J. Likungu

Inhibition of noradrenaline release from the sympathetic nerves of the human saphenous vein by presynaptic histamine H3 receptors.

SummaryThe human saphenous vein was used to examine whether presynaptic histamine receptors can modulate noradrenaline release and, if so, to determine their pharmacological characteristics. Strips of this blood vessel were incubated with [3H]noradrenaline and subsequently superfused with physiological salt solution containing desipramine and corticosterone. Electrically (2 Hz) evoked 3H overflow was inhibited by histamine and the H3 receptor agonist R-(−)-α-methylhistamine. Histamine-induced inhibition of electrically evoked tritium overflow was not affected by α2-adrenoceptor blockade by rauwolscine. S-(+)-α-methylhistamine (up to 10 μmol/l) as well as the histamine H1 and H2 receptor agonists 2-(2-thiazolyl)ethylamine (up to 3 μmol/l) and dimaprit (up to 30 μmol/l), respectively, were ineffective. The selective histamine H3 receptor antagonist thioperamide abolished the inhibitory effect of histamine. The histamine H2 and H1 receptor antagonists ranitidine and pheniramine, respectively, did not affect the histamine-induced inhibition of evoked tritium overflow. The present results are compatible with the suggestion that the sympathetic nerves of the human saphenous vein are endowed with inhibitory presynaptic histamine receptors of the H3 class.

Effects of selective h5-HT1B (SB-216641) and h5-HT1D (BRL-15572) receptor ligands on guinea-pig and human 5-HT auto- and heteroreceptors.

Human cerebral cortical slices and synaptosomes, guinea-pig cerebral cortical slices and human right atrial appendages were used to study the effects of SB-216641, a preferential h5-HT1B receptor ligand, and of BRL-15572, a preferential h5-HT1D receptor ligand, on the presynaptic h5-HT1B and h5-HT1B-like autoreceptors in the human and guinea-pig brain preparations, respectively, and on the presynaptic h5-HT1D heteroreceptors in the human atrium. The brain preparations, preincubated with [3H]serotonin ([3H]5-HT), and the segments of atrial appendages, preincubated with [3H]noradrenaline, were superfused with modified Krebs’ solution and tritium overflow was evoked electrically (human and guinea-pig cerebral cortex slices and human atrial appendages) or by high K+ (human cerebral cortex synaptosomes). The electrically evoked tritium overflow from guinea-pig cerebral cortex slices was reduced by the 5-HT receptor agonist 5-carboxamidotryptamine (5-CT). This effect was not modified by BRL-15572 (2μM; concentration 154 times higher than its Ki at h5-HT1D receptors) but was antagonized by SB-216641 (0.1μM; concentration 100 times higher than its Ki at h5-HT1B receptors; apparent pA2 8.45). SB-216641 (0.1μM) by itself facilitated, whereas BRL-15572 (2μM) did not affect, the evoked overflow. In human cerebral cortex slices SB-216641 (0.1μM) also facilitated, and BRL-15572 (2μM) again failed to affect, the electrically evoked tritium overflow. In human cerebral cortical synaptosomes, 5-CT reduced the K+-evoked tritium overflow. This response was unaffected by BRL-15572 (300nM) but antagonized by SB-216641 (15nM; drug concentrations 23 and 15 times higher than their Ki at h5-HT1D and h5-HT1B receptors, respectively). Both drugs, given alone, did not modify the K+-evoked tritium overflow. In human atrial appendages, the electrically evoked tritium overflow was inhibited by 5-HT in a manner susceptible to antagonism by BRL-15572 (300nM; 23 times Ki at h5-HT1D receptors) but not by SB-216641 (30nM; 30 times Ki at h5-HT1B receptors). Both drugs by themselves did not change the electrically evoked tritium overflow. In conclusion, SB-216641 behaves as a preferential antagonist at native human 5-HT1B receptors and BRL-15572 as a preferential antagonist at native human 5-HT1D receptors. These compounds are clearly useful tools for the differentiation between human 5-HT1B and 5-HT1D receptors in functional studies.

Inhibition of noradrenaline release from the sympathetic nerves of the human saphenous vein via presynaptic 5-HT receptors similar to the 5-HT1D subtype

SummaryThe human saphenous vein preincubated with [3H]noradrenaline was used to determine the pharmacological properties of the release-inhibiting presynaptic serotonin (5-HT) receptor on the sympathetic nerves. The overflow of tritium evoked by transmural electrical stimulation (2 Hz) was concentration-dependently inhibited by drugs known to stimulate 5-HT receptors in the following rank order: oxymetazoline ≥ 5-HT ≥ 5-carboxamidotryptamine = 5-methoxytryptamine = sumatriptan > tryptamine > N,N(CH3)2-5-HT = yohimbine = 8-hydroxy-2-(di-n-propylamino)-tetraline. The potencies of these agonists in inhibiting overflow were significantly correlated with their affinities for 5-HT1B and 5-HT1D binding sites, but not with those for 5-HT1A or 5-HT1C binding sites. 5-Aminotryptamine, methysergide, ipsapirone, cyanopindolol, SDZ 21009 and metergoline dit not produce a significant inhibition. Metitepine and methysergide antagonized the inhibitory effect of 5-HT, whereas spiroxatrine, propranolol, ketanserin and ICS 205-930 did not.These data exclude the idea that the inhibitory presynaptic 5-HT receptor on the sympathetic nerves belongs to the 5-HT2 and 5-HT3 receptor class; the pattern of agonist potencies suggests that the receptor is very similar to the 5-HT1D receptor subtype.

Tissue resident C reactive protein in degenerative aortic valves: correlation with serum C reactive protein concentrations and modification by statins

Objective: To assess aortic valve probes for valvar C reactive protein (CRP) presence, the relation between valvar and serum CRP, and a possible modification of CRP by statin medication. Setting: Tertiary referral centre. Patients and design: End stage, degenerative valve tissue was taken from 81 patients, 57 with non-rheumatic aortic valve stenosis (AS) and 24 with degenerative aortic valve bioprosthesis (BP). Five non-stenosed valves served as controls. Tissue from four non-implanted bioprostheses was also examined. The presence and location of CRP was analysed by use of immunostaining and morphometry. Serum CRP concentrations were measured preoperatively. Results: The majority of AS and BP valves exhibited CRP labelled cells, predominantly localised to the valvar fibrosa. The expression of CRP was much higher in BP than in AS (by a factor of 3.7, p  =  0.03). Notably, non-stenosed aortic valves and non-implanted bioprostheses did not have CRP signalling. Serum CRP was also increased with BP (by a factor of 2.5, p  =  0.02) and was significantly correlated with valvar CRP expression (r  =  0.54, p < 0.001). The main finding in patients with (n  =  26) and without statin treatment (n  =  55) was that both valvar CRP expression (p  =  0.02) and serum CRP concentrations (p  =  0.04) were lower in the statin treated group. Conclusions: CRP was found in a large series of degenerative aortic valves, more often in bioprostheses than in native cusps. Serum CRP concentrations may reflect inflammatory processes within the aortic valve. The association of statin treatment with decreases in both valvar and serum CRP concentrations may explain known pleiotropic effects of statins in patients with aortic stenosis.

N-Type Calcium Channels Control Sympathetic Neurotransmission in Human Heart Atrium

BACKGROUND Because knowledge about the type of calcium channels involved in action potential-induced norepinephrine release from the human peripheral sympathetic nervous system is sparse, we investigated which types of calcium channels are functionally important in the sympathetic nerves of human cardiac tissue. METHODS AND RESULTS In superfused segments of human right atrial appendages, the type of calcium channels that control [(3)H]norepinephrine release evoked by transmural electrical stimulation was determined. [(3)H]norepinephrine release was almost abolished by 0.2 micromol/L omega-conotoxin GVIA (a selective blocker of N-type channels) but was not modified by 0.1 micromol/L omega-agatoxin IVA (a selective blocker of P- and Q-type channels). Mibefradil (a T-type and N-type calcium channel blocker) at concentrations of 0.3 to 3 micromol/L reduced the evoked tritium overflow in a frequency- and calcium-dependent manner, whereas 0.1 to 10 micromol/L amlodipine, diltiazem, and verapamil (selective blockers of L-type channels) were ineffective. CONCLUSIONS Norepinephrine release from cardiac sympathetic nerves is triggered by Ca(2+) influx via N-type but not L- and P/Q-type calcium channels. The inhibitory effect of mibefradil on norepinephrine release at clinically relevant concentrations is probably due to its blocking action on N-type Ca(2+) channels. This property of mibefradil is unique among the calcium channel blockers that have been or still are therapeutically applied and may considerably contribute to its slight negative chronotropic effect in vivo.

Presynaptic imidazoline receptors and non-adrenoceptor[3H]-idazoxan binding sites in human cardiovascular tissues

1 In segments of human right atrial appendages and pulmonary arteries preincubated with [3H]‐noradrenaline and superfused with physiological salt solution containing desipramine and corticosterone, the involvement of imidazoline receptors in the modulation of [3H]‐noradrenaline release was investigated. 2 In human atrial appendages, the guanidines aganodine and DTG (1,3‐di(2‐tolyl)guanidine) which activate presynaptic imidazoline receptors, inhibited electrically‐evoked [3H]‐noradrenaline release. The inhibition was not affected by blockade of α2‐adrenoceptors with 1 μM rauwolscine, but antagonized by extremely high concentrations of this drug (10 and/or 30 μM; apparent pA2 against aganodine and DTG: 5.55 and 5.21, respectively). 3 In the presence of 1 μM rauwolscine, [3H]‐noradrenaline release in human atrial appendages was also inhibited by the imidazolines idazoxan and cirazoline, but not by agmatine and noradrenaline. The inhibitory effects of 100 μM idazoxan and 30 μM cirazoline were abolished by 30 μM rauwolscine. 4 In the atrial appendages, the rank order of potency of all guanidines and imidazolines for their inhibitory effect on electrically‐evoked [3H]‐noradrenaline release in the presence of 1 μM rauwolscine was: aganodineBDF 6143 [4‐chloro‐2‐(2‐imidazolin‐2‐yl‐amino)‐isoindoline]>DTGclonidine>cirazoline>idazoxan (BDF 6143 and clonidine were previously studied under identical conditions). This potency order corresponded to that previously determined at the presynaptic imidazoline receptors in the rabbit aorta. 5 When, in the experiments in the human pulmonary artery, rauwolscine was absent from the superfusion fluid, the concentration‐response curve for BDF 6143 (a mixed α2‐adrenoceptor antagonist/imidazoline receptor agonist) for its facilitatory effect on electrically‐evoked [3H]‐noradrenaline release was bell‐shaped. In the presence of 1 μM rauwolscine, BDF 6143 and cirazoline concentration‐dependently inhibited the evoked [3H]‐noradrenaline release. 6 In human atrial appendages, non‐adrenoceptor [3H]‐idazoxan binding sites were identified and characterized. The binding of [3H]‐idazoxan was specific, reversible, saturable and of high affinity (KD: 25.5 nM). The specific binding of [3H]‐idazoxan (defined by cirazoline 0.1 mM) to membranes of human atrial appendages was concentration‐dependently inhibited by several imidazolines and guanidines, but not by rauwolscine and agmatine. In most cases, the competition curves were best fitted to a two‐site model. 7 The rank order of affinity for the high affinity site (in a few cases for the only detectable site; cirazoline=idazoxan>BDF 6143>DTGclonidine) is compatible with the pharmacological properties of I2‐imidazoline binding sites, but is clearly different from the rank order of potency for inhibiting evoked noradrenaline release from sympathetic nerves in the same tissue. 8 It is concluded that noradrenaline release in the human atrium and, less well established, in the pulmonary artery is inhibited via presynaptic imidazoline receptors. These presynaptic imidazoline receptors appear to be related to those previously characterized in rabbit aorta and pulmonary artery, but differ clearly from I1 and I2 imidazoline binding sites.

Inhibition of noradrenaline release via presynaptic 5-HT1Da receptors in human atrium

In segments of human right atrial appendages preincubated with [3H]noradrenaline and superfused with physiological salt solution containing desipramine and corticosterone, we determined the effects of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists on tritium overflow evoked by transmural electrical stimulation (2 Hz).Tritium overflow was inhibited by 5-HT, 5-carboxamidotryptamine (5-CT), 5-methoxytryptamine (5-McOT), 5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)-1H indole succinate (RU 24969) and sumatriptan. Yohimbine and oxymetazoline (in the presence of idazoxan) also inhibited tritium overflow. The inhibitory potency of the drugs was significantly correlated with their affinity for 5-HTID receptors in human brain and for cloned human 5-HT1Dα and 5-HT1Dβ receptors, but not with their affinity for 5HT1B, 5-HT1E, 5-HT1F, 5-HT2A, 5-HT2B, 5-HT2C, 5-HT3, 5-HT5A, 5-HT5B and 5-HT7 receptors. The potency order 5-CT >5-HT >5-MeOT is opposite to the order of affinities reported for 5-HT6 binding sites. The preferential 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetraline (up to 0.3 μM) and the selective 5-HT4 receptor agonist cisapride (up to 1 μM) failed to inhibit tritium overflow. L-694,247, a potent 5-HTIDβ receptor agonist, did not inhibit tritium overflow, but counteracted the inhibitory effect of 5-HT. Ketanserin at a concentration which should block 5-HT1Da but not 5-HT1Dβ receptors and methiothepin at a concentration which may be assumed to block both 5-HT1Dα and 5-HT1Dβ receptors antagonized the inhibitory effect of 5-HT. Propranolol and ondansetron did not modify the 5-HT-induced inhibition of release. In conclusion, noradrenaline release in human right atrial appendages is inhibited via 5-HT receptors which are located on the noradrenergic axon terminals. These inhibitory presynaptic 5-HT receptors belong to the 5-HTID subfamily. The ability of ketanserin to antagonize the inhibitory effect induced by activation of these receptors suggests that they can be subclassified as 5-HT1Dα.

Modulation of noradrenaline release from the sympathetic nerves of the human saphenous vein and pulmonary artery by presynaptic EP3- and DP-receptors.

1 Spirally cut strips of the human saphenous vein and pulmonary artery were used to determine the pharmacological properties of the presynaptic prostanoid receptors involved in the modulation of sympathetic [3H]‐noradrenaline release. Strips preincubated with [3H]‐noradenaline were superfused with physiological salt solution containing inhibitors of uptake1 and uptake2 and rauwolscine to eliminate involvement of presynaptic α2‐adrenoceptors. Tritium overflow was evoked by transmural electrical stimulation (standard frequency: 2 Hz). 2 In the saphenous vein, prostaglandin E2 (PGE2) inhibited the electrically‐evoked tritium overflow; at the highest concentration investigated, tritium overflow was inhibited by more than 75% and the pEC50 value was 7.00. These effects were mimicked by prostaglandin E1, the EP1/EP3 receptor agonist, sulprostone and the EP2/EP3 receptor agonist, misoprostol with the rank order (pEC50): sulprostone (8.60) > PGE1 (7.25) > misoprostol (6.96). This rank order of potency suggests that the inhibitory effect of the drugs is mediated by presynaptic EP3‐receptors. In contrast, PGF2α did not inhibit evoked tritium overflow; the IP/EP1 receptor agonist iloprost and the stable thromboxane A2 analogue U 46619 (9,11‐dideoxy‐11α,9α‐epoxy‐methanoprostaglandin F2α) produced inhibition only at concentrations above 1 μm. 3 The EP1‐receptor antagonist, AH 6809 (6‐isopropoxy‐9‐oxoxanthene‐2‐carboxylic acid) had no effect on the evoked tritium overflow nor did it modify the inhibitory effect of PGE2, further excluding involvement of inhibitory presynaptic EP1‐receptors. 4 PGD2 caused a facilitation of evoked tritium overflow in the saphenous vein; this facilitation is probably mediated by presynaptic DP‐receptors, since it was abolished by the selective DP‐receptor antagonist, BW A868C (3‐benzyl‐5‐(6‐carboxyhexyl)‐1‐(2‐cyclohexyl‐2‐hydroxyethylamino)hydantoin). 5 In the pulmonary artery, sulprostone (pEC50 value 8.35), misoprostol (7.70) and PGE2 (6.80) inhibited electrically‐evoked tritium overflow. This rank order of potency is consistent with the involvement of inhibitory presynaptic EP3‐receptors. 6 These results suggest that the sympathetic nerve fibres of both human saphenous vein and pulmonary artery are endowed with presynaptic inhibitory EP3 receptors. The EP3‐receptors do not interact with the α2‐autoreceptors. In addition, the human saphenous vein seems to be endowed with presynaptic facilitatory DP‐receptors.

Transit time flow measurement in on-pump and off-pump coronary artery surgery

OBJECTIVE Transit time flow measurement is frequently used during coronary artery bypass with and without cardiopulmonary bypass to detect graft dysfunction resulting from technical errors. METHODS Intraoperative transit time flow measurement measurements of 896 patients requiring surgery for double- or triple-vessel disease were reviewed retrospectively. Six-hundred and ninety-five patients were operated on-pump (Group A: coronary artery bypass with cardiopulmonary bypass), and 201 patients off-pump (Group B: coronary artery bypass without cardiopulmonary bypass). Transit time flow measurement measurements were analyzed for mean flow (mL/min). In total, measurements of 2247 grafts were analyzed. RESULTS Transit time flow measurement flows were lower in coronary artery bypass without cardiopulmonary bypass patients (left internal thoracic artery to left anterior descending artery: Group A, 37 [31, 40] mL/min vs Group B, 24 [20, 26] mL/min; saphenous vein graft to left anterior descending artery: Group A, 46.5 [40, 56] mL/min vs Group B, 21 [14, 57] mL/min. Troponin I release was reduced in the coronary artery bypass without cardiopulmonary bypass patients, with median values of 7.8 [7.0, 8.3] microg/L in Group A and 1.2 [0.9, 2.3] microg/L in Group B. CONCLUSION Evaluation of transit time flow measurement is valuable in determining coronary graft patency after coronary artery bypass with cardiopulmonary bypass and coronary artery bypass without cardiopulmonary bypass. Decreased troponin I release suggests a myocardial benefit of coronary artery bypass without cardiopulmonary bypass compared to coronary artery bypass with cardiopulmonary bypass, although the intraoperative transit time flow measurement flow measurements are markedly lower.

Presynaptic imidazoline receptors and α2-adrenoceptors in the human heart: discrimination by clonidine and moxonidine

The involvement of presynaptic α2-autoreceptors and imidazoline receptors in the modulation of noradrenaline release was investigated in strips from human atrial appendages preincubated with [3H]noradrenaline and superfused with medium containing desipramine and corticosterone. Electrical impulses were applied transmurally at 2 Hz. The imidazoline derivatives moxonidine and clonidine reduced the evoked tritium overflow in a concentration-dependent manner. Moxonidine was 18-fold more potent than clonidine. The concentration-response curve for moxonidine, but not for clonidine was shifted to the right by the α2-adrenoceptor antagonist rauwolscine. The apparent pA2 value of rauwolscine against moxonidine was 8.41. An inhibitory effect was also observed for the imidazoline derivative BDF 6143 (4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline), a mixed α2-adrenoceptor antagonist/imidazoline receptor agonist; BDF 6143 was about 2-fold more potent than clonidine. Rauwolscine (1 μM) did not substantially shift the concentration-response curve of BDF 6143.It is concluded that noradrenaline release in the human atrium is inhibited not only via presynaptic α2-autoreceptors but also via presynaptic non-I1, non-I2 imidazoline receptors. The failure of rauwolscine to antagonize the inhibitory effect of clonidine suggests that clonidine preferentially stimulates the presynaptic imidazoline receptors; the α2-adrenoceptor component of its action is probably suppressed by an inhibitory interaction between the two receptors. In contrast, moxonidine acts via presynaptic α2-autoreceptors, leaving the presynaptic imidazoline receptor unaffected.