Photoaffinity labeling of opioid receptor with morphine-7,8-oxide (Morphine epoxide)

The opioid receptor mediating inhibitory action of morphine in the electrically stimulated guinea pig ileum was irreversibly photoinactivated by morphine epoxide (3 × 10⁻⁶). Morphine epoxide (up to 3 × 10⁻⁵ M) did not influence the responses of rat vas deferens (ϵ-receptor) or rabbit vas deferens (κ-receptor) to electrical stimulation. Effective concentrations of morphine epoxide were much lower in the guinea pig ileum (μ-receptor) than in the mouse vas deference (δ-receptor). The inhibitory action of [Met]-enkephalin on the twitch responses of the rat vas deferens and mouse vas deferens to electrical stimulation were not influenced after irradiation in the presence of morphine epoxide (3 × 10⁻⁶ M). Therefore, morphine epoxide is probably a useful probe for photoaffinity labeling of the μ-receptor in vitro.     

Effects of Cholinoceptor Blocking Drugs,Adrenoceptor Stimulants,and Calcium Antagonists on the Transmurally Stimulated Guinea-pig Urinary Bladder in Vitro and in Vivo

Abstract The effects of different drugs on the response to transmural electrical stimulation of the guinea-pig urinary bladder were studied in vitro and in vivo. In vitro, atropine (3.0 × 10^-8-5.9 × 10^-4M) did not influence the contractions. When used in high concentrations (>5.2 × 10^-5M), PR 197, another anticholinergic compound, reduced the responses by 25-40%, probably by a non-specific action. Noradrenaline (2.0×10^-6-2.0 × 10^-4M) and isoprenaline (2.0 × 10^-8-2.0 × 10^-4M) had concentration-related inhibitory effects that could be blocked by propranolol (5.2 × 10^-6M). Adenosine (2.0 × 10^-2M) inhibited the response by 27±3% (mean±S.E.M., n = 9). Theophylline (2.0 × 10^-3-6.0 × 10^-4M) had no consistent effects. The calcium antagonist nifedipine (1.2 × 10^-6-1.7 × 10^-5M) reduced the contractions by 25-50%; verapamil (2.2 × 10^-5-4.4 × 10^-4M) was little effective. In vivo, atropine (10 mg/kg) reduced the contractions by 55±5 % (n= 10), whereas PR 197 (5 mg/kg) almost completely suppressed the responses. Noradrenaline (20-100 μg/kg) and isoprenaline (20-300 μg/kg) also caused a marked inhibition that could be blocked by propranolol (0.25-2.0 mg/kg). Theophylline (5 and 10 mg/kg) had a weak (10-20%) inhibitory effect. Adenosine (3.0 mg/kg) reduced the contractions by 47±4% (n= 14); in guinea-pigs pretreated with atropine (10 mg/kg), adenosine produced a further 10 to 20% decrease of the responses. Verapamil (0.5-2.0 mg/kg) had no consistent effect, whereas nifedipine (0.1-0.2 mg/kg) caused an inhibition of 20-50%. The results suggest that β-adrenoceptor stimulants, and drugs with a combined anticholinergic and non-specific action, can effectively suppress the electrically evoked contractions in the guinea-pig urinary bladder.     

Modulation of adenosine 3',5'-monophosphate contents of rat peritoneal macrophages mediated by beta2-adrenergic receptors.

The modulation of cAMP contents of rat peritoneal macrophages by β-adrenergic stimulants and blocking agents were studied. The maximum increase of cAMP contents induced by isoproterenol, epinephrine, norepinephrine and hexoprenaline (a selective β₂-adrenergic stimulant) was about 170 ~ 200 per cent and about 100 per cent by salbutamol (a selective β₂-adrenergic stimulant) above the basal level. The activity of phenylepherine, a α-adrenergic stimulant, was very weak. The concentration giving a half maximum stimulation was as follows: isoproterenol; 6.3 × 10^?8 M, hexoprenaline; 8.9 × 10^?8 M, salbutamol; 3 × 10^?7 M, epinephrine; 5.6 × 10^?7 M, and norepinephrine; 5.6 × 10^?6 M. Taking propranolol as a standard for comparison, antagonists of the isoproterenol induced increase in cAMP in macrophages ranged in their minimum effective concentrations as follows: bufetolol; 1. metoprolol (a selective β₁-adrenergie blocking agent); 1000, practolol (a selective β₁-adrenergic blocking agent); > 1000, while in rat hearts bufetolol; 1, metoprolol and practolol; 100. The increase of cAMP by 10^?5 M epinephrine or 10^?5 M hexoprenaline in rat peritoneal macrophages was blocked by bufetolol at a concentration of 10^?7 M or 10^?6 M, but not by practolol at a concentration of 10^?4 M. Phentolamine, a α-adrenergic blocking agent, showed no antagonistic activity against isoproterenol in rat peritoneal macrophages.These observations suggest that the increases in cAMP in rat peritoneal macrophages by catecholamines are mediated by β₂-adrenergic receptors.     

Involvement of Opioid Mechanisms in Peripheral Motor Control of Detrusor Muscle

Abstract: Isometric recordings of mechanical activity in muscle strips from rat and human detrusor were performed and the effect of μ- and δ-opioid receptor stimulation and blockade on detrusor contraction induced by electrical field stimulation was tested. Stimulation of the opioid μ-receptor with morphine (10^-13-10^-4 M) and DAGO (10^-13-10^-6 M) had no significant effect on electrical field stimulation except at one concentration of morphine (10^-6 M). Naloxone (10^-10-10^-5 M) caused a significant facilitation of the electrical field stimulation-induced contraction, which was counteracted by morphine (10^-8 M) and the δ-agonist DPDPE (10^-8 M) in both rat and human detrusor. Addition of atropine (10^-6 M) or hexamethonium chloride (10^-6 M) or spantide (10^-6 M) did not alter the facilitating effect of naloxone in the rat detrusor. Hexamethonium (10^-5 M) decreased the facilitating effect of naloxone on electrical field stimulationinduced contractions in the human detrusor, indicating involvement of ganglionic mechanisms. In human detrusor about 15% of the contractile response was found to be atropine-resistant (10^-6 M) and one third of this was found to be resistant to tetrodotoxin (1.5 × 10^-6 M). The atropine resistant-response in human detrusor was facilitated by naloxone to the same extent as the atropine-sensitive part. Adrenergic blockade per se, achieved with phentolamine mesylate (10^-6 M) and propranolol (10^-6 M), caused a significant facilitation of the electrical field stimulation-induced contraction in the rat detrusor but did not affect the facilitating effect of naloxone (10^-13-10^-5 M). Addition of the δ-agonist DPDPE (10^-13-10^-5 M) did not significantly change the detrusor response to electrical field stimulation in rat detrusor, but the δ-antagonist naltrindole (10^-13-10^-5 M) caused a significant facilitation at the highest concentration. In human detrusor, DPDPE 10^-8 M significantly reduced the response induced by electrical field stimulation. Conclusion: Blockade of the opioid receptors with naloxone facilitates the detrusor contraction elicited by electrical field stimulation. The existence of a tonic, inhibitory action on the detrusor nerve mediated detrusor contraction by opioid μ- and possibly δ-receptors is suggested.     

Mechanism of action of histamine in the estrogen-primed rat uterus

Histamine produced a dose-dependent relaxation of uterine strips obtained from the estrogen-primed rat uterus. The responses to histamine were blocked competitively by metiamide (10^?8 ?10^?8M), a specific H₂-receptor antagonist. Propranolol, a selective β-receptor blocker also produced competitive antagonism of the responses to histamine in the same dose range (10^?8?10^?6 M). The pA₂ value obtained for metiamide (8.9) was not significantly different from that obtained for propranolol (8.6). Nialamide (2.2 × 10^?6 M), the monoamine oxidase inhibitor, and cocaine (4.3 × 10^?6 M), the selective noradrenaline uptake blocker, potentiated the responses to histamine. However bretylium (2.4 × 10^?5 M), an adrenergic neuron blocker inhibited the responses to histamine. The combined effect of tyramine and histamine was found to be additive. Our data suggest that the histamine-induced relaxation of rat uterus may be produced through the stimulation of presynaptic H₂-receptors which causes the release of noradrenaline. The released noradrenaline acts on the postsynaptic β-receptors and produces relaxation of the rat uterus.     

The Effect of a Calcium-antagonistic Drug,Nifedipine, on the Rat Atrial Action Potential at Different Calcium Levels

Abstract: Increasing concentrations of calcium in the Ringer solution, [Ca⁺⁺]_O, increased the maximum rate of rise (dv/dt) of the intracellularly recorded action potential of electrically stimulated, isolated rat left atria; while a drug with calcium-antagonistic activity, nifedipine 100 μg/l, reduced dv/dt. The duration of the action potential was shortened with increasing [Ca⁺⁺]_O. A further reduction in the time for 50 and 90 per cent of the repolarization was observed after the administration of nifedipine. All effects of nifedipine on the action potential were most pronounced at the lowest [Ca⁺⁺]_O studied (2.0 meq./l). At the highest [Ca⁺⁺]_O used (8.0meq./l) the effects of this drug were negligible. It is concluded that in the present in vitro experiments, a Ca⁺⁺ current seems to influence the rapid phase of depolarization of the rat left atrium.     

Inhibition of ATP-Induced Contraction in the Guinea-pig Urinary Bladder in Vitro and in Vivo

Abstract Contractions were elicited by adenosine 5′-triphosphate (ATP) in the guinea-pig urinary bladder in vitro and in vivo. In isolated detrusor strips, tetrodotoxin (3.1 × 10^-6-4.4 × 10^-5M) did not affect contractions induced by a submaximum concentration (10^-3M) of ATP, nor did atropine (1.7 × 10^-6 - 2.1 × 10^-4M), or the anticholinergic agent PR 197 within the concentration range 2.6 × 10^-8 - 2.6 × 10^-5M. In higher concentrations (5.2 × 10^-5-2.6 × 10^-4M), PR 197 inhibited the ATP-response by 60-70% in a way that was not clearly concentration-related. Isoprenaline (10^-7-2.0 × 10^-5M) and noradrenaline (2.5 × 10^-6-10^-4M) reduced the ATP-induced contractions by up to 79%. The effects of the amines were abolished by propranolol (5.2 × 10^-6-3.8 × 10^-5M). Adenosine, 1.0-2.0 × 10^-2M, reduced the ATP-response by about 50%; in lower concentrations, it had no effect. Nifedipine, 7.8 × 10^-7-1.2 × 10^-5M, reduced the responses by 15-795%. Indomethacin (≤2.0 × 10^-4M), and theophylline (2.0 × 10^-4M) had no consistent effects on ATP-induced contractions. Exposure of the preparations to a calcium-free medium reduced and abolished the ATP-response within 60 min. Intravenous injection of ATP (1 -20 mg/kg) caused a rapid and transient increase in intravesical pressure in the anaesthetized guinea-pig. The effect of ATP (3mg/kg) was reduced by atropine(5-10 mg/kg) by approximately 35%. PR 197 (2.5-5 mg/kg) abolished the ATP-response. Isoprenaline (5-100 pg/kg) caused a 53-95% inhibition that could be blocked by propranolol (1 mg/kg). The inhibiting effect of noradrenaline (10-100,μg/kg) could not be blocked by propranolol (1 mg/kg). Adenosine (1.5-3.0 mg/kg) given immediately before ATP completely inhibited the ATP-response. Nifedipine, 0.1-0.2 mg/kg, reduced the ATP-induced contraction by 34 to 100%. It is concluded that the ATP-induced contraction is elicited by a direct effect on the smooth muscle cell. It can be inhibited non-specifically by drugs with different modes of action.     

Dual Effect of the Muscarinic Agonist McN-A-343 on Vascular Neuroeffector Transmission

Abstract: The site and mechanism of action of McN-A-343 (4-m-chlorophenylcarbamoyloxy)-2-butynyltrime-thylammonium chloride) on sympathetic neuroeffector transmission in the rabbit isolated pulmonary artery was studied. Low concentrations (10^?6 — 3 × 10^?5 M) of McN-A-343 and cocaine enhanced (up to 210 and 236%, respectively) the contractions evoked by electrical-field stimulation, while higher concentrations (10^?4 — 3 × 10^?4 M) inhibited them. McN-A-343 (10^?4 M) caused an initial transitory potentiation (222% of control) of the stimulation-evoked contractions followed by an inhibition. In the presence of cocaine (3 × 10^?5 M), the potentiation caused by McN-A-343 was prolonged and the secondary inhibitory phase was thus abolished. Physostigmine (10^?5 — 10^?4 M), hexamethonium (10^?5 M), atropine (3 × 10^?7 M), suprofen (10^?5 M), and 4-aminopyridine did not alter the effect of McN-A-343 (10^?4 M). Cocaine (3 × 10^?5 M)and(+)-amphetamine (10^?5 M) reversed the McN-A-343-evoked block, while they did not alter the inhibition caused by tetracaine (3.2 × 10^?5 M). Atropine (3 × 10^?7 M) had no effect on the McN-A-343-induced block, while 4-aminopyridine (10^?4 M) caused a partial and transitory reversal. On the aorta McN-A-343 (10^?4 M) did not alter the contractile concentration-response curve of (—)-noradrenaline (10^?9 — 3 × 10^?4 M), while that of serotonin (10^?8 — 3 × 10^?5 M) was moved to the right in a competitive manner. McN-A-343 (10^?4 M) did not alter the contractions evoked by noradrenaline (10^?7 M) during the period corresponding to the stimulation-evoked enhancement and subsequent inhibition. McN-A-343 (10^?4 M) slightly antagonized the contractions caused by tyramine (10^?6 — 10^?3 M) and carbachol (10^?6 — 10^?3 M). It is concluded that McN-A-343 enhances stimulation-evoked transmitter release by a presynaptic facilitatory action mediated via receptors localized on the outer surface of adrenergic neurones and to a lesser extent by inhibition of noradrenaline re-uptake. The enhancement does not involve presynaptic nicotine or muscarine receptors. Furthermore, McN-A-343 inhibits transmitter release by acting as an adrenergic neurone blocking agent at an intraneuronal site. The inhibition does not involve presynaptic muscarine inhibitory receptors and is prostaglandin-independent.     

The effect of LB-46, a beta-adrenoreceptor blocking drug, on isolated normal and ouabain-intoxicated Purkinje fibers of the dog

Effects of 4-(2-hydroxy-3-isopropylaminopropoxy)-indole (LB-46), a potent β-adrenergic blocking agent, were investigated on the transmembrane electrical properties of the canine His-Purkinje system. The drug produced a competitive blockade of the effect of epinephrine on automaticity of these fibers in 5 × 10^?9 to 1 × 10^?8 M concentrations. The direct effects of LB-46 on single cell action potential were produced only in concentrations larger than 1 × 10^?5 M and were characterized by a decrease in resting potential, maximum rate of rise of TMP (vmax), action potential duration, excitability and membrane responsiveness. The drug produced a consistent decrease in the effective refractory period in the His-Purkinje fibers. The effect of LB-46 on conduction and propagation of premature impulses was more pronounced than on primary beats. In similar experiments, the drug was found effective in protecting the His-Purkinje system against ouabain, but the β-receptor blocking action could clearly be dissociated from such an antidigitalis action. It is suggested that the direct transmembrane effects of LB-46 on excitability, membrane stabilization, vmax and conduction may all contribute to its anti-arrhythmic action against the toxicity of ouabain.     

The effects of veratridine and BDF 9148 on the action potentials and contractility of the rat right ventricle

• 1.1. The effects of veratridine, BDF 9148 and lignocaine on the action potentials and contractile force of the electrically-driven rat right ventricle have been determined.
• 2.2. Veratridine at 10⁻⁷-10⁻⁶M and BDF 9148 at 10⁻⁷-10⁻⁵M had no effect on the threshold potential or amplitude but prolonged the ventricular action potentials.
• 3.3. In contractility studies, veratridine at 10⁻⁷-10⁻⁶M augmented the cardiac stimulation responses and the augmenting effects with 3 × 10⁻⁷ and 10⁻⁶ M were greater at 2 than 4 Hz. In the presence of veratridine at 3 × 10⁻⁶M, the ventricle would not pace.
• 4.4. BDF 9148 at 10⁻⁷-10⁻⁵M augmented the cardiac stimulation responses and the augmenting effects with 10⁻⁷ and 3 × 10⁻⁷M were greater at 2 than 4 Hz, and the effect was maximal at 3 × 10⁻⁷M and submaximal at 10⁻⁵ M. The effects of BDF 9148 at 10⁻⁵ were not readily reversible.
• 5.5. Lignocaine at 10⁻⁴ M had no effect on the ventricular action potential duration but decreased the threshold potential and amplitude and also reduced the cardiac stimulation force responses. In the presence of lignocaine, the augmenting effects of veratridine and BDF 9148 on ventricular force were reduced.
• 6.6. In summary this study has shown that BDF 9148 prolongs the action potential and augments the contractile force responses of the rat right ventricle by a lignocaine-sensitive mechanism. BDF 9148 or similar drugs may have potential as positive inotropes in the treatment of heart failure.

     

Effect of various drugs on electrical and mechanical parameters of atrial muscle

The contractile amplitude and simultaneously recorded maximal rate of rise of depolarization (MRD) of the intracellular action potential were compared in electrically-driven rabbit and guinea-pig atria. β-receptor blocking agents (propranolol, practolol and oxprenolol), carbamyl choline and sodium pentobarbital were pefused through the tissue bath to determine which parameter was a more sensitive criterion of the direct effects of these agents on heart. Propranolol (10⁻⁶ M) decreased the contractile amplitude, but had no effect or caused a small increase in MRD in rabbit atria. A higher concentration depressed both MRD and contractile amplitudes. In guinea-pig atria oxprenolol (10⁻⁶ M) elicited similar results, practolol (10⁻⁵ M) depressed the MRD but not the contractile amplitude and propranolol (10⁻⁵ M) had no significant effect on either contraction or MRD. Carbamyl choline and sodium pentobarbital both decreased contractile amplitudes while not affecting the MRD. We conclude that the most sensitive criterion of the direct effects of these drugs on heart depends upon the agent studied and upon experimental conditions.     

Dual Inhibitory Action of ATP on Adrenergic Neuroeffector Transmission in Rabbit Pulmonary Artery

The aim of this study was to determine whether the inhibitory action of ATP on sympathetic neuroeffector transmission in the isolated pulmonary artery is due to ATP itself or one of its dephosphorylated breakdown products, ADP, AMP or adenosine. Furthermore, the mechanism of the inhibitory action was investigated. ATP (10^?6?3 × 10^?4 M), the degradation-resistant ATP-analogue, β, γ-methylene-5′-triphosphate (10^?5?3 × 10^?4 M), ADP (10^?6?3 × 10^?4 M), AMP (10^?5?3 × 10^?4 M), adenosine (10^?5?3 × 10^?4 M) and 2-chloroadenosine (10^?7?3 × 10^?4 M) reduced the contractions evoked by field-stimulation. This was also the case for prostaglandin E₂ (3 × 10^?9?3 × 10^?7 M), while prostaglandin F_2α(1.4 × 10^?8 M) slightly augmented the neurogenic response. The time course of the inhibitory effect of purinergic compounds on the stimulation evoked contractions was studied. In the case of ATP and ADP the inhibition was biphasic: an initial marked block (1 min. after drug addition) which in the continued presence of either compound recovered partially 10 min. later and then remained almost constant for another 90 min. The other purinergic agents caused a monophasic reduction. In the presence of indomethacin (5 × 10^?5 M), ATP and ADP also reduced the neurogenic contractions in a monophasic manner. Indomethacin did not alter the β,γ-methylene-5′-triphosphate-induced inhibition. Dilazep (3 × 10^?6 M) plus deoxycoformycin (3.6 × 10^?6 M), augmented the inhibitory effect of ATP. In contrast, theophylline (5 × 10^?5 M) did not alter the effect of ATP. The inhibitory effect of ATP (10^?4 M) on stimulation-evoked contractions was inversely proportional to the extracellular Ca²⁺ concentration (0.3–5.2 mM) and to frequency of stimulation (3–15 Hz). These results suggest that ATP initially causes a presynaptic inhibition of noradrenaline release evoked by field-stimulation. This phase I block is probably mainly due to an ADP-mediated short-lasting release of prostaglandins of the E type. The continuous inhibition (phase II) is probably due to ATP and its metabolites, possibly mainly adenosine. The phase II inhibition may possibly involve a decreased entry of Ca²⁺ into adrenergic nerve terminals during depolarization.     

Effects of pirmenol HCl on electrophysiologic properties of cardiac Purkinje fibers

We studied the effects of pirmenol hydrochloride (CL 845), (cis-(±)-α-[3-(2,6-dimethyl-1-piperidinyl)propyl]-α-phenyl-2- Pyridinemethanol monohydrochloride, on the electrophysiologic properties of canine cardiac Purkinje fibers having normal (fast response) action potentials. CI 845 ? 1 × 10^?6 M depressed both maximum upstroke velocity of phase 0 and automaticity. CI 845 ? 1 × 10^?5 M significantly decreased action potential amplitude and duration measured at 50% repolarization and prolonged action potential duration measured at full repolarization. In addition, this concentration depressed membrane responsiveness and prolonged the effective refractory period and conduction time. All changes were reversible following superfusion with drug-free Tyrode solution. Increasing extracellular potassium ([K[su+]₀) from 4 mM to 6 mM did not potentiate the CI 845-induced changes. CI 845 1 × 10^?5 M decreased automaticity of slow response action potentials studied in a Na⁺-free solution but had no effect on the action potential characteristics of these spontaneously dischargin fibers. In blood superfusion studies, plasma levels of 0.1–3.0 μg/ml CI 845 affected the action potential characteristics in a manner similar to concentrations ranging from 1 × 10^?6 to 1 × 10^?5 M CI 845 in Tyrode solution. At plasma levels ? 1.1 μg/mg, CI 845 induced a significant prolongation of the electrocardiogrpahic PR interval. These studies indicate that CI 845 has effects on the action potential and ECG similar but not identical to those of ‘local anesthetic’ antiarrhythmic agents.     

Studies on the action and mechanism of action of oxyfedrine on isolated tracheal chains

Oxyfedrine, a β-sympathomimetic drug, did not affect isolated rat and rabbit trachea in concentration from 2.86 × 10^?8 to 2.86 × 10^?4 M, but on the guinea-pig trachea, it caused a dose dependent relaxation of natural tone in lower concentrations (1.79 × 10^?7 to 2.86 × 10^?6 M. In higher concentrations (1.14 × 10^?5 to 2.86 × 10^?4 M), however, a contraction was observed, which was also dose dependent. This contraction was not affected by atropine, lysergic acid diethylamide or by pretreatment with reserpine but was blocked by antihistaminics (isothipendyl and clemastine). Adrenaline, noradrenaline, phenylephrine and isoprenaline did not contract the guinea-pig trachea, whereas contractions were observed after high concentrations of norephedrine, amphetamine, ephedrine and tyramine. These contractions were also unaffected by reserpine pretreatment.It is concluded that the contraction of the guinea-pig trachea by oxyfedrine is related to its structural relationship to the phenylethylamines and might be due to histamine release, an action on histamine receptors or a histamine-like action.     

Pharmacology of B-HT 920 in some Isolated Smooth Muscles of the Guinea-pig

Abstract: The effects of B-HT 920 were investigated on four isolated preparations from the guinea-pig, namely the aorta, trachea, ileum and vas deferens. The latter three preparations were studied during electrical field stimulation, which induced contractions by activating cholinergic neurones (trachea and ileum) or adrenergic neurones (vas deferens), respectively. Comparative studies were also made with clonidine and noradrenaline. In ileum and trachea B-HT 920 was almost equipotent with noradrenaline to inhibit the electrically induced contractions. In these tissues, B-HT 920 also displayed almost the same maximal effect as noradrenaline. Clonidine also inhibited the contractions in ileum and trachea; the drug was slightly more potent than noradrenaline. However, in contrast to the intrinsic activity of B-HT 920 that of clonidine was only submaximal. In vas deferens both B-HT 920 and clonidine induced inhibition of contractions on electrical field stimulation at low concentrations. In this organ, both drugs were capable of inducing complete inhibition of the contractile response. In aorta B-HT 920 as well as clonidine were only weak agonists in comparison to noradrenaline. The α₂-blocker, yohimbine, completely blocked the effect of B-HT 920 in ileum at low concentrations (1 × 10^?7 M). Remarkably, however, the inhibitory action of B-HT 920 in trachea was only marginally affected even by high concentrations of yohimbine (1 × 10^?6 M). It is suggested from the present results that B-HT 920 can induce inhibition of both cholinergic and adrenergic neurotransmission presumably by inducing selective stimulation of prejunctional α₂-receptors. In fact, the selectivity of B-HT 920 seems to be comparable to that of clonidine for the α₂-receptor. However, the mode of action of B-HT 920 in trachea may be somewhat uncertain since its effect was not inhibited by yohimbine.     

Enhancement of the electrically induced release of norepinephrine from the rat portal vein: mediation by beta 2-adrenoceptors.

The effect of β-adrenoceptors agonist ont the electrically induced release of ³H-NE from the superfused rat portal vein was studied. It was observed that the β-adrenoceptor agonists isoproterenol and terbutaline produced a concentration dependent enhancement of the field-stimulation induced release of NE. In contrast to what was seen with drugs possesing β₂-activity, the β₁-agonist, dobutamine, was inactive. The enhancing effect of both isoproterenol and terbutaline was reduced or completely blocked by the β₁–β₂-antagonist, propranolol or the β₂-antagonist butoxamine but not by the β₁-antagonist, practolol. In the concentration used none of the three β-antagonists had any effect themselved in altering the stimulation induced release of NE. Epinephrine (EPI) at a concentration of 10^?8 M produced an enhancement of the electrically induced release of NE. The effects of EPI and terbutaline were additive with phenoxybenzamine. Higher concentrations (i.e., 10^?7 M) or norepinephrine (10^?8 or 10^?7 M) reduced the enhancement induced by the simultaneous administration of phenoxybenzamine. It is concluded that β-adrenoceptors of the β₂ type are present on adrenergic neurons innervating the rat portal vein. Activation of these receptors results in an enhancement of the electrically induced release of NE. Our data are consistent with the hypothesis of Stjärne and Brundin which suggests that the β-adrenoceptor mediated enhancement of NE release is an example of hormonal modulation of adrenergic transmission mediated by ciculating EPI of adrenal medullary origin.     

Effects of prostaglandin E1 and indomethacin on the stimulation- evoked overflow of 3H-noradrenaline from guinea-pig taenia caecum

Prostaglandin E₁ (5.8 × 10^?8 M) markedly and reversibly reduced the stimulation-evoked overflow of total tritium, while it had no effects on basal outflow. Indomethacin (8.4 × 10^?6 M) increased the stimulation-evoked overflow of total tritium at low frequencies (2–5 Hz), while it had no effect at high frequencies of stimulation (more than 10 Hz). It was concluded that endogenous prostaglandin E₁ also plays a regulatory role in adrenergic inhibitory neurotransmission by inhibiting the noradrenaline release from adrenergic nerve terminals of the guinea-pig taenia caecum.     

Relaxations of guinea-pig fundic strip by adenosine, adenine nucleotides and electrical stimulation: antagonsism by theophylline and desensitization to adenosine and its derivatives

Adenosine and related compounds produced a concentration-dependent relaxation of the guinea-pig fundic strip in non-treated preparations and in preparations treated with atropine, tetrodotoxin (TTX), guanethidine, reserpine or 6-hydroxydopamine (6-OH-DA). The relative potencies in descending order were ATP ? ADP + adenosine. Theophylline (THEO) at low concentrations (25–75 μM) selectively antagonized the inhibitions produced by ATP, ADP and adenosine without antagonizing those due to noradrenaline (NA), isoprenaline (ISO) and papaverine (PAPAV). Schild plots yielded apparent pA₂ values for THEO of1.4 × 10^?5M, 1.8 × 10^?5 M and 2.8 × 10^?5 M for ATP, ADP and adenosine respectively, with slopes which were not significantly different from 1 (p + 0.05). These results suggest competitive antagonism. Theophylline was a more specific antagonist of the relaxations induced by the purines than were compounds such as quinidine, phentolamine or 2,2′-pyridylisatogen (PIT). In atropine (1 × 10^?7to 5 × 10^?7 M) pretreated fundic strips, transmural or periarterial nerve stimulation produced relaxations which were frequency-dependent. The effects of periarterial nerve stimulation were abolished by phentolamine plus propranolol, by guanethidine, by bretylium, by TTX and by pretreatment with reserpine or 6-OH-DA. The responses to transmural nerve stimulation were abolished only by TTX. The relaxations induced by transmural nerve stimulation were reduced or abolished by THEO, whereas those produced by periarterial nerve stimulation were not affected. Theophylline was more selective than quinidine or phentolamine in blocking such responses. Exposure of the tissues to high concentrations of ATP, ADP or adenosine caused tachyphylaxis to the relaxations induced by the purines as well as those to transmural nerve stimulation, while the responses to NA, ISO, PAPAV or periarterial nerve stimulation were either augmented or unaffected. The effects of desensitization were reversible. This results suggests that the response to transmural nerve stimulation may be due to neural release of adenosine or a related compound. These results are discussed in relation to the release of ATP or related nucleotides from purinergic nerves since the responses to the purines and those to transmural nerve stimulation were reduced or abolished by either THEO or by desensitization with ATP, ADP or adenosine. The possibility is raised that an adenine nucleotide or nucleoside released from nerves might have a physiological function in the “receptive relaxation” of the stomach following stimulation of the pharynx and upper oesophagus by passage of a bolus of food.     

Effects of prostaglandin E1 on the positive inotropic actions of noradrenaline,nerve stimulation and calcium in the isolated blood-perfused papillary muscle of the dog

The effects of PGE₁ on the dog heart were studied using the blood-perfused sinus node papillary muscle preparations isolated separately from the same animal. PGE₁ administered into the papillary muscle artery as bolus injections in doses of 1–1000 ng caused a dose-dependent increase of the developed tension and dT/dt of the papillary muscle. The effect was not inhibited by the ß-adrenoceptor blocking agent pindolol. PGE₁ injected into the sinus node artery in dosesof 3–300 ng did not change the rate of contraction of the sinus node preparation. PGE₁ in the blood concentrations of 4.4 × 10^?9 to 1.7 × 10^?7 M enhanced the positive inotropic response to noradrenaline and field stimulation as well as to calcium. The influence of PGE₁ on the positive inotropic effect of perivascular nerve stimulation was not consistent: the action of perivascular nerve stimulation was enhanced by PGE₁ in the majority of preparations but was reduced in one third of preparations. PGE₁ in the same blood concentrations as used in the papillary muscle significantly depressed the positive chronotropic responses to noradrenaline and dopamine. The present results indicate that PGE₁ induces multiple actions on the dog heart. Its predominant effect on the ventricular myocardium appears to be enhancement of the adrenergic stimuli probably via the facilitation of calcium movement through the myocardial cell membrane. In addition, PGE₁ may decrease the sensitivity of ß-adrenoceptors to adrenergic stimuli in the sinus node.     

Cyclic AMP and Ca-binding in Microsomal Fractions Isolated from Rabbit Colon Smooth Muscle

Abstract From a homogenate of rabbit colon muscle two ATP dependent Ca-accumulating microsomal fractions were isolated by differential centrifugation on a sucrose density gradient at 35 % and 35-45 % sucrose. Adenylate cyclase and phosphodiesterase activities were found in the fractions. The Ca-accumulation and the ATPase activity of these fractions were stimulated by cyclic AMP (10-⁵ M) at an ATP concentration of 0.35 mM ATP. In the presence of higher concentrations of ATP (5 mM) cyclic AMP had no effect on the Ca-binding. The higher concentration of ATP markedly increased the cyclic AMP formation in relation to the activity found at the lower concentration of ATP. Isoprenaline (2 × 10-⁶ M) stimulated the Ca-accumulation in the 35-45 % fraction and increased the hydrolysis of ATP. These effects were absent in the fraction isolated at 35 % sucrose. In the former fraction isoprenaline also stimulated the adenylate cyclase activity at 0.35 mM but not at 5 mM ATP. Both the effect of isoprenaline on the Ca-binding and the adenylate cyclase activity were inhibited by the adrenergic β-receptor blocking agent sotalol. In the 35-45 % fraction papaverine (1 × 10-³ M) stimulated the Ca-accumulation and inhibited the phosphodiesterase activity. It is suggested that cyclic AMP and agents which influence the cyclic AMP metabolism in the microsomes may have a regulatory role on the Ca-binding of the microsomes.