ACh-induced Responses Research Articles

Diversity of mechanical responses and their possible underlying mechanisms in the proboscis muscles ofBusycon canaliculatum

Mechanical responses of the radular protractor and retractor, the odontophore retractor and the radular sac muscles ofBusycon canaliculatum were compared. The radular protractor responded to both ACh and high K salines with similar slow, smooth contractures showing no evidence of fast twitch activity. The radular sac, odontophore retractor, and radular retractor muscles responded to low K salines with bursts of fast twitches at a mechanical threshold below that for responses in the radular protractor. With high K salines these three muscles showed inactivation of fast twitch activity and replacement by slow maintained tonic force. With rare exceptions, the ACh responses of all four muscles consisted of slow, maintained tonic contractures with no fast twitch activity, although individual muscles differed in their ACh sensitivity. A scheme is presented to explain the mechanical modus operandi of this complex organ by the co-operative actions of these four physiologically diverse muscles. It is proposed that fast twitch responses depend upon the activity of fast transient Ca channels showing strong voltage sensitivity and ready voltage inactivation. It is proposed that maintained tonic contractures in all the muscles depends upon the activity of slow long-lasting voltage-dependent Ca channels which only open with substantial membrane depolarization. It is suggested that K-induced and ACh-induced responses may activate a similar cellular Ca pool but by different membrane transduction routes.

Unusual responses of proboscis muscles ofBusycon canaliculatum to some calcium antagonist agents

K- and ACh-induced responses of the radular sac, odontophore retractor, and radular retractor muscles ofBusycon canaliculatum were found to be strongly dependent upon [Ca]0. Diltiazem had strong positive inotropic and chronotropic actions on fast twitch activity in the odontophore retractor and radular protractor muscles. K-induced tonic force in these muscles was partly inhibited by diltiazem but only at very high concentrations. ACh responses in all muscles were eliminated by diltiazem. Nifedipine enhanced fast twitches and tonic force in response to high K, and induced persistent spontaneous fast twitch discharges. Nifedipine inhibited ACh-induced tonic force, but induced rhythmic bursts of fast twitches persisting long after nifedipine washout. Verapamil strongly inhibited K- and ACh-induced tonic force in all three muscles at high concentration, but stimulated fast twitch responses and converted ACh contractures into fast twitch activity. Sucrose gap studies showed that nifedipine and diltiazem reduced K- and ACh-induced tension and depolarization. Paradoxically, verapamil reduced K- and ACh-induced tension but significantly enhanced their induced depolarizations. Diltiazem, nifedipine and verapamil did not act like slow Ca channel antagonists in these muscles. This may reflect differences in channel structure between molluscs and mammals, or differences in the cellular calcium release pathways operated by such channels in molluscan and mammalian muscle. These ‘Ca-ant-agonists’ appeared to act as agonists of fast twitch activity in these muscles and antagonists of the ACh-induced calcium release pathway for tonic force development.

Metabolic pathways of concanavalin action on chemical sensitivity at the neuronal membrane

Concanavalin A (Co A) has been shown to degress response produced at the membrane of dialyzed snail neurons by acetylcholine (ACh) and γ-aminobutyric acid (GABA) and blocked by ouabain. The inhibitory effect of Co A on such response was suppressed by lowering the temperature to 5°C. Co A produced no noticeable effect on ACh-induced response insensitive to ouabain and inhibited ACh-induced outflow of Rb+ (86Rb) from neurons previously enriched by Rb ions. Phosphorylation of neuronal membrane fractions occurred under the action of Co A. In addition, intracellular concentration of cyclic adenosine monophosphate (cAMP) declined while that of cyclic guanosine monophosphate (cGMP) increased. It is postulated that cAMP-dependent membrane phosphorylation underlies sensitivity to ACh and GABA at the neuronal membrane.

Endothelium-dependent relaxation, cholesterol content and high energy metabolite balance in Watanabe hyperlipemic rabbit aorta

Functional and metabolic parameters of thoracic aorta from Watanabe heritable hyperlipemic (WHHL) rabbits (aged 11–14 months) were investigated in vitro. The aortic preparations, normally responsive to noradrenaline, showed a diminished response to the endothelium-dependent agent, acetylcholine, in comparison with control preparations from age-matched New Zealand rabbits (maximal relaxation: 33 ± 4% in WHHL vs. 52 ± 2% in controls; P < 0.005). ATP relaxant effect (only partially endotheliumdependent) was unimpaired in WHHL aorta, and it was much higher than in controls (maximal response: 63 ± 6% vs. 37 ± 3%, respectively; P < 0.005). The response to NaNO 2, an endothelium-independent relaxant, was unchanged in WHHL aortas. Acetylcholine-induced response was found to be inversely related to the degree of total cholesterol infiltration in aorta ( r = −0.62, P < 0.05). No correlation was observed between either total serum cholesterol or triglycerides and ACh-induced response. Furthermore, the concentration of adenine nucleotides and nucleosides in the aortic tissue of WHHL rabbits was lower than in controls, indicating a loss of energy balance. The results indicate a functional damage induced by genetic hyperlipidemia on endothelium-dependent relaxation and an impairment of energy-rich phosphate metabolism of the aortic wall. The relationship between functional and metabolic parameters is not yet clarified.

Activation and desensitization mechanisms of muscarinic current response in single pancreatic acinar cells of rats.

1. In single, enzymatically dissociated, rat pancreatic acinar cells both ACh stimulation and IP3 (inositol 1,4,5-trisphosphate) injection can evoke Ca(+)-dependent transient current responses. However, exogenously applied IP3 (10 microM) gradually loses its ability to induce the Ca2(+)-dependent response (an increase in [Ca2+]i) during cell incubation with a saline solution. 2. Administration of IP4 (inositol 1,3,4,5-tetrakisphosphate, 10 microM) together with the IP3 (the injection of IP3-IP4 mixture) allows partial recovery of the response, but not full replication of the response induced by ACh (0.2 microM). Injection of IP4 alone never induces the current response. 3. The sensitivity of IP3 recovers after short-term administration of ACh (0.2 microM), and in turn, the ACh-induced response is augmented by the presence of internal IP3. These results suggest that a synergism between IP3 and another ACh-induced substance plays an important role in muscarinic Ca2+ signalling. 4. ACh-induced responses are inhibited by pre-incubation (10 min) with an activator of protein kinase C, TPA (12-O-tetradecanoylphorbol-13-acetate, 16 nM), or augmented by pre-incubation (10 min) with an inhibitor, H-7 (1-(5-isoquinoline-sulphonyl)-2-methylpiperazine, 10 microM), whilst IP3-induced responses are unaffected by that with both agents. These results indicate that protein kinase C acts negatively on the signalling elements prior to the formation of IP3. 5. The oscillatory responses, induced by cell dialysis with a nominally Ca2(+)-free (ca 1-10 microM) solution containing GTP gamma S (100 microM), are unaffected by the pre-treatment with TPA or H-7. In addition, these responses and/or those triggered by short-term stimulation with ACh and internal GTP gamma S are not influenced by external ACh. On the other hand, the oscillatory responses recorded in acinar cells pre-treated with H-7 are tightly controlled by external ACh. 6. Taken together these results suggest that activation of protein kinase C does not affect the activity of GTP-binding protein, but disconnects the link between the muscarinic ACh receptor and GTP-binding protein, or inhibits ACh binding to the receptor, in rat pancreatic acinar cells.

Effect of pirenzepine and gallamine on cardiac and pulmonary muscarinic receptors in the rabbit.

1. The effect of muscarinic antagonists considered to be selective for M1 receptors (pirenzepine) and for M2 receptors (gallamine) were studied on bronchoconstriction and bradycardia elicited by stimulation of the vagal nerves and by i.v. acetylcholine (ACh) in anaesthetized rabbits. 2. Pirenzepine was equipotent as an antagonist of ACh-induced responses at postjunctional muscarinic receptors in the heart, lung and blood vessels, whereas gallamine was at least ten times less potent at pulmonary and vascular muscarinic receptors. Thus, gallamine never caused complete inhibition of bronchoconstrictor or hypotensive responses to i.v. ACh, whereas doses of pirenzepine in excess of 1 mumol kg-1 abolished all muscarinic responses. 3. In the lung, both antagonists inhibited bronchoconstriction caused by vagal stimulation and ACh-induced bronchoconstriction to the same extent (pirenzepine, mean ED50 65 +/- 22 and, 130 +/- 28 nmol kg-1 respectively; gallamine, ED50 greater than 10,000 nmol kg-1 for both responses). Enhancement of vagally-induced bronchoconstriction was never observed. 4. In the heart, however, both pirenzepine and gallamine were ten times less potent as antagonists of vagally-induced bradycardia than of ACh-induced bradycardia. This differential blockade was unaltered by propranolol (1 mg kg-1) pretreatment. 5. It is concluded that there is no evidence for M1 or M2 muscarinic receptors in the pulmonary innervation of the rabbit and the potency of the antagonists in abolishing in abolishing vagally-induced bronchoconstriction was consistent with blockade of M3 muscarinic receptors on airway smooth muscle. 6. The results suggest that M2 muscarinic receptors may exert an inhibitory effect on transmission in the parasympathetic nerves innervating the heart in the rabbit. Blockade of such neuronal receptors would increase transmitter output to the atrial cells and explain the low potency of both antagonists in abolishing vagally-induced bradycardia in the rabbit.

Platelet-activating factor-induced contraction of guinea-pig lung parenchymal strips: possible involvement of arachidonate metabolites.

The identification of the mediators possibly involved in platelet-activating factor (PAF)-induced contraction of guinea-pig lung parenchymal strips (GPLP) was attempted by means of antagonists and inhibitors. Histamine, serotonin, acetylcholine (ACh) or other transmitters released from the nerve terminals are not likely to play a role in this response, since specific antagonists and tetrodotoxin did not affect the contraction. PAF antagonists (brotizolam and WEB 2086) produced a concentration-dependent inhibition of the contraction. Inhibitors of TXA2 synthesis (dazoxiben) and of 5-lipoxygenase (nordihydroguaiaretic acid and AA 861) and antagonists of TXA2 (ICI 159995) and peptidoleukotrienes (L 649923 and LY 171883, but not FPL 55712) produced a significant inhibition of the PAF-induced response at concentrations which did not reduce the ACh-induced response. These results suggest that arachidonate metabolites, both of the cyclo-oxygenase and of the lipoxygenase pathway, are determinants of the PAF-induced contraction of GPLP.

Role of calcium ions in processes of serotonin-induced modulation of neuronal response to acetylcholone application in Helix pomatia

The role of calcium ions in modulating serotonin action on acetylcholine (ACh) response in nonidentified and identified (LPa3 and RPa3) neurons ofHelixpomatia was investigated using voltage-clamping at the neuronal membrane. Exposure for 1 min to serotonin prior to ACh application reduced response to ACh in neuron LPa3 and raised it in RPa3. The same two patterns of modulating ACh-induced response were produced by extracellular application of theophylline and dibutyryl c-AMP. Injecting calcium ions into neuron LPa3 led to reinforcement of ACh-induced current in the presence of serotonin, thus changing the pattern of serotonin-induced modulation of ACh response in this unit. In neuron RPa3, the same process enhanced the serotonin-induced modulating effect on ACh response but without changing the pattern of modulation, while injected EDTA produced the reverse effects. Increased intracellular concentration of calcium ions brought about a reduction in the degree of serotonin-induced modulation of ACh response in neuron RPa3. Possible reasons are discussed for changes in serotonin-induced bimodal modulation of ACh response in test neurons produced by altering the extracellular concentration of calcium ions.

Electromechanical uncoupling in a molluscan muscle examined by the sucrose gap technique. The effect of calcium antagonist and agonist agents.

Membrane potential and tension of Busycon radular protractor muscles were studied by sucrose gap methods. Excitation-contraction (EC) coupling was examined in response to acetylcholine (ACh) and high K which depolarized the fibres and induced tension, but without action potential firing. Potassium depolarization did not follow predictions expected from the Nernst equation at low and very high K levels, and maximum tension was found at about 100 mM K. EC coupling was very sensitive to [Ca]0. Ca-free media eliminated K- and ACh-induced tension but with normal depolarization, showing full electromechanical uncoupling. Ionophore A23187 enhanced K- and ACh-induced responses and X-537A enhanced ACh responses, demonstrating acute dependence of activation on [Ca]0 in this muscle. The calcium antagonists nifedipine and nisoldipine reduced tension in the muscle only at very high concentrations, and both agents slightly reduced K- and ACh-induced depolarization. Verapamil reduced K- and ACh-induced tension but paradoxically it enhanced the depolarizing actions of these agents leading to electromechanical uncoupling. Abscisic acid (ABA) enhanced ACh- and K-induced tension and simultaneously enhanced their depolarizing actions. Ionophores and ABA appear to enhance calcium influx which may secondarily influence sodium influx. Calcium antagonists have no consistent actions on this muscle, suggesting that calcium channel activity of the radular protractor may be different from that seen in mammalian visceral muscles.

Heterogeneous distribution of muscarinic receptors in the rabbit saphenous artery.

1. The properties of the muscarinic receptors in the rabbit saphenous artery were determined from electrical and mechanical responses of smooth muscle cells produced by acetylcholine (ACh). The inhibitory action of atropine and pirenzepine on the ACh-induced responses was also studied. 2. ACh produced a transient hyperpolarization of the membrane and inhibited the noradrenaline (NA)-induced contraction. These effects of ACh were apparent only when the endothelial cells were intact. 3. The ACh-induced transient hyperpolarization was antagonized by atropine or pirenzepine, with similar potencies (the ID50 values were about 2 x 10(-8) M for both antagonists). 4. The ACh-induced inhibition of the contraction to NA was antagonized by atropine more preferentially than by pirenzepine (the ID50 values were 2 x 10(-8) M for atropine and 10(-6) M for pirenzepine). 5. The excitatory junction potential (e.j.p.) evoked by perivascular nerve stimulation was inhibited by ACh (above 10(-8) M). The ACh-induced inhibition of the e.j.p. was antagonized by atropine more preferentially than by pirenzepine (the ID50 values were 3 x 10(-8) M for atropine and 6 x 10(-6) M for pirenzepine). 6. It is concluded that in the rabbit saphenous artery, two subtypes of muscarinic receptor (M1 and M2) are located on the endothelial cells. Stimulation of each subtype releases a different substance, i.e., a hyperpolarizing substance (M1-subtype) or a relaxant substance (M2-subtype). In prejunctional nerve terminals, the muscarinic receptors responsible for inhibiting the release of transmitter substances are of the M2-subtype.

Action of enflurane on cholinergic transmission in identified Aplysia neurones.

Effects of enflurane on the cholinergic transmission in Aplysia neurones were studied by current and voltage clamp methods. Acetylcholine (ACh) evoked three types of postsynaptic responses on different identified neurones: (1) a depolarizing response due to an increase in Na and K conductances (D-response), (2) a fast hyperpolarizing response due to an increase in C1 conductance (C1-response), and (3) a slow hyperpolarizing response due to an increase in K conductance (K-response). Enflurane altered neither the action potential nor the membrane resistance of the neurones but depressed the three ACh-induced responses, non-competitively, in a dose-dependent manner. The K-response was less suppressed than the other two. Blockade of the closed state of ion channel was suggested by a reduction in the first ACh response evoked 1 min after administration of enflurane. The anaesthetic facilitated the decay of the neurally evoked e.p.s.c. and i.p.s.c. in suggesting a reduction in the mean open time of the postsynaptic ion channel. It is concluded that enflurane depresses excitatory and inhibitory cholinergic transmission by reducing the postsynaptic currents.

Effects of Ketanserin on the Pacemaker Activity and Contractility in the Isolated, Blood-Perfused Dog Atrium

Effects of ketanserin on the pacemaker activity and atrial contractility and on 5-hydroxytryptamine (5-HT)-induced cardiac responses were investigated in the isolated, blood-perfused dog atrium. Ketanserin (1-300 micrograms) injected into the sinus node artery evoked a transient positive followed by a negative chronotropic effect, and a dose above 30 micrograms of ketanserin produced a dose-dependent negative inotropic effect with a little positive one. Ketanserin-induced negative chronotropic and inotropic effects were not affected by atropine in a dose which blocked ACh-induced responses. Propranolol inhibited positive inotropic responses to ketanserin and norepinephrine and significantly augmented the negative chronotropic and inotropic responses to ketanserin. Imipramine did not affect the transient positive followed by negative chronotropic and the negative inotropic responses to ketanserin but it induced the positive cardiac responses following the negative ones. Tetrodotoxin, phentolamine, and diphenhydramine did not modify the effects of ketanserin. From these results, it is concluded that ketanserin might induce the direct negative chronotropic and inotropic effects and the indirect effects by catecholamine release. Ketanserin-induced catecholamine release might not be due to tyramine-like or nicotine-like action. Ketanserin significantly inhibited a low dose (3 micrograms) of 5-HT-induced negative chronotropic and inotropic effects, suggesting the possibility of 5-HT2 receptors in the isolated dog atrium.

Effects of methionine-enkephalin and substance P on the chemosensory discharge of the cat carotid body

The effects of methionine enkephalin (ME) and substance P (SP) were tested on the chemosensory discharge of the cat carotid body-nerve preparation in vitro. ME superfused in concentrations of 10 −8 to 10 −5 M depressed the sensory discharge, an effect followed by receptor excitation (rebound). Bolus application of ME (30 ng to 3.0 μ) induced variable effects (excitation or depression) on the discharge, excitation being more pronounced with the smaller doses. Superfusions with SP (10 −8 to 10 −5 M) either excited or depressed the discharge, excitation being more pronounced with higher SP concentrations (i.e. 10 −6 M). Bolus applications of SP (43 ng to 0.5 μg) also excited or depressed the sensory discharge. These variations may be dose-dependent. Superfused ME (10 −6 M) significantly depressed the chemoreceptor response to hypoxia (100% N 2) and hypercapnia (6% CO 2, pH 7.43). The responses to NaCN and acidity (pH 6.0) were marginally depressed. Superfused SP (10 −6 M) clearly depressed the responses to hypoxia, those to hypercapnia and NaCN were marginally affected but the effects of acidity were not altered. When the peptides were tested against the receptor responses to exogenously applied putative neurotransmitters (ACh, dopamine — DA), it was found that ME tended to depress both the ACh and DA actions whereas SP (10 −6 M) tended to increase their effects. Superfusions with naloxone (10 −6 M) increased the basal chemosensory discharge and this enkephalin blocker partially relieved the depressant effect of ME on the ACh-induced response. It is concluded that carotid body chemoreceptors have excitatory and inhibitory reactive sites to both ME and SP although their precise location is still unknown.

Selective blocking action of tetraethylammonium ion on three types of acetylcholine receptors in Aplysia ganglion cells.

The ganglion cells of Aplysia have three types of acetylcholine (ACh)-receptors in their membranes. The activation of one type produces Na+-dependent depolarization (DNa-type). The other receptors are HK- and HC1-types, the activation of which produces either K+- and Cl- dependent hyperpolarization, respectively. ACh-induced membrane current (IACh) was measured under the voltage clamp at different holding potential levels (V) of the membrane. The effects of tetraethylammonium (TEA) were analyzed quantitatively on each type of receptor activity by observing the changes in dose-inhibition curves and IACh-V relations obtained under voltage clamp. TEA exhibited a dose dependent block on both DNa- and HK-type of receptor activities but no effect on HCl-type. The mode of blocking action was found to be competitive in HK-type but noncompetitive in DNa-type. In the DNa-type receptor, 3 mM TEA shifted the reversal potential of ACh-induced current response from +5 to +40 mV, and caused a decrease in the slope of the IACh-V curve. In HK-type, TEA simply decreased the slope of IACh-V curve without changing the reversal potential of the ACh-induced response. It was concluded that TEA competed that with ACh for common binding sites at the receptor of the HK-type, but not of the DNa-type, and that in DNa-type, TEA made a binding with an allosteric site of the ACh-receptor and depressed the increase in membrane permeability toward Na+ as well as K+.

The effects of L-alanine and acetylcholine on membrane potential, 45Ca2+ and 86Rb+ efflux and amylase secretion in the isolated mouse pancreas.

A comparative study was made of the effects of acetylcholine (ACh) and the neutral amino acid L-alanine on membrane potential, 45Ca2+ and 86Rb+ fractional efflux, and amylase secretion in segments of the isolated mouse pancreas. Both ACh and L-alanine evoked similar dose-dependent depolarizations of pancreatic acinar cells. ACh, but not L-alanine, caused a marked reduction in input resistance and evoked electrical uncoupling of acinar cells at high concentrations. Measurements of 45Ca2+ release from pre-labelled tissue showed that ACh increased the fractional efflux of 45Ca2+ whereas L-alanine had no such effect. Both ACh and L-alanine stimulated the release of 86Rb+ from pre-labelled tissues. The L-alanine-evoked 86Rb+ efflux was markedly reduced by pre-treatment with 10(-3) M ouabain which had no effect on the ACh-induced response. L-alanine had no significant effect on amylase secretion whereas ACh elicited a marked increase in amylase output. These results show that although both ACh and L-alanine evoke depolarization of the pancreatic acinar cell membrane, the mechanisms underlying their actions on the cells are different.

Effects of onium derivatives on Na-, Cl-, and K-dependent acetylcholine receptors.

Various types of acetylcholine (ACh) receptor in the pleural or abdominal ganglion cells of Aplysia were classified into three types depending on the ion species involved in the electrogenesis of their responses. The Na-dependent receptor was designated the DNa-type since the response was a depolarization (D) produced by an increase in permeability, mainly toward Na+, although other ions were partially involved. Similarly, the Cl- or K-dependent receptor was designated the HCl- or HK-type since the response was a hyperpolarization (H) produced by an exclusive increase in permeability toward Cl- or K+. ACh-induced responses were evaluated by an increase in membrane conductance. The DNa-type receptor could be readily activated by tetramethylammonium (TMA) and choline, but not by decamethonium (Deca). Instead, it was blocked competitively either by hexamethonium (Hexa) or by Deca. The HCl-type receptor could be readily activated by TMA, choline, and Deca. Hexa did not block this type of receptor. The HK-type receptor was neither activated nor blocked by TMA, choline, Hexa, and Deca. The above findings suggested that there is a certain structural similarity in the ACh-binding site between DNa-type and HCl-type receptors, although the ion species involved in the electrogenesis of each response were totally different. The structure of the ACh-binding site at the HK-type receptor seemed considerably different from that of either DNa-type or HCl-type.

Hyperpolarization caused by external high potassium in snail neurons.

Two-thirds of the tested subesophageal ganglion cells of Japanese snail, Euhadra peliomphala, was hyperpolarized when they were perfused by high K-Ringer. This hyperpolarization was accompanied with a marked decrease in membrane resistance and was independent of cell types classified by ACh-induced response. Pentobarbital was found to convert high K-induced hyperpolarization into depolarization, or to augment K-depolarization. The high K-induced hyperpolarization was considered as the result of summated IPSP's elicited by the presynaptic inhibitory fibers which were primarily depolarized by high K. Pentobarbital may remove this synaptic inhibition and disclose the original K-depolarization underlying the cell. In Cl-free media, high K caused a marked depolarization instead of hyperpolarization, or augmented the depolarization observed in normal Ringer. This suggested that the subsynaptic membrane became permeable to Cl ions during the inhibitory presynaptic activity when high K was applied. ACh was found not to be responsible for this synaptic inhibition, because d-tubocurarine, which is known to block the Cl-dependent ACh response of the snail neurons, did not affect the high K-induced hyperpolarization. The possibility of indirect action of high K on the membrane potential through synaptically mediated inhibition was discussed as a cause of its hyperpolarizing effect.

モルモット摘出心・房標本の経壁刺激効果およびNicotine反応におよぼす1-ethyl-4-(2-morpholinoethyl)-3，3-diphenyl-2-pyrrolidinone hydrochloride hydrate(Doxapram)の影響

It has been reported that doxapram exhibits a remarkable stimulating effect on respiration in humans and various experimental animals. The present experiment was an attempt to investigate whether or not doxapram inhibits an atrial arrest induced by transmural stimulation and exogenously applied nicotine. Doxapram and dimorpholamine used as comparative agents showed transient and slightly positive responses followed by a negative one in the atrium preparation. Transmural stimulation under a condition of 30 V intensity with 0.3 msec duration at a frequency of 10 Hz for 2 sec caused an atrial arrest for about 3 sec followed by negative chronotropic and inotropic responses, and a positive one. All responses caused by transmural stimulation were hardly affected by pretreatment with doxapram at a concentration of 10(-5) g/ml or less, while dimorpholamine at a high concentration (10(-5)g/ml) showed an inhibition of atrial arrest and following negative responses. Negative chronotropic and inotropic responses caused by an application of nicotine were significantly inhibited by pretreatment with doxapram or dimorpholamine. Both doxapram and dimorpholamine at higher concentrations also inhibited the positive responses of the atrium caused by nicotine. Neither doxapram nor dimorpholamine affected the ACh-induced responses. NA-induced responses were uneffected by pretreatment with doxapram, while the responses were slightly potentiated by dimorpholamine. The action mechanisms of doxapram and dimorpholamine are discussed.

The action of some cholinergic blockers on carotid body chemoreceptors In vivo

Chemosensory discharges were recorded in vivo from fine filaments of the carotid nerve containing a few or single active units; their frequency was used as an index of receptor activity. The effects of hemicholinium, atropine, mecamylamine and hexamethonium on the chemosensory discharges elicited by ACh and NaCN were studied in 46 adult cats. Hemicholinium in low concentrations (0.5–1 mg/kg, i.v.) did not change the resting discharge rate or the chemoreceptor response to ACh and NaCN. One-2 h after injections of hemicholinium (2–5 mg/kg, i.v.) the responnse to NaCN was markedly depressed, while that to ACh remained unchanged. Larger doses of the drug (10 mg/kg, i.v.) depressed or blocked the responses to both chemicals. These experiments indicate that transmitter substances (probably ACh) stored in the cells are depleted or decreased by the presence hemicholinium, which blocks ACh synthesis. Atropine in low concentrations (0.2–1 mg/kg, i.v.) depressed the receptor response to ACh without affecting the discharges elicited by NaCN. On the other hand, relatively large doses of the drug (2–10 mg, i.v. and i.a.) blocked or depressed both the basal chemoreceptor discharges and the responses to ACh and NaCN. This depressant effect of atropine on the response to both chemicals lasted for 10–20 min. Mecamylamine (2–10 mg, i.a.) blocked the chemoreceptor response to ACh and NaCN. Naturally occuring chemoreceptor discharges were blocked transiently by the drug. Hexamethonium (2–10 mg, i.a.) was ineffective on the chemoreceptor response to ACh and NaCN in 25% of the chemosensory units examined, and in 50% of them the drug depressed or blocked the ACh-induced response without affecting the NaCN-induced effect. In 25% of the units, both the response to ACh and NaCN was blocked or depressed by the drug. Results indicate the approximate population of nicotinic cholinoceptive sites of chemosensory nerve terminals. It is concluded that a cholinergic mechanism is probably responsible for chemoreceptor impulse initiation. Exogenous ACh appears to be acting on extrasynaptic cholinergic receptors, probably located in the stem of the non-myelinated sensory endings, while natural stimuli and NaCN are probably acting on the glomus cells to liberate ACh, which exerts its effects on synaptic cholinergic sites.

Radiation and Whole Light Effects on Molluscan Nerve Cells in Vitro

Acute effects of low-dose X-radiation and light illumination on the isolated ganglion cells of Aplysia were analyzed by means of intracellular recording of their synaptic activities. The acetylcholine (ACh)-induced depolarization associated with an increase in membrane conductance was not significantly affected by X-radiation up to 9300 R. Orthodromically activated EPSP's were not markedly altered by X-ray up to 4500 R, but they were slightly enhanced with the higher dose. The threshold firing level of the soma was not affected by a total dose of 13,000 R. Membrane resting potential (MRP) was hyperpolarized 1 to 3 mV during exposure to 1700 to 5000 R but was depolarized 1 to 3 mV during exposure to more than 13,000 R. It was concluded that the central nervous system of Aplysia is much more radioresistant than that of mammals. Light illumination produces either an increase or a decrease in frequency of the spontaneous spike discharge. It was found that the light does not alter the ACh-induced responses of ...