Spontaneous Excitatory Junction Potentials Research Articles

Effect of normorphine and enkephalin on spontaneous potentials in the vas deferens

Spontaneous excitatory junction potentials were recorded intracellularly from the smooth muscle of the mouse vas deferens. Exposure of the vas deferens to either normorphine (1 μM) or methionine-enkephalin (0.5 μM) did not alter the frequency or amplitude distribution of the spontaneous potentials. The possible mechanisms by which the narcotic agonists depress the release of noradrenaline on nerve stimulation without inhibiting the spontaneous release of noradrenaline are discussed.

AN ELECTROPHYSIOLOGICAL ANALYSIS OF THE EFFECTS OF AMINE‐UPTAKE BLOCKERS AND α‐ADRENOCEPTOR BLOCKERS ON ADRENERGIC NEUROMUSCULAR TRANSMISSION

1 An electrophysiological study has been made of the effects of either blocking noradrenaline (NA) uptake or alpha-adrenoceptors on conduction in adrenergic preterminal axons and on NA release. 2 The excitatory junction potential (e.j.p.) evoked by a single stimulus increased slightly in duration (maximum 20%) in the presence of high concentrations of desipramine or cocaine (larger than or equal to 1 mug/ml) but there was no change in the spontaneous miniature excitatory junction potential (m.e.j.p.s); the single compound preterminal action potential was decreased in amplitude by a maximum of 10%. The e.j.p., m.e.j.p. and the terminal action potential were not altered by lower concentrations of these drugs (less than mug/ml). 3 The increased decline of the e.j.p. amplitude observed during the first few hundred impulses at high frequencies (10 Hz) in the presence of desipramine or cocaine was accompanied by a similar decline in the amplitude of the preterminal compound action potential, suggesting that the latter gave rise to the former. 4 These observations suggest that the action on post-synaptic alpha-adrenoceptors of NA released by single impulses is terminated by diffusion, and that any NA which is subsequently taken up into nerves is metabolized. 5 All the alpha-adrenoceptor blocking drugs tested reversed the normal depression in e.j.p. amplitude observed during the first few hundred impulses at high frequencies to facilitation; this was unaccompanied by any changes in the preterminal compound action potential. 6 Alpha-Adrenoceptor blocking drugs did not alter the potentiating effect which a conditioning impulse had on the amplitude of the e.j.p. evoked by a subsequent test impulse. The facilitated release of NA during trains of impulses was quantitatively predicted in terms of the addition of the individual potentiations introduced by each impulse in the train. 7 It is suggested that if there is an auto-inhibition of NA release, then it is unlikely that the pre- and post-synaptic alpha-adrenoceptors are identical.

Action of botulinum toxin on transmission from sympathetic nerves to the vas deferens.

Botulinum toxin (Type A) depressed or abolished transmission from postganglionic nerves to smooth muscle of isolated preparations of guinea-pig and mouse vas deferens. The time course of blockade was 2 to 6 times slower than that observed with the same concentration of the same batch of toxin on the rat diaphragm. Spontaneous excitatory junction potentials were still observed after 7 h exposure, indicating that the smooth muscle membrane was still able to respond to noradrenaline. Depression of transmission is probably due to a presynaptic action of unknown nature.

Nerve‐mediated excitation and inhibition of the smooth muscle cells of the avian gizzard

1. The electrical events evoked in smooth muscle cells of the chick and pigeon gizzards by vagal, perivascular sympathetic and transmural stimulation were recorded with intracellular micro-electrodes.2. Single stimulating pulses applied to the extrinsic nerves, or to intrinsic fibres, produced excitatory junction potentials (E.J.P.s) which were blocked by hyoscine. Repetitive stimulation caused facilitation of E.J.P.s and, at higher frequencies, summation. When the tissue was well oxygenated, maximal stimulation evoked action potentials and the tissue contracted. If the tissue was allowed to become anoxic, action potentials were blocked and thus junctional transmission could be observed uncomplicated by tissue contraction.3. Cholinergic E.J.P.s appeared to be due to stimulation of both post-ganglionic neurones and the pre-ganglionic input to post-ganglionic neurones, since ganglion-blocking drugs abolished the late phase of complex E.J.P.s while leaving unaffected their initial components.4. Repetitive perivascular sympathetic stimulation, after cholinergic E.J.P.s had been blocked by hyoscine, evoked a slow, long-lasting depolarization of the muscle cells. This depolarization was blocked by guanethidine (5 x 10(-6) g/ml.) and was mimicked by noradrenaline (10(-7) g/ml.).5. Under non-anoxic conditions, the tissue underwent rhythmical contractions which were associated with action potential firing. When the tissue was anoxic action potentials were not seen, but spontaneous membrane depolarizations resembling E.J.P.s were observed. Spontaneous miniature excitatory junction potentials (M.E.J.P.s) were observed only infrequently whether the tissue was well oxygenated or not.6. Single stimulating pulses applied to extrinsic or intrinsic nerves frequently evoked inhibitory junction potentials (I.J.P.s) in smooth muscle cells of the gizzards of both birds. I.J.P.s were blocked by tetrodotoxin (10(-7) g/ml.). Repetitive stimulation of inhibitory nerves gave rise to a membrane hyperpolarization; when stimulation was stopped the membrane potential showed rebound depolarization.7. These results are discussed in terms of the autonomic innervation of the avian gizzard, and possible explanations of the various types of activity observed are considered.

Electrical properties of the smooth muscle membrane of the guinea-pig vas deferens.

1. The effects of polarizing current on single smooth muscle cells of the isolated guinea-pig vas deferens have been studied by means of a bridge circuit in which the same electrode was used to pass current and record membrane potential.2. Most cells had polarization resistances from 15 to 37 MOmega.3. Depolarizing currents caused graded spikes up to a maximum amplitude of 75 mV.4. Self-regenerative spikes occurred in some cells if the membrane was depolarized by > 35 mV.5. Currents of long duration and sufficient intensity caused a repetitive discharge.6. Spikes arising from excitatory junction potentials were always ;all-or-none' in contrast to the graded responses to intracellular stimulation.7. Electrotonic potentials declined much more rapidly than spontaneous excitatory junction potentials (EJPs); thus the time course of the EJP cannot be determined by the passive electrical properties of the smooth muscle cell membrane.

The determination of serum iron after the intravenous injection of iron-dextran.

A rapid technique for the determination of iron in serum, after the intravenous injection of iron-dextran, is presented. Dithionite is used for the reduction of ferric to ferrous iron. The use of a detergent solution enables protein precipitation to be omitted. The iron reagent, 4: 7-diphenyl-1: 10-phenanthroline, and its ferrous complex are both soluble in the dilute detergent solution. Lipaemic serum can be used without affecting the results. The recovery of added ferric iron, as iron-dextran, is quantitative.