Beta 1- and beta 2-adrenoceptor-mediated relaxation in human internal mammary artery and saphenous vein: unchanged beta- and alpha-adrenoceptor responsiveness after chronic beta 1-adrenoceptor blockade

1. We have recently reported that patients taking beta 1-adrenoceptor-selective antagonists exhibit marked sensitization of beta 2-adrenoceptor responses but unaltered beta 1-adrenoceptor responses in the heart, both in vitro and in vivo. We therefore investigated beta 1- and beta 2-adrenoceptor-mediated relaxant responses in rings of human internal mammary artery and saphenous vein without endothelium, taken from beta 1-blocked and non-beta-blocked patients undergoing coronary artery bypass graft surgery, for comparison. We also examined alpha 1-adrenoceptor-mediated contraction in these vessels, to determine whether beta 1-blockade had any cross-regulatory effect. 2. Following alpha-blockade with 10 microM phenoxybenzamine, both noradrenaline adrenaline produced concentration-dependent relaxations in both blood vessels, their effects being mediated predominantly through beta 2-adrenoceptors; a lesser beta 1-adrenoceptor component to relaxation was also found in internal mammary artery and a minor beta 1-adrenoceptor component was present in saphenous vein. No differences were found in beta 1- or in beta 2-adrenoceptor-mediated vasorelaxation between beta 1-blocked and non-beta-blocked patients. 3. Methoxamine produced concentration-dependent contractions in both blood vessels, and the potency and efficacy were not significantly different between vessels from beta 1-blocked and from non-beta-blocked patients. 4. These findings indicate that, in these tissues, which possess a relatively minor beta 1-adrenoceptor component in contrast to myocardial tissue, chronic beta 1-blocker treatment does not alter either beta 1- or beta 2-adrenoceptor responses. Likewise, in such tissues, alpha 1-adrenoceptor responses are unaffected by prior beta 1-blockade.

Human S31 serotonin receptor clone encodes a 5-hydroxytryptamine1E-like serotonin receptor

We reported recently the molecular cloning of a genomic fragment, designated S31, that has an open reading frame of 1095 nucleotides, encoding a protein of 365 amino acids. Amino acid similarity analysis suggested that the S31 protein could be a guanine nucleotide-binding protein-coupled receptor pertaining to the serotonin receptor subfamily. Expression of the S31 open reading frame in murine L cells confirmed this, because it led to the appearance of serotonin-mediated inhibition of adenylyl cyclase activity, which was absent in the recipient L cells. We now report some aspects of the pharmacological profile of this receptor. We found that the relative potencies with which 5-hydroxytryptamine, 5-carboxamidotryptamine, methysergide, ergotamine, 8-hydroxydipropylaminotetralin, and trifluoromethylphenylpiperazine promote inhibition of adenylyl cyclase are as follows: 5-hydroxytryptamine >> methysergide >> ergotamine >> trifluoromethylphenylpiperazine > or = 8-hydroxydipropylaminotetralin > 5-carboxyamidotryptamine. This corresponds to the rank order of potencies assigned for these drugs for the 1E subtype of serotonin receptors discovered by Leonhardt and collaborators in human brain [J. Neurochem. 53:465-471 (1989)].

Effects of 6-hydroxydopamine on pre- and post-junctional 5-HT1-like receptor-mediated responses in dog saphenous vein

To investigate whether 5-HT1-like receptor-mediated inhibition of adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation occurs in nerves or smooth muscle of saphenous vein, infusions of 6-hydroxydopamine (6-OHDA) were administered to dogs with the aim of inducing sympathetic nerve damage. The effects of 6-OHDA on other 5-HT1-like receptor-mediated responses at the pre- and post-junctional level were investigated for comparison by studying 5-hydroxytryptamine (5-HT)-induced inhibition of 3H-noradrenaline release and contraction of smooth muscle respectively. Disruption of nerve function by 6-OHDA was revealed by the lack of catecholaminergic fluorescence and neurogenic contractile responses in saphenous veins from dogs treated with 6-OHDA. In addition, severe impairment of neuronal uptake mechanisms were apparent since basal efflux of 3H-noradrenaline, electrically-evoked release of 3H-noradrenaline and remaining 3H-noradrenaline content were considerably reduced. Some 3H-noradrenaline was taken up and released in 6-OHDA-treated tissues which is consistent with the existence of nerve varicosities resistant to the present dosing regime of 6-OHDA, an observation substantiated by electron microscopy studies showing inconsistent lesions of nerve terminals. 6-OHDA pre-treatment potentiated the smooth muscle contractile responses mediated by 5-HT1-like receptors as well as potentiating 5-HT-evoked inhibition of prostaglandin E2-stimulated cyclic AMP accumulation. It did not, however, affect 5-HT-induced inhibition of 3H-noradrenaline release. The present results suggest that inhibition of cyclic AMP accumulation by 5-HT occurs predominantly in smooth muscle.

Sumatriptan contracts large coronary arteries of beagle dogs through 5-HT1-like receptors

The effects of 5-hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT), methysergide and sumatriptan were studied on endothelium-denuded rings of beagle dog large coronary arteries. Submicromolar concentrations of the compounds contracted the rings with the order of potency 5-CT > 5-HT > sumatriptan = methysergide. Concentrations greater than 2 microM of both 5-HT and 5-CT, and 60 mumol/l methysergide also caused concentration-dependent relaxation. Sumatriptan did not cause relaxation. Peak intrinsic activities relative to the plateau contraction to sumatriptan (1.00), were 5-CT 0.47, 5-HT 0.87 and methysergide 0.51. Ketanserin 1 mumol/l affected neither contractile responses nor relaxant responses to 5-CT, methysergide and sumatriptan and only caused marginal blockade of the contractile effects of 5-HT. Methiothepin 200 nM shifted the concentration-contractile response curves by around 2 log units, as expected from its affinity for 5-HT1-like receptors. The rank order of contractile potency of the agonists, the antagonism by methiothepin and the resistance to blockade by ketanserin are consistent with a nearly exclusive involvement of 5-HT1-like receptors. Isolated large coronary arteries from beagle dogs may be a suitable model for the study of human coronary artery 5-HT1-like receptors that are involved in the spasm observed with 5-HT and sumatriptan.

Pharmacology of portal-systemic collaterals in portal hypertensive rats: role of endothelium

The portal-systemic collateral circulation of portal hypertensive rats was studied. The collaterals were perfused through the mesenteric vein with Krebs solution, which was allowed to escape through the jugular veins. The portal-collateral resistance can be quantitated from slopes of the pressure-flow relationships. In collaterals perfused at constant flow, both norepinephrine (NE) and 5-hydroxytryptamine (5-HT) increased the perfusion pressure. Phentolamine caused surmountable antagonism of the constrictor effects of NE, suggesting an involvement of alpha-adrenoceptors. The effects of 5-HT were competitively blocked by the 5-HT2 receptor-selective antagonist ICI 169,369. Isoproterenol dilated NE-preconstricted collaterals. The effect of isoproterenol was blocked by propranolol, demonstrating that the effect was mediated by beta-adrenoceptors. Acetylcholine (ACh) dilated NE-preconstricted collaterals. The dilatation effect of ACh was absent in collaterals in which the endothelium was removed. The competitive inhibitor of the nitric oxide synthase, N omega-nitro-L-arginine (L-NNA), increased collateral resistance and prevented the ACh-induced dilatation of the collaterals. The constrictor response to L-NNA and the blockade of the ACh-induced relaxation by both L-NNA and removal of endothelium are consistent with an involvement of nitric oxide. This experimental model can thus be used to explore the pathophysiological and the pharmacological properties of the collateral venous bed in portal hypertensive states.

Molecular cloning of a human serotonin receptor (S12) with a pharmacological profile resembling that of the 5-HT1D subtype

We report the molecular cloning of a fragment of human genomic DNA called S12, containing an open reading frame of 1170 nucleotides, which encodes a receptor for serotonin of 390 amino acids. The receptor function of the S12 protein was demonstrated by functional expression in mouse LS12 cells obtained by stable transfection of Ltk- cells, and LM5S12 cells, derived from LM5 cells (Ltk- cells previously transfected with the M5 muscarinic acetylcholine receptor). Adenylyl cyclase studies showed that the S12 receptor is able to mediate inhibition of adenylyl cyclase in response to serotonin in both types of cells. As studied in LM5S12 cells, the S12 receptor did not promote Ca2+ mobilization from internal stores, nor did it significantly modulate the sustained increase in [Ca2+]i elicited by stimulation of the phospholipase C stimulating M5 acetylcholine receptor. The pharmacologic profile of S12 as seen in adenylyl cyclase assays is as follows: (EC50 in nM): serotonin, full agonist (37 nM), 5-carboxamidotryptamine, full agonist (10 nM), sumatriptan, full agonist (50 nM), metergoline, partial agonist (10 nM), methysergide, partial agonist (40 nM), yohimbine, partial agonist (150 nM), metitepin, antagonist (KB = 0.7 to 1.2 nM). We propose that the human S12 serotonin receptor is a receptor of the 5-hydroxytryptamine1D subtype.

A 5-HT4-like receptor in human left atrium

The effects of 5-hydroxytryptamine (5-HT) on left atrial preparations obtained from 5 patients with terminal heart failure who were undergoing heart transplant surgery were investigated. The preparations were paced under isometric conditions. In the presence of (-)-pindolol 1 mumol/l (to block beta-adrenoceptors) and cocaine 6 mumol/l (to block tissue uptake of 5-HT) 5-HT increased contractile force with a pEC50 of 7.0. The maximum effect of 5-HT amounted to 24.5% of that caused by a maximally effective concentration of (-)-isoprenaline (200 mumol/l) and 25% of that caused by 6.75 mmol/l CaCl2. The effects of 5-HT were competitively antagonised by 3 alpha-tropanyl-1H-indole-3-carboxylate (ICS 205-930) with a pKB of 6.8. The effects of 5-HT on cyclic AMP levels and cyclic AMP-dependent protein kinase activity were also studied using left atrial tissues from one of the patients; 5-HT increased the cyclic AMP content and stimulated the kinase. The results are consistent with the existence of a human left atrial 5-HT receptor which is similar to the recently identified human right atrial 5-HT receptor that resembles the 5-HT4 receptor. The left atrial 5-HT4-like receptor is functional in tissues obtained from patients with terminal heart failure.

Molecular cloning of a human gene (S31) encoding a novel serotonin receptor mediating inhibition of adenylyl cyclase

We report the molecular cloning of human gene (S31) containing an open reading frame of 1095 nucleotides, which encodes a protein of 365 amino acids. The encoded protein contains seven hydrophobic putative transmembrane domains considered the hallmark of G protein-coupled receptors. The amino acid sequence shows highest homology to receptors for serotonin (5-hydroxytryptamine). Expression of this receptor in murine Ltk- cells conferred upon these cells the ability to respond to serotonin by inhibition of adenylyl cyclase. No response was observed to isoproterenol, epinephrine, histamine, dopamine or melatonin in the transfected cells. We propose that the human gene S31 encodes a novel serotonin receptor.

The antimigraine drugs ergotamine and dihydroergotamine are potent 5-HT1C receptor agonists in piglet choroid plexus

1. Fozard & Gray (1989) proposed that migraine is mediated by stimulation of 5-HT1C receptors. We have examined the interaction of two effective anti-migraine agents, ergotamine and dihydroergotamine (DHE), with these receptors. Binding (inhibition of labelling by [3H]-mesulergine) and agonist activity (phosphoinositide hydrolysis) were measured in piglet choroid plexus, a tissue rich in 5-HT1C receptors. 2. The pKD for [3H]-mesulergine binding was 8.4. Ergotamine and DHE both inhibited [3H]-mesulergine binding with a pKD of 7.1. This was similar to the potency of m-chlorophenylpiperazine (m-CPP) (pKD 7.4) and rather less than that of 5-hydroxytryptamine (5-HT) (pKD 8.1). 3. Both ergotamine and DHE were full agonists (pEC50S 7.5 and 7.6 respectively) with potencies similar to that of 5-HT (pEC50 7.7) and greater than that of m-CPP (pEC50 7.1). Mesulergine 10(-7) M produced near-parallel rightward shifts of the concentration-response curves for all these agents of 1.8-2.2 log units, consistent with an action of the agonists at the same receptor. 4. There was no effect of prazosin, spiperone, mepyramine or atropine on the phosphoinositide hydrolysis induced by ergotamine, ruling out an action via alpha 1-adrenoceptors, 5-HT2, histamine H1, or muscarinic receptors. 5. It is concluded that, together with 5-HT, ergotamine and DHE are the most potent 5-HT1C agonists reported so far. These findings do not support the theory that 5-HT1C receptor activation causes migraine.

A 5-HT4-like receptor in human right atrium

The effects of 5-carboxamidotryptamine (5-CT) and the gastrokinetic benzamides renzapride and cisapride on contractile force were investigated using isolated paced right atrial appendages from patients treated with beta-adrenoceptor blocking agents who were undergoing open heart surgery. These effects were compared to those of 5-hydroxytryptamine (5-HT). The effects of the drugs on atrial cyclic AMP levels and cyclic AMP-dependent protein kinase ratios were also investigated. The drugs all increased contractile force of rank order of potency was 5-HT greater than renzapride greater than cisapride greater than 5-CT. The maximum responses, expressed as a fraction of the response to 200 mumol/l (-)-isoprenaline, were 5-HT 0.6, 5-CT 0.6, renzapride 0.4 and cisapride greater than or equal to 0.2, suggesting that the latter two are partial agonists. 5-HT, 5-CT and renzapride but not cisapride caused significant shortening of time to peak force. The effects of the four drugs were blocked by mumolar concentrations of ICS 205-930, suggesting an involvement of 5-HT4 receptors. As expected of partial agonists both renzapride and cisapride caused simple competitive antagonism of the positive inotropic effects of 5-HT. The estimated equilibrium dissociation constants pKP (-log mol/l KP) were 6.7 for renzapride and 6.2 for cisapride. 5-CT at concentrations up to 10 mumol/l did not antagonise the effects of 5-HT. In the presence of (+/-)-propranolol 0.4 mumol/l, 5-HT 10 mumol/l, 5-CT 100 mumol/l, renzapride 10 mumol/l and cisapride 40 mumol/l significantly increased cyclic AMP levels. 5-HT and renzapride also significantly increased cyclic AMP-dependent protein kinase activity, whereas 5-CT caused only marginal stimulation and cisapride was ineffective. The results confirm the existence of a human right atrial 5-HT receptor that is similar in nature to, but not necessarily identical with, the 5-HT4 receptor of mouse embryonic colliculi neurones. The main difference is that in human right atrium the benzamides are less potent and efficacious than 5-HT and that cisapride is less potent and less efficacious than renzapride while in mouse embryonic colliculi these two benzamides are equipotent with and more efficacious agonists than 5-HT. We designate the human right atrial 5-HT receptor 5-HT4-like. The human right atrial 5-HT4-like receptor greatly resembles porcine sinoatrial and left atrial 5-HT4-like receptors and also appears to be similar to 5-HT4-like receptors of guinea-pig ileum and rat oesophagus.

Regional differences of beta 1- and beta 2-adrenoceptor-mediated functions in feline heart. A beta 2-adrenoceptor-mediated positive inotropic effect possibly unrelated to cyclic AMP

The effects of (-)-adrenaline and (-)-noradrenaline were studied on isolated preparations of kitten heart. To define the contribution of beta 1-adrenoceptors (beta 1AR) and beta 2-adrenoceptors (beta 2AR) we used as tools the highly beta 1AR-selective antagonist CGP 20,712 A and non-linear analysis of antagonism. The beta 2AR-mediated responses to the catecholamines, disclosed by CGP 20,712 A, were verified by blockade with the beta 2AR-selective ICI 118,551. The relative density and contribution of beta 1AR and beta 2AR to (-)-adrenaline- and (-)-noradrenaline-induced adenylyl cyclase stimulation was also estimated in right ventricular membranes. 1. In the sinoatrial pacemaker (-)-adrenaline caused positive chronotropic effects through both beta 1AR and beta 2AR while (-)-noradrenaline does so predominantly through beta 1AR. During beta 1AR blockade (-)-adrenaline did cause the same maximum effects through beta 2AR as (-)-noradrenaline did through beta 1AR. 2. In left atria (-)-adrenaline caused positive inotropic effects predominantly through beta 1AR. CGP 20,712 A also uncovered a beta 2AR component at high (-)-adrenaline concentrations comprising one third of the maximum beta 1AR-mediated response. 3. Receptor binding assays revealed that 80% of right ventricular beta AR were beta 1AR and 20% beta 2AR. Consistent with this finding, around 80% of the adenylyl cyclase stimulation by both (-)-noradrenaline and (-)-adrenaline was mediated through beta 1AR, around 20% through beta 2AR. The positive inotropic effects of (-)-noradrenaline appeared to be nearly exclusively mediated through beta 1AR in right ventricular papillary muscles. 4. The positive inotropic effects of (-)-adrenaline were quite variable with regard to beta 1AR and beta 2AR in right ventricular papillary muscles. Although beta 1AR-mediated effects are predominant in many muscles with only a small contribution of beta 2AR, in some muscles beta 2AR mediated around 50% of the maximum effect elicited through beta 1AR. In 3 out of 17 muscles beta 2AR mediated the same maximum effect of (-)-adrenaline as beta 1AR. 5. On occasion, we found marked beta AR heterogeneity amongst two muscles from the same right ventricle. One muscle only exhibited beta 1AR-mediated effects of (-)-adrenaline whereas in the other muscle maximal effects could be elicited through beta 2AR.(ABSTRACT TRUNCATED AT 400 WORDS)

Some aspects of heart beta adrenoceptor function

Heart rate and force can be increased by noradrenaline and adrenaline through an interaction with both beta 1-adrenoceptors (beta 1AR) and beta 2-adrenoceptors (beta 2 AR). Several ionic currents (I) can flow upon beta AR activation: ICa (through either beta 1AR or beta 2AR), INa, IK, and ICl. Calcium currents (ICa) can be increased directly by the alpha s unit of a GTP binding protein, Gs, or by coupling of Gs to adenylyl cyclase with subsequent formation of cyclic AMP, release of the catalytic unit of cyclic AMP-dependent protein kinase, and phosphorylation of calcium channels and other proteins. Chronic exposure (days or months), but not acute exposure (hours), to a catecholamine downregulates human heart beta 1AR. Acute desensitization partially uncouples human heart beta AR from the adenylyl cyclase. Both acute and chronic desensitization reduce positive inotropic responses to catecholamines. In human heart, catecholamine-induced activation of one beta 2AR causes the production of at least four times more cyclic AMP than activation of one beta 1AR. Chronic treatment of patients with beta 1AR-selective blockers paradoxically induces selective inotropic beta 2AR hyperresponsiveness, presumably by increasing coupling of beta 2AR to Gs. Several partial agonists with high affinity for heart beta 1AR and beta 2AR cause stimulant effects that are resistant to blockade of beta 1AR and beta 2AR. Such nonconventional partial agonists could perhaps interact with beta AR that resemble beta 3 adrenoceptors.

Primate vascular responses to octimibate, a non-prostanoid agonist at the prostacyclin receptor

1. Octimibate is a potent inhibitor of human platelet aggregation, and appears to act (at least in part) through the prostacyclin receptor, as described in the preceding paper. Here, the vascular effects, both in vitro and in vivo, of octimibate have been compared to those of the stable prostacyclin (PGI2) mimetic, iloprost. Since octimibate shows extensive species variation and is potent at inhibiting platelet aggregation in primates, all of the experiments reported here have been carried out with primate tissue or in vivo in cynomolgus monkeys. 2. Activation of adenylyl cyclase in human lung membranes appears to involve stimulation of the vascular PGI2 receptor. Octimibate, as well as iloprost, stimulates adenylyl cyclase in this preparation. The EC50 values for iloprost and octimibate are 50 nM and 340 nM respectively. These values are similar to those seen with human platelet membranes. As with platelets, the maximal activation achievable with octimibate is 60% of that seen with iloprost. This result suggests that octimibate is a partial agonist for stimulation of adenylyl cyclase. 3. Iloprost (10-100 nM) relaxes human coronary and mesenteric artery precontracted with KCl, and also relaxes cynomolgus monkey aorta precontracted with phenylephrine. Octimibate appears to be a partial agonist for relaxation of human coronary artery precontracted with KCl; the intrinsic activity of octimibate (10 microM) is 0.15 compared to iloprost, and octimibate surmountably antagonizes the relaxant effects of iloprost with a Kp of 200 nM. Octimibate (up to 10 microM) evokes only weak relaxation of human mesenteric artery (precontracted with KCl) and cynomolgus monkey aorta (precontracted with phenylephrine). 4. The effects of iloprost and octimibate were compared in vivo in cynomolgus monkeys. In addition to inhibiting ex vivo platelet aggregation, both compounds cause hypotension with little effect on heart rate. The dose-response curves for inhibition of ex vivo platelet aggregation and a fall in mean arterial blood pressure were compared. The dose-separation (i.e., the relative differences in effective concentrations) for the two responses is similar with both iloprost and octimibate. 5. Since the pern; beral resistance vessels are intimately involved in regulation of systemic arterial blood pressure, the effects of both agents were tested on human peripheral resistance vessels (150-400pm diameter) in vitro. These vessels are relaxed by both iloprost and octimibate following precontraction with KCI. The IC50 value for iloprost is 44nM, and 1.7 microM octimibate evokes 50% of the maximal relaxation obtained with iloprost. Thus, the relative potencies of the two compounds in relaxing human subcutaneous resistance vessels are similar to their relative potencies in inhibiting platelet responses. This result correlates with the lack of platelet versus vascular selectivity seen with the in vivo monkey studies. 6. These results suggest that octimibate, a partial agonist at the prostacyclin receptor, is unable to discriminate between platelet and vascular prostacyclin receptors in primates.

Octimibate, a potent non-prostanoid inhibitor of platelet aggregation, acts via the prostacyclin receptor

1. Octimibate, 8-[(1,4,5-triphenyl-1H-imidazol-2-yl)oxy]octanoic acid, is reported to have antithrombotic properties. This is in addition to its antihyperlipidaemic effects which are due to inhibition of acylCoA:cholesterol acyltransferase (ACAT). The aim of this study was to investigate the mechanism of the antithrombotic effect of octimibate, and to determine whether the effects of octimibate are mediated through prostacyclin receptors. 2. In suspensions of washed (plasma-free) human platelets, octimibate is a potent inhibitor of aggregation; its IC50 is approx. 10 nM for inhibition of aggregation stimulated by several different agonists, including U46619 and ADP. The inhibitory effects of octimibate on aggregation are not competitive with the stimulatory agonist; the maximal response is suppressed but there is no obvious shift in potency of the agonist. In platelet-rich plasma, octimibate inhibits agonist-stimulated aggregation with an IC50 of approx. 200 nM. 3. Octimibate also inhibits agonist-stimulated rises in the cytosolic free calcium concentration, [Ca2+]i, in platelets. Both Ca2+ influx and release from intracellular stores are inhibited. The effects of octimibate on aggregation and [Ca2+]i are typical of agents that act via elevation of adenosine 3':5'-cyclic monophosphate (cyclic AMP). Similar effects are seen with forskolin, prostacyclin (PGl2) and iloprost (a stable PGl2 mimetic). 4. Octimibate increases cyclic AMP concentrations in platelets and increases the cyclic AMP-dependent protein kinase activity ratio. Octimibate stimulates adenylyl cyclase activity in human platelet membranes, with an EC50 of 200 nM. The maximal achievable activation of adenylyl cyclase by octimibate is 60% of that obtainable with iloprost. Octimibate has no effect on the cyclic GMP-inhibited phosphodiesterase (phosphodiesterase-ITI), which is the major cyclic AMP-degrading enzyme in human platelets.5. Octimibate inhibits, apparently competitively, the binding of [3H]-iloprost (a stable PGl2 mimetic) to platelet membranes; the estimated Ki is 150 nm. 6. The platelets of different species show considerable differences in the apparent potency of their inhibition of aggregation by octimibate; platelets from cynomolgus monkeys are 3 fold more sensitive than those from humans, while rat, cat and cow platelets are 50, 100, and 250 fold less sensitive than human platelets. The sensitivity of these different species to iloprost, however, varies over a range of only 10 fold with no obvious difference between primates and non-primates. 7. Octimibate appears to be a potent agonist (aggregation), or partial agonist (adenylyl cyclase), at prostacyclin receptors and is the first non-prostanoid agent of this type to be identified. The species differences in relative potency of octimibate and iloprost may reflect the existence of receptor subtypes.

5-HT4-like receptors in mammalian atria

Atrial myocardium of man and pig possess receptors that mediate positive inotropic effects and/or positive chronotropic effects of 5-hydroxytryptamine (5-HT). These 5-HT receptors are blocked with moderate affinity (pKB = 6.7-6.9) by 3 alpha-trophanyl-1H-indole-3-carboxylate (ICS 205930) but not by antagonists of alpha- and beta-adrenoceptors, or 5-HT1 subtypes, 5-HT2 and 5-HT3 receptors. In human and porcine atrium the receptors also mediate 5-HT-induced increases of both cyclic AMP levels and cyclic AMP-dependent protein kinase activity. Human and porcine atrial 5-HT receptors are also partially activated by the benzamides renzapride and cisapride albeit with lower potency and efficacy than 5-HT. The properties of these atrial 5-HT receptors resemble those of "so called" 5-HT4 receptors, positively coupled to the adenylyl cyclase of mouse embyonic colliculi neurons. However, for colliculi 5-HT4 receptors the benzamides have greater efficacy and ICS 205930 lower affinity than for atrial 5-HT receptors. The atrial 5-HT receptors of man and pig are, therefore, designated 5-HT4-like receptors. Guinea-pig night atria appear to have a mixture of 5-HT3- and 5-HT4-like receptors involved in the mediation of positive chronotropic effects of 5-HT.

Piglet sinoatrial 5-HT receptors resemble human atrial 5-HT4-like receptors

5-Hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT) and the gastrointestinal kinetic benzamides renzapride and cisapride caused tachycardia in spontaneously beating right atria of piglet in the presence of 400 nmol/l (+/-)-propranolol and 6 mumol/l cocaine. The maximum tachycardia caused by agonists, compared to that evoked by 200 mumol/l (-)-isoprenaline, was 63% for 5-HT, 50% for 5-CT, 50% for renzapride and 28% for cisapride. The rank order of potency was 5-HT greater than renzapride greater than cisapride greater than 5-CT. The effects of the agonists, but not those of (-)-isoprenaline, were antagonised by 3 alpha-tropanyl-1H-indole-3-carboxylic acid (ICS 205930); the pKB of ICS 205930 (vs 5-HT) was 6.9. These characteristics suggest that piglet sinoatrial 5-HT receptors are similar to "so-called" 5-HT4 receptors previously described in mouse colliculi neurons. Piglet sinoatrial 5-HT4-like receptors resemble the human atrial 5-HT receptors that mediate positive inotropic effects of 5-HT.

A 5-hydroxytryptamine receptor in human atrium

1. The effects of 5-hydroxytryptamine (5-HT) were investigated on right atrial appendages obtained from patients treated with beta-adrenoceptor blocking agents who were undergoing open heart surgery. Atrial strips were paced under isometric conditions. 2. 5-HT increased contractile force to approximately one half of the force produced by a saturating concentration of (-)-isoprenaline. Both 5-HT and (-)-isoprenaline accelerated the onset of relaxation, as indicated by an abbreviation of time to peak force. 3. The effects of 5-HT were resistant to blockade by 0.4 microM (+/-)-propranolol, 1 microM (-)-pindolol, 0.4 microM methiothepin, 4 microM yohimbine, 0.4 microM ketanserin, 10 microM phenoxybenzamine, 1 microM methysergide, 2 microM MDL 72222 and 20 microM granisetron. 4. Cocaine 6 microM potentiated the effects of 5-HT, increasing the pEC50 from 6.6 to 7.4. The inotropic potency of 5-HT is five times greater than that of (-)-noradrenaline. 5. ICS 205930 antagonized competitively the effects of 5-HT with a pKB of 6.7. 6. In the presence of 0.4 microM (+/-)-propranolol, 10 microM 5-HT increased both adenosine 3':5' cyclic-monophosphate (cyclic AMP) levels and cyclic AMP-dependent protein kinase activity by approximately one half and two thirds respectively, of the corresponding effects of 200 microM (-)-isoprenaline. 7. Both the increase in cyclic AMP levels and the stimulation of protein kinase activity are consistent with the inotropic effects of 5-HT being mediated by cyclic AMP-dependent phosphorylation of Ca2+ channels and of proteins involved in contraction and relaxation. 8. The human atrial 5-HT receptor resembles the neuronal 'so called' 5-HT4 receptor of rodents both in increasing cyclic AMP levels and in its affinity for ICS 205930.

Selective beta 1-adrenoceptor blockade enhances positive inotropic responses to endogenous catecholamines mediated through beta 2-adrenoceptors in human atrial myocardium

We determined the relative contribution of beta 1- and beta 2-adrenoceptor stimulation to the positive inotropic responses of human atrial myocardium to catecholamines. (-)Norepinephrine produced stimulation predominantly through beta 1-receptors and (-)epinephrine through both beta 1- and beta 2-receptors. However, there were marked differences in the responses of tissues from patients treated with the beta 1-selective antagonist atenolol compared with non-beta-blocker-treated patients; surprisingly, beta 2-mediated responses were enhanced, and beta 1-mediated responses were unaltered. There was an enhanced responsiveness to (-)epinephrine (atenolol treated: -log M EC50, 7.57 +/- 0.07; non-beta-blocker treated: -log M EC50, 6.77 +/- 0.17; p less than 0.001), and the relative importance of beta 2-adrenoceptor stimulation was increased for both (-)norepinephrine and (-)epinephrine. In tissues from atenolol-treated patients, salbutamol, a beta 2-selective partial agonist, had an enhanced potency and a greater intrinsic activity (atenolol treated: -log M EC50, 7.13 +/- 0.09; intrinsic activity, 0.86 +/- 0.04; non-beta-blocker treated: -log M EC50, 5.76 +/- 0.44; intrinsic activity, 0.39 +/- 0.13). We investigated possible mechanisms underlying the enhanced responsiveness to beta 2 stimulation. Determination of beta 2-adrenoceptor affinity for salbutamol showed no change of affinity in atenolol-treated patients. Responses of the tissues to the cyclic AMP analogue dibutyryl cyclic AMP were not different between atenolol-treated and non-beta-blocker-treated patients. The results suggest that chronic blockade of beta 1-adrenoceptors causes enhanced coupling of beta 2-adrenoceptors to adenylate cyclase or to other mechanisms leading to increased contractile force.

Catecholamines relax portal and mesenteric veins from normal and portal hypertensive rats

The relaxation by catecholamines of superior mesenteric veins and portal veins, precontracted by 65 mM KCl, was studied after a 2-h exposure to phenoxybenzamine. The veins were obtained from portal-hypertensive and sham-operated rats. The beta 2-selective antagonist ICI 118551 attenuated the relaxation caused by epinephrine and isoproterenol but did not change the relaxation caused by norepinephrine in superior mesenteric veins. The beta 1-specific antagonist CGP 20712A reduced the relaxation caused by norepinephrine but only marginally attenuated those caused by epinephrine and isoproterenol in superior mesenteric veins. The combination of ICI 118551 and CGP 20712A abolished the relaxation caused by the catecholamines in superior mesenteric veins. The relaxation caused by norepinephrine, epinephrine, and isoproterenol was markedly reduced by ICI 118551 in portal veins. The pattern of antagonism was similar in veins from portal-hypertensive and sham-operated rats. The results suggest that both beta 1- and beta 2-adrenoceptors mediate relaxation induced by catecholamines in superior mesenteric veins. Relaxation of portal veins appears to be mediated mostly by beta 2-adrenoceptors.

Dimethylation of the activator ICI 169,369 results in a high-affinity partial deactivator, ICI 170,809, of the arterial 5-hydroxytryptamine2 receptor system

We investigated the mode of action of the potent antagonist ICI 170,809 in the 5-hydroxytryptamine (5-HT)2 receptor system of arterial smooth muscle. We used isolated preparations from rat tail artery and calf coronary artery with the endothelium rubbed off. In tail artery ICI 170,809 (0.3-30 nM) antagonized surmountably and nearly competitively the contractile effects of 5-HT (pKB = 10.0) and partially prevented the depression of 5-HT-induced contractions caused by methysergide. Increasing methysergide concentrations gradually prevented the protective effect of ICI 170,809. The combination of 30 nM ICI 170,809 with 300 nM of its demethylated analog ICI 169,369 (pKB = 8.8) caused surmountable blockade of the effects of 5-HT as expected from competition of the three drugs for the same receptor. In calf coronary artery ICI 170,809 (1-100 nM) reduced the maximum contractile response to 5-HT by 35% and caused competitive antagonism (pKB = 10.4) of the remaining 65% of the responses to 5-HT. ICI 169,369 (100 nM) completely prevented the depression of the maximum response to 5-HT caused by ICI 170,809. Methysergide (3 nM) depressed the maximum response to 5-HT by 65 and 30% in the absence and presence of ICI 170,809. The results are consistent with the existence of two interconvertible states R in equilibrium R' of the 5-HT2 receptor. The equilibrium of R in equilibrium R' is shifted toward R' by methysergide greater than ICI 170,809 much greater than ICI 169,369.

Is there a third heart beta-adrenoceptor?

A series of partial agonists with high affinity for myocardial beta 1- and beta 2-adrenoceptors cause stimulant effects in heart that are resistant to blockade of beta 1- and beta 2-adrenoceptors. The concentrations of partial agonist that cause stimulant effects greatly exceed those that cause blockade. Alberto Kaumann suggests that such non-conventional partial agonists, often analogues of pindolol, may act through a third heart beta-adrenoceptor, which resembles the beta 3-adrenoceptor of white adipocytes and smooth muscle of airways and ileum.

A comparison of the effects of adrenaline and noradrenaline on human heart: the role of beta 1- and beta 2-adrenoceptors in the stimulation of adenylate cyclase and contractile force

The stimulant effects of adrenaline and noradrenaline on contractile force and adenylate cyclase, mediated through beta 1 and beta 2-adrenoceptors, are analysed in isolated atrial and ventricular myocardium of man. The tissues were obtained from patients without advanced heart failure undergoing heart surgery. Usually, both adrenaline and noradrenaline stimulated adenylate cyclase predominantly through ventricular and atrial beta 2-adrenoceptors. Because the relative density of beta 2-adrenoceptors is usually smaller than that of beta 1-adrenoceptors, stimulation of one beta 2-adrenoceptor leads to the production of up to 10 times more cyclic AMP molecules than does stimulation of one beta 1-adrenoceptor. Adrenaline and noradrenaline maximally enhance contractile force through both atrial and ventricular beta 1-adrenoceptors. Adrenaline can also maximally enhance contractile force through atrial beta 2-adrenoceptors. In the ventricle, adrenaline increases force via beta 2-adrenoceptors by up to 60% of its maximal beta 1 response. Noradrenaline can increase atrial and ventricular contractile force through beta 2-adrenoceptors but only at high concentrations. Unexpectedly, in atria from patients treated with the beta 1-selective antagonist atenolol, contractile responses to adrenaline are markedly and selectively augmented through activation of beta 2-adrenoceptors. In atria from atenolol-treated patients equi-inotropic concentrations of adrenaline and noradrenaline acting through beta 2 and beta 1-adrenoceptors, respectively, cause similar increases of cyclic AMP and of cyclic AMP-dependent protein kinase activity.

Neuronally released (-)-noradrenaline relaxes smooth muscle of calf trachea mainly through beta 1-adrenoceptors: comparison with (-)-adrenaline and relation to adenylate cyclase stimulation

The nature of the receptors that mediate the relaxation of smooth muscle by field stimulation, (-)-noradrenaline and (-)-adrenaline was investigated in calf tracheal smooth muscle. The relation between relaxation, stimulation of the adenylate cyclase and density of beta-adrenoceptor subtypes was studied with the help of antagonists of beta 1- and beta 2-adrenoceptors. The question of the existence of catecholamine-containing nerves was also investigated. (1) Nerves with varicosities exhibiting catecholaminergic fluorescence were observed between bundles of smooth muscle cells. (2) Consistent with the existence of adrenergic nerves (-)-noradrenaline was also found. The content of (-)-noradrenaline (1 microgram.g-1 w.w.) was the same in smooth muscle strips from the sublaryngeal region and from the region close to the bifurcation of the calf trachea. (-)-Adrenaline was not detected. (3) Smooth muscle relaxation by low (-)-noradrenaline concentration (0.6-2 nmol/l) was mediated through beta 1-adrenoceptors. Low concentrations of (-)-adrenaline (0.06-1 nmol/l) relaxed through beta 2-adrenoceptors. High concentrations of (-)-noradrenaline and (-)-adrenaline also caused relaxation through beta 2- and beta 1-adrenoceptors respectively. (4) Field stimulation caused relaxation which was half maximal at 0.2-0.8 Hz. Blockade of beta 1-adrenoceptors strongly attenuated the relaxant response to field stimulation and shifted the frequency-relaxation curves to 4 times higher frequencies. These results are consistent with a beta 1-adrenoceptor-mediated relaxation caused by (-)-noradrenaline released from sympathetic nerve endings at low stimulation frequencies. (5) Blockade of beta 2-adrenoceptors failed to reduce smooth muscle relaxation caused by field stimulation at low stimulation frequencies (0.1-1 Hz). However, after beta 1-adrenoceptor blockade, additional blockade of beta 2-adrenoceptors reduced the relaxant effects observed at high frequencies (2-400 Hz). The results suggest that high concentrations of endogenous (-)-noradrenaline cause relaxation through beta 2-adrenoceptors. (6) Binding experiments with 3H-(-)-bupranolol and 3H-ICI 118,551 revealed between 10,000 and 20,000 beta-adrenoceptors per smooth muscle cell of which 3/4 were beta 2 and 1/4 beta 1. The equilibrium dissociation constant of (-)-adrenaline for both beta 1- and beta 2-adrenoceptors and of (-)-noradrenaline for beta 1-adrenoceptors was 1 mumol/l. The affinity of (-)-noradrenaline for beta 2-adrenoceptors was 10 to 20 times lower than for beta 1-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoradiographical localization of binding sites for porcine [125I]endothelin-1 in humans, pigs, and rats: functional relevance in humans

Endothelin-1 (ET-1) is a recently discovered 21 amino acid peptide with potent vasoconstrictor properties. So far, its expression has been found only in porcine aorta, whereas its putative role as the endothelium-derived constricting factor (EDCF) would require it to be expressed and active in most vascular beds. We have used quantitative receptor autoradiography on pig, rat, and some human tissues to determine the distribution and localization of specific binding sites for ET-1. In some cases where binding sites were found, studies were performed to determine whether these are likely to be functional receptors. Binding sites for ET-1 have been found in heart (nerves greater than atria greater than ventricle greater than coronary arteries), kidney (glomeruli greater than papilla), adrenal (zona glomerulosa greater than medulla), cerebellum, spinal cord, gut, spleen, and lung. The binding of [125I]ET-1 was displaced at all these sites by unlabeled ET-1 but not by nitrendipine, apamin, and other vasoconstrictor peptides. ET-1 contracted strips of human coronary artery at an EC50 of 15 nM, with a maximal contraction 130% that of K+. A positive inotropic effect was found in strips of human atria (EC50 = 1 nM), which was not blocked by alpha- or beta-blockade. The widespread distribution of its binding sites suggests a more extensive role than control of vascular tone.