Current spread in the smooth muscle of the guinea-pig vas deferens

Electrophysiology of Syncytial Smooth Muscle

As in other excitable tissues, two classes of electrical signals are of fundamental importance to the functioning of smooth muscles: junction potentials, which arise from neurotransmission and represent the initiation of excitation (or in some instances inhibition) of the tissue, and spikes or action potentials, which represent the accomplishment of excitation and lead on to contractile activity. Unlike the case in skeletal muscle and in neurons, junction potentials and spikes in smooth muscle have been poorly understood in relation to the electrical properties of the tissue and in terms of their spatiotemporal spread within it. This owes principally to the experimental difficulties involved in making precise electrical recordings from smooth muscles and also to two inherent features of this class of muscle, ie, the syncytial organization of its cells and the distributed innervation they receive, which renders their biophysical analysis problematic. In this review, we outline the development of hypotheses and knowledge on junction potentials and spikes in syncytial smooth muscle, showing how our concepts have frequently undergone radical changes and how recent developments hold promise in unraveling some of the many puzzles that remain. We focus especially on computational models and signal analysis approaches. We take as illustrative examples the smooth muscles of two organs with distinct functional characteristics, the vas deferens and urinary bladder, while also touching on features of electrical functioning in the smooth muscles of other organs.

Investigation of the Syncytial Nature of Detrusor Smooth Muscle as a Determinant of Action Potential Shape

Unlike most excitable cells, certain syncytial smooth muscle cells are known to exhibit spontaneous action potentials of varying shapes and sizes. These differences in shape are observed even in electrophysiological recordings obtained from a single cell. The origin and physiological relevance of this phenomenon are currently unclear. The study presented here aims to test the hypothesis that the syncytial nature of the detrusor smooth muscle tissue contributes to the variations in the action potential profile by influencing the superposition of the passive and active signals. Data extracted from experimental recordings have been compared with those obtained through simulations. The feature correlation studies on action potentials obtained from the experimental recordings suggest the underlying presence of passive signals, called spontaneous excitatory junction potentials (sEJPs). Through simulations, we are able to demonstrate that the syncytial organization of the cells, and the variable superposition of the sEJPs with the "native action potential", contribute to the diversity in the action potential profiles exhibited. It could also be inferred that the fraction of the propagated action potentials is very low in the detrusor. It is proposed that objective measurements of spontaneous action potential profiles can lead to a better understanding of bladder physiology and pathology.

Release of dopamine beta-hydroxylase and chromogranin A upon stimulation of the splenic nerve

Two proteins present in noradrenergic vesicles of the splenic nerve (dopamine beta-hydroxylase and chromogranin A) are released into the perfusate from the spleen when the splenic nerve is stimulated. Experiments in which drugs were added to the perfusion fluid showed that the proteins were released from terminals of the splenic nerve. There was a correlation between the amounts of the proteins released and the quantity of noradrenaline released; and the release process was dependent upon calcium. It is suggested that the proteins are released from the large dense-cored vesicles present in the terminals of the splenic nerve, and that secretion from these vesicles occurs by exocytosis.

Post- and prejunctional consequences of ecto-ATPase inhibition: electrical and contractile studies in guinea-pig vas deferens

At sites of purinergic neurotransmission, synaptic ecto-ATPase is believed to limit the actions of ATP following its neural release. However, details of the modulation by this enzyme of the ATP-mediated conductance change and the possible mechanisms mediating this modulation remain unelucidated. We have addressed these issues by studying the effect of ARL 67156, a selective ecto-ATPase inhibitor, on ATP-mediated electrical and contractile activity in the sympathetically innervated guinea-pig vas deferens. ARL 67156 at 100 mum significantly potentiated the amplitude of spontaneous excitatory junction potentials (SEJPs) by 81.1% (P < 0.01) and prolonged their time courses (rise time by 49.7%, decay time constant by 38.2%; P < 0.01). Moreover, the frequency of occurrence of SEJPs was strikingly increased (from 0.28 +/- 0.13 to 0.90 +/- 0.26 Hz; P < 0.01), indicating an additional, primarily presynaptic, effect of ecto-ATPase inhibition. The frequency of occurrence of discrete events (DEs), which represent nerve stimulation-evoked quantal release of neurotransmitter, was also increased ( approximately 6-fold; P < 0.01), along with the appearance of DEs at previously 'silent' latencies. Purinergic contractions of the vas deferens were potentiated significantly (P < 0.01) by ARL 67156; these potentiated contractions were suppressed by the A1 agonist adenosine (P < 0.01) but left unaffected by the A1 antagonist 8-phenyltheophylline (8-PT). Our results indicate (i) that ecto-ATPase activity, in addition to modulating the ATP-mediated postjunctional conductance change, may regulate transmitter release prejunctionally under physiological conditions, and (ii) that the prejunctional regulation may be mediated primarily via presynaptic P2X, rather than A1, receptors.

Spatial and temporal coordination of junction potentials in circular muscle of guinea-pig distal colon

1. In isolated, stretched, flat-sheet preparations of guinea-pig distal colon, simultaneous intracellular recordings were made from pairs of circular muscle (CM) cells to map the region of smooth muscle at which spontaneous junction potentials (sJPs) were coordinated in both space and time. 2. Spontaneous inhibitory junction potentials (sIJPs) and excitatory junction potentials (sEJPs) were recorded from all animals with varying frequencies and amplitudes (up to 25 mV). 3. Large amplitude (> or = 9 mV) sIJPs or sEJPs with near-identical amplitudes and time courses were recorded synchronously from two CM cells, even when the two electrodes were separated by up to 11 mm in the circumferential axis and < or = 4 mm in the longitudinal axis. However, smaller (< 9 mV) sIJPs or sEJPs were less coordinated and exhibited greater variability in their times to peak. 4. The standard deviation (S.D.) for the time difference between the peaks of sJPs was related to the amplitude of the events and the distance between the electrodes. The S.D. for large amplitude JPs (approximately 30 ms), which was less than that for small JPs (approximately 150 ms), remained constant across the circumferential axis (at least up to 6 mm), but declined rapidly for distances > or = 2 mm in the longitudinal axis. 5. Current injection (up to 8 nA) into a single CM cell elicited electrotonic potentials in neighbouring CM cells, only when the two electrodes were separated by less than 100 microm circumferentially. Beyond 50 microm electronic potentials were rarely detected. 6. Tetrodotoxin (TTX; 1 microM) abolished all sJPs, whereas hexamethonium (300 microM) either abolished, or substantially reduced all sJPs. 7. Nitro-L-arginine (L-NA; 100 microM) abolished the slow repolarisation phase of sIJPs without any apparent effect on the amplitude of sIJPs. Apamin abolished the fast, initial component of sIJPs, suggesting synchronous release of two inhibitory neurotransmitters during the sIJP. Atropine (1 microM) abolished sEJPs. 8. No sJPs were recorded from the CM layer when it was separated from the myenteric plexus. 9. In conclusion, sIJPs and sEJPs in colonic CM occur synchronously over large regions of the smooth muscle syncitium. The results are discussed in relation to the idea that spontaneous junction potentials in colonic CM are not monoquantal events, but are generated by the simultaneous release of transmitter from many release sites, due to the synchronous activation of many enteric motor neurons.

Electrical coupling in smooth muscles. Is it universal?

There is strong experimental evidence for electrical coupling in all types of smooth muscle. In some publications, and particularly in physiological textbooks, smooth muscles are still divided into those that are electrically coupled and those that are not. In this article we review the evidence for the universal presence of coupling in smooth muscles and the underlying mechanism, which, in most cases, appears to be gap junctions. We propose a classification of smooth muscles based on the mechanisms that initiate their activity. The two main types of smooth muscle according to this classification are neurogenic (e.g., iris, arterioles, vas deferens) and myogenic (e.g., urinary bladder, intestine, most blood vessels).

Analysis of synaptic quantal depolarizations in smooth muscle using the wavelet transform

The time-frequency characteristics of synaptic potentials contain valuable information about the process of neurotransmission between nerves and their target organs. For example, at the synapse between autonomic nerves and smooth muscle, two central issues of neurophysiology, i.e., 1) the probability of neurotransmitter release and 2) the quantal behavior of transmission can be deduced from analysis of the rising phases of evoked excitatory junction potentials (eEJP's) recorded from smooth muscle. eEJP rising phases are marked by prominent inflexions, which reflect these features of neuronal activity. Since these inflexions contain time-varying frequency information, we have applied recent techniques of time-frequency analysis based upon wavelet transforms to eEJP's recorded from the guinea-pig vas deferens in vitro. We find that these techniques allow accurate and convenient characterization of neuronal release sites, and that their probability of release falls between 0.001-0.004. We have also analyzed eEJP's recorded in the presence of the chemical 1-heptanol, which reveals quantal depolarizations. These results have helped clarify the nature of the quantal depolarizations that underly eEJP's. The present method offers significant advantages over those previously employed for these tasks, and holds promise as a novel approach to the analysis of synaptic potentials.

Quantal evoked depolarizations underlying the excitatory junction potential of the guinea-pig isolated vas deferens

1. The effects of a putative gap junction uncoupling agent, heptanol, on the intracellularly recorded junction potentials of the guinea-pig isolated vas deferens have been investigated. 2. After the stimulation-evoked excitatory junction potentials (EJPs) had been suppressed by heptanol (2.0 mM) to undetectable levels, a different pattern of evoked activity ensued. This consisted of transient depolarizations that were similar to EJPs in being stimulus locked and in occurring at a fixed latency, but differed from EJPs in that they occurred intermittently and had considerably briefer time courses. 3. Analysis of the amplitudes and temporal parameters of the rapid residual depolarizations revealed a close similarity with spontaneous EJPs (SEJPs). There was no statistically significant difference between the rise times, time constants of decay and durations of the rapid residual depolarizations and of SEJPs. 4. Selected evoked depolarizations were virtually identical to SEJPs occurring in the same cell. Evoked depolarizations of closely similar amplitude and time course also occurred, usually within a few stimuli of each other. 5. These depolarizations appear to represent the individual quantal depolarizations that normally underlie the EJP and are therefore termed 'quantal excitatory junction potentials' (QEJPs) to distinguish them from both EJPs and SEJPs. 6. We examined the possibility that heptanol revealed QEJPs by disrupting electrical coupling between cells in the smooth muscle syncytium. Heptanol (2.0 mM) had no effect on the amplitude distribution, time courses, or the frequency of occurrence of SEJPs. Intracellular input impedance (Rin) of smooth muscle cells was left unaltered by heptanol. 7. 'Cable' potentials of the vas deferens, recorded using the partition stimulation method, also remained unchanged in the presence of heptanol. Thus, heptanol appeared not to modify syncytial electrical properties of the smooth muscle in any significant way. 8. Our observations show directly that the quantal depolarizations underlying the EJP in syncytial smooth muscle are SEJP-like events. However, no unequivocal statement can be made about the mechanism by which heptanol unmasks QEJPs from EJPs.

Effects of heptanol on the neurogenic and myogenic contractions of the guinea-pig vas deferens

1. The effects of the putative gap junction uncoupler, 1-heptanol, on the neurogenic and myogenic contractile responses of guinea-pig vas deferens were studied in vitro. 2. Superfusion of 2.0 mM heptanol for 20-30 min produced the following reversible changes in the biphasic neurogenic contractile response (8 trials): (i) suppression of both phases; (ii) delayed development of both the first as well as the second phase, accompanied by complete temporal separation of the two phases; (iii) prominent oscillations of force during the second (noradrenergic) phase only. 3. To eliminate prejunctional effects of heptanol, myogenic contractions were evoked by field stimulation of the vas in the presence of suramin (200 microM) and prazosin (1 microM). Heptanol (2.0 mM) abolished these contractions reversibly. 4. These results show that (i) heptanol inhibits both excitatory junction potential (EJP)-dependent and non EJP-dependent contractions of the vas; (ii) a postjunctional site of action of heptanol, probably intercellular uncoupling of smooth muscle cells, contributes to the inhibition of contraction.

Effects of heptanol on electrical activity in the guinea-pig vas deferens

1. The effects of the putative intercellular uncoupling agent I-heptanol on electrical activity in the guinea-pig vas deferens were studied by use of intracellular and extracellular recording techniques. 2. At concentrations of 0.5, 1 and 2 mM, heptanol rapidly, monotonically and reversibly attenuated intracellularly recorded excitatory junction potential (e.j.p.) amplitude without affecting its time course, while spontaneous excitatory junction potentials (s.e.j.ps) were left unaffected. 3. Heptanol did not affect either the extracellularly recorded evoked excitatory junction current (e.j.c.), or the nerve terminal impulse that preceded it. These observations indicate that heptanol does not affect nerve impulse conduction, neurotransmitter release, or the postjunctional receptors involved in the production of the e.j.p. 4. E.j.ps appear to be suppressed by heptanol due to its intercellular uncoupling effects. Therefore, functional intercellular coupling may be necessary for the generation of the e.j.p. in smooth muscle.

Autonomic neuromuscular transmission at a varicosity

This review attempts to clarify the definition of what constitutes an autonomic neuromuscular function formed by a varicosity. Ultrastructural studies of serial sections through varicosities, partly or wholly bare of Schwann cell covering, show that areas of close apposition occur between varicosities and muscle cell membrane that vary between 20 and 150 nm, depending on the muscle considered. Consideration of the diffusion of purine transmitters and their receptor kinetics after secretion in a packet show that the number of purinergic receptor channels opened at a site of 150 nm apposition by a varicosity is about 15% of that at a site of 50 nm apposition. These results, together with the analysis of the stochastic fast component and the deterministic slow components of the rising phase of the EJP suggest that the stochastic fast component is due to varicosities that form especially close appositions (20-50 nm), whereas the deterministic slow component is due to the large number of varicosities at distances up to about 150 nm. Varicosities forming appositions of 20-150 nm with muscle cells several hundred micrometers long possess junctional receptor types distinct from extrajunctional receptors. According to this argument, then, there are two different classes of varicosities: one that gives rise to a relatively large junctional current and another that is responsible for a very small junctional current. Present evidence suggests that two subclasses of varicosities can be discerned amongst the varicosities that generate large junctional currents. One of these subclasses of varicosity possesses relatively few post-junctional receptors compared with the amount of transmitter reaching the receptors from the varicosity, so that the junctional current generated is determined by the size of the receptor population; in this case, the size of the transmitter packages released from these varicosities is unknown and the size of the junctional current is relatively constant. The other subclass of varicosity possesses large receptor patches, sufficient to accommodate the largest amounts of transmitter released from the varicosities: in this case, the size of the transmitter packages is shown to be highly non-uniform. These speculations await confirmation by direct labelling of the receptor patches beneath varicosities, a possibility that is likely to be realized in the near future.

An electron-microscopic study of smooth muscle cell dye coupling in the pig coronary arteries. Role of gap junctions

Arterial smooth muscles behave like a syncytium, since they are electrically coupled. It is generally assumed that electrical coupling and dye coupling are mediated by gap junctions. No gap junctions could be detected by transmission electron microscopy in media of coronary arteries. We looked for the presence of gap junction protein in vascular smooth muscle by immunohistochemistry with light microscopy. Immunohistologically detectable connexin is expressed by smooth muscle cells of the media of pig coronary arteries, where staining occurs as a discrete punctation. We investigated the dye coupling in strips of pig coronary artery. The fluorescent dye lucifer yellow was microiontophoretically injected into a smooth muscle cell through an intracellular microelectrode. The dye was visualized on the entire strip, then on semithin sections with a fluorescence microscope, and at the ultrastructural level by using an anti-lucifer yellow antibody revealed by the protein A-gold technique. In all the tissues examined, the cells were dye-coupled. We conclude that in arterial media the smooth muscle cells are dye-coupled, despite the absence of detectable gap junctions by transmission electron microscopy, and suggest that dye coupling could occur via isolated gap junction channels.

On the factors which determine the time-courses of junction potentials in the guinea-pig vas deferens

The time-courses of junction potentials and junction currents at the sympathetic neuroeffector junction of guinea-pig vas deferens were investigated using simultaneous intracellular and focal extracellular recording. Excitatory junction potentials produced in the smooth muscle cells following electrical stimulation of the sympathetic nerves had duration of 600-1000 ms and were prolonged in time-course compared with the underlying excitatory junction currents which had durations of 40-150 ms. The time-course of the excitatory junction potential could be predicted accurately from the time-course of the underlying excitatory junction current by assuming that the smooth muscle cells were isopotential during the excitatory junction potential. In contrast, the time-course of the spontaneous excitatory junction potential, produced by the action of a single quantum of transmitter, was identical to the time-course of the underlying spontaneous excitatory junction current, both events having durations of 40-150 ms. Local application of 10(-4) M adenosine 5'-triphosphate in the vicinity of an intracellular microelectrode produced depolarizations ("adenosine 5'-triphosphate potentials") of durations ranging between 400 and 2500 ms. When adenosine 5'-triphosphate was applied close to the rim of an extracellular electrode, negative-going signals ("adenosine 5'-triphosphate currents") were produced which reflected the time-course of postjunctional membrane currents evoked by adenosine 5'-triphosphate. Adenosine 5'-triphosphate currents were abolished by alpha, beta-methylene adenosine 5'-triphosphate (10(-6) M) but were unaffected by tetrodotoxin (3 x 10(-7) M or 10(-6) M) and the alpha-adrenoceptor blockers prazosin (10(-6) M) and phentolamine (10(-6) M). The time-courses of adenosine 5'-triphosphate currents were similar to the time-courses of excitatory junction currents recorded in the same preparation. Adenosine 5'-triphosphate currents were generally brief compared with simultaneously recorded adenosine 5'-triphosphate potentials which had durations or greater than or equal to 1000 ms. The time-courses of relatively brief adenosine 5'-triphosphate potentials (duration less than or equal to 800 ms) followed the time-courses of the underlying adenosine 5'-triphosphate currents. These results indicate that the time-course of decay of the excitatory junction potential in the guinea-pig vas deferens is determined mainly by the passive membrane properties of the smooth muscle cells, the duration of underlying transmitter action being brief. However, the factors determining the time-course of adenosine 5'-triphosphate potentials are less clear, and may include passive membrane properties and diffusion of locally applied adenosine 5'-triphosphate.(ABSTRACT TRUNCATED AT 400 WORDS)

Simultaneous intracellular and focal extracellular recording of junction potentials and currents, and the time course of quantal transmitter action in rodent vas deferens

Simultaneous recordings were made of spontaneous excitatory junction potentials and the underlying spontaneous excitatory junction currents in guinea-pig and mouse vas deferens using adjacent intracellular and focal extracellular electrodes. Concurrent spontaneous excitatory junction potentials and spontaneous excitatory junction currents were observed in a small proportion of smooth muscle cells penetrated intracellularly within 50-200 microns of the extracellular electrode. These simultaneous events had identical variations in time course, indicating that they were caused by the same transmitter release event. Their amplitudes were not related. Concurrent spontaneous excitatory junction potentials and currents had identical durations, rise times and time constants of decay, showing that the spontaneous excitatory junction potential reflects the time course of quantal transmitter action. In contrast, spontaneous "discrete events" obtained by differentiating the rising phases of spontaneous excitatory junction potentials with respect to time were brief compared with the underlying currents. Excitatory junction potentials evoked by electrical stimulation of the hypogastric nerve were prolonged compared to the underlying excitatory junction currents. The peaks in the first time differential of individual excitatory junction potentials (evoked discrete events) were brief compared to corresponding excitatory junction currents. It is concluded that at the neuroeffector junction of the rodent vas deferens the membrane potential response to a quantum of spontaneously released transmitter is a good estimate of the duration of transmitter action, in accordance with some of the predictions for three-dimensional syncytial tissues. The first time differential of the membrane potential, the "discrete event", does not reflect the time course of spontaneous or evoked quantal transmitter action in these syncytial tissues.

Lack of involvement of Ca2+ conductance change in ATP-induced depolarization of the guinea-pig vas deferens

The double sucrose-gap method was used to examine the electrical responses of the guinea-pig vas deferens to ATP and their possible dependence on external Ca2+. Normally ATP induced a depolarization and an increase in membrane conductance and both effects were concentration-dependent. The reversal potential of the 10(-4) M ATP-induced depolarization was 27.1 mV positive to the resting membrane potential of the tissue. This value was quite similar to that previously obtained for the 3 X 10(-5) M ATP-induced depolarization. The smooth muscle membrane was depolarized by 5.9 mV in a Ca-free medium, in which ATP also caused a depolarization, associated with an increase in membrane conductance. The reversal potential of the depolarization induced by ATP (10(-4) M) in the Ca-free medium was 26.5 mV positive to the resting membrane potential. The results suggest that, in this tissue, ATP induces membrane depolarization with little effect on Ca2+ conductance.

Evidence for an increase in membrane conductance during adenosine triphosphate-induced depolarization in the guinea-pig vas deferens

The effects of exogenously applied adenosine triphosphate (ATP) on the smooth muscle of guinea-pig vas deferens were studied with the double sucrose-gap method. ATP evoked a membrane depolarization which was associated with a decrease in the size of electrotonic potentials. Conditioning hyperpolarization induced by current application caused an increase in the magnitude of the ATP-induced depolarization; the larger the conditioning hyperpolarization, the greater the ATP-induced depolarization. These results are discussed with respect to the ionic mechanism of the electrical event in response to ATP in this tissue.

Transmitter secretion from individual varicosities of guinea-pig and mouse vas deferens: highly intermittent and monoquantal

A modification of "classical" electrophysiological techniques was used to characterise the secretory activity of individual release sites of the sympathetic nerves of guinea-pig and mouse vas deferens. The rising phases of the intracellularly recorded excitatory junction potentials of a smooth muscle cell, were electrically differentiated, and fluctuations in the rate of rise recorded as phasic peaks (in the dV/dt of excitatory junction potentials), termed "discrete events". Experimental factors which may influence discrete events were examined systematically, and criteria established to recognize a discrete event as the "image" of transmitter secreted from a particular release site. To determine the quantal content of a stimulus-evoked discrete event, it was compared with discrete events occurring spontaneously in the same cell. Furthermore, the frequency dependence of the probability of occurrence of discrete events was compared with that of the evoked fractional secretion of tritium-labelled noradrenaline, to find out if release sites from which noradrenaline is secreted share the characteristics of those secreting the (still unknown) transmitter causing discrete events. The results obtained permit the following tentative conclusions: Both in guinea-pig and mouse vas deferens stimulus-induced secretion of transmitter from a single varicosity of the sympathetic nerves is highly "intermittent". Transmitter secretion from a varicosity is basically monoquantal. Spontaneous secretion of transmitter quanta occurs from "random" sites, but a nerve impulse causes secretion of a quantum from a "preferred site" of a varicosity: during a stimulus train quanta are secreted in "complementary pairs". Secretion of the first quantum in a "pair" does not lead to "autoinhibition" of the site from which it was released, but induces a short-lasting facilitation. (5) Some characteristic features of the secretion of the neurotransmitter causing discrete events, seem to apply to the secretion of noradrenaline from noradrenergic nerves also.

Discrete events measure single quanta of adenosine 5'-triphosphate secreted from sympathetic nerves of guinea-pig and mouse vas deferens

Intracellularly recorded excitatory junction potentials in smooth muscle cells, and the first time differentials of their rising phases ("discrete events") were used to analyse transmitter secretion from the postganglionic sympathetic nerves of guinea-pig and mouse vas deferens. The aim was to determine whether the transmitter causing these responses is noradrenaline or adenosine 5'-triphosphate (ATP). Depletion of the noradrenaline stores following treatment with reserpine reduced the frequency of occurrence, but not the amplitude, of spontaneous junction potentials and discrete events. Nerve stimulation could still evoke "fast" junction potentials and discrete events, normal in appearance, but "slow" junction potentials were reduced in amplitude and had shorter times to peak. In contrast, desensitization of ATP receptors by alpha, beta-methylene ATP abolished spontaneous and stimulus-induced "fast" (but not "slow") junction potentials and discrete events, reversibly. It is concluded that it is not noradrenaline, but ATP or some related compound which causes spontaneous and "fast" stimulus-induced junction potentials, and discrete events. The present and earlier data show that discrete events reflect the secretion of individual quanta of ATP (or quanta of "ATP + noradrenaline", if both are secreted in parallel from the same vesicle) from postganglionic sympathetic nerve terminals in guinea-pig and mouse vas deferens.

Neural coordination of excitation of ferret trachealis muscle

The distribution of junction potentials and synaptic potentials to muscle cells and ganglion cells in the ferret trachealis muscle-nerve plexus preparation was studied with local electrical stimulation of branches of the laryngeal nerve or the interganglionic nerve trunk. Stimulations evoked excitatory junction potentials in muscle cells and fast excitatory postsynaptic potentials in ganglion (AH) cells located throughout the preparation, regardless of the location of the stimulating electrode. Evoked excitatory junction potentials were nearly simultaneous in widely separated muscle cells, suggesting that excitation of different muscle cells is coordinated. The apparent conduction velocity to muscle cells after a local nerve stimulation was 0.1-0.2 m/s. Dissemination of input to ganglion cells and muscle cells was dependent on the integrity of the interganglionic nerve trunk. There was evidence based on analysis of conduction velocities that coupling of electrical activity in different AH ganglion cells and muscle cells was related to the following: 1) an interlacing arrangement of myelinated preganglionic neurons that enter the plexus from multiple branches of the laryngeal nerve, and 2) interlacing neural circuits characterized by synapses between neurons whose cell bodies lie in different ganglia.

Electrophysiology of neurones of the inferior mesenteric ganglion of the cat

Intracellular recordings were obtained from cells in vitro in the inferior mesenteric ganglia of the cat. Neurones could be classified into three types: non-spontaneous, irregular discharging and regular discharging neurones. Non-spontaneous neurones had a stable resting membrane potential and responded with action potentials to indirect preganglionic nerve stimulation and to intracellular injection of depolarizing current. Irregular discharging neurones were characterized by a discharge of excitatory post-synaptic potentials (e.p.s.p.s.) which sometimes gave rise to action potentials. This activity was abolished by hexamethonium bromide, chlorisondamine and d-tubocurarine chloride. Tetrodotoxin and a low Ca2+ -high Mg2+ solution also blocked on-going activity in irregular discharging neurones. Regular discharging neurones were characterized by a rhythmic discharge of action potentials. Each action potential was preceded by a gradual depolarization of the intracellularly recorded membrane potential. Intracellular injection of hyperpolarizing current abolished the regular discharge of action potential. No synaptic potentials were observed during hyperpolarization of the membrane potential. Nicotinic, muscarinic and adrenergic receptor blocking drugs did not modify the discharge of action potentials in regular discharging neurones. A low Ca2+ -high Mg2+ solution also had no effect on the regular discharge of action potentials. Interpolation of an action potential between spontaneous action potentials in regular discharging neurones reset the rhythm of discharge. It is suggested that regular discharging neurones were endogenously active and that these neurones provided synaptic input to irregular discharging neurones.

Evidence that ionic channels associated with the muscarinic receptor of smooth muscle may admit calcium

1 The actions of carbachol on the membrane potential and conductance of smooth muscle of the guinea-pig intestine were investigated using microelectrode recording and the double sucrose-gap method in solutions in which calcium was the only cation creating an inwardly-directed electrochemical gradient.2 In a calcium chloride solution containing a small amount of potassium but no sodium and buffered to physiological pH (Ca Locke) the membrane was hyperpolarized to more than -80 mV. Carbachol (2 x 10(-7)-10(-4)M) depolarized the membrane and increased the membrane conductance.3 By passing current the membrane potential of the smooth muscle cells could be varied. In Ca Locke the depolarization produced by carbachol was shown to be reduced if the membrane was depolarized. The relationship between the size of the carbachol depolarization and the membrane depolarization was linear, giving an apparent reversal potential for carbachol depolarization some 20 mV positive to the resting membrane potential, as measured by extracellular electrodes in the sucrose gap.4 Carbachol depolarization was reduced if the calcium concentration was reduced below 2.5 mM by replacing calcium with Tris, but the depolarization in 2.5 mM Ca and in Ca Locke (100 mM Ca) were of similar size. In Ca-free Na-free solution with EGTA, carbachol depolarization was soon abolished.5 In the sucrose-gap when the chloride gradient across the cell membrane was reversed by replacing the chloride of Ca Locke by an impermeant anion the membrane depolarized. Carbachol now contracted the muscle but produced a hyperpolarization.6 These results are consistent with the hypothesis that activation of muscarinic receptors opens ionic channels which at least in the solutions used, can admit sufficient calcium ions to depolarize the cell and cause tension development.

Neurophysiology of adult male Schistosoma mansoni

From the studies on the neurophysiology of schistosomes it appears that in spite of the unique hexilaminar arrangement of the tegument's outer membrane, it has biophysical properties not markedly different from those of a variety of other multi-dimensional syncytia. The close electrical coupling of the muscle and tegument must be taken into account when one attempts to define sites and modes of action of drugs which affect motor activity. Agents which disrupt muscle function in the schistosome may exert their action indirectly by way of an effect on the tegumental membrane. The syncytial nature of the musculature and the possibility that longitudinal contraction waves are myogenic suggests that neurotransmitters may simply function as modulators of muscle activity as is the case for many vertebrate visceral muscles. External recordings indicate the presence of a variety of electrically active tissues within the schistosomes. There is no clear correlation of this activity with longitudinal muscle activity or with active membrane responses in this muscle. From this it would appear the bulk of this activity may have its origins in tissues other than the longitudinal muscle such as other muscle groups, nerve trunks, or the peripheral nerve net.

Microelectrode studies of the tegument and sub-tegumental compartments of male Schistosoma mansoni: an analysis of electrophysiological properties

Standard intracellular microelectrode techniques were used to determine the electrical properties of the tegument and sub-tegumental regions in male Schistosoma mansoni. Three distinct compartments of electrical potential were observed. The resting potentials recovered in these compartments of -45.9 +/- 2.5 mV (Eteg), -22.0 +/- 1.1 mV (E2) and -4.7 +/- 03 mV (E3) corroborate those previously reported by Fetterer, Pax & Bennett (1980) and Bricker, Pax & Bennett (1981). Input resistance was measured in each compartment and was found to be 4. 5 M omega (tegument), 9.2 M omega (E2) and 3.5 M omega (E3). Time-constants for the tegument, E2 and E3 were 0.24 +/- 0.01 msec, 0.25 +/- 0.01 msec and 0.13 +/- 0.01 msec, respectively. Multiple electrode experiments revealed that the tegument and E2 compartment are electrical syncytia with similar current-spreading capabilities. Low resistance pathways also appear to connect the tegument and E2 region, since electrotonic signals initiated in either of those compartments experience only a 15-25% reduction upon passing into the other. Injecting large (greater than 200 nA) depolarizing current pulses into the tegument or E2 compartment often resulted in the initiation of active membrane responses. These spikes were highly variable, ranging from 4 to 75 mV in magnitude (occasionally overshooting zero potential by as much as 25 mV) and from 10-40 msec in duration. The responses were not actively propagated along the parasite, and their decay over distance was approximately equal to that predicted on the basis of length constant values obtained from electronic signals. The addition of a non-diffusible solute to the recording medium resulted in a significant reduction in the current-spreading capacity of both the tegument and E2 compartment. Coupling ratios between the tegument and E2 compartment. Coupling ratios between the tegument and E2 compartment were decreased, and the input resistance for both compartments increased, while resting potentials remained constant. Active responses could not be evoked in schistosomes exposed to the hyperosmotic medium.

Measurement of intracellular chloride in guinea-pig vas deferens by ion analysis, 36chloride efflux and micro-electrodes

1. Cl-sensitive micro-electrodes were used to measure the intracellular Cl activity (a(Cl) (i)) in smooth muscle cells of the guinea-pig vas deferens. The values obtained were compared with those of intracellular Cl (Cl(i)) found by both ion analysis and (36)Cl efflux.2. Various combinations of filling solution for recording membrane potential (E(m)), and type of micro-electrode were tested. The most successful, which allowed continuous recording of a(Cl) (i) for several hours, was a double-barrelled electrode using the reference liquid ion exchanger (RLIE; Thomas & Cohen, 1981). However, a(Cl) (i) measured both by simultaneous impalements of separate cells with Cl-sensitive and conventional micro-electrodes, and by double-barrelled micro-electrodes, was about 42 mM in normal Krebs solution. This is five times higher than the value from a passive distribution. E(Cl) was about -24 mV, more than 40 mV positive to E(m).3. On complete removal of extracellular Cl (Cl(o)), a(Cl) (i) fell to an apparent level of about 3 mM. If this represents interference from other anions, the maximum error in E(Cl) measured in normal Krebs solution is 2.5 mV. Replacement of Cl(o) caused a rapid increase in a(Cl) (i). This must be caused by an active transport of Cl(-) ions into the cell against their electrochemical gradient.4. The stabilized values of a(Cl) (i) measured at different levels of Cl(o) agree surprisingly well with a(Cl) (i) estimated from ion analysis and (36)Cl efflux, assuming that the intracellular activity coefficient was the same as measured in the normal Krebs solution. The relationship of a(Cl) (i) to Cl(o) was hyperbolic.5. It is concluded that Cl-sensitive micro-electrodes accurately measure a(Cl) (i) in smooth muscle cells. The remarkable agreement between the direct and indirect methods of measuring Cl(i) suggests that Cl(-) ions are not bound to a significant extent and that the compartment seen by the micro-electrodes is probably representative of the whole cell.

Spontaneous and evoked excitatory junction potentials in rat tail arteries

1. The frequency, amplitude and time course of spontaneous excitatory junction potentials (s.e.j.p.s) and their relationship to the time course and amplitude of evoked excitatory junction potentials (e.j.p.s) were examined. 2. The frequency and amplitude of s.e.j.p.s varied dramatically between cells. There was good correlation between their rise and decay times. 3. The amplitude and time course of e.j.p.s also varied between cells. E.j.p.s with large amplitudes and fast time courses were recorded in cells with high s.e.j.p. frequencies. 4. Active responses propagated only for very limited distances. 5. The frequency of s.e.j.p.s decreased after reserpine and 6-hydroxydopamine (6-OHDA) treatments, suggesting that s.e.j.p.s were related to spontaneous release of noradrenaline from nerve terminals.

Length-tension relations of smooth muscle from normal and denervated rat urinary bladders

Urinary bladders of rats were denervated by bilateral excision of the pelvic ganglion and removed 10 days after the operation. They were filled with 0.75 ml saline and a longitudinal muscle strip was marked out, measured and dissected out. Strips from normal bladders filled with the same volume were used as controls. Denervated bladders were 4-5 times heavier than control bladders. Muscle strips from denervated bladders showed, in contrast to controls, marked phasic spontaneous contractions which were unaffected by tetrodotoxin, indicating a myogenic origin. Active tension in response to AC stimulation was measured at different lengths. In relation to the in situ length (Lin situ) at 0.75 ml the denervated strips had to be stretched to much greater extent than controls in order to reach optimum length (L0) for force development. Furthermore, the denervated strips shortened less in relation to Lin situ than the controls. If active length-tension relations were expressed in relation to L0, the difference between denervated and control strips was abolished. Maximal active force was the same for denervated and control strips. Water content increased significantly in denervated bladders. The results suggest a remodelling of the smooth muscle structure in denervated bladders; the characteristics of the contractile machinery seem, however, to be unaltered.

Some properties of spontaneous excitatory junction potentials recorded from arterioles of guinea-pigs

1. Spontaneous excitatory junction potentials were recorded from electrically short segments of arterioles taken from the intestinal submucosa of guinea-pigs. 2. Histograms of the amplitudes of these spontaneous potentials were unimodal; their amplitudes often corresponded with the amplitudes of the smallest evoked potentials recorded from the same preparation. 3. The time courses of both spontaneous and evoked potentials were very similar and it is suggested that evoked potentials are made up by the simultaneous occurrence of several spontaneous potentials. 4. The mean quantal content of evoked potentials was always far fewer than the number of varicosities present in the preparations. 5. It is suggested that during neuromuscular transmission, transmitter is released at relatively few sites throughout the ground plexus for each nerve impulse.

The passive membrane properties and excitatory junction potentials of the guinea pig deferens

1. Electrotonic potentials were recorded from the superficial smooth muscle cells of the guinea-pig vas deferens using the method of Abe & Tomita (1968), in response to low-amplitude, long-duration (greater than or equal to 2 sec) pulses. 2. Averaging techniques were used to increase the signal/noise ratio, and the intracellularly recorded electrotonic potentials were corrected for extracellular voltage drop across the bath series resistance. 3. Since the length of tissue in the stimulating and recording compartments affects the time course of electrotonic potentials (see Appendix and Bywater & Redman, 1978) the passive membrane properties were measured with known amounts of tissue in these two compartments. 4. The length constant (lambda) was 0.86 mm and the membrane time constant (tau m) 270 msec. 5. Excitatory junction potentials (e.j.p.s) were recorded and averaged in response to field stimulation of intact branches of the hypogastric nerve. The mean time constant of the exponential decay phase of the e.j.p. (288 msec) was similar to the membrane time constant (tau m = 270 msec). 6. As the e.j.p.s showed little change in amplitude or time constant of decay when recorded up to several millimetres from the stimulating electrode it was assumed that the tissue was isopotential during the e.j.p., and an estimate was made of the time course of the underlying junctional current. 7. The estimated time course of the junctional current during an e.j.p. was similar to the observed time course of a spontaneous junction potential (s.e.j.p.). 8. As the time course of the junctional current during an s.e.j.p.is similar to the time course of the potential change it is likely that the factors which determine the time current underlying the s.e.j.p. also determine the time course of the e.j.p. current.

Electrophysiological properties of human oviduct smooth muscle cells in dissociated cell culture

Intracellular recordings were made from human oviduct smooth muscle maintained in cell culture. Solitary cells isolated from one another and cells in contact with one another retained electrical properties of smooth muscle in vivo. Membrane potential of solitary cells and connected cells was -35 mV. Connected cells formed electrotonic junctions which transmitted current from one cell to another. This current spread was responsible for differences in input resistance and time constant in solitary cells, 66 Momega and 96 msec, compared to connected cells, 26 Momega and 56 msec. All cells expressed delayed rectification to depolarizing current pulses. Some cells generated action potentials spontaneously or in response to intracellular current pulses. Action potentials were abolished by cobalt or by EGTA. Slow wave potentials, 5 . 20 mV in amplitude, occurred continuously once every 15 to 45 seconds in connected cells.

An analysis of excitatory junctional potentials recorded from arterioles

1. Arterioles were impaled with two independent micro-electrodes, one to pass current and the other to record membrane potential. 2. When current was injected into one branch of an arteriole, a membrane potential change could be detected either in the same branch or in an adjoining branch indicating that the arteriolar smooth muscle cells were electrically connected. 3. Fine dissection of the arteriolar tree gave short segments of arteriole which appeared to behave electrically as short cables with sealed ends. 4. Analysis of the electrotonic potentials recorded from isolated segments of arterioles allowed a determination of the arteriole cable properties. 5. Using the data from the cable analyses it was concluded that the junctional current underlying an excitatory junction potential has a duration that is brief when compared with that of the potential.

Neuromuscular transmission in arterioles of guinea-pig submucosa

1. Intracellular recordings were made from arterioles lying in the submucosa of guinea-pig small intestine. 2. Low frequency perivascular nerve stimulation evoked subthreshold excitatory junction potentials which facilitated. 3. Higher frequency stimulation caused summation of excitatory junction potentials and the initiation of muscle action potentials. 4. Arteriolar constriction was only observed following the initiation of a muscle action potential.

A comparison of the electrical properties and morphological characteristics of the smooth muscle of the rat and guinea-pig vas deferens

Microelectrodes were used to compare a variety of electrophysiological parameters of the rat and guinea-pig vas deferens. In comparison to the guinea pig, spontaneous junction potentials in the rat tissue were of shorter duration and occurred with greater frequency and amplitude. Action potentials induced by nerve stimulation could be observed in the smooth muscle of both species. However, in the rat tissue the majority of action potentials were generated in the impaled cell while 60% of the action potentials in the guinea-pig vas deferens were propagated. When current was intracellularly applied, spike potentials could be induced in approximately 90% of the cells of the rat vas deferens but in less than 10% of the cells of the guinea-pig vas deferens. The space constant was 1.48 mm for the guinea-pig vas deferens, but less than 0.5 mm for the rat vas deferens. Electromicroscopic examination of the homologous tissues indicates that the differences in electrical properties can be accounted for in part by differences in morphology. The incidence and intimacy of neuromuscular contacts was greater in the rat vas deferens while the incidence of nexuses between smooth muscle cells was greater in the guinea-pig tissue.

Some properties of the smooth muscle of mouse vas deferens

1. Contractions of the mouse vas deferens in response to electrical stimulation differ form those recorded form the guinea-pig vas deferens in that they are abolished by tetrodotoxin. 2. Changes in membrane potentials were recorded form the smooth muscle of both preparations in response to stimulation with current pulses applied by an intracellular electrode and by alrge extracellular plate electrodes. 3. Both preparations behaved similarly in response to intracellular stimulation. Electrotonic potentials in response to extracellular current pulses spread in a longitudinal direction in the guinea-pig vas deferens in accordance with the cable-like properties of this preparation. In contrast, no longitudinal spread of eletrotonus was observed in the mouse vas deferens. 4. Responses to nerve stimulation differed in the two preparations. In the guinea-pig, single stimuli caused excitatory junction potentials (e.j.p.s) which gave rise to action potentials. Some cells from the mouse vas deferens showed similar e.j.p.s and action potentials, although the threshold for the initiation of action potentials was lower and more variable. 5. The majority of cells in the mouse vas deferens failed to show action potentials in response to a single stimuli even though the amplitude of e.j.p.s was from 35 to 40 mV. This was probably due to the large resting membrane potentials of these cells, as all-or-nothing action potentials could be evoked if successive e.j.p.s were allowed to sum with each other or if a depolarizing current pulse was applied at the peak of an e.j.p. 6. The nature of the response to nerve stimulation recorded from differnt cells in the mouse vas deferens could be correlated with the amplitude and time course of the response of the same cell to intracellular stimulation. 7. It is concluded that individual smooth muscle cells in both preparations are probably coupled electrically but that there are few, if any, low resistance pathways in the longitudinal direction in the mouse vas deferens.

Cell membrane structure of vascular smooth muscle of circle of Willis

Cell membranes of vascular smooth muscles of the circle of Willis were studied in thin sections and freeze-replicas. The cell membranes were differentiated into a caveolae intracellulares zone and caveolae-free zone, both of which were generally arranged in an alternate manner and parallel to the major axis of the smooth muscle cell. In the former zone, the caveolae intracellulares, about 600 A in diameter, were neatly oriented in one to several rows running parallel to the longitudinal axis of the muscle cell with a center-to center distance of about 800 A. The latter zone was of variable width and smooth, apart from membrane particles or scattered caveolae, and corresponded mainly to the dense area and partially to the myofibril area beneath the cell membrane. Membrane particles were generally more numerous on face A than on face B, and their average number per micronm2 was about twice as many inside the rows of the caveolae as outside. Rosette formations of membrane particles were often evident at the stomal rims of the caveolae. Adherentes and gap junctins were occasionally found on the caveolae-free areas which often protruded externally. Tight junctions appeared as a collection of scattered strands, which frequently showed free ends and were parallel to each other and also to the major axis of the smooth muscle cell.

Membrane properties of the smooth muscle cells of the rat anococcygeus muscle

1. The membrane properties of the rat anococcygeus muscle, during rest and activity, were investigated with micro-electrodes and partition stimulation. 2. Intercellular current spread occurred within the muscle and the mean length constant was 2-7 mm. The membrane showed rectification to depolarizing pulses. 3. The mean resting potential was --62-1 mV and the input resistance was 23-0 MOMEGA. Stimulation of intramural nerves produced depolarization to --21 mV and a 10% reduction in input resistance. Displacement of the membrane potential indicated that the transmembrane potential at the peak of the response was independent of the membrane potential. 4. Noradrenaline also produced depolarization and this was accompanied by a decrease in membrane resistance as indicated by a reduction in amplitude of the electrotonic potential. 5. It was concluded that the muscle possesses cable properties and that the action of the transmitter, noradrenaline, is to increase membrane permeability so that the membrane potential moves towards an equilibrium potential.

Junction potentials in response to ortho- and anti-dromic stimulation of hypogastric nerve in mouse vas deferens

The interaction of the junction potentials in response to ortho- and antidromic hypogastric nerve stimulation in mouse vas deferens was studied, using an extracellular recording method. Ortho-dromic repetitive hypogastric nerve stimulation (10 Hz, 5 min) simultaneously depressed the amplitude of the junction potentials in response to both ortho- and anti-dromic hypogastric nerve stimulation (post-te-tanic depression). No time-lag in recovery from the post-tetanic depression was observed between the junction potentials recorded from two separate electrodes, indicating that the proximodistal axonal flow of available transmitter was not involved in the recovery process. Double shocks, with intervals from 10 msec to 1 sec, were applied to the hypogastric nerve. The junction potentials in response to ortho- and ortho-dromic or anti- and anti-dromic double shock were markedly facilitated. On the contrary, the junction potentials in response to ortho- and anti-dromic double shocks were not facilited. The findings indicate that facilitation of the junction potentials is produced by the impulses propagated in the same direction along the terminal axon and also that the origin of the facilitation may be at a pre-junctional site.

Current spread in the smooth muscle of the rabbit aorta

1. The electrical responses of the smooth muscle cells of the rabbit aorta to both extracellular and intracellular stimulation were studied using the partitioned chamber and Wheatstone bridge method.2. No spontaneous electrical activity was recorded when the tissue was soaked in either isotonic or hypertonic Krebs solutions, and strong depolarizing currents also failed to trigger action potentials in either solution.3. The circular muscle of the aorta has cable properties. Mean values in isotonic Krebs solution were 2.1 mm for space constant and 433 msec for time constant.4. The input resistance (mean 12 MOmega) measured with the Wheatstone bridge method was considerably smaller than that calculated from values measured with the partitioned chamber method.5. Electrotonic potentials could be recorded from the smooth muscle of ;injury bundles' although their amplitude was smaller than that from the intact bundle.6. High concentrations of noradrenaline readily induce oscillatory potentials from the aorta in both isotonic and hypertonic Krebs solutions. It was estimated by simultaneous recording with two micro-electrodes that noradrenaline-induced oscillatory potential can conduct in both longitudinal and transverse directions of the smooth muscle.7. These results suggest that the smooth muscle of the aorta behaves like a syncytium or single unit muscle and activation of cells on the inner surface of the media can be induced both by electrotonic current spread and by propagation of oscillatory potentials from the outer cells directly activated by the transmitter.