Appearance Of Action Potentials Research Articles

Osmotic and Salt Stresses Modulate Spontaneous and Glutamate-Induced Action Potentials and Distinguish between Growth and Circumnutation in Helianthus annuus Seedlings.

Action potentials (APs), i.e., long-distance electrical signals, and circumnutations (CN), i.e., endogenous plant organ movements, are shaped by ion fluxes and content in excitable and motor tissues. The appearance of APs and CN as well as growth parameters in seedlings and 3-week old plants of Helianthus annuus treated with osmotic and salt stress (0–500 mOsm) were studied. Time-lapse photography and extracellular measurements of electrical potential changes were performed. The hypocotyl length was strongly reduced by the osmotic and salt stress. CN intensity declined due to the osmotic but not salt stress. The period of CN in mild salt stress was similar to the control (~164 min) and increased to more than 200 min in osmotic stress. In sunflower seedlings growing in a hydroponic medium, spontaneous APs (SAPs) propagating basipetally and acropetally with a velocity of 12–20 cm min−1 were observed. The number of SAPs increased 2–3 times (7–10 SAPs 24 h−1plant−1) in the mild salt stress (160 mOsm NaCl and KCl), compared to the control and strong salt stress (3–4 SAPs 24 h−1 plant−1 in the control and 300 mOsm KCl and NaCl). Glutamate-induced series of APs were inhibited in the strong salt stress-treated seedlings but not at the mild salt stress and osmotic stress. Additionally, in 3-week old plants, the injection of the hypo- or hyperosmotic solution at the base of the sunflower stem evoked series of APs (3–24 APs) transmitted along the stem. It has been shown that osmotic and salt stresses modulate differently hypocotyl growth and CN and have an effect on spontaneous and evoked APs in sunflower seedlings. We suggested that potassium, sodium, and chloride ions at stress concentrations in the nutrient medium modulate sunflower excitability and CN.

Roles of specific Kv channel types in repolarization of the action potential in genetically identified subclasses of pyramidal neurons in mouse neocortex.

The action potential (AP) is a fundamental feature of excitable cells that serves as the basis for long-distance signaling in the nervous system. There is considerable diversity in the appearance of APs and the underlying repolarization mechanisms in different neuronal types (reviewed in Bean BP. Nat Rev Neurosci 8: 451-465, 2007), including among pyramidal cell subtypes. In the present work, we used specific pharmacological blockers to test for contributions of Kv1, Kv2, or Kv4 channels to repolarization of single APs in two genetically defined subpopulations of pyramidal cells in layer 5 of mouse somatosensory cortex (etv1 and glt) as well as pyramidal cells from layer 2/3. These three subtypes differ in AP properties (Groh A, Meyer HS, Schmidt EF, Heintz N, Sakmann B, Krieger P. Cereb Cortex 20: 826-836, 2010; Guan D, Armstrong WE, Foehring RC. J Neurophysiol 113: 2014-2032, 2015) as well as laminar position, morphology, and projection targets. We asked what the roles of Kv1, Kv2, and Kv4 channels are in AP repolarization and whether the underlying mechanisms are pyramidal cell subtype dependent. We found that Kv4 channels are critically involved in repolarizing neocortical pyramidal cells. There are also pyramidal cell subtype-specific differences in the role for Kv1 channels. Only Kv4 channels were involved in repolarizing the narrow APs of glt cells. In contrast, in etv1 cells and layer 2/3 cells, the broader APs are partially repolarized by Kv1 channels in addition to Kv4 channels. Consistent with their activation in the subthreshold range, Kv1 channels also regulate AP voltage threshold in all pyramidal cell subtypes.

Electrophysiological characterization of murine HL-5 atrial cardiomyocytes

HL-5 cells are cultured murine atrial cardiomyocytes and have been used in studies to address important cellular and molecular questions. However, electrophysiological features of HL-5 cells have not been characterized. In this study, we examined such properties using whole cell patch-clamp techniques. Membrane capacitance of the HL-5 cells was from 8 to 62 pF. The resting membrane potential was -57.8 +/- 1.4 mV (n = 51). Intracellular injection of depolarizing currents evoked action potentials (APs) with variable morphologies in 71% of the patched cells. Interestingly, the incidence of successful, current-induced APs positively correlated with the hyperpolarizing degrees of resting membrane potentials (r = 0.99, P < 0.001). Only a few of the patched cells (4 of 51, 7.8%) exhibited spontaneous APs. The muscarinic agonist carbachol activated the acetylcholine-activated K(+) current and significantly shortened the duration of APs. Immunostaining confirmed the presence of the muscarinic receptor type 2 in HL-5 cells. The hyperpolarization-activated cation current (I(f)) was detected in 39% of the patched cells. The voltage to activate 50% of I(f) channels was -73.4 +/- 1.2 mV (n = 12). Voltage-gated Na(+), Ca(2+), and K(+) currents were observed in the HL-5 cells with variable incidences. Compared with the adult mouse cardiomyocytes, the HL-5 cells had prolonged APs and small outward K(+) currents. Our data indicate that HL-5 cells display significant electrophysiological heterogeneity of morphological appearance of APs and expression of functional ion channels. Compared with adult murine cardiomyocytes, HL-5 cells show an immature phenotype of cardiac AP morphology.

Repetitive action potentials induced in Arabidopsis thaliana leaves by wounding and potassium chloride application

A procedure (named prick-and-drop treatment) generating action potentials in the plant model Arabidopsis thaliana is described here, opening new possibilities for the analysis and understanding of the genuine nature of this phenomenon. Bioelectrical measurements were made on the petiole of the C24 ecotype with extracellular electrodes. The stimulating method includes wounding of a small area of the distal part of the leaf and subsequent addition of a drop of different kinds of solutions. The treatment inducing action potentials (APs) consists in a drop of KCl (> 0.25 M) applied on a wound, moreover the frequency of APs increased with the augmentation of the KCl concentration. Two molar sorbitol solutions were ineffective in causing APs, indicating that the effect of KCl solution is not due to the osmotic effect. Neither pricking nor addition of KCl alone caused repetitive APs (RAP), suggesting that the diffusion of the KCl solution into the apoplastic space through the wounding area is essential. The bioelectrical responses induced with 1 M KCl consisted of 35 % of ultra-rapid APs (propagation rates > 4 mm·s –1). The other APs propagated at 1.7 ± 0.9 mm·s –1. Variation potentials (VPs) were also observed, and were characterised by a larger wave (longer event duration) with a slower propagation velocity (0.3 ± 0.1 mm·s –1) than the APs. A dark phase inserted during the light period led to a decrease of the amplitudes and the frequency of appearance of APs, but their velocity stayed unchanged. Our experiments demonstrate the efficiency of our method (> 90 %) to induce spontaneous APs in A. thaliana for more than 2 h.

Unregulated Insulin Release in Neonatal Hyperinsulinism; an Overview of Functional Studies Reporting the Loss of Depolarization-Response Coupling Events • 997

In insulin-secreting β-cells, depolarization-response coupling is dependent upon the operation of function of ion selective channels. In response to glucose, a depolarization of the cell membrane results from ATP-sensitive K+ (KATP) channel closure. This in turn generates openings from voltage-dependent Ca2+ channels, and the appearance of action potentials associated with the accelerated influx of Ca2+. The subsequent rise in cytosolic Ca2+ then leads to insulin release. Diazoxide is an agonist of KATP channels and thus inhibits the release of insulin in normal β-cells. In persistent hyperinsulinaemic hypoglycaemia of infancy (PHHI) gene variations in the subunits of KATP channels have been linked to the disease. In our studies, we have investigated the functional properties of β-cells isolated from 29 patients with PHHI following pancreatectomy. 5/29 patients were from families with a previous history of PHHI, and 28/29 patients had a severe form of the disease that was either unresponsive or incompletely responsive to medical therapy with either diazoxide or somatostatin, used in combination with an elevated glucose infusion rate. In 8/29 subjects defects in the SUR1 gene have been identified. Studies of ion channel function and cytosolic Ca2+ signalling events were performed on isolated β-cells using patch-clamp techniques and cell microfluorimetry proceedures, respectively. The results of these experiments were correlated to insulin secretion studies in vitro and in vivo. These investigations revealed that loss of KATP channel function prevails inβ-cells isolated from PHHI patients. In many cases there was a direct correlation between defects in KATP channel activity and a spontaneous depolarisation of the cell membrane, leading to the appearance of action potentials. This inappropriate influx of Ca2+ is suggested to underpin the uncontrolled release of insulin from PHHI β-cells and is supported by[1] direct measurements of Ca2+-dependent exocytosis (through capacitance changes in isolated cells) and [2] by showing that removal of external Ca2+ or the addition of Ni2+ to the isolated cells, both lowers cytosolic Ca2+ levels and inhibits insulin secretion. These novel studies of isolated β-cells from patients with neonatal hyperinsulinsm [1] document the condition as a novel ion channel disorder and[2] provide important clues to the development of new lines of medical therapy with which to treat this condition.

Oscillations in cytoplasmic free calcium concentration in human pancreatic islets from subjects with normal and impaired glucose tolerance.

Plasma insulin levels in healthy subjects oscillate and non-insulin-dependent diabetic patients display an irregular pattern of such oscillations. Since an increase in cytoplasmic free Ca2+ concentration ([Ca2+]i) in the pancreatic beta cell is the major stimulus for insulin release, this study was undertaken to investigate the dynamics of electrical activity, [Ca2+]i-changes and insulin release, in stimulated islets from subjects of varying glucose tolerance. In four patients it was possible to investigate more than one of these three parameters. Stimulation of pancreatic islets with glucose and tolbutamide sometimes resulted in the appearance of oscillations in [Ca2+]i, lasting 2-3 min. Such oscillations were observed even in some islets from patients with impaired glucose tolerance. In one islet from a diabetic patient there was no response to glucose, whereas that islet displayed [Ca2+]i-oscillations in response to tolbutamide, suggesting that sulphonylurea treatment can mimic the complex pattern of glucose-induced [Ca2+]i-oscillations. We also, for the first time, made patch-clamp recordings of membrane currents in beta-cells in situ in the islet. Stimulation with glucose and tolbutamide resulted in depolarization and appearance of action potentials. The islet preparations responded to stimulation with a number of different secretagogues with release of insulin. The present study shows that human islets can respond to stimulation with glucose and sulphonylurea with oscillations in [Ca2+]i, which is the signal probably underlying the oscillations in plasma insulin levels observed in healthy subjects. Interestingly, even subjects with impaired glucose tolerance had islets that responded with oscillations in [Ca2+]i upon glucose stimulation, although it is not known to what extent the response of these islets was representative of most islets in these patients.

Oscillations in cytoplasmic free calcium concentration in human pancreatic islets from subjects with normal and impaired glucose tolerance

Plasma insulin levels in healthy subjects oscillate and non-insulin-dependent diabetic patients display an irregular pattern of such oscillations. Since an increase in cytoplasmic free Ca2+ concentration ([Ca2+]i) in the pancreatic beta cell is the major stimulus for insulin release, this study was undertaken to investigate the dynamics of electrical activity, [Ca2+]i-changes and insulin release, in stimulated islets from subjects of varying glucose tolerance. In four patients it was possible to investigate more than one of these three parameters. Stimulation of pancreatic islets with glucose and tolbutamide sometimes resulted in the appearance of oscillations in [Ca2+]i, lasting 2–3 min. Such oscillations were observed even in some islets from patients with impaired glucose tolerance. In one islet from a diabetic patient there was no response to glucose, whereas that islet displayed [Ca2+]i-oscillations in response to tolbutamide, suggesting that sulphonylurea treatment can mimic the complex pattern of glucose-induced [Ca2+]i-oscillations. We also, for the first time, made patch-clamp recordings of membrane currents in beta-cells in situ in the islet. Stimulation with glucose and tolbutamide resulted in depolarization and appearance of action potentials. The islet preparations responded to stimulation with a number of different secretagogues with release of insulin. The present study shows that human islets can respond to stimulation with glucose and sulphonylurea with oscillations in [Ca2+]i, which is the signal probably underlying the oscillations in plasma insulin levels observed in healthy subjects. Interestingly, even subjects with impaired glucose tolerance had islets that responded with oscillations in [Ca2+]i upon glucose stimulation, although it is not known to what extent the response of these islets was representative of most islets in these patients.

Influence of oxytocin on integration of postsynaptic potentials in molluscan neurons

1. Intracellular recordings were made from identified and non-identified neurons in perioesophageal ganglionic ring with buccal ganglia of the mollusc, Helix pomatia. The influence of oxytocin (OXT) on neuronal integration: space and temporal summations of postsynaptic potentials (PSPs) in various neurons was investigated. The obtained data indicated that these PSPs were cholinergic PSPs. 2. Ten minute exposure to 10 −8M OXT had no effects on the resting membranes, but triggered secondary mechanisms, which lead to enhancement of the excitatory PSP (EPSR) amplitudes and the decrease of the decay time constant (τ EPSP) obtained from the falling phase of the EPSP. 3. The enhancement of the EPSP amplitude and the decrease of (τ EPSP) after OXT application evoked the appearance of action potential under space summation of two spontaneous EPSPs and made easier the appearance of action potential under temporal summation of EPSPs produced by paired afferent stimuli, when the corresponding interstimuli interval was smaller than (τ EPSP) in the presence of OXT. 4. Ten minute exposure to 10 −8M OXT made the integrated amplitude of the excitatory acetylcholine response and the inhibitory dopamine response in the neuron E5 more positive only when the interval between applications of these mediators was smaller than the time constant of desensitization of acetylcholine receptors in the presence of OXT. 5. The pharmacological studies showed that drugs, which elevate intracellular cyclic AMP levels, mimicked the influence of OXT on integration of PSPs in the investigated neurons.

Teaching Methods of Performing Ukemi in Judo by Means of EMG Analysis

The performances of YOKO-UKEMI styles during falls in judo were analyzed by means of electromyogram and cinematography for to developments to protect the head from falls shock.The subjects of the study were three well-trained players and five beginners in judo who volunteered to cooperate. These performance were classified in three ways: 1) falling from a squatting position, 2) falling after being lifted 70cm from the floor with supine position, 3) falling after hizaguruma. The subjects were instructed to keep pressing the mat after slapping hand (press-type ukemi) and to lift up the hand (pull-type ukemi) immediately upon body contact with mat after slapping hand.For the purpose of the falling test, the relationship between elbow, shoulder muscles and neck muscles were examined by used recording VTR and EMG. The subjects elbow and shoulder joints were flexed and extended quickly. During this test, they were sitting in a chair, eyes closed.For comparative examination of ukemi styles, one group of junior high school students were instructed to look at the knots in their belts during performance of ukemi, and another group of them to perform the pull-type ukemi.The results are as follows;1) After contacting the hand to the mat, the electrical discharges of left and right muscle sternocleidmastoids appeared more markedly in both well-trained players and beginners when performing the pull-type ukemi than the press-type ukemi. This result suggests that the occiput of subjects were protected from falls shock to the mat by pull-type ukemi.2) During the pull-type ukemi neck flexions appeared greatly in both well-trained players and begunners.3) After contraction of muscle biceps brachii with elbow flexion in a sitting position discharges of action potential of left and right muscle sternocleidomastoids were 69±21 msec (mean±s. d.), and after contraction of muscle deltoid anterior portion with shoulder flexion in the same position discharges of action potential of left and right muscle sternocleidomastoids were 42±21 msec. The appearance of action potential of left and right muscle sternocleidomastoids were observed in all of the subjects of the study. After contraction of muscle triceps brachii with elbow extension in the sitting position discharges of action potential of left and right muscle splenius were 82±18 msec in four out of eight subjects, and after contraction of muscle deltoid posterior portion with shoulder extension in the same position, discharges of action potential of muscle splenius were 70±20 msec in five out of eight subjects. Splenius muscle activity was possibly absent in the well-trained as compared to the beginers.4) The present experiments indicated that the junior high school students could learn to perform the ukemi safely sooner if taught to do it in the pull-type ukemi than if instructed in the more traditional way to keep their eyes on their belt-knots when falling.

Comparative analysis of the action of a vasopressin analogue on functionally different neurons of the edible snail

Application of desglycine-argininvasopressin (DG-AVP) had a differing effect on different types of cell isolated from the central nervous system of the edible snail. We observed in the neurosecretory cells an increase in the frequency of spontaneous impulse activity and, as a rule, the appearance of bursts of impulses, most frequently of synaptic origin with the exception of PPal neurons and one of the neurosecretory cells of the left parietal ganglion. The intensity of the burst activity of these cells was due to an increase in the amplitude of the waves of the membrane potential. The activation of the neurosecretory cell system brought about an increase in the frequency and amplitude of the EPSPs in secondary-sensory neurons, this leading to an increased probability of the appearance of action potentials in them. On prolonged action the spontaneous EPSPs in these cells began to be arranged in bursts. The excitability and membrane resistance of these cells remained constant. The DG-AVP had no effect on the primary-sensory neurons and motoneurons.

Selective induction of brain type II Na+ channels by nerve growth factor.

Cells derived from a rat pheochromocytoma (PC12 cells) can generate an action potential only upon treatment with nerve growth factor. Using electrophysiological methods, we found that the appearance of action potentials in nerve growth factor-treated PC12 cells can be explained by an increase in the density of Na+ channels. The functional properties of Na+ channels in PC12 cells are similar to those described for peripheral nerves but appear to be different from Na+ channels synthesized in Xenopus oocytes injected with brain type II Na+ -channel mRNA. To determine if PC12 cells express the brain type II Na+ -channel gene, we performed RNase-protection analyses using probes that can distinguish between the brain type I and type II Na+ -channel mRNAs. The results from these studies indicate that undifferentiated PC12 cells express the type II but not the type I Na+ -channel gene. Treatment with nerve growth factor increases expression of the type II Na+ -channel gene but has no effect on type I gene expression. Our findings suggest that Na+ -channel excitability in PC12 cells is due to the specific induction of the brain type II gene by nerve growth factor.

Intracellular recordings from bovine anterior pituitary cells: modulation of spontaneous activity by regulators of prolactin secretion.

The electrophysiological properties of cultured bovine anterior pituitary cells were examined using intracellular recordings. Although all cells had high input resistances (mean 332 M omega), membrane potentials (Vm) showed a wide distribution (-32 to -80 mV). In 9 cells with low Vm (mean -38 mV) spontaneous Ca2+-dependent action potentials were observed (mean frequency 3.4 hertz). In the majority of silent cells (mean Vm -58 mV), application of TRH caused a reduction in membrane resistance and a transient hyperpolarization of Vm. This initial response was followed by a small, sustained depolarization. In 4/18 cells this depolarization led to the appearance of action potentials in previously silent cells. In 15/18 cells application of dopamine caused an increase in the membrane conductance and led to a sustained hyperpolarization of 30-35 mV. The response reversibly blocked the propagation of action potentials in spontaneously active cells. This effect could also be reversed by the dopamine receptor antagonists, sulpiride and haloperidol. In all cells, elevating the extracellular potassium concentration caused a concentration-dependent depolarization and decrease in the membrane input resistance. In some cells this effect was associated with an increased frequency of action potentials. Electrophysiological responses to TRH, dopamine, and elevated potassium were correlated with changes in the release of PRL. It is concluded that by their effects on Vm these factors modulate spontaneous electrical properties and may regulate the entry of calcium necessary for hormone secretion from lactotrophs.

Electrical activity of the sinus gland of the terrestrial isopod, Oniscus asellus: Characteristics of identified potentials recorded extracellularly from neurosecretory terminals

Spontaneously occurring neurosecretory action potentials recorded extracellularly from the sinus gland (SG) of the terrestial isopod, Oniscus asellus, are of 5 types (A through E) identified by their amplitudes and patterns of discharge. Type A have the largest (200–450 μV) and type E the smallest (25–50μV) amplitude. Types A, B and C originate from the bulb of the SG, and discharge at high frequencies (30–60 Hz) in coordinated bursts ranging from seconds to several minutes in duration. Coordination of their discharges suggests a mechanism for synchronizing bursting activity among different cell types. Types D and E originate from the lateral extension of the SG, and discharge at low frequencies (0.5–1.0 Hz) for prolonged periods (5–10 min). Their activity is not synchronized with discharges of other potentials. Following transection of the brain through the lateral part of the central protocerebral neuropile, A, B and C potentials are eliminated whereas D and E potentials remain active. This result suggests A, B and C potentials arise from neurosecretory cells (NSCs) whose cell bodies are located in the medial protocerebrum, and D and E potentials arise from NSCs identified in the optic lobe. Alterations in the appearance of action potentials following exposure to salines deficient in Na + or Ca 2+, or containing tetrodotoxin or cobalt, reveal that A and B potentials are primarily Ca 2+ dependent whereas C potentials are both Ca 2+ and Na + dependent.

TSH-stimulated electrical excitation in thyroid cells

This report demonstrates TSH-stimulated electrical excitation in cultured porcine thyroid cells. TSH depolarizes the thyroid cell membrane potentials and causes the appearance of action potentials, which occur in a burst. The burst is preceded by depolarization and after the burst, during which usually 2 spikes are seen, a repolarization occurs. This TSH-induced electrical excitation is associated with iodide discharge.

Appearance of calcium action potentials in crayfish slow muscle fibres under conditions of low intracellular pH.

1. The intracellular pH (pHi) of crayfish slow flexor muscle fibres was measured using recessed-tip pH micro-electrodes. To study the electrophysiological effects of intracellular acidification, pHi was lowered to values between the normal 7 . 2 and 6 . 3 by removal of external NH4Cl after a 20--30 min exposure. 2. During intracellular acidification, the fibres became capable of generating all-or-none Ca action potentials, rather than the normal small graded responses; no change in resting potential or input resistance accompanied this change in excitability. 3. All-or-none spikes appeared at pHi = 6 . 4--6 . 5. The spikes disappeared when pHi was increased again following the addition of 10 mM-bicarbonate to the external solution. CO2-saturated saline, which decreased pHi to 6 . 4, also caused the appearance of all-or-none action potentials. 4. The appearance of action potentials was correlated with a decrease in delayed rectification as pHi fell, as indicated by the response to depolarizing current pulses and by constant-current I--V plots in solutions in which all Ca ions had been replaced by Co. 5. External tetraethylammonium (TEA), at concentrations which reproduced the pHi = 6 . 4 effects on the I--V relation, also caused the appearance of all-or-none action potentials. 6. It is concluded that low pHi in these fibres partially blocks (or causes a depolarizing shift in the voltage dependence of) the voltage-sensitive outward K current which normally shunts the inward Ca current and prevents the generation of action potentials.

Development of electrophysiological and biochemical membrane properties during differentiation of embryonic skeletal muscle in culture.

Newly fused chick myotubes undergo simultaneous and rapid changes in cell membrane properties during synchronous differentiation in culture. These changes are coordinately regulated and include increases in acetylcholine receptor, acetylcholinesterase, and resting potential, as well as the appearance of action potentials in discrete membrane areas upon stimulation. Subsequently, the acetylcholine receptor reaches maximal levels, whereas the development of electrical properties is marked by a further increase in resting potential, changes in the characteristics of the elicited action potential, and the recruitment of additional membrane areas for action potential generation. Maturation of electrical excitability, marked by the acquisition of the ability to fire repetitively and to conduct action potentials along the membrane, occurs well after resting potential has reached a maximum. During post-maturational development, myotubes exhibit spontaneous electrical and contractile activity, and levels of acetylcholine receptor accessible to externally applied 125I-labeled alpha-bungarotoxin decrease markedly. It is suggested that electrophysiological membrane maturation is autonomously regulated with no requirement for neuronal intervention and involves the coordinated biosynthesis of discrete membrane components and their subsequent organization in the myotube membrane.

Electro- and pharmacomechanical coupling in the smooth muscle cells of the rabbit ear artery.

A contraction of the rabbit ear artery can be induced by depolarizing the cells with a K-rich solution if Ca is present. 10(-9)-10(-6) M noradrenaline and 10(-8)-10(-7) M histamine cause a contraction of this tissue without modifying the membrane potential. If the histamine concentration exceeds 10(-7) M some depolarization of the membrane also occurs. Both noradrenaline and histamine also induce a contraction in Ca-free medium, even if La is present. None of these stimuli produces action potentials or fluctuations of the membrane potential. Besides these tonic contractions, the ear artery can also produce phasic contractions when 10 mM TEA is added to the medium. Such contractions are caused by the appearance of action potentials which are Ca dependent and which are similar to those appearing in visceral smooth muscle. A study of 45Ca fluxes has revealed that K depolarization and noradrenaline cause only a small increase in 45Ca uptake by the cells, while noradrenaline also releases cellular Ca, even in Ca-free medium. A comparison of tension development and 45Ca release induced by noradrenaline in Ca-free medium suggests that Ca extrusion could be very efficient in the rabbit ear artery and that it could play a direct role in its relaxation.

Excitation-contraction coupling in multiunit tracheal smooth muscle during metabolic depletion: induction of rhythmicity

Multiunit canine tracheal smooth muscle responded to carbachol with graded depolarization and tonic contraction. The same concentration of carbachol, after metabolic depletion by substrate removal, produced rhythmic contractions and action potentials. Similar mechanical effects were also observed with acetylcholine or histamine. These effects were reversed by reintroducing glucose or beta-hydroxybutyrate, but not by 3-O-methylglucose, which is not metabolized; hence, the structural requirements for glucose, per se, or any osmotic effect were ruled out. Sensitivity to extracellular Ca2+ was increased. A Ca2+-influx blocker, D-600, in low concentration (2 X 10(-8) M) abolished the rhythmic contractions without affecting the tonic contraction. Progressive metabolic depletion in presence of carbachol led to fluctuations in membrane potential with a crest of depolarization and appearance of action potentials, each of which resulted in a small contraction. Many of the small contractions partially fused to form the major rhythmic contractions which appeared at a frequency of one per minute. Rhythmicity could not be produced by increasing extracellular K+ concentration (20-120 mM) in presence of atropine (13(-7) M), but instead a tonic contraction occurred. These results suggest changes in excitation-contraction coupling mechanism with agonists like acetylcholine, carbachol, or histamine during substrate deprivation.

The distribution of acetylcholine receptors on muscle fibres of regenerating salamander limbs.

1. Muscle fibres developing during limb regeneration were examined for responsiveness to acetylcholine (ACh) applied iontophoretically along the membrane. 2. Fibres which were uninnervated as well as those with non-transmitting synapses had high over-all sensitivities, with only minor variations along their length. 3. Functionally innervated fibres in which depolarization did not yet elicit action potentials had high over-all sensitivities, even when the synaptic potentials ahd amplitudes of 40-50 mV. In these the membrane in the vicinity of synapses tended to have sensitivities above the background level. 4. Upon the appearance of action potentials, several weeks after fibre innervation, the responsiveness to ACh began to decline in synapse free regions of the membrane. In mature muscle the sensitivity to ACh is restricted to sites of synaptic contact.

Effect of adequate stimulation on the discharge pattern of an isolated frog muscle spindle

Discharges from an isolated frog muscle spindle during mechanical stimulation of varied amplitude, velocity, and shape were investigated. The firing rate during a linear increase in strength of the stimulus is determined by its amplitude, whereas the change in firing rate is determined by the rate of increase of amplitude. With sinusoidal stimulation the firing rate apparently reproduces stimulus shape, i.e., the muscle spindle is sensitive not only to amplitude and velocity, but also to acceleration of the stimulus. Sensitivity to acceleration is most probably due to the change in threshold of appearance of action potentials observed during variation of the speed of stretching.