Kyozo Koketsu

INCREASE OF ACETYLCHOLINE‐RECEPTOR SENSITIVITY BY ADENOSINE TRIPHOSPHATE: A NOVEL ACTION OF ATP ON ACh‐SENSITIVITY

1 The sensitivity of the nicotinic acetylcholine (ACh)‐receptor, measured as the amplitude of ACh‐current induced by iontophoretic application of ACh to the frog skeletal muscle endplate, was increased by the action of adenosine triphosphate (ATP) 2 This potentiation was not due to the effect of ATP on ACh‐esterase, since the increase of the sensitivity could also be demonstrated by use of carbachol (CCh) 3 Kinetic analysis of the effect of ATP on the dose‐response curve of CCh‐current suggests that ATP increases the ACh‐sensitivity by acting on the allosteric site of receptor‐ionic channel complex without changing the affinity of ACh for its recognition site 4 The equilibrium potential and the life‐time of the endplate current (e.p.c.) are not altered by the presence of ATP 5 These results suggest that ATP increases the ACh‐sensitivity by increasing either the conductance of unit channels or the total number of available channels.

Modulatory actions of ATP on membrane potentials of bullfrog sympathetic ganglion cells

Adenosine triphosphate (ATP) depolarized the membrane of bullfrog sympathetic ganglion cells by decreasing resting K+ conductance. ATP also depressed the maximum amplitude of after-hyperpolarization of action potentials. Voltage-clamp study revealed that ATP markedly suppressed the TEA-insensitive K+ current which appeared to correspond to the M-current, while it affected less significantly on the delayed rectifier K+ current. It was suggested that ATP depolarized resting membrane by suppressing resting K+ conductances, including the M-current, and also depressed the after-hyperpolarization of action potentials by suppressing both the M-current and delayed rectifier K+ current.

Effect of adenosine triphosphate on the sensitivity of the nicotinic acetylcholine-receptor in the bullfrog sympathetic ganglion cell

1 The effects of adenosine triphosphate (ATP) and related compounds on the sensitivity of the nicotinic acetylcholine (ACh)‐receptor of bullfrog sympathetic ganglion cells were analysed electro physiologically. 2 ATP in concentrations between 0.05 and 2 mm increased the amplitudes of the potentials and currents induced by ACh, and carbachol‐induced currents. 3 Compared with ATP, ADP was less potent in producing augmentation of the carbachol‐induced current by one order of magnitude. AMP, cyclic AMP and adenosine had no appreciable effect. 4 Analysis of this ATP effect, based on Michaelis‐Menten type kinetics, revealed that ATP increased the maximum response (Vmax) of the dose‐response curve of ACh currents without an appreciable effect on the affinity (Km) of ACh for its receptor. 5 It is suggested that ATP increased the receptor sensitivity by acting on an allosteric site of the nicotinic ACh receptor‐ionic channel complex which, thus, may be linked to an ATP receptor, probably of the P2‐receptor type (Burnstock, 1981).

PRESYNAPTIC MUSCARINIC RECEPTORS INHIBITING ACTIVE ACETYLCHOLINE RELEASE IN THE BULLFROG SYMPATHETIC GANGLION

1 The effects of bethanechol and atropine on the release of acetylcholine (ACh) from bullfrog sympathetic preganglionic nerve terminals were examined electrophysiologically. 2 Bethanechol (1 mm) caused no depolarization of sympathetic preganglionic nerve terminals, whereas carbachol or ACh in the same concentration induced marked depolarizations of these terminals. 3 Bethanechol (10 μm) depressed the amplitude of fast excitatory postsynaptic potentials (e.p.s.ps) recorded in low Ca2+‐high Mg2+ solution, without depolarizing ganglion cells. The quantal content measured from these fast e.p.s.ps by the variance method showed a significant reduction. 4 Amplitudes of both miniature e.p.s.ps and ACh‐potentials induced by iontophoresis of ACh were not affected by addition of bethanechol (10 μm). 5 The depressant effect of bethanechol (10 μm) on fast e.p.s.ps disappeared in the presence of atropine (3 μm). 6 Atropine (3 μm) increased the quantal content measured from fast e.p.s.ps recorded in low Ca2+‐high Mg2+ solution. 7 The depressant effect of bethanechol (10 μm) on fast e.p.s.ps was unaffected by α‐adrenoceptor blocking agents (phenoxybenzamine (10 μm) or phentolamine (10 μm)). 8 These results suggest that presynaptic nerve terminals in bullfrog sympathetic ganglia possess a muscarinic receptor which inhibits active release of ACh.

The effect of adrenaline on the electrogenic Na+ pump in cardiac muscle cells.

Electrogenic Na+ pump currents during K+-activated hyperpolarizations of bullfrog atrium muscle fibres are increased by adrenaline. The log dose-response relation between these currents and activating K+ concentrations is expressed by a sigmoidal curve, which is shifted in parallel to the left by adrenaline. It is suggested that adrenaline increases the rate of Na+ extrusion without increasing the Na/K coupling ratio and the total number of pumping sites.

The effect of adrenaline on sympathetic ganglion cells of bullfrogs

Abstract Amphibian sympathetic ganglion cells produce hyperpolarizing and depolarizing responses to adrenaline. Two kinds of adrenergic receptors responsible for the production of these responses appear to occur at different sites on the cell surface. The hyperpolarizing response is produced by changes in the sodium pump, presumably an electrogenic sodium pump, whereas the depolarizing response is produced by changes in the membrane permeability to certain ions.

5-hydroxytryptamine controls ACh-receptor sensitivity of bullfrog sympathetic ganglion cells.

Experimental evidences showing that 5-hydroxytryptamine (5-HT) is directly interacting with nicotinic acetylcholine (ACh) receptors and thereby depresses the sensitivity of these receptors to ACh, are presented by making use of bullfrog sympathetic ganglion cells and frog skeletal muscle endplates. It was suggested that 5-HT might decrease the affinity of ACh to nicotinic receptor sites, since the mode of 5-HT action was comparable to that of D-tubocurarine action.

Modulation of voltage-dependent currents by muscarinic receptor in sympathetic neurones of bullfrog.

The muscarinic actions of acetylcholine (ACh) on the action potentials of bullfrog sympathetic ganglion cells were studied with voltage-clamp experiments. The slow inward current (Isi) carried by Ca2+ was markedly depressed by ACh. ACh also markedly depressed the time-dependent outward current following Isi. The outward current was composed of two components, a TEA-sensitive rectifier K+ current (IK1) and a TEA-insensitive slow rectifier K+ current (IK2). Both of these components were depressed by ACh.

Voltage-clamp studies of a slow inward current in bullfrog sympathetic ganglion cells

Voltage-dependent inward membrane currents of bullfrog sympathetic ganglion cells were analyzed with the voltage clamp method. Two distinct inward currents, a tetrodotoxin (TTX)-sensitive fast inward current (INa) and a TTX-insensitive slow inward current (Isi), were recorded in Ringer solution. The Isi was markedly depressed by removal of external Ca2+ and by the addition of Mn2+, Co2+ or D-600. This suggests that the Isi is, almost exclusively, due to Ca2+. These results indicate that under physiological conditions a Ca2+ influx is induced during initiation of action potentials in these ganglion cells.

Effects of histamine on acetylcholine release in bullfrog sympathetic ganglia

The effects of histamine on the release of acetylcholine (ACh) from bullfrog sympathetic preganglionic nerve terminals were examined by means of intracellular microelectrode techniques. Low concentrations of histamine (1, 3 muM) increased the amplitude of fast excitatory postsynaptic potentials (fast EPSPs) and ACh quantal content, while high concentrations (100, 300 muM) decreased the amplitude and content. Amplitudes of miniature EPSPs and ACh potentials were not affected by histamine (0.3-300 muM). The facilitatory effect of histamine on fast EPSPs disappeared in the presence of mepyramine, whereas the depressant effect of histamine on fast EPSPs disappeared in the presence of cimetidine. These results suggest that histamine has facilitatory and depressant actions on ACh release. The facilitatory action is probably mediated by the H1-receptor and the depressant action by the H2-receptor, both of which are located at the presynaptic nerve terminals of bullfrog sympathetic ganglia.