Influence of lead on rat thoracic aorta contraction and relaxation.

Low levels of lead, but not high levels, produce hypertension. This mystery has not yet been resolved. In this study we compared the in vitro vasoresponsiveness in rat thoracic aorta to low dose (10(-8) mol/L) and high dose (10(-5) mol/L and 10(-4) mol/L) lead acetate. In addition to the direct response to lead, we examined reactivity to norepinephrine, acetylcholine, isoproterenol, phorbol ester, and calcium in the presence and absence of lead. Neither low-dose nor high-dose lead directly affected aortic contractile or relaxant responses. However, lead, only at the highest concentration (10(-4) mol/L), increased the contractions to calcium at all submaximal calcium concentrations. We conclude that low-dose lead must increase blood pressure indirectly through a humoral effect. The reasons for the failure of high-dose lead to influence blood pressure remain to be explored.

A 15-nonstereogenic carbocyclic analogue of prostacyclin: effects on human platelets and uterine artery.

Platelet-suppressant and vasodilator effects of the (+) enantiomer of 13,14-dihydro-15,16,17,18,19,20-hexanor-14-(1-hydroxycyclohexyl)++ +carbocyclin (MM706), a carbocyclic analogue of prostacyclin with the 15-hydroxyl group located at a symmetrically substituted carbon atom were studied on human platelets and isolated uterine artery. In washed platelets it inhibits platelet aggregation with an IC50 value of 77 nM as compared to IC50 = 1.3 nM for PGI2. In the presence of indomethacin its potency like that of PGI2 decreases about 2-fold. Also, MM706 like PGI2 fails to inhibit the norepinephrine (NE)-stimulated contractions of isolated human uterine artery in the absence of indomethacin. Pretreatment of the artery with indomethacin (3 microM) makes it sensitive to both MM706 and PGI2, the EC50 values being 99 and 3.1 nM, respectively. The results show that MM706 is an effective inhibitor of platelet aggregation and NE-stimulated contractions of the indomethacin-treated uterine artery.

Quest for agonist and antagonist selectivity at muscarinic receptors in guinea-pig smooth muscles and cardiac atria.

Potencies of 11 muscarinic agonists in eliciting contraction of smooth muscle in guinea-pig ileum, trachea, urinary bladder and uterus and in inhibiting the rate of contractions of cardiac atria were compared. While acetylcholine (ACh) was the most potent agonist on the ileum, uterus and cardiac atria, cis-L(+)-dioxolane was equally as potent as ACh on the ileum and more potent on the urinary bladder and trachea. Compared to ACh, methylfurmethide, oxotremorine, acetoxybut-2-inyl-trimethylammonium and cis-L(+)-dioxolane acted weakly on the atria. Aceclidine, arecoline and acetyl-beta-methylcholine displayed selectivity for the urinary bladder and pilocarpine for the tracheal and urinary bladder smooth muscles. Oxotremorine had very low activity on the uterus. The stereoselectivity of muscarinic ACh receptors (mAChRs) for cis-L(+)-and cis-D(-)-dioxolane was low in the urinary bladder and uterus and high in the ileum and trachea. Most antagonists showed little selectivity between different organs, but S(-)-phenylcyclohexylglycoloyl choline was 6 times more active on the urinary bladder than on the ileum and AF-DX 116 was 12-30 times more active on the atria than on the smooth muscles. Among the N-alkyl derivatives of benzilylcholine, the octyl derivative as 400 times more active on the ileum than on the atria, while among the N-alkyl derivatives of QNB, the N-decyl derivative was 41 times more active on the ileum. The observed differences in the potency of various agonists and their stereoisomers on different smooth muscles cannot be explained by differences in the accessibility of receptors or in receptor reserve.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of the neuromuscular blocking drug atracurium with muscarinic acetylcholine receptors.

On isolated rat heart atria, atracurium competitively antagonized the negative chronotropic effect of methylfurmethide, shifting the concentration-response curve to the right without diminishing the agonist's maximal effect; Kd calculated from dose ratios was 3.0 mumol/l. On the longitudinal muscle of rat ileum, atracurium antagonized the effect of methylfurmethide in a non-competitive manner; at 50 mumol/l atracurium, the maximum response to methylfurmethide was diminished by about 50%. Atracurium antagonized the binding of (3H)quinuclidinyl benzilate [3H)QNB) to muscarinic binding sites in the atria, ileal longitudinal muscle and cerebellum with IC50 values of 5-8 mumol/l, and in brain cortex of 25 mumol/l. Atracurium was little efficient, however, in antagonizing the binding of N-(3H-methyl) scopolamine [3H)NMS) to muscarinic binding sites. Complete blockade was not achieved at concentrations up to 1 mmol/l. Concentrations required to diminish the binding by 50% were 10 - 1000 times higher for (3H)NMS than for (3H)QNB. Atracurium brought about the dissociation of (3H)QNB-receptor complexes, but its effect was considerably stronger at a concentration of 30 mumol/l than at 1 mmol/l. Atracurium slowed down the dissociation of (3H)QNB-receptor complexes observed after the addition of atropine. The effects of atracurium on the dissociation of (3H)NMS-receptor complexes were similar to those on (3H)QNB-receptor complexes, but a high concentration of atracurium (1 mmol/l) produced a transient increase in (3H)NMS binding preceding its subsequent dissociation. Although the observations of the antagonism by atracurium of the effect of methylfurmethide on the heart atria, and of the inhibition of the specific binding of (3H)QNB to the atria, ileal smooth muscle, cerebellum and brain cortex are compatible with the assumption of a competitive interaction, the discrepancy between the effects of atracurium on the binding of (3H)QNB and (3H)NMS indicates that atracurium does not bind to the same binding site as (3H)QNB and (3H)NMS. It appears that most effects of atracurium on muscarinic receptors are allosteric and that both negative and positive cooperatives play a role in interactions between atracurium and muscarinic ligands.

Positive cooperativity in the binding of alcuronium and N-methylscopolamine to muscarinic acetylcholine receptors.

The effect of the neuromuscular blocker alcuronium on the binding of N-[3H]-methylscopolamine [( 3H]NMS) and l-[3H]quinuclidinylbenzilate ([3H]QNB) to muscarinic binding sites in rat heart atria, longitudinal smooth muscle of the ileum, cerebral cortex, cerebellum, and submaxillary glands was measured using filtration techniques. In the presence of 10(-5) M alcuronium, the binding of [3H]NMS (which was present at a subsaturating concentration of 2 x 10(-10) M) was increased 5.3-fold in the atria and smooth muscle and 3-fold in the cerebellum; no increase was observed in the brain cortex and salivary glands. The binding of [3H]NMS was inhibited at 10(-3) M and higher concentrations of alcuronium. The rates of [3H]NMS association to and dissociation from muscarinic binding sites in the atria were diminished by 10(-5) M alcuronium. Scatchard plots of [3H]NMS binding data obtained with and without 10(-5) M alcuronium indicated that the maximum number of binding sites was not altered by the drug, whereas the apparent Kd for [3H]NMS was diminished. In contrast to [3H] NMS, the effects of alcuronium on the binding of [3H]QNB were only inhibitory. The concentration of alcuronium required to diminish the binding of [3H]QNB by 50% (IC50) was 4-7 microM in the atria, ileal smooth muscle, and the cerebellum, 140 microM in the brain cortex, and 1200 microM in the parotid gland. The results suggest that the binding of low concentrations of alcuronium to muscarinic receptors in the heart, ileal smooth muscle, and cerebellum allosterically increases the affinity of muscarinic receptors towards [3H]NMS, although not [3H]QNB. At high concentrations, alcuronium inhibits the binding of muscarinic ligands, presumably by competition for the classical muscarinic binding site. Positive cooperativity induced by alcuronium appears to be specific for the m2 (cardiac) subtype of muscarinic receptors.

Stabilization of antagonist binding to cardiac muscarinic acetylcholine receptors by gallamine and other neuromuscular blocking drugs.

The effects of neuromuscular blocking drugs and muscarinic agonists and antagonists on the dissociation of [3H]quinuclidinylbenzilate ([3H]QNB) from muscarinic receptors was studied on rat atrial homogenates. In typical experiments the investigated drug was added to the homogenate equilibrated with [3H]QNB and the amount of undissociated [3H]QNB receptor complexes was measured 40 min later. The antagonists atropine and pirenzepine, agonists carbamoylcholine and methylfurmethide and neuromuscular blockers pancuronium, d-tubocurarine and decamethonium caused a concentration-dependent dissociation of [3H]QNB from the receptors, which may be explained by their competition with [3H]QNB for the same (primary) binding sites. In accordance with this, these drugs did not affect the dissociation of [3H]QNB elicited by an excess of atropine, which indicates that the kinetics of dissociation of the [3H]QNB receptor complex remained unchanged in their presence. Neuromuscular blockers alcuronium, gallamine and to a lesser degree tercuronium differed from the other drugs in that 1) their effect on [3H]QNB dissociation was biphasic, being higher at their low (10(-6) to 10(-5) M) than at their high concentrations (10(-4) to 10(-3) and that 2) at high concentrations they strongly inhibited the dissociation of [3H]QNB receptor complexes elicited by the excess of atropine. Their behavior may be rationalized by assuming that at low concentrations they bind to the primary binding site making rebinding of once dissociated [3H]QNB molecules improbable (competitive mechanism), whereas at high concentrations they also act on a secondary (allosteric) binding site stabilizing the [3H]QNB receptor complexes by slowing their off-kinetics.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of the neuromuscular blocking drugs alcuronium, decamethonium, gallamine, pancuronium, ritebronium, tercuronium and d-tubocurarine with muscarinic acetylcholine receptors in the heart and ileum.

Neuromuscular blocking drugs have a high affinity for muscarinic acetylcholine receptors in the heart atria and ileal smooth muscle. In experiments on homogenates, alcuronium, gallamine, pancuronium, tercuronium and ritebronium inhibited the binding of the muscarinic antagonist (3H)quinuclidinyl benzilate (QNB) to rat heart atria with IC50 values of 0.15-0.53 mumol X 1(-1) and to ileal longitudinal muscles with IC50 values of 0.12-0.45 mumol X 1(-1). d-Tubocurarine and decamethonium inhibited (3H)QNB binding to these tissues with IC50 values of 6.2-8.5 mumol X 1(-1). For each neuromuscular blocking drug, the IC50 values were virtually identical for (3H)QNB displacement in the homogenates of the atria and of the ileal muscle. Alcuronium and gallamine differed from the other blocking agents in that they produced less steep (3H)QNB displacement curves both in the atria and the ileal muscle; Hill coefficients for the binding of alcuronium and gallamine to atrial and ileal homogenates were lower than unity. On isolated atria, gallamine, pancuronium, ritebronium and tercuronium antagonized the inhibition of tension development caused by the muscarinic agonist, methylfurmethide, with Kd values which were of the same order of magnitude as the IC50 values for the displacement of (3H)QNB binding to homogenates; the Kd of alcuronium was 12.5 times higher. d-Tubocurarine and decamethonium did not antagonize the effects of methylfurmethide at concentrations up to 100 mumol X 1(-1). On isolated ileal longitudinal muscle, gallamine and pancuronium antagonized the effects of methylfurmethide with Kd values that were 53 times and 100 times higher than their respective Kd values in the atria.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholinoreceptors of the dentis retractor muscle of the sea urchin Strongylocentrotus intermedius.

Neostigmine increases and cholinergic blocking agents block contractions of Strongylocentrotus intermedius dentis retractor muscle caused by either acetylcholine or indirect stimulation. Both innervated and non-innervated parts of the muscle are sensitive to low acetylcholine concentration suggesting that there are both synaptic and extra-synaptic cholinoreceptors. Nicotinomimetics are more potent than muscarinomimetics. Both d-tubocurarine and atropine are weak blocking agents as compared to pentadecamethylene-bis-trimethylammonium or hexadecamethylene-bis-trimethylammonium, Kd being 1 X 10(-4) to 5 X 10(-5) and 5 X 10(-7) to 4 X 10(-7) M respectively. Disulphide bonds and carboxylic (or phosphate) groups were revealed in dentis retractor muscle cholinoreceptors using drugs modifying receptor structure. Cholinoreceptors of both nicotinic and muscarinic type were revealed in oesophagus longitudinal muscles of Strongylocentrotus intermedius.

The muscle chemoreceptors of the ascidian Halocynthia aurantium.

The body wall muscle of the ascidian, Halocynthia aurantium, is sensitive to both nicotinomimetics and muscarinomimetics. Atropine (4 X 10(-11 M) and benzilylcholine mustard (1 X 10(-5) M for 15 min) selectively block responses to muscarinomimetics. d-Tubocurarine (2 X 10(-6) M) and hexadecamethylene-bis-trimethylammonium (2 X 10(-7) M) prevent contractions induced by both nicotinomimetics and muscarinomimetics. In the cold season of the year the sensitivity of the muscle to muscarinomimetics disappears while responses to nicotinomimetics do not change throughout the year. The cooling of the water in the experimental bath results in an increase in the muscle contractions caused by muscarinomimetics while the responses to nicotinomimetic propionylcholine do not change. Catecholamines induce a decrease of the ascidian muscle responses to acetylcholine, cGMP and papaverine produce a similar effect.

[Action of polymethylene-bis-quaternary compounds on the muscarinic and nicotinic cholinoreceptors]. / Deistvie nekotorykh polimetilen-bis-chetvertichnykh soedinenii na muskarinovye i nikotinovye cholinoretseptory.

The action of compounds with general formula (formula: see text) on the frog heart ventricle, cat blood pressure, guinea pig ileum and frog rectus abdominis was studied. With dioxolane radicals (type F-2268) a strong muscarinomimetic action on the cat arterial blood pressure and guinea pig ileum was observed, with maximum marked action at n = 10, which was more pronounced at an even than at odd number of methylene groups. On the frog heart the compounds with an odd number of "n" elicited an atropine-like action. The compounds with pentyl radicals produced no effect on blood pressure and a weak cholinolytic effect on the frog heart. On the ileum they exhibited a cholinomimetic effect. All compounds studied acted as noncompetitive cholinolytics on the frog rectus.

[Study of skeletal muscle cholinoreceptors using alkylating agents]. / Izuchenie kholinoretseptorov skeletnoi myshtsy s pomoshch'iu alkiliruiushchikh agentov.

The ability of decamethonium containing beta-clorethylamino groups to alkylate the nicotinic cholinoreceptors of the frog tonic muscles was studied. D-tubocurarine prevented the action of the alkylating decamethonium (AD). The latter equally inhibited the effects of carbacholine and tetramethylammonium. The degree of alkylation did not change with pH varying from 6 to 11. AD did not produce any parallel shifts, but inhibited at once the maximal response to carbacholine both of the frog intact muscle and of a single tonic fibre. It is suggested that decamethonium blocks the cholinoreceptors anionic sites, which are represented by the carboxylate, or phosphate anions. The frog tonic muscle probably fails to posses any spare receptors.

[Comparative findings on the stereoselectivity of cholinoreceptors]. / Sravnitel'nye dannye o stereoselektivnosti kholinoretseptorov

The potency of the optical isomers of the muscarinomimetic agent 2-methyl-4-dimethylaminomethyl-1.3-dioxolane methiodide (F-2268) was compared on cholinoreceptors, (ChR) of different animals. The greatest difference between optical isomers was observed on the muscarinic ChR of guinea pig ileum smooth muscle cis-L(+)isomer being more than hundred times as potent as cis-D(-)isomer. On the ChR of muscarinic type in the holothuria Cucumaria japonica retractor muscle, cis-L(+)isomer was 25 times as efficient as cis-D(-)isomer. On the ChR of sea urchin and sipunculid locomotor muscles, optical isomers differ only 3 to 5 times. There was no difference between the effect of optical isomers on the ChR of muscarinic type which mediate hyperpolarization in the neurones of the gastropod mollusc Planorbarius corneus. This suggest that some changes in ChR stereoselectivity may occur in the course of evolution.