Long-term activation of protein kinase C by nicotine in bovine adrenal chromaffin cells

Previous results from our laboratory suggest that long-term treatment of primary cultured bovine adrenal medullary (BAM) chromaffin cells with nicotine or phorbol 12-myristate 13-acetate, either of which directly activates protein kinase C (PKC), increases the mRNA levels encoding catecholamine-synthesizing enzymes and proenkephalin. In the present study, we have examined the effects of nicotine on BAM cell PKC activity with special emphasis on long-term effects. Nicotine increased particulate PKC activity in a concentration-dependent manner when measured using in vitro enzyme assay with histone as the substrate. This effect is mediated through nicotinic cholinergic receptors, because 1,1-dimethylphenylpiperazinium, a nicotinic agonist, had a similar effect. In addition, chlorisondamine, a specific nicotine-receptor blocking drug, antagonized the effect of nicotine. Nicotine also increased specific [3H]phorbol 12,13-dibutyrate ([3H]PdBu) binding within 1 min, the effect of which was maximal between 3 and 12 min. This effect was reversed by chlorisondamine similarly after 12 min and after 18 h of nicotine treatment, indicating that continual nicotinic-receptor occupancy is required for persistent PKC activation. Compared to PKC activation, the onset of nicotine-stimulated diacylglycerol production was slow, and it was observed after 12 min of incubation with nicotine. The diacylglycerol levels, specific [3H]PdBu binding, and PKC activity remained significantly elevated for at least 18 h with continuous nicotine incubation. Furthermore, nicotine increased the PKC immunoreactivity of a particulate protein with a molecular mass of 82 kDa in the western blot. These results suggest that nicotinic-receptor activation increases PKC activity and immunoreactivity in BAM cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine A1 receptors mediate inhibition of tachykinin release from perifused enteric nerve endings

A perifused preparation of guinea pig myenteric nerve varicosities (synaptosomes) was used to determine the characteristics of evoked tachykinin release and the inhibition of such release by adenosine analogues. Release of substance P-like immunoreactivity (SP-LI) and neurokinin A-like immunoreactivity (NKA-LI) was evoked by elevated extracellular [K+] in a reversible and repeatable manner. This release was completely abolished in the absence of extracellular Ca2+. Perifusion in the presence of 5'-N-ethylcarboxamidoadenosine (NECA), a nonselective A1/A2 adenosine receptor agonist, decreased K(+)-evoked release of SP-LI and NKA-LI compared with that in the absence of the nucleoside. Similar decrements in peptide release were obtained with N6-cyclopentyl adenosine (CPA), a selective A1 agonist, and 2-[p-(2-carboxyethyl)]phenethylamino-5'-N-ethyl-carboxamidoadenosi ne (CGS 21680), a selective A2 agonist. Response to all nucleosides was graded. Potency order of adenosine analogues was CPA greater than NECA much greater than CGS 21680. Inhibition due to the nucleosides was diminished in the presence of the highly selective A1-receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) while perifusion in the presence of DPCPX alone did not alter evoked release of either peptide. These findings provide direct measurements of inhibitory effects of adenine nucleosides on the release, from enteric nerve endings, of endogenous neuromediators SP and NKA. The findings also directly demonstrate the presence of functional adenosine receptors of the A1 subtype on enteric nerve endings coupled negatively to release of tachykinins. The presence of A2 receptors on enteric nerve endings is neither supported nor excluded.

Ganglionic nicotinic receptor agonists exhibit anti-muscarinic effects in guinea-pig olfactory cortical brain slices

The action of some nicotinic acetylcholine receptor agonists was re-examined on the surface field potentials (N-waves) evoked by electrical stimulation of the lateral olfactory tract in guinea-pig olfactory cortical brain slices. Bath superfusion of nicotine or the nicotinic stimulants dimethylphenylpiperazinium (DMPP), lobeline, cytisine, tetramethylammonium or suberyldicholine (up to 100 microM) had little or no effect on the extracellular N-wave amplitude, or the membrane potential, input resistance or excitability of olfactory neurones recorded intracellularly. In contrast, the muscarinic agonists, carbachol or oxotremorine-M consistently depressed the field in a reversible dose-dependent manner. Interestingly, in the presence of the ganglionic stimulants DMPP (n = 6 slices) or lobeline (n = 5 slices) (10-50 microM), the effects of carbachol or oxotremorine-M were antagonized in a weak competitive-type manner (pA2 values = 5.58 and 5.63 respectively, estimated from Schild plots, constrained to unity slope). This anti-muscarinic action was unaffected by d-tubocurarine or hexamethonium. Nicotine, cytisine, tetramethylammonium and suberyldicholine showed much weaker and inconsistent carbachol-blocking effects. Combination of DMPP with atropine produced dose ratio shifts close to those predicted for a common-site interaction of two competitive antagonists. In conclusion, consistent pre- or postsynaptic nicotinic agonist actions could not be detected in olfactory cortex slices; however, some ganglionic nicotinic agonists were shown to exhibit significant anti-muscarinic effects on this preparation. We suggest this action might be due to a direct atropine-like mechanism.

Functional innervation of pig tracheal smooth muscle: neural and non-neural mechanisms of relaxation

In pig tracheal smooth muscle (TSM), the isometric tension responses to electrical field stimulation (EFS) were compared to exogenous peptides, vasoactive intestinal polypeptide (VIP) and calcitonin gene-related peptide (CGRP), as well as to the nicotinic agonist dimethylphenyl piperazinium chloride (DMPP). The objectives of this study were to understand the mechanisms of nonadrenergic, noncholinergic relaxations of pig TSM and identify some putative candidate substances mediating the neural inhibitory response. In strips of TSM obtained from 6- to 12-week-old pigs set up in vitro in organ baths in Kreb's solution, EFS resulted in frequency-dependent contractions that were abolished by 1 microM atropine or 0.1 microM tetrodotoxin (TTX). Addition of DMPP (20-100 microM) resulted in a transient, atropine-sensitive contraction, which quickly desensitized. EFS failed to elicit any further contractions, but the tissues responded to carbachol. In tissues exposed to DMPP and contracted with carbachol, EFS elicited frequency-dependent relaxations that were unaffected by 1 microM propranolol, abolished by TTX and partially inhibited by omega-conotoxin. At the peak of carbachol-induced contraction, addition of DMPP resulted in rapid relaxation reversing spontaneously to base line, with no significant relaxation to further addition of DMPP. DMPP-induced relaxations were unaffected by TTX or omega-conotoxin. These tissues readily relaxed to 10(-8) M VIP. In carbachol precontracted tissues, VIP elicited concentration-dependent relaxations that quickly desensitized. However, these tissues readily relaxed to DMPP. CGRP produced only weak relaxations.(ABSTRACT TRUNCATED AT 250 WORDS)

Nicotinic and muscarinic modulation of 5-hydroxytryptamine (5-HT) release from porcine and canine small intestine

Strips of porcine and canine small intestine were incubated in vitro and the release of 5-hydroxytryptamine (5-HT) was determined by HPLC with electrochemical detection. The spontaneous outflow of 5-HT from the porcine and canine small intestine largely reflects calcium-dependent 5-HT secretion from enterochromaffin cells which are under a spontaneous neuronal, excitatory input as indicated by the inhibitory effect (30-40%) of tetrodotoxin. In both species, nicotine enhanced the release of 5-HT in a concentration-dependent manner by a maximum of about 50% at 100 microM. This effect was blocked by the nicotine receptor antagonist hexamethonium, but not by the subtype-selective nicotine receptor antagonist alpha-bungarotoxin. The effect of nicotine was rapidly desensitized. The presence of tetrodotoxin abolished the effect of nicotine on 5-HT release in canine tissue but not in porcine tissue. The presence of the muscarine receptor antagonist scopolamine prevented the effect of nicotine on 5-HT release from canine tissue, but significantly enhanced 5-HT release from porcine tissue. The muscarine receptor agonist oxotremorine inhibited 5-HT release from porcine tissue, but increased 5-HT release from canine tissue. However, in the presence of tetrodotoxin, oxotremorine enhanced 5-HT release in tissue from both species. In conclusion, activation of nicotine receptors induce the release of 5-HT from porcine and canine small intestine. In the dog, the effect of nicotine is mediated via the release of acetylcholine which then stimulates 5-HT release via muscarine receptors on the enterochromaffin cells. In the pig, the stimulatory effect of nicotine appears to be located directly on the enterochromaffin cells. In addition, activation of neuronal muscarine receptors in the porcine small intestine induced the release of a previously unidentified neurotransmitter which inhibited 5-HT release. Nicotine, via cholinergic interneurons, also appears to induce the release of this inhibitory neurotransmitter which opposes the direct stimulatory action of nicotine on 5-HT release.

Phorbol esters cause preferential secretion of norepinephrine from bovine chromaffin cells

Differential secretion of norepinephrine and epinephrine was studied in cultured bovine chromaffin cells. Nicotinic agonists and 55 mM K+ evoked a slightly greater release of norepinephrine than of epinephrine: The percentage of norepinephrine secreted was 1.5 to two times greater than the percentage of epinephrine secreted. In contrast, when the cells were treated with phorbol 12,13-dibutyrate, the percentage of norepinephrine released was six to eight times greater than that of epinephrine released. Similar results were obtained in experiments with cultures highly enriched in either norepinephrine-containing cells or epinephrine-containing cells. In response to 55 mM K+, catecholamine release from norepinephrine-containing cells was two times greater than that from epinephrine-containing cells. In response to phorbol 12,13-dibutyrate, secretion from norepinephrine-containing cells was 13 times greater than that from epinephrine-containing cells. These results suggest that protein kinase C plays a specific role in the regulation of catecholamine secretion from norepinephrine-containing cells.

Presence of Na+/Ca2+ exchange activity and its role in regulation of intracellular calcium concentration in bovine adrenal chromaffin cells

The presence of a Na+/Ca2+ exchanger in bovine adrenal chromaffin cells was demonstrated by measuring the efflux of 45Ca2+ which had been preloaded into cells by a brief depolarization. The efflux of 45Ca2+ was dependent on extracellular Na+ (Na+o); 45Ca2+ efflux was significantly decreased by replacing Na+o with N-methylglucamine (NMG), or Li+. Replacement of Na+o by NMG increased the resting intracellular Ca2+ concentration ([Ca2+]i) of freshly isolated chromaffin cells. This could be reversed by adding Na+, suggesting that Na+/Ca2+ exchanger activity was involved in maintaining [Ca2+]i at its resting level. The initial rate of Na(+)-dependent [Ca2+]i recovery after Ca2+ loading by depolarization was dependent on the level of [Ca2+]i. There was an apparent linear relationship between the activity of the Na+/Ca2+ exchanger and [Ca2+]i both in the presence and absence of Na+o. When cells were treated with other stimuli, including 10 microM DMPP or 40 mM caffeine, the ability of the stimulated cells to decrease [Ca2+]i was significantly reduced upon replacing Na+o with NMG. Our data show that the Na+/Ca2+ exchanger is one of the major pathways for regulating [Ca2+]i in chromaffin cells in both resting and stimulated states.

Studies on the functional role of alpha-adrenoceptors in the cardiac sympathetic ganglia of the dog

Inhibitory alpha-adrenoceptors were studied in cardiac ganglia of pentobarbital-anesthetized dogs. Blockade of alpha 1- or alpha 2-adrenoceptors augmented preganglionic nerve stimulation induced tachycardia without altering the response to postganglionic nerve stimulation. The effect produced by blockade of ganglionic alpha 1-adrenoceptors with terazosin had different frequency-response characteristics from, was of smaller magnitude than, and was additive with the effect produced by blockade of ganglionic alpha 2-adrenoceptors with rauwolscine. The response to activation of ganglionic nicotinic cholinergic receptors in the absence of electrical stimulation of the preganglionic nerve was not affected by blockade of either alpha 1- or alpha 2-adrenoceptors. The response to nicotinic cholinergic receptor activation during periods of continuous preganglionic nerve stimulation was augmented following blockade of alpha 2-adrenoceptors but unaffected by alpha 1-adrenoceptor blockade. These results suggest that there are two different inhibitory pathways involving alpha-adrenoceptors in mammalian sympathetic ganglia and provide evidence that these inhibitory pathways are operative under the experimental conditions of ganglionic transmission.

The source of calcium for muscarinic-mediated catecholamine release from cat adrenals

1. In view of conflicting reports on the source of Ca2+ needed to trigger the secretory response to muscarinic stimulation of chromaffin cells, we have reinvestigated this problem in the cat adrenal gland perfused with oxygenated Krebs solution at 37 degrees C. Above a basal rate of secretion of 60 ng/30 s of total catecholamines, 5 s pulses of 100 microM-methacholine evoked 10-fold increases of secretion. This response was entirely mediated by muscarinic receptors, since it was blocked by submicromolar concentrations of atropine but not by d-tubocurarine. 2. Delayed application of methacholine pulses after Ca2+ removal from the Krebs solution led to a progressive decline of the secretory response with a t1/2 of 15 s. Secretion was blocked by 85% after a 60 s period of Ca2+ deprivation; extension of the external Ca2+ (Ca2+o) wash-out period up to 5 min did not further reduce the secretory response. 3. When EGTA (1 mM) was present in the 0 Ca2+ solution, the rate of decline of methacholine responses, as a function of the time of exposure to 1 mM-EGTA, was similar to that obtained with 0 Ca2+. Again, about 15-20% of the secretory response was resistant even to prolonged periods of washing out with the 0 Ca(2+)-EGTA solution. 4. The Ca2+ ionophore ionomycin (1 microM) first decreased and then accelerated the rate of decline of methacholine responses upon Ca2+o wash-out. Particularly relevant is the complete blockade of secretion when the Ca2+o wash-out is performed in the presence of this ionophore. This suggests the existence of a small intracellular functional Ca2+ store sensitive to ionomycin. 5. After abolition of the secretory response through 60 s periods of wash-out with a 0 Ca(2+)-EGTA-ionomycin solution, followed by delayed 5 s methacholine pulses after Ca2+o reintroduction, the glands instantly recovered their normal muscarinic-mediated secretory response. This suggests that upon muscarinic stimulation, Ca2+ required by the secretory machinery to trigger such response immediately comes from extracellular sources. How Ca2+o gains the cell interior so fast upon muscarinic stimulation is unknown; we have previously suggested that the muscarinic receptor in the cat chromaffin cell could be coupled to an ionophore channel which might be chemically activated by muscarinic agonists. 6. Secretory responses to 5 s pulses with 35 or 100 mM-K+ declined faster (t1/2 of 3 and 6 s, respectively) upon Ca2+o wash-out than those of methacholine.(ABSTRACT TRUNCATED AT 400 WORDS)

Beta subunits determine the time course of desensitization in rat alpha 3 neuronal nicotinic acetylcholine receptors

Standard two-electrode voltage-clamp techniques were used to investigate some of the pharmacological and functional properties of two types of rat neuronal nicotinic acetylcholine receptors expressed in Xenopus oocytes after pairwise injection of alpha 3 beta 4 or alpha 3 beta 2 mRNAs. Currents of several microA amplitude were elicited by fast application of micromolar concentrations of either acetylcholine (ACh) or 1,1-dimethyl-4-piperazine (DMPP). The activation of either receptor type by DMPP showed cooperativity (Hill coefficient, n greater than or equal to 1.7) with a half-maximal activation concentration (EC50) of 15-30 microM. In alpha 3 beta 4 receptors, ACh displayed cooperativity (n = 1.8) but was less efficacious than DMPP, yet its EC50 was about equal to that of DMPP. Finally, in alpha 3 beta 2 receptors, ACh was much less efficacious and had a much lower EC50. Desensitization induced by either DMPP or ACh was slow in alpha 3 beta 4 nicotinic ACh receptors but was rapid and extensive in alpha 3 beta 2 receptors, causing a significant proportion of the response to wane within the first few seconds of agonist application.

Evidence for the existence of a urethro-urethral excitatory reflex in urethane anesthetized rats: involvement of peripheral ganglionic structures

In urethane-anesthetized rats urethral distension with saline at the level of the external urethral sphincter elicited a series of slow phasic contractions with an amplitude between two and 18 mm. Hg. Contractions having an amplitude lower than four mm Hg were unaffected by i.v. or topical hexamethonium (HEX) or by topical tetrodotoxin (TTX). HEX or TTX transiently abolished or reduced the distension-evoked contractions having an amplitude between four and 18 mm. Hg. Administration of d-tubocurarine (d-Tc), atropine, bilateral section of pudendal nerves, spinalization (T12-S1) or bilateral removal of the major pelvic ganglia did not modify the distension-induced rhythmic urethral activity. In rats desensitized to capsaicin (four days before), both amplitude and frequency of urethral contractions did not differ from control value. In rats pretreated with atropine, administration of HEX still inhibited the distension-induced urethral contractions. Intravenous dimethylphenylpiperazinium (DMPP) triggered a rapid phasic contraction with an amplitude ranging between 15 and 25 mm. Hg. This effect was inhibited by previous administration of HEX. In accordance with histochemical studies showing spherical ganglionic cell bodies between the outermost striated muscle layer and the smooth muscular coat, present data indicate that distension of the external urethral sphincter reflexly activates urethral contractions, possibly through a local mechanism.

Effects of calcitonin gene-related peptide on longitudinal muscle and myenteric plexus of guinea pig ileum

Effects of rat calcitonin gene-related peptide (rCGRP) on the longitudinal muscle-myenteric plexus (lm-mp) have been investigated in the present study. rCGRP was shown to have a biphasic effect on the nonstimulated lm-mp, i.e., a transient contraction followed by a longer lasting relaxation. This biphasic action was dose-dependent over the concentration range of 10(-10) to 10(-5) M. Tetrodotoxin (1 microM), atropine (1 microM), 2-chloroadenosine (1 microM) and clonidine (1 microM) inhibited the stimulatory effect. Tetrodotoxin and clonidine also partially inhibited the relaxing effect of rCGRP. This biphasic action of rCGRP was also seen on the contraction of the lm-mp elicited by electrical field stimulation. rCGRP showed only a relaxing effect on the nonstimulated plexus-free longitudinal muscle. rCGRP (2.6 x 10(-10) to 2.6 x 10(-7) M) inhibited both the phasic and tonic contractions induced by histamine in a competitive way and showed a more potent effect on the tonic contraction than that on the phasic contraction. rCGRP at the same dose range also inhibited the tonic but not phasic contraction evoked by membrane depolarization with the maximal stimulation of KCl (30 mM). rCGRP inhibited KCl-, but not histamine-induced, extracellular calcium-dependent contraction. The peptide did not affect the dose-response curve of oxotremorine. These results suggest that rCGRP may exert its stimulatory actions on the lm-mp via actions on the plexus nerves, whereas the depressant actions may be mediated via the nerves, as well as directly on the smooth muscle.

Presynaptic facilitation by the new nootropic drug nebracetam, of ganglionic muscarinic transmission in the dog cardiac sympathetic ganglion

Effects of nebracetam (4-aminomethyl-1-benzylpyrrolidine-2-one hemifumarate, WEB 1881 FU, CAS 118607-07-1), a new nootropic drug, on impulse transmission in the cardiac sympathetic ganglia were studied in spinal dogs by monitoring heart rate as an indicator of the ganglionic function. The ganglionic stimulants were given directly into the cardiac sympathetic ganglia through the right subclavian artery (i.a.). Nebracetam, 5 mg/kg, i.v. caused a slight and temporal increase in heart rate. After nebracetam, the frequency-response curves of heart rate for preganglionic stellate stimulation (0.25-4 Hz) were not altered in the untreated and atropine-pretreated animals, but the curves (2.5-40 Hz) were shifted to the left in the hexamethonium-pretreated animals. The enhancement of ganglionic muscarinic transmission was dose-dependent on nebracetam i.v. at doses ranging from 0.5 to 15 mg/kg, with a maximal effect at 5 mg/kg. This enhanced muscarinic transmission by nebracetam was almost abolished after subsequent administration of pirenzepine 0.5 mg/kg i.v. The enhancement in the muscarinic transmission by nebracetam was also eliminated after depletion of acetylcholine at preganglionic sites caused by treatment with hemicholinium-3 in combination with preganglionic stimulation. Furthermore, nebracetam failed to affect dose-dependent post-ganglionic stimulation by McN-A-343 (1-32 micrograms), 1,1-dimethyl-4-phenylpiperazinium (1-32 micrograms) and angiotensin II (0.1 and 0.2 micrograms) administered i.a. directly to the ganglia. These results suggest that nebracetam facilitates the ganglionic muscarinic transmission through acting on presynaptic sites.

Characterization of somatodendritic neuronal nicotinic receptors located on the myenteric plexus

The effects of nicotine and dimethylphenylpiperazinium (DMPP) on resting and stimulation-evoked release of [3H]-acetylcholine ([3H]ACh) from cholinergic interneurons and neuro-effector neurons of the ileal longitudinal muscle and the responses of the smooth muscle to nicotinic agonists were studied. (-)-Nicotine was 15 times more effective than (+)-nicotine in releasing ACh. Since tetrodotoxin (1 microM) completely antagonized the effect of nicotinic agonists, the site of action of the nicotinic agonists studied was on the somatodendritic nicotinic receptors. The electrical field stimulation-evoked release was not affected by nicotinic agonists and antagonists, indicating that the axon terminals of cholinergic interneurons are not equipped with nicotinic receptors. This preparation proved to be useful to study the effect of nicotinic agonists on somatodendritic receptors, to determine the affinity constants of nicotinic antagonists, and to characterize these receptors. The rank order of antagonists was d-tubocurarine = mecamylamine greater than pipecuronium greater than pancuronium greater than vecuronium greater than hexamethonium; the apparent affinity constants (KD) were 1.15, 1.55, 3.06, 3.98, 13.59 and 32.88 microM, respectively. alpha-Bungarotoxin had no antagonistic activity at all. This finding indicates that nicotine and the endogenous ligand ACh act via a postsynaptic, somatodendritic nicotinic receptor that is pharmacologically similar to those located on the axon terminals of sympathetic neurons or in ganglions, but is dissimilar to those located at the postsynaptic site of the neuromuscular junction.

Differential secretion of enkephalin-like peptides from bovine adrenal chromaffin cells

Stimulation of chromaffin cells in culture with 1,1-dimethyl-4-phenylpiperazinium (DMPP) or depolarizing concentrations of K+ resulted in a significant secretion of high and low molecular weight enkephalin-like peptides (ELPs) into the culture medium. BioGel P-10 column chromatography was used to characterize the ELPs in chromaffin cell extracts and in culture media before and after stimulation with either DMPP or K+. DMPP (50 microM) stimulation produced a significant secretion of primarily low molecular weight (less than 3 kDa) ELPs whereas 56 mM K+ caused a secretion of both high and low molecular weights ELPs. The expected decrease in cellular content of low molecular weight peptides was not observed regardless of stimulation type. Our results support the hypothesis that the precursor/product ratio of secreted ELPs is dependent upon the nature of the chromaffin cell stimulus. Moreover the cellular content of low molecular weight ELPs is not depleted with either type of stimulation.

Conductance and kinetic properties of single nicotinic acetylcholine receptor channels in rat sympathetic neurones

1. The unitary conductance of nicotinic acetylcholine (ACh) receptor channels in rat sympathetic neurones has been studied. Conductance estimates varied from 26-48 pS with a mean of 36.8 pS in 1 mM-Ca2+. The main conductance level varied from patch to patch and the presence (or absence) of additional conductance levels also varied. 2. The channels showed large open channel noise and experiments with 300 mM-NaCl in the patch pipette substantially increased the open channel noise. The appearance of detectable step-like transitions within this noise strongly suggested the existence of closely spaced discrete levels. 3. Removal of divalent cations from the external solution increased the unitary channel conductance. Altering the main permeant ion in divalent-free solutions gave the following conductance sequence: K+ (93 pS) greater than Cs+ (61 pS) greater than Na+ (51 pS) greater than Li+(23 pS). 4. Replacement of Na+ by Cs+ in the external solution considerably reduced the current evoked by ACh in whole-cell recordings and the channel-opening frequency in outside-out patches. 5. The kinetic properties of channels activated by ACh and 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) were also studied. At low concentrations of ACh and DMPP the gap distributions were complex and best fitted by the sum of four exponential components. Individual activations (bursts) were interrupted by the two shortest closed periods the briefer of which had time constants of 36 microseconds for ACh and 67 microseconds for DMPP. 6. The distribution of burst lengths had two components for each agonist, each component making up about 50% of the total area under the distribution. For ACh, the time constant of the longer component (12.2 ms) was similar to the decay time constant of excitatory postsynaptic potentials (EPSCs) at similar temperature and potential. For DMPP the time constant of the longer component was 17.6 ms. 7. The relative number of brief gaps per long burst was much larger for ACh than for DMPP. Therefore the corrected mean open time for ACh (0.86 ms) was much shorter than that for DMPP (2.3 ms). 8. In terms of receptor mechanism, the values of the channel opening equilibrium constant (beta/alpha) estimated from these numbers (ACh, 23; DMPP, 25) suggest that both agonists are efficaceous. 9. DMPP is a potent blocker of the channel with an equilibrium dissociation constant (KB) of around 50 microM and blockage gaps of around 1 ms duration. ACh also blocks the channel but with a higher KB of around 470 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

The Pharmacology of Cholinoceptors on the Somatic Muscle Cells of the Parasitic Nematode Ascaris Suum

1. Acetylcholine (ACh) elicited depolarization and an increase in input conductance of the somatic muscle cells of the parasitic nematode Ascaris suum. 2. The relative potency of nicotinic and muscarinic agents was studied in this preparation. The order of potency of these compounds was metahydroxyphenylpropyltrimethylammonium (HPPT)> 1,1 dimethyl-4-phenylpiperazinium>(DMPP)> ACh> carbachol> nicotine> tetramethylammonium (TMA+)> muscarone> furtrethonium> arecoline. Decamethonium was also a weak agonist. McN-A-343 elicited a very weak depolarization at concentrations above 1 mmoll−1. Bethanechol and methacholine were without effect up to 1 mmoll−1. Pilocarpine and muscarine elicited a slight hyperpolarization of up to 3 mV with a threshold for the response of around 500 μmoll−1. Oxotremorine (1 mmoll−1) was without effect. 3. The nitromethylene insecticide 2(nitromethylene)tetrahydro 1,3-thiazine (NMTHT), an agonist at insect nicotinic receptors, was without effect on Ascaris muscle cells up to 1 mmoll−1. 4. Mecamylamine and benzoquinonium were the most potent antagonists of the acetylcholine response. The order of potency of the other antagonists was tetraphenylphosphonium (TPP) > quinacrine > pancuronium, curare> trimethaphan> atropine chlorisondamine, decamethonium > hexamethonium > dihydro-/3-ery throidine. 5. The agonist profile of the Ascaris muscle cell ACh receptor clearly indicates that it is nicotinic. The potency of ganglionic and neuromuscular nicotinic receptor antagonists in Ascaris does not enable a further subclassification of this nicotinic receptor. The Ascaris nicotinic receptor seems to possess some of the pharmacological properties of each type of vertebrate nicotinic receptor. The pharmacology of the Ascaris nicotinic receptor is discussed in relation to that of nicotinic receptors in other invertebrate preparations and in vertebrate preparations.

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Calcium promotes the accumulation of polyphosphoinositides in intact and permeabilized bovine adrenal chromaffin cells

1. Because cellular pools of phosphatidylinositol phosphate and phosphatidylinositol bisphosphate turn over rapidly during phospholipase C stimulation, the continuing production of inositol phosphates requires continuing synthesis from phosphatidylinositol of the polyphosphoinositides. In the present study in adrenal chromaffin cells, we examined the effects of nicotinic stimulation and depolarization in intact cells and micromolar Ca2+ in permeabilized cells on the levels of labeled polyphosphoinositides. We compared the effects to muscarinic stimulation in intact cells and GTP gamma S in permeabilized cells. 2. Nicotinic stimulation, elevated K+, and muscarinic stimulation cause similar production of inositol phosphates (D. A. Eberhard and R. W. Holz, J. Neurochem. 49:1634-1643, 1987). Nicotinic stimulation and elevated K+ but not muscarinic stimulation increased the levels of [3H]inositol-labeled phosphatidylinositol phosphate by 30-60% and [3H]phosphatidylinositol bisphosphate by 25-30%. The increase required Ca2+ in the medium, was maximal by 1-2 min, and was not preceded by an initial decrease in phosphatidylinositol phosphate and phosphatidylinositol bisphosphate. 3. In digitonin-permeabilized cells, Ca2+ caused as much as a twofold increase in [3H]phosphatidylinositol phosphate and [3H]phosphatidylinositol bisphosphate. Similarly, Ca2+ enhanced the production of [32P]phosphatidylinositol phosphate and [32P]phosphatidylinositol bisphosphate in the presence of [gamma-32P]ATP. In contrast, GTP gamma S in permeabilized cells decreased polyphosphoinositides in the presence or absence of Ca2+. 4. The ability of Ca2+ to increase the levels of the polyphosphoinositides decayed with time after permeabilization. The effect of Ca2+ was increased when phosphoesterase and phospholipase C activities were inhibited by neomycin. 5. These observations suggest that Ca2+ specifically enhances polyphosphoinositide synthesis at the same time that it activates phospholipase C.

Pharmacology of nicotinic receptor-mediated inhibition in rat dorsolateral septal neurones

1. Intracellular electrophysiological techniques were employed to investigate the effects of nicotinic receptor stimulation on rat dorsolateral septal nucleus (DLSN) neurones in a submerged rat brain slice preparation. 2. Acetylcholine (in the presence of the muscarinic antagonist, atropine), nicotine or dimethylphenylpiperazinium (DMPP), applied either by pressure ejection or superfusion, produced predominantly a membrane potential hyperpolarization. 3. Following concentration-response comparisons, DMPP appeared to exhibit fewer desensitizing properties and greater efficacy than nicotine with half-maximal hyperpolarizing responses attainable at 3 and 10 microM, respectively. 4. Pharmacological analyses revealed that the agonist-induced membrane hyperpolarization was sensitive to antagonism by mecamylamine (50-100 microM) and neuronal bungarotoxin (0.2-0.3 microM), but not alpha-bungarotoxin (0.5-1.0 microM), curare (10-50 microM) or dihydro-beta-erythroidine (50-100 microM). 5. Hyperpolarizing responses to DMPP were found to reverse near the equilibrium potential for potassium and were sensitive to changes in extracellular potassium concentration as predicted by the Nernst equation. Under single-electrode voltage clamp, application of DMPP produced an outward current (75-100 pA) which approached reversal at around -88 mV. These findings indicated that the hyperpolarizing response to nicotinic receptor stimulation was mediated by changes in membrane permeability to potassium. 6. DMPP-induced membrane hyperpolarization resulted from a direct action on postsynaptic DLSN neurones since the response persisted under conditions of superfusion with calcium-free/high-magnesium media or tetrodotoxin; both conditions blocked orthodromically induced neurotransmission. The hyperpolarizing response remained unaltered in TTX but was diminished in calcium-free/high-magnesium media. Further studies revealed blockade of the DMPP response following intracellular injection of EGTA. This response was also sensitive to antagonism by various calcium-dependent potassium channel blockers including apamin, barium and tetraethylammonium. 7. Our studies reveal a novel class of CNS nicotinic receptor whose action upon stimulation by an agonist results in a membrane hyperpolarization via a calcium-dependent increase in potassium ion conductance.

Separation of two pathways for calcium entry into chromaffin cells

1. The effects of various drugs on 45Ca + 40Ca uptake into cultured bovine adrenal chromaffin cells evoked by 1,1-dimethyl-4-phenylpiperazinium (DMPP) or high K, were studied. In the presence of 1 mM external 40Ca, with 45Ca as a radiotracer, unstimulated cells took up an average of 0.13 fmol/cell 40Ca and 772 c.p.m./10(6) cells of 45Ca (n = 76). Upon stimulation with DMPP (100 microM for 60 s) or K (59 mM for 60 s), Ca uptake increased to 0.92 and 1 fmol/cell, respectively. 2. Flunarizine behaved as a potent blocker of both DMPP- and K-evoked Ca uptake (IC50 of 1.76 and 1.49 microM, respectively for DMPP and K). A similar picture emerged with Cd ions, though Cd exhibited an IC50 against K (1.86 microM) slightly lower than the IC50 against DMPP (8.14 microM). 3. Clear cut differences were observed with amiloride, guanethidine, nimodipine and nisoldipine which behaved as selective blockers of DMPP-mediated Ca uptake responses: IC50 values to block DMPP effects were 290, 27, 1.1 and 1.63 microM respectively for amiloride, guanethidine, nimodipine and nisoldipine. Amiloride blocked K-evoked Ca uptake by only 35% and guanethidine did not affect it. Nisoldipine inhibited K-evoked Ca uptake only partially at low concentrations (about 30%); a second blocking component was observed at the highest concentration used (10 microM). At 10 microM, nimodipine blocked K-evoked Ca uptake by 50%. 4. Thus, it seems that the nicotinic receptor mediated Ca uptake pathway can be pharmacologically separated from the K-activated pathway. The results are compatible with the hypothesis that in cultured bovine adrenal chromaffin cells, stimulation of nicotinic receptors recruits a single type of Ca channel which is sensitive to flunarizine, Cd, amiloride, guanethidine, nimodipine and nisoldipine. The results also suggest that K depolarization might be recruiting in addition to this channel, another Ca channel which is highly sensitive to Cd and flunarizine, resistant to nisoldipine, nimodipine and amiloride, and insensitive to guanethidine.

Bioassay of nitric oxide released upon stimulation of non-adrenergic non-cholinergic nerves in the canine ileocolonic junction

1. The release and the nature of the inhibitory non-adrenergic non-cholinergic (NANC) neurotransmitter was studied in the canine ileocolonic junction. A circular muscle strip of the canine ileocolonic junction served as donor tissue in a superfusion bioassay in which rings of rabbit aorta with the endothelium removed served as detector tissue. 2. The ileocolonic junction released a labile factor with vasodilator activity upon stimulation of non-adrenergic non-cholinergic (NANC) nerves in response to electrical impulses and the nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP). This release was respectively frequency- and concentration-dependent. 3. The release was reduced by the blocker of neuronal conductance, tetrodotoxin, and by the inhibitor of the nitric oxide (NO) biosynthesis NG-nitro-L-arginine. The biological activity was enhanced by superoxide dismutase and eliminated by haemoglobin. Hexamethonium abolished only the release in response to DMPP. 4. Injection of adenosine 5'-triphosphate (ATP) or vasoactive intestinal polypeptide (VIP) onto the cascade induced relaxations of the rabbit aorta but they were different from those induced by NO or the transferable factor. 5. Based on organ bath experiments in which the reactivity of different parts of the circular smooth muscle layer of the ileocolonic junction was investigated, a muscle strip of superficial circular muscle with submucosa was chosen as the detector strip in the bioassay cascade. 6. The ileocolonic junction dose-dependently relaxed in response to nitroglycerin and NO. NO was much more potent in the rabbit aorta than in the canine ileocolonic junction. 7. In conclusion, our results demonstrate the release of a transferable vasorelaxant factor in response to NANC nerve stimulation which behaves pharmacologically like NO but not like ATP or VIP. Therefore, we suggest that NO or a NO releasing substance is the inhibitory NANC neurotransmitter in the canine ileocolonic junction.

Effects of glycerolformal on sympathetic neurotransmission in the isolated rabbit heart

Glycerolformal (CAS 5464-28-8; a mixture of 1,3-dioxan-5-ol and 1,3-dioxolane-4-methanol) used as an organic solvent or vehicle for drugs has been shown to possess its own toxicopharmacological activities. The aim of the present investigation was to determine the effects of glycerolformal on sympathetic neurotransmission in the isolated rabbit heart. At concentrations between 0.05 and 1 mmol/l glycerolformal inhibits both the neuronal and extraneuronal uptake of noradrenaline and its metabolism degradation which could explain the initial positive inotropic action of glycerolformal on the heart by increasing noradrenaline concentration in the synaptic cleft. However, the preponderant effect of glycerolformal was an inhibition of noradrenaline release, resulting in a myocardial depression which may explain the hypotension observed in the anesthetized rat. Hence, it is important to take into account the interference effects of glycerolformal with other molecules, when used as solvent or vehicle for drugs.

Both alpha- and beta-subunits contribute to the agonist sensitivity of neuronal nicotinic acetylcholine receptors

A family of genes has been identified that encodes subunits of nicotinic acetylcholine receptors (nAChRs) and is expressed in the nervous system. Functional neuronal nAChRs can be expressed in Xenopus oocytes by injection of RNA encoding 1 of 2 different beta-subunits (beta 2, beta 4) in pairwise combination with RNA encoding 1 of 3 different alpha-subunits (alpha 2, alpha 3, alpha 4). We examined the sensitivity of these 6 different alpha- beta-subunit combinations to the nicotinic agonists ACh, nicotine, cytisine, and 1,1-dimethyl-4-phenylpiperazinium (DMPP). Each subunit combination displayed a distinct pattern of sensitivity to these 4 agonists. The alpha 2 beta 2 combination was 5-fold more sensitive to nicotine than to acetylcholine, while the alpha 3 beta 2 combination was 17-fold less sensitive to nicotine than to ACh, and the alpha 3 beta 4 combination was equally sensitive to both nicotine and ACh. nAChRs composed of alpha 2, alpha 3, or alpha 4 in combination with beta 2 were 14-100-fold less sensitive to cytisine than to ACh. In contrast, nAChRs composed of alpha 2, alpha 3, or alpha 4 in combination with beta 4 were 3-17-fold more sensitive to cytisine than to ACh. The alpha 2 beta 2, alpha 3 beta 2, and alpha 3 beta 4 combinations were each equally sensitive to DMPP and ACh, while the alpha 2 beta 4, alpha 4 beta 2, and alpha 4 beta 4 combinations were 4-24-fold less sensitive to DMPP than to ACh. We also demonstrated that these differences are neither a consequence of variation in the relative amounts of RNA injected nor an artifact of oocyte expression. The oocyte system can accurately express ligand-gated ion channels because mouse muscle nAChRs expressed in oocytes display pharmacological properties similar to those reported for these receptors expressed on BC3H-1 cells. We conclude that both the alpha- and the beta-subunits contribute to the pharmacological characteristics of neuronal nAChRs.

The differential contribution of endogenous prostaglandins to the release of acetylcholine from the myenteric plexus of the guinea-pig ileum

1. Prostaglandin E (PGE) may be essential for maintaining the sensitivity of the myenteric plexus of guinea-pig ileum to nicotine. The contributions of prostaglandins to nervous activity evoked by different stimuli have now been investigated by measuring the amount of acetylcholine (ACh) released from the myenteric plexus of the guinea-pig ileum. 2. The amount of ACh released in response to dimethylphenylpiperazinium (DMPP) or substance P was depressed to about 40% of control by 2.8 microM indomethacin (Ind), whereas the release of ACh induced by 5-hydroxytryptamine (5-HT) was not affected. The inhibitory effects of Ind were overcome by 14.3 nM PGE2. 3. Mepacrine 5 microM, an inhibitor of phospholipase A2, depressed the release of ACh in response to DMPP and substance P to the same extent as Ind. These inhibitory effects of mepacrine were overcome by arachidonic acid (10 microM), but not by arachidonic acid plus Ind. The release of ACh evoked by 5-HT or electrical field stimulation (EFS) was also inhibited to about 60% of control by mepacrine but these inhibitions were overcome by arachidonic acid (10 microM) either in the absence or the presence of Ind. 4. The results suggest that endogenous prostaglandins and arachidonic acid contribute to the maintenance of the excitability of the myenteric plexus by DMPP and substance P. By contrast, the release of ACh induced by 5-HT and EFS may be regulated by arachidonic acid and not by prostaglandins.