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.     

Frequency-Dependence of 3H-Noradrenaline Release from Rabbit Pulmonary Artery: Effect of α-Adrenoceptor Antagonists and Inhibitors of Transmitter Inactivation

Abstract: The aim of this study was to examine the modulating role of presynaptic α₂-adrenoceptors on transmitter release from vascular sympathetic neurones. This was done by examining the influence of removal of inactivation pathways on the effect of α-adrenoceptor antagonists on the release of transmitter from noradrenergic neurones. The rabbit main pulmonary artery preloaded with ³H-noradrenaline (³H-NA) was used. The artery was stimulated with 300 pulses at various frequencies (1, 3, 10 and 30 Hz). Pargyline (3 × 10^?4M) increased the stimulation-evoked ³H-overflow at 1 and 3 Hz and decreased it at 30 Hz. U-0521 (3′,4′-dihydroxy-2-methylpropiophenone; 3 × 10^?6M) enhanced the overflow at 1 Hz and had no effect at 3–30 Hz. Corticosterone (4 × 10^?5M) did not alter the stimulation-evoked ³H-overflow at 1–30 Hz. Cocaine (3 × 10^?6M) enhanced the ³H-overflow slightly at 1–30 Hz. At 3 × 10^?5M, cocaine enhanced ³H-overflow at 1 Hz and reduced it at 30 Hz. Neither corticosterone (4 × 10^?5M) nor propranolol (10^?7M) modified this effect of cocaine. Propranolol (10^?7M) alone decreased the ³H-overflow at 30 Hz and had no effect at 1–10 Hz. Phenoxybenzamine (10^?6M) and chlorpromazine (3 × 10^?6M) potentiated the stimulation-evoked ³H-overflow at 1–30 Hz. Phentolamine (10^?6M) decreased the ³H-overflow at 1 Hz and enhanced it at 3–30 Hz. Rauwolscine (10^?6M) enhanced the stimulation-evoked ³H-overflow maximally at 10 Hz and had no effect at 1 and 30 Hz. Cocaine (3 × 10^?5M), cocaine (3 × 10^?5M) + corticosterone (4 × 10^?5M), pargyline (3 × 10^?4M) and U-0521 (3 × 10^?6M), but not corticosterone (4 × 10^?5M), increased the rauwolscine-induced enhancement at 1 and 3 Hz, but had no influence at 10 and 30 Hz. It is concluded that at a low frequency (1 Hz) of nerve stimulation, inhibition of either neuronal uptake, extraneuronal uptake, monoamine oxidase (MAO) or catechol-O-methyltransferase (COMT) causes an enhancement in transmitter overflow that is due to removal of inactivation pathways. The enhancement is not mediated via facilitatory β-adrenoceptors. At higher frequencies (3–30 Hz) inhibition of either neuronal uptake or MAO causes a sufficient amount of transmitter in the neuromuscular gap to activate the presynaptic α-adrenoceptor mediated negative feed-back system causing a decrease in transmitter release. α- Adrenoceptor antagonists enhance the stimulation-evoked release of ³H-NA only when the junctional gap concentration of transmitter is sufficient to trigger the presynaptic α-adrenoceptor auto-inhibitory system.     

Inhibition by (−)-Deprenyl of Agonist-Evoked Contractions in Rabbit Aorta

Abstract: The effect of (?)-deprenyl, a relatively selective MAO-B inhibitor, was examined for its ability to inhibit the contractions of rabbit isolated aorta evoked by various agonists and potassium. (?)-Deprenyl (10^?5?3 × 10^?4 M) antagonized the contractions evoked by noradrenaline (10^?8?3 × 10^?4 M); pA₂: 5.10. The antagonism was reversible. It was attenuated by cocaine (3x M); pA₂: 4.38, unchanged by corticosterone (4 × 10^?5 M); pA₂ 4.79 and enhanced by cocaine (3 × 10^?5 M) plus corticosterone (4 × 10^?5 M); pA₂: 5.48. (+)-Deprenyl (10^?6?10^?4 M) did not alter the contractions evoked by noradrenaline (3 × 10^?9?10^?4 M). Clorgyline (10^?5 and 10^?4 M) antagonized the noradrenaline-evoked contractions. Pargyline (10^?4 and 3 × 10^?4 M) had no effect. (?)-Deprenyl (10^?5?3 × 10^?4 M) antagonized the contractions evoked by phenylephrine (10^?8?10^?4 M); pA₂: 5.10. Removal of the endothelium did not alter the antagonism; pA₂: 5.35. (?)-Deprenyl (10^?5?3 × 10^?4 M) antagonized the contractions evoked by either 5-hydroxytryptamine (3 × 10^?8?3 × 10^?4 M); pA₂: 4.61 or by histamine (10^?6?3 × 10^?2 M); pA₂: 4.84. (?)-Deprenyl (3 × 10^?4 M) caused a non-competitive antagonism of the contractions evoked by potassium (1.5-5.5 × 10^?2 M). It is concluded that (?)-deprenyl is a weak inhibitor of postjunctional α-adrenoceptors, 5-hydroxytryptamine (5-HT₂) receptors, and histamine (H₁) receptors.     

Possible muscarinic regulation of catecholamine secretion mediated by cyclic GMP in isolated bovine adrenal chromaffin cells

Catecholamine secretion and cyclic GMP levels were measured in chromaffin cells isolated from bovine adrenal medulla. Acetylcholine (ACh) and nicotine, but not muscarine, induced 8- to 10-fold increases in catecholamine secretion, with respective ED₅₀ values of 10 and 2 M. Cyclic GMP levels were also increased from 3- to 5-fold in the presence of ACh, and this stimulation was mimicked by muscarine but not by nicotine. Half-maximum stimulations of cyclic GMP levels with ACh and muscarine were observed at 0.1 and 0.3 M respectively. The order of potency of various cholinergic drugs for cyclic GMP stimulation was as follows: ACh > oxotremorine > methacholine > muscarine > carbamylcholine > furthretonium > arecholine > bethanechol. Pilocarpine, McN-A-343, and AHR-602 were inactive at concentrations between 10^?8 and 10^?3 M. Isobutylmethylxanthine (1 mM), a specific phosphodiesterase inhibitor, caused a 7-fold increase in cyclic GMP and potentiated 3-fold the stimulation of cyclic GMP by ACh. The nicotine-induced catecholamine secretion was inhibited 19 and 33 per cent by the co-stimulation of the muscarinic receptor with 0.2 and 0.5 M ACh, respectively. Isobutylmethylxanthine (1 mM) also caused a 44 per cent inhibition of nicotine-induced catecholamine secretion, and its effect was additive to that of ACh. Atropine (0.1 M) selectively abolished the inhibition caused by ACh. Similar inhibitions were also obtained in the presence of exogenous dibutyryl cyclic GMP or 8-bromo cyclic GMP. These data indicate that the nicotinic stimulation of catecholamine secretion from bovine adrenal chromaffin cells may be regulated by cyclic GMP via the stimulation of a muscarinic receptor.     

Effect of Dopexamine Hydrochloride on Contractions of Rabbit Isolated Aorta Evoked by Various Agonists

The inhibitory affinity of dopexamine hydrochloride to postsynaptic adrenoceptors, cholinoceptors, 5-hydroxytryptamine and histamine receptors was studied in rabbit isolated aorta. Dopexamine (10^?7–10^?5M) antagonized competitively the contractions of rabbit aorta evoked by noradrenaline (pA₂: 6.60). Neither cocaine plus corticosterone nor cocaine, corticosterone plus propranolol altered the inhibition (pA₂: 6.77 and 6.63, respectively). The antagonism of dopexamine against noradrenaline-evoked contractions was the same after 1 and 4 hr of pretreatment with dopexamine. In the presence of cocaine plus corticosterone, dopexamine antagonized the contractions evoked by phenylephrine (pA₂: 6.94). Removal of endothelium did not influence this antagonism (pA₂: 7.06). Dopexamine (10^?7–10^?5M) did not antagonize the contractions of aorta evoked by histamine (3×10^?7–6 × 10^?5M) and by 5-hydroxytryptamine (3 × 10^?7 –3 × 10^?4M). Dopexamine (10^?8 and 10^?7M) did not alter the contractions of endothelium-free aorta evoked by carbachol. Dopexamine (10^?7–10^?5M) slightly enhanced the contractions of aorta evoked by potassium (10^?2–5.5 × 10^?2M). These results suggest that dopexamine is an α₁-adrenoceptor antagonist. Furthermore, dopexamine has no affinity to cholinoceptors, histamine and 5-hydroxytryptamine (5-HT₂) receptors and is apparently not a calcium antagonist.     

Effect of Cocaine and Corticosterone on the Metabolism of 3H-Noradrenaline Released from Rabbit Isolated Aorta and Adventitia

Abstract: The effect of cocaine and corticosterone (neuronal and extraneuronal uptake inhibitors, respectively) on the metabolism of ³H-noradrenaline (³H-NA) released spontaneuosly or by electrical-field stimulation was studied in the rabbit isolated aorta and tunica adventitia preloaded with ³H-NA. The spontaneous outflow of tritium from untreated aorta consisted of ³H-NA (25%); ³H-3,4-dihydroxyphenylglycol (³H-DOPEG; 24%); ³H-3,4-dihydroxymandelic acid (³H-DOMA; 8%); ³H-O-methylated plus deaminated compounds (³H-OMDA; 45%); and ³H-normetanephrine (³H-NMN; 1%). The percentage distribution of ³H-NA and its ³H-metabolites in this outflow was not altered by either cocaine (3 × 10^?5 M), corticosterone (4 × 10^?5 M), or cocaine (3 × 10^?5 M) + corticosterone (4 × 10^?5 M). The spontaneous ³H-outflow from untreated tunica adventitia did not differ from that of aorta. Cocaine (3 × 10^?5 M) + corticosterone (4 × 10^?5 M) did not alter the composition of ³H-NA and its ³H-metabolites in the spontaneous outflow from adventitia. The stimulation-evoked ³H-overflow from untreated rabbit aorta preloaded with ³H-NA consisted of ³H-NA (40%); ³H-DOPEG (15%); ³H-DOMA (2%); ³H-OMDA (50%); and ³H-NMN (3%). Cocaine (3 × 10^?5 M) decreased the percentage recovered as ³H-DOPEG, increased ³H-NMN and had no effect on the other ³H-metabolites. Corticosterone (4 × 10^?5 M) had no effect on the percentage distribution of ³H-NA and its ³H-metabolites. Cocaine (3 × 10^?5 M) + corticosterone (4 × 10^?5 M) increased ³H-NA, decreased ³H-DOPEG and had no effect on the percentage distribution of the other ³H-metabolites. In the case of adventitia, cocaine (3 × 10^?5 M) + corticosterone (4 × 10^?5 M) only decreased the percentage recovered as ³H-DOPEG, without any significant effect on ³H-NA and its other ³H-metabolites. It is concluded that inhibition of neuronal plus extraneuronal uptake of H-NA by cocaine plus corticosterone, respectively does not fully prevent metabolism of ³H-NA released either spontaneously or by electrical-field stimulation.     

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.     

The inhibitory action of pgf2α on release of [3H]noradrenaline enhanced by α2-adrenoceptor blockade,sodium-pump inhibition and 4-aminopyridine in the main pulmonary artery of the rabbit

Large concentrations of prostaglandin PGF_2α inhibited the stimulation (2 Hz) evoked release of [³H]noradrenaline from the isolated main pulmonary artery of the rabbit (the inhibition caused by 3 × 10^?5 M PGF_2α was 62%). Furthermore, PGF_2α inhibited the release evoked by stimulation when it was enhanced by different procedures. During blockade of presynaptic α₂-adrenoceptors by 3 × 10^?7 M yohimbine, which by itself enhanced the overflow of [³H]NA in response to stimulation, the inhibitory action of PGF_2α was more pronounced (78.2%). In tissue in which the Na⁺-pump was inhibited (K⁺-free treatment) where the overflow of ³H was markedly increased, PGF_2α exerted nearly equal inhibition of transmitter release to that observed in control experiments (64.3%). The inhibitory effect of PGF_2α on the stimulation-evoked release of [³H]NA was less pronounced (32.1%) in the presence of 10^?4M 4-aminopyridine (a blocker of K⁺ -channels).     

A comparison between ATP and bradykinin as possible mediators of the responses of smooth muscle to non-adrenergic non-cholinergic nerves

The effects of ATP, bradykinin (BK) and electrical stimulation of intramural non-adrenergic non-cholinergic nerves (NS) were compared in four smooth muscle preparations. In the guinea-pig taenia caeci and rat duodenum, ATP (10^?7-5 × 10^?5 M) and BK (5 × 10^?10-10^?7 M) closely mimicked the response to NS. The relaxations to BK, but not to ATP or NS, were inhibited by carboxypeptidase B (3–15 U/ml) and apamin (10^?8-5 × 10^?8 M) prevented the relaxations to all three stimuli. BK contracted the guinea-pig distal colon whereas ATP and NS caused inhibition. In the guinea-pig bladder, ATP and NS induced rapid phasic contractions whereas BK caused tonic contractions. In the latter two preparations, incubation with indomethacin failed to reveal any BK relaxationic In view of its failure to mimic the nerve-mediated response in two of the tissues, and of its selective inhibition by carboxypeptidase B in the other two, BK is less likely than ATP to be the transmitter in non-adrenergic non-cholinergic nerves supplying smooth muscle.     

Effect of Dopexamine Hydrochloride on Sympathetic Neuroeffector Transmission in Rabbit Isolated Pulmonary Artery

Abstract: The effects of dopexamine hydrochloride on sympathetic neuroeffector transmission were studied in rabbit isolated pulmonary artery. Short-term exposure of dopexamine (10-⁸x10^-7 M) and cocaine (10^-6-3x10^-5 M), but not desipramine (3x10^-9-3x10^-7 M), to the artery enhanced the contractions evoked by electrical-field stimulation. Corticosterone (4x10^-5 M), corticosterone (4x10^-5 M) plus cocaine (3x10^-8 M), but not cocaine (3x10^-5 M), attenuated the enhancement seen with dopexamine. High concentrations of dopexamine (10^-5-3x10^-5 M), cocaine (10^-4 M), and desipramine (10^-6-10^-5M) decreased the stimulation-evoked contractions. Dopexamine (10^-7-3x10^-5 M), but neither cocaine nor desipramine, caused an increase in resting tension that waned with time. Corticosterone (4x10^-5 M), but not cocaine (3x10^-5 M), attenuated the increase in resting tension. Propranolol (10^-6 M) did not alter the enhancing and inhibitory effects of dopexamine. A single concentration (10^-7 and 10^-6 M) of either dopexamine or desipramine caused a time-dependent biphasic response as regards the repetitive stimulation-evoked contractions of pulmonary artery: initial enhancement followed by inhibition. The inhibitory effect of dopexamine (10^-6 M) and desipramine (3x10^-6 M) seen after prolonged exposure was almost irreversible and partially reversible, respectively, by washing the preparations with drug-free salt solution. Cocaine caused a monophasic steady-state response: either enhancement (10^-5 M) or inhibition (2x10^-4 M). In both cases, the onset was rapid. The reduction caused by cocaine (2x10^-4 M) and by prazosin (10^-9 M) was fully reversed. Dopexamine (10^-5 M) antagonized competitively the contractions evoked by noradrenaline (3x10^-9-10^-4 M). It is concluded that (1) the dopexamine-induced enhancement of neurogenic contractions is not due to either inhibition of neuronal and extraneuronal uptake of noradrenaline or an agonist action on prejunctional β₂-adrenoceptors; (2) that the dopexamine-induced inhibition of stimulation-evoked contraction is due to an inhibition of postjunctional α₁-adrenoceptors; and (3) that the dopexamine-induced increase in resting tension is due to its metabolite methyldopexamine.     

Effect of ω‐Conotoxin GVIA on Noradrenaline Release from Postganglionic Sympathetic Neurones in Rabbit Aorta

Abstract: The aim of the present work was to examine the effect of the selective N‐type calcium blocking agent ω‐conotoxin GVIA on stimulation‐evoked release of noradrenaline from sympathetic nerves in rabbit isolated aorta with regard to stimulation frequency, extracellular Ca²⁺ concentration, and transmitter uptake. Rings of rabbit isolated aorta were preloaded with (‐)‐³H‐noradrenaline and the fractional ³H‐overflow evoked by electrical‐field stimulation was determined by liquid scintillation spectrometry. ω‐Conotoxin GVIA (3×10^?10– 3×10^?8 M) did not alter the spontaneous ³H‐outflow. ω‐Conotoxin GVIA (3×10^?10– 3×10^?8 M) caused a slowly developing reduction of stimulation‐evoked ³H‐overflow at 1 and 30 Hz. The Emax for the ω‐conotoxin‐induced inhibition was less (70%) at 30 Hz than that (96%) seen at 1 Hz. Short‐term incubation with ω‐conotoxin GVIA caused a subsequent steady‐state inhibition. The inhibitory action of ω‐conotoxin GVIA (3×10^?10– 3×10^?9 M) was inversely related to the extracellular Ca²⁺ concentration (6.5×10^?4– 2.7×10^?3 M). Cocaine (3×10^?5 M) plus corticosterone (4×10^?5 M), neuronal and extraneuronal uptake inhibitors, respectively, did not alter the inhibitory effect of ω‐conotoxin GVIA (3×10^?9 M) on ³H‐overflow evoked by stimulation at a frequency of either 1 or 30 Hz. It is concluded that ω‐conotoxin GVIA acts on prejunctional N‐type calcium channels to inhibit stimulation‐evoked noradrenaline release from sympathetic neurone terminals in rabbit aorta. At a high frequency, another subtype calcium channel may possibly be involved. The action of ω‐conotoxin GVIA is independent of neuronal and extraneuronal uptake mechanisms for noradrenaline, but dependent on the amount of Ca²⁺ to be transported across the neurilemma from the extracellular space into the neurone.     

Muscarinic M1- and M2-receptors mediating opposite effects on neuromuscular transmission in rabbit vas deferens

Twitch contractions of the rabbit vas deferens elicited by electrical field stimulation were inhibited by tetrodotoxin, guanethidine, bretylium and alpha,beta-methylene-ATP but were unaffected by hexamethonium, physostigmine, 1,1-dimethyl-4-phenylpiperazinium and prazosin, suggesting that they resulted from ATP released following postganglionic sympathetic nerve stimulation. McN-A-343 inhibited but carbachol and several other muscarinic agonists potentiated the twitch contractions; these effects were not modified by hexamethonium or physostigmine. Muscarinic agonists had no effect on the tension in unstimulated organs whereas contractions elicited by ATP, noradrenaline and KCl were potentiated by carbachol but remained unaffected by McN-A-343. The responses of the twitch contractions to McN-A-343 and carbachol were inhibited to different degrees by antimuscarinic drugs: the affinity (pA2) of atropine, secoverine and himbacine against McN-A-343 and carbachol was similar. However, pirenzepine, telenzepine, trihexyphenidyl, dicyclomine and hexahydro-sila-difenidol displayed preferential antagonism of the responses to McN-A-343 whereas the converse was true for AF-DX 116 and gallamine. The highly significant correlation between the pA2 values obtained for 10 antagonists against carbachol responses in rabbit vas deferens and rat left atrium suggests that the receptors may be similar. The data support the presence of a presynaptic M1-receptor mediating inhibition and a postsynaptic, cardiac-like M2-receptor responsible for enhancing neurogenic contractions in rabbit vas deferens.     

Influence of TMB-8 on secretion of catecholamines from the perfused rat adrenal glands

An attempt was made to investigate the effect of TMB-8 [3,4,5-trimethoxybenzoate-8 (N,N-diethylamino) octyl ester], which is known to be an inhibitor of intracellular Ca²⁺ release, on catecholamines (CA) secretion evoked by Ach, excess K⁺, DMPP, McN-A-343 and caffeine from the isolated perfused rat adrenal glands and to clearify its mechanism of action. The pretreatment with a low dose of TMB-8 (10 μM) for 20 min led to marked inhibition in CA secretion evoked by Ach (5.32 mM), excess K⁺ (56 mM), DMPP (100 μM), McN-A-343 (100 μM) and BAY-K 8644 (10^?5M). Caffeine-induced CA secretion was similar to that of control only during the first periods (0–3 min) but thereafter marked inhibition in CA secretion evoked by caffeine was observed during the rest periods up to 30 min. The increased moderate concentration of TMB-8 (30 μM) caused the result similar to that of 10 μM TMB-8. However, in adrenal glands preloaded with a high dose of TMB-8 (100 μM), CA releases evoked by Ach, excess K⁺, DMPP, McN-A-343 and caffeine were almost completely blocked by the drug. These experimental data demonstrate that TMB-8 may inhibit cholinergic receptor-mediated and also depolarization-dependent CA secretion, suggesting that these TMB-8 effects seem to be mediated through inhibiting influx of extracellular calcium into the rat adrenal medullary chromaffin cells as well as reducing the release of calcium from intracellular sources.     

Inhibitory effects of GABA,L-glutamic acid and nicotine on the potassium-evoked release of subsaance P in substantia nigra slices of the rat

Rat substantia nigra slices were superfused with a physiological medium containing a diluted substance P (SP) antiserum, bacitracin and serum albumin to measure SP released in superfusates. As shown by measuring the degradation of a SP-labelled derivative incubated with cerebellar slices, this medium prevented the enzymatic inactivation of SP. Potassium (K⁺, 50 mM) and veratridine (5 × 10^?5M) stimulated SP release and these effects were respectively prevented in absence of calcium and in presence of tetrodotoxin (5 × 10^?7 M). GABA (5 × 10^?5 M) nicotine (10^?6 M) and L-glutamic acid (5 × 10^?5 M) reduced the K⁺ (50 mM)-evoked release of SP. In contrast, glycine (5 × 10^?5 M), oxotremorine (5 × 10^?5 M), D-glutamic acid (5 × 10^?5 M) and serotonin (5 × 10^?5 M) were without effect. Pempidine (10^? M) prevented the inhibitory effect of nicotine (10^? M) on the K⁺-evoked release of SP. Glutamic acid diethyl ester (10^?4 M) completely abolished the L-glutamic acid-induced inhibition of the K⁺-evoked release of SP. Picrotoxin (5 × 10^?5 M) did not influence the L-glutamic acid inhibitory effect excluding the intervention of GABAergic mechanisms.     

Effect of dantrolene sodium on the contractility of rabbit jejunum in vitro

The effect of dantrolene sodium on the spontaneous contractions of rabbit jejunum was studied in vitro. Dantrolene sodium (4.5 × 10^?6 to 4.5 × 10^?4 M) reversibly decreased the amplitude of contractions in a dose-dependent manner. ED₅₀ was found to be about 7.9 × 10^?5 M. Its effect was biphasic in that a period of potentiation preceded that of suppression of contractions. Lowering or increasing (2.5 fold in each direction) the calcium concentration of bathing media did not affect the suppression of contraction caused by dantrolene sodium to any significant degree. Caffeine but not quinne was found to be able to restore the activity of the intestine to normal after a 50% inhibition caused by dantrolene sodium. Dantrolene sodium, verapamil and nifedipine were able to shift the dose-response curves of calcium in potassium-depolarized rabbit jejunum to the right and pA₂ values were found to be 4.18, 7.76 and 8.47 respectively. These data indicate that the effect of dantrolene on smooth muscle is mediated via inhibition of calcium movement across the membrane.     

Separate receptors mediating the positive inotropic and chronotropic effect of histamine in guinea-pig atria

The direct positive inotropic effect of histamine was studied on paced left atrial preparations from guinea pigs. Histamine (10^?8 to 10^?4 M) increased the maximum tension developed in left atria incubated at 35°C and driven at 2 Hz. The maximum increase in tension was 60% of that observed with norepinephrine. Metiamide (3 × 10^?5 M; a specific H₂-receptor antagonist) did not alter the inotropic response of left atria to histamine. However, tripelennamine (a typical H₁-receptor antagonist) competitively shifted the histamine inotropic dose—response curve to the right at concentrations from 10^?8 to 10^?7 M. Higher concentrations (3 × 10^?7 and 10^?6 M) caused little further additional shift to the right. The positive chronotropic effect of histamine on spontaneously beating atria was competitively antagonized by metiamide (10^?6 and 3 × 10^?6 M). These results demonstrate that in guinea-pig atria histamine increases myocardial contractility by an interaction with receptors closely related to classical H₁-receptors while its chronotropic effects is mediated by interaction with H₂-receptors.     

CAPTOPRIL INHIBITS BOTH DOPAMINERGIC AND CHOLINERGIC NEUROTRANSMISSION IN THE CENTRAL NERVOUS SYSTEM

1. The present study was performed to investigate the effects of captopril on both dopaminergic and cholinergic neurotransmission in the rat central nervous system . 2. Slices of rat striatum were prepared and prelabelled with [³H]-dopamine or [³H]-choline. Slices were continuously super-fused with Krebs'-Ringer solution and electrical stimulation (1 Hz) was performed . 3. Captopril significantly inhibited stimulation-evoked [³H]-dopamine release from rat striatal slices in a concentration-dependent manner (S2/S1 ratios: control 0.83S±0.018 (n= 6); 1×10^?5 mol/L captopril 0.597±0.035 (n= 6; P<0.05); 5 × 10^?5 mol/L captopril 0.561 ± 0.041 (n = 6; P<0.05)). However, the basal release of [³H]-dopamine was not affected by captopril . 4. Captopril also reduced stimulation-evoked [³H]-acetyl-choline release in the striatum (S2/S1 ratios: control 0.891 ±0.016 (n= 6); 1 × 10^?5 mol/L captopril 0.794±0.011 (n= 6; P<0.05)) . 5. These results show that captopril inhibits the release of both dopamine and acetylcholine in the rat striatum. Although the mechanisms underlying the neurosuppressive effects of captopril remain to be determined, the findings suggest that the inhibition of dopaminergic and cholinergic neurotransmission may be related to the central action of the angiotensin-converting enzyme inhibitor .     

Effects of 5-(N,N-diethylamino)-n-pentyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-5) on cardiovascular systems

The effects of 5-(N,N-diethylamino)-pentyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-5) were studied pharmacologically on smooth muscle, skeletal muscle, blood vessel and cardiac preparations. In all cases, TMB-5 inhibited muscle contractions induced by muscle stimulants such as acetylcholine, norepinephrine, KCl and BaCl₂, indicating that the muscle inhibition induced by TMB-5 is unrelated to specific receptors. TMB-5 was found to be most potent in inhibiting skeletal muscles (at 10^?6 –10^?5 M level) and least effective inhibiting smooth muscles (at 10^?4 –10^?3 M level). The potency of vascular inhibition was in between these two levels (at 10^?4 M level). The ability of TMB-5 to raise the threshold of cardiac arrhythmias was quite good at 7 × 10^?7 –7 × 10^?6 M. It is concluded that TMB-5 could be a good antiarrhythmic agent with some skeletal muscle relaxation action.     

Effects of morphine on the rat striatal dopamine metabolism.

Concomitant with total suppression of the spontaneous unitary discharges of neurons in the rat corpus striatum, intracarotidly injected morphine (5 mg/kg) was also found to increase the levels of dopamine, homovanillic acid and cyclic AMP by 80, 65 and 46 per cent respectively, measured 5 min after injection. This provides further support to the hypothesis that the nigrostriatal dopaminergic pathway is stimulated by acutely administered morphine. Morphine (10^?5-10^?3 M) did not alter the activity of striatal tyrosine aminotransferase. The drug, added in vitro (10^?6-10^?4 M) or by intracarotid injection (5 mg/kg) did not affect the activity of striatal tyrosine hydroxylase. Moreover, morphine (10^?4 M) did not interfere with the inhibitory effects of dopamine (10^?6-10^?4 M) on striatal tyrosine hydroxylase. However, it significantly potentiated the stimulatory effects of cyclic AMP on this enzyme. Morphine (10^?5-10^?4) was also found to have no effect on the spontaneous or K⁺-stimulated release of dopamine from striatal homogenate and synaptosomes. However, in the presence of 5 × 10^?5 M and 10^?4 M morphine, the uptake of dopamine by striatal homogenate was inhibited by 14 and 33 per cent respectively. With synaptosomal preparations, dopamine uptake was inhibited by 17 per cent in the presence of 10^?4 M morphine—the inhibition being competitive with dopamine with an apparent K_i of 0.41 mM. The inhibition of dopamine uptake caused by 10^?4 M morphine in either preparation was not reversed by the addition of 10^?4 M naloxone. It was concluded that the increase in dopaminergic activity following acute treatment of morphine is probably due to (1) prolongation of the effect of dopamine on the post-synaptic neurons resulting in increased production of cyclic AMP which in turn potentiates dopamine synthesis and (2) decrease in presynaptic cystosol dopamine which is normally a feedback inhibitor of tyrosine hydroxylase thus leading to increased synthesis of dopamine.     

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.