aim of the present study was to elucidate the NPY receptor subtypes responsible for mediating these three responses. NPY, peptide YY (PYY) and pro³⁴NPY (a NPY Y₁ receptor agonist) induced equipotent and equally strong concentration-dependent contractions of guinea pig basilar arteries. NPY_13-36 (a NPY Y₂ receptor agonist), however, caused only weak contraction with significantly lower potency. The NPY-induced contraction was significantly inhibited by the selective NPY Y₁ receptor antagonist BIBP3226 (1 μM). NPY, PYY and pro³⁴NNPY but not NPY_13-36 significantly potentiated the NA-induced contraction in guinea pig mesenteric arteries. The potentiation was significantly inhibited by BIBP3226 (IpM). In precontracted guinea pig basilar arteries, ACh induced a concentration-dependent relaxation which was significantly inhibited by NPY, PYY and NPY_13-36 but not by pro³⁴NPY. BIBP3226 had no significant effect on the NPY-induced inhibition of the relaxation. These results suggests that the NPY Y, receptors mediate the direct contraction and the potentiation of the NA-induced contraction but not the inhibition of the ACh-induced relaxation. This effect seems to be mediated by another NPY receptor subtype, presumably by the Y₂ receptor, as judged from the agonist potency order.     

Magnesium sulphate infusion suppresses the cardiac release of noradrenaline during a handgrip stress test

BACKGROUND: Magnesium has several important cardiovascular effects, but its effect on cardiac sympathetic efferent neuron activity has not been clarified. Objectives: To examine the effect of magnesium sulphate infusion on cardiac sympathetic efferent postganglionic neuronal liberation of noradrenaline. PATIENTS AND METHODS: Twenty-two patients who underwent cardiac catheterization were randomly allocated to the control group or the magnesium group. Plasma noradrenaline and adrenaline concentrations in the aorta and the coronary sinus were measured. Noradrenaline or adrenaline release from the heart was calculated by dividing the difference in noradrenaline or adrenaline concentration between the aorta and the coronary sinus by that of the aorta. After baseline blood sampling, the control patients and the patients in the magnesium group received intravenous infusion of saline or magnesium sulphate (10 mmol). All patients were then subjected to 3 min of handgrip exercise stress test to augment sympathetic efferent neuronal activity, and the blood sampling was repeated. RESULTS: Although blood pressure was increased by the handgrip stress test, there were no differences in heart rate and blood pressure between the two groups, both at baseline and during the handgrip stress test. The plasma noradrenaline and adrenaline concentrations and noradrenaline or adrenaline release from the heart did not differ between the two groups in the baseline condition. However, the handgrip stress increased plasma noradrenaline concentrations and the cardiac noradrenaline release was increased in the control group, whereas the cardiac noradrenaline release was not increased by the handgrip stress in the magnesium group (P<0.02). CONCLUSIONS: These data indicate that magnesium sulphate infusion suppresses the release of catecholamines by the heart, which is an indirect index of sympathetic efferent neuronal activity.     

Postmortem functional changes in coronary and cerebral arteries from humans and monkeys

Contractile responses to 30 mmol . litre-1 K+ of helical strips of coronary arteries from human cadavers did not change within 5 h after death; however, they were suppressed 8 h after death. In coronary arteries from monkey cadavers, the K+-induced contractions did not significantly differ within the first 5 h, but were suppressed 12 h after death. On the other hand, K+-induced contractions were retained without deterioration in cerebral artery strips from human cadavers 20 to 24 h after death and those from monkey cadavers 8 to 16 h. Acetylcholine caused contractions of human coronary arteries, but caused only a relaxation of monkey coronaries which was abolished by rubbing off the endothelium. These responses were attenuated by no more than K+-induced contractions up to 12 h after death. Maximum contractions induced by noradrenaline, histamine and serotonin remained the same in human coronary arteries for 3 to 5 h after death. Similar magnitudes of contraction were elicited by noradrenaline in human cerebral arteries up to 20 h after death. It appears that the reactivity of human coronary arteries to K+ and other vasoconstrictor agents used is normally retained for at least 6 h after death and that of human cerebral arteries up to 24 h.     

Role of endothelial cells in relaxation of isolated arteries by bradykinin.

Bradykinin elicits relaxation of isolated transverse rings of canine coronary, celiac, superior mesenteric, renal, splenic, pulmonary, gastric, and femoral arteries. After endothelial cells of the vessel wall are removed by rubbing of the intimal surface, canine arteries fail to relax upon addition of bradykinin. The endothelium-dependent relaxation of canine arteries remains intact after treatment with cyclooxygenase inhibitors (indomethacin and flurbiprofen), and this argues against mediation by prostaglandins. When they are stimulated with bradykinin, endothelial cells of canine arteries appear to release a substance mediating vascular smooth muscle relaxation. In contrast, preparations of arteries of cats (superior mesenteric) and rabbits (superior mesenteric and celiac) may be rubbed on the intimal surface without a consistent loss of sensitivity to the relaxing effects of bradykinin. In addition, relaxation of the cat and rabbit arteries is completely blocked by cyclooxygenase inhibitors. Preliminary studies indicate that bradykinin relaxes human arteries in an endothelium-dependent manner and that this effect is not mediated by prostaglandins. We have previously reported that arteries of all species tested require the presence of endothelial cells for relaxation in response to acetylcholine and we have also demonstrated, using the rabbit aorta, that this effect is mediated by the release of an uncharacterized substance from these cells that relaxes vascular smooth muscle. We conclude that bradykinin relaxes canine and human arteries via a similar mechanism but that it relaxes cat and rabbit arteries by stimulating release of prostaglandins from as yet undefined cell types.     

Neonatal administration of a specific neuropeptide Y antiserum alters the vasopressin response to haemorrhage and the hypothalamic content of noradrenaline in rats

The possibility that neuropeptide Y (NPY) exerts organizational effects on central noradrenergic systems was investigated by treating newborn rats with subcutaneous injections of a specific NPY-antiserum. Three months later, neuroendocrine function was determined by measuring plasma vasopressin following haemorrhage, since this response is known to be regulated by ascending noradrenergic pathways. Basal mean arterial pressure, heart rate and plasma vasopressin were similar in both control (normal rabbit serum-treated) and NPY-antiserum-treated rats. Treatment with this antiserum resulted in an impaired vasopressin response to haemorrhage, although the haemodynamic changes observed after this hypovolaemic challenge were similar to control rats. NPY and noradrenaline content in the hypothalamus and brainstem were examined at the end of these experiments. NPY-like immunoreactivity was similar in both groups of animals. However, electrochemical detection of noradrenaline after HPLC revealed significantly higher levels in the hypothalamus, but not brainstem, of NPY-antiserum-treated rats. The presence of enduring changes in noradrenaline levels and neurohypophyseal function following neonatal treatment with NPY-antiserum suggests a role for NPY in postnatal organization of the rat hypothalamus.     

Functional and non-functional nerve-smooth muscle transmission in the renal arteries of the newborn and adult rabbit and guinea-pig

Isolated renal arteries from newborn and adult guinea-pigs failed to respond to electrical stimulation of the intramural nerves, using parameters identical to those which elicit large neurogenic contractions in renal arteries from newborn and adult rabbits. The threshold stimulation frequency was lower and responses were greater in adult compared with neonatal rabbits. Comparisons of noradrenaline EC50 values showed that guinea-pig renal arteries are significantly less sensitive to noradrenaline that rabbit vessels. Quantitative fluorescence histochemistry of the perivascular adrenergic nerves in renal arteries revealed that the density of innervation gradually declines in the guinea-pig but substantially increases in the rabbit during postnatal development. The lack of neurogenic response of the guinea-pig renal artery, despite the existence of a varicose, adrenergic nerve plexus, is discussed.     

Comparison of rabbit coronary arterial relaxation induced by acetylcholine and lemakalim: activation of ATP sensitive potassium channels.

STUDY OBJECTIVE--The purpose was to assess the role of ATP sensitive potassium channels (KATP) in endothelium dependent vasodilatation induced by acetylcholine, or endothelium independent vasodilatation induced by lemakalim in rabbit coronary arteries. DESIGN--The effect of glibenclamide, a specific inhibitor of KATP, on coronary artery relaxation induced by acetylcholine or lemakalim was investigated. The relaxing effectiveness of acetylcholine and lemakalim on coronary arteries precontracted with KCl (K+) or prostaglandin F2 alpha (PGF2 alpha) was compared. EXPERIMENTAL MATERIALS--Left epicardial coronary arteries from male New Zealand white rabbits (2.5-3.0 kg), killed by an overdose of pentobarbitone, were dissected free of connective tissue. Rings suspended in organ baths for the measurement of isometric tension. MEASUREMENTS AND MAIN RESULTS--K+ (30 mmol.litre-1) and PGF2 alpha (3 mumols.litre-1) caused comparable contraction (p greater than 0.05) in endothelium intact or endothelium denuded coronary arterial rings. Acetylcholine induced relaxation was greater in endothelium intact rings precontracted with PGF2 alpha than with K+ and was abolished by the removal of endothelium. Relaxations induced by acetylcholine (0.1 and 0.3 mumol.litre-1) were reduced from 82(SEM 2.7)% and 93(2.8)% to 71(2.4)% and 82(2.7)% (p less than 0.05), and to 63(3.2)% and 79(4.5)% (p less than 0.05 or less than 0.01) by glibenclamide (3 and 10 mumols.litre-1) respectively in PGF2 alpha precontracted rings; and also attenuated (p less than 0.05 or less than 0.01) in K+ precontracted rings. Lemakalim induced relaxation was greater in endothelium denuded rings precontracted with PGF2 alpha than with K+, and was markedly reduced by glibenclamide (p less than 0.01). CONCLUSIONS--These results suggest that activation of KATP may partially be involved in endothelium dependent relaxation induced by acetylcholine in rabbit coronary arteries. Lemakalim-induced endothelium independent relaxation results mainly from activation of KATP.     

Mechanical properties and effects of sympathetic co-transmitters on human coronary arteries and veins

Active isometric wall tension was studied at different levels of passive wall tension in isolated circular 2 mm long segments of human epicardial coronary arteries and veins, and maximum active wall tension was calculated to 6.60 mN/ mm for arteries and 0.86 mN/mm for veins. Vasomotor responses to sympathetic co-transmitters were studied at resting tension and after precontraction with U46619. Noradrenaline (NA) and adenosine 5-triphosphate (ATP) induced strong contractions of veins, whereas relaxant responses dominated in arteries. Isoprenaline potently relaxed all arteries and veins. Prazosin and rauwolscine in a concentration of 10^–7 M both competitively antagonized NA-induced contraction of arteries and veins. For uridine 5-triphosphate (UTP), relaxant responses were demonstrated in most arteries but only some veins. Neuropeptide Y (NPY) elicited no observable vasomotor responses in either arteries or veins. Mechanical removal of the arterial endothelium did not significantly alter relaxant responses to NA, ATP, UTP or isoprenaline. In conclusion, ₁-and ₂-adrenoceptors mediating contraction and -adrenoceptors mediating relaxation seem to be present in both human epicardial coronary arteries and veins. When applied to isolated epicardial coronary vessels, NA and ATP had a stronger influence on vasomotor tone than NPY and UTP, mediating strong contraction of veins but mainly relaxation of coronary arteries, that was independent of an intact endothelium.     

Potency and receptors involved in coronary vasoconstriction caused by neuropeptide Y(NPY)

Using anesthetized dogs, the coronary vascular effects of neuropeptide Y (NPY) were studied and the action of alpha- or serotonergic receptor blockade on the action of NPY was evaluated. To demonstrate the biological significance of the action of NPY, the vasoconstrictor potencies of NPY and norepinephrine were compared. One to 5 nmol of intracoronary NPY reduced coronary flow in a dose-dependent manner. The action started rather gradually and lasted for 10 min or more. Since perfusion pressure and central venous pressure were unchanged, the decrease in coronary flow should be a result of coronary vasoconstriction. Intracoronary norepinephrine infusion caused vasodilatation but when dogs were pretreated with 0.5 to 1.0 mg/kg of systemic propranolol, a vasoconstrictor effect was observed at a 5 times higher dose than with NPY. Furthermore, the action of NE was only transient, lasting for 30 sec or less. The vasoconstrictor action of NPY was not antagonized by phentolamine or by ketanserin. Since NPY is an endogenous polypeptide found in the sympathetic nerve terminals around coronary arteries, it may participate in the regulation of coronary flow.     

Hemodynamic effects of magnesium sulfate on the normal human heart

Because of its antiarrhythmic properties,¹ parenteral magnesium sulfate has been widely used in the last decades in treating several supraventricular or ventricular arrhythmias^2,3 even in patients with cardiovascular diseases. Recently, intravenous magnesium sulfate has proved effective in treating arrhythmias associated with acute myocardial infarction⁴ and long QT syndrome.⁵ However, a systematic evaluation of its effects on cardiovascular hemodynamics has not been reported. Recent experimental⁶ and clinical⁷ observations have suggested that magnesium sulfate may exert a vasodilator effect on human coronary arteries. The present study investigates the effects of magnesium sulfate infusion on coronary and systemic hemodynamics in humans.     

The Gender Differences in the Relaxation to Levosimendan of Human Internal Mammary Artery

Purpose The mechanism of the vasorelaxation to levosimendan varies depending on the vascular bed and species studied. Here, we examined the vasorelaxation to levosimendan as well as its modification by various potassium channel antagonists in human internal mammary artery (IMA) obtained from male and female patients. Methods IMA grafts were supplied from 27 male and 19 age-matched female patients undergoing coronary bypass operation. The contraction to noradrenaline and relaxation to levosimendan were studied in IMA rings obtained from both gender. The relaxations to levosimendan were also assessed in the presence of glibenclamide (10 μM), an adenosine triphosphate-sensitive potassium channel (K_ATP) blocker, or charybdotoxin (100 nM), a calcium-activated potassium channel (K_Ca) blocker, or 4-aminopyridine(1 mM), a voltage-sensitive potassium channel (K_v) inhibitor. Results Concentration–response curves to noradrenaline were not different in IMA rings from either gender. Pretreatment with levosimendan (3 × 10⁻⁷ M) slightly modified the contractions to noradrenaline in both gender. Levosimendan (10⁻⁹–10⁻⁵ M) produced concentration-dependent relaxation in IMA rings, contracted by noradrenaline (5 × 10⁻⁶ M), from males and females. The vasodilatory effects of levosimendan were more pronounced in the arteries from males (83%) than females (69%), in term of the maximal relaxation (E _max). Charybdotoxin and glibenclamide significantly inhibited the relaxation to levosimendan in the arteries from males but not in those of females. Conclusions The vasodilating efficacy of levosimendan and its relaxation mechanism differs between the arteries from males and females, which may have clinical consequences in the treatment of heart failure.     

Plant-derived estrogens relax coronary arteries in vitro by a calcium antagonistic mechanism

OBJECTIVES

To investigate the potential for plant derived estrogens (phytoestrogens) genistein, phloretin, biochanin A and zearalanone to relax rabbit coronary arteries in vitro and to determine the mechanism(s) of such relaxation.

BACKGROUND

Epidemiological data suggests a reduction in the incidence of coronary heart disease in humans who have a high intake of phytoestrogens.

METHODS

Isolated rabbit coronary artery rings were suspended in individual organ baths, precontracted with potassium chloride (30mM), and the relaxing effects and mechanisms of relaxation to genistein, phloretin, biochanin A and zearalanone were determined by measurement of isometric tension.

RESULTS

Genistein, phloretin and biochanin A induced significant gender-independent relaxation in rings with and without endothelium. Inhibition of nitric oxide and prostaglandin synthesis with L-NAME and indomethacin had no effect on genistein-induced relaxation. Relaxation was unaffected by the specific estrogen receptor antagonist ICI 182,780, the ATP-sensitive potassium channel inhibitor glibenclamide and the potassium channel inhibitor, barium chloride. Calcium concentration-dependent contraction curves in high potassium depolarization medium were significantly shifted to the right and downward after incubation with genistein and zearalanone. An inhibitory effect of genistein (2 μM) on L-type calcium current in guinea-pig ventricular myocytes confirmed a calcium antagonist relaxing mechanism of action. In healthy volunteers, plasma genistein levels of approximately 2 μM are achieved after ingestion of a commercially available soy protein drink (Supro) containing 37 mg genistein.

CONCLUSIONS

This study demonstrates that phytoestrogens induce endothelium-independent relaxation of coronary arteries; the mechanism involves calcium antagonism. These mechanisms may contribute to the potential long-term cardiovascular protective effect of these substances.     

Studies on the mechanism of neuropeptide Y induced potentiation of neurogenic vasoconstriction in the isolated rabbit ear artery

Neuropeptide Y (NPY) has been shown to potentiate the response of the isolated rabbit ear artery to field stimulation, provided that the vascular endothelium is intact. This report describes efforts to elucidate the mechanism by which NPY produces this effect. The response of the perfused rabbit ear artery to brief intermittent field stimulation was significantly enhanced by NPY (100 nmol/L). The administration of indomethacin (10 mumol/L) to inhibit cyclooxygenase did not affect the ability of NPY to potentiate neurogenic vasoconstriction. Blockade of calcium channels with nifedipine (1 mumol/L) produced a partial attenuation of the NPY effect. Our studies therefore suggest that an arachidonic acid metabolite is not involved in NPY induced potentiation of vascular neuroeffector transmission in the rabbit ear artery. On the other hand, the translocation of extracellular calcium may play a role in this process.     

Diffusion through the vessel wall of transmitter released by sympathetic stimulation. A fluorescence and Autoradiography study.

The disposition of sympathetic transmitter within the wall of rat femoral and rabbit saphenous arteries has been studied during sympathetic stimulation. Using a histochemical fluorescence technique, sharply delineated bead-like fluorescent spots representing sympathetic nerve terminals were found at the adventitio-medial junction. In specimens excised during sympathetic nerve stimulation, the entire media was veiled by fluorescent material representing noradrenaline which had diffused into that layer after release. The localization of the nerve terminals in controls as well as diffusion of transmitter during stimulation in the rat femoral artery was corroborated by autoradiography of tritiated noradrenaline.     

Distribution of the neuropeptide galanin in the cat heart and coexistence with vasoactive intestinal peptide, substance P and neuropeptide Y.

The neuropeptide galanin (GAL) has been detected in the peripheral and central nervous systems. However, little is known about its distribution and localization in heart, and the possible coexistence of GAL with other neuropeptides in the heart is not established. The present immunocytochemical study describes the distribution of GAL in nerves of the feline heart and its colocalization with vasoactive intestinal peptide (VIP), substance P (SP), and neuropeptide Y (NPY). GAL-like immunoreactivity was widely distributed in the atrial and ventricular myocardium and around coronary arteries. Colocalization of GAL with VIP, SP and NPY was observed in many nerve fibers. Further, GAL and NPY were colocalized in nerve cell bodies of intracardiac ganglia. Since these neuropeptides have been found to be associated with sensory and autonomic innervation in the heart, the present findings provide evidence that GAL is shared by functionally different neuronal populations in the heart and that GAL may participate in controlling cardiac function by combined action with other neuropeptides.