Increase In Sodium Conductance Research Articles

Short P-Wave Duration is a Marker of Higher Rate of Atrial Fibrillation Recurrences after Pulmonary Vein Isolation: New Insights into the Pathophysiological Mechanisms Through Computer Simulations.

BackgroundShort ECG P‐wave duration has recently been demonstrated to be associated with higher risk of atrial fibrillation (AF). The aim of this study was to assess the rate of AF recurrence after pulmonary vein isolation in patients with a short P wave, and to mechanistically elucidate the observation by computer modeling.Methods and ResultsA total of 282 consecutive patients undergoing a first single‐pulmonary vein isolation procedure for paroxysmal or persistent AF were included. Computational models studied the effect of adenosine and sodium conductance on action potential duration and P‐wave duration (PWD). About 16% of the patients had a PWD of 110 ms or shorter (median PWD 126 ms, interquartile range, 115 ms–138 ms; range, 71 ms–180 ms). At Cox regression, PWD was significantly associated with AF recurrence (P=0.012). Patients with a PWD <110 ms (hazard ratio [HR], 2.20; 95% CI, 1.24–3.88; P=0.007) and patients with a PWD ≥140 (HR, 1.87, 95% CI, 1.06–3.30; P=0.031) had a nearly 2‐fold increase in risk with respect to the other group. In the computational model, adenosine yielded a significant reduction of action potential duration 90 (52%) and PWD (7%). An increased sodium conductance (up to 200%) was robustly accompanied by an increase in conduction velocity (26%), a reduction in action potential duration 90 (28%), and PWD (22%).ConclusionsOne out of 5 patients referred for pulmonary vein isolation has a short PWD which was associated with a higher rate of AF after the index procedure. Computer simulations suggest that shortening of atrial action potential duration leading to a faster atrial conduction may be the cause of this clinical observation.

Evaluation Of Electrophysiological Changes In Primary Spinal Cord Neuronal Cultures Exposed To ALS-CSF Using Microelectrode Array.

Evaluation Of Electrophysiological Changes In Primary Spinal Cord Neuronal Cultures Exposed To ALS-CSF Using Microelectrode Array.

Functional Interaction between the Scaffold Protein Kidins220/ARMS and Neuronal Voltage-Gated Na+ Channels

Kidins220 (kinase D-interacting substrate of 220 kDa)/ankyrin repeat-rich membrane spanning (ARMS) acts as a signaling platform at the plasma membrane and is implicated in a multitude of neuronal functions, including the control of neuronal activity. Here, we used the Kidins220(-/-) mouse model to study the effects of Kidins220 ablation on neuronal excitability. Multielectrode array recordings showed reduced evoked spiking activity in Kidins220(-/-) hippocampal networks, which was compatible with the increased excitability of GABAergic neurons determined by current-clamp recordings. Spike waveform analysis further indicated an increased sodium conductance in this neuronal subpopulation. Kidins220 association with brain voltage-gated sodium channels was shown by co-immunoprecipitation experiments and Na(+) current recordings in transfected HEK293 cells, which revealed dramatic alterations of kinetics and voltage dependence. Finally, an in silico interneuronal model incorporating the Kidins220-induced Na(+) current alterations reproduced the firing phenotype observed in Kidins220(-/-) neurons. These results identify Kidins220 as a novel modulator of Nav channel activity, broadening our understanding of the molecular mechanisms regulating network excitability.

O3-C-3. Electrophysiological study of familial amyotrophic lateral sclerosis with expansions in the C9ORF72

A hexanucleotide repeat expansion in the chromosome 9 open reading frame 72 (C9ORF72) gene was recently discovered as a cause underlying familial amyotrophic lateral sclerosis (ALS), and considered as the most common genetic cause of sporadic ALS in Caucasian populations. We investigated electrophysiological features of an ALS patient with expansion in the C9OFR72.The patient was a 63-year-old female and had family history of ALS. She developed bulbar palsy and subsequent limb weakness for 9 months. On admission, muscle atrophy in the tongue and four limbs, and pyramidal signs in the cranial, cervical, and lumbar segments were seen. She showed neither dementia nor Parkinsonism. On electromyogram, fasciculations were detected in the tibialis anterior. Axonal excitability testing showed greater strength-duration time constant, greater changes in depolarizing threshold electrotonus, and greater supernormality in recovery cycle, which indicate increased sodium conductance and reduced potassium conductance. The above findings suggest increased axonal excitability in this case. Motor axonal hyperexcitability is considered as one of the major prognostic factors in sporadic ALS. This might enhance neuronal death also in FALS with the C9ORF72 repeat expansion.

1090 INTEGRATING GENETICS AND GENOMICS IN PRIMARY ALDOSTERONISM

Primary aldosteronism (PA) is the most common form of endocrine hypertension, due in most cases to aldosterone producing adenoma (APA) or bilateral adrenal hyperplasia (BAH). Screening for PA is of major importance, given the severe cardiovascular outcome of aldosterone excess that is independent of blood pressure levels. Yet, the pathogenic mechanisms leading to aldosterone hypersecretion and cell proliferation are largely unknown. Transcriptome analyses have highlighted the contribution of different signaling pathways converging upon increased expression of CYP11B2 coding for aldosterone synthase. Less is known with regard to the mechanisms underlying nodulation. Recent evidence indicates a link between APA formation and peritumoral tissue remodelling and ZG hyperplasia with the re-expression of genes involved in foetal adrenal development that could underlie excessive cell proliferation and APA formation. Recently, a few recurrent somatic mutations of the KCNJ5 gene, coding for the potassium channel Kir3.4, have been implicated as a cause of APA, while inherited mutations of the same gene were identified in familial hyperaldosteronism type 3. Somatic KCNJ5 mutations are found in ∼34% of unselected patients with APA; germline KCNJ5 mutations, however, are not similarly causative for BAH. Mutations lie near or within the selectivity filter of the Kir3.4 channel and result in a loss of channel selectivity, with increased sodium conductance leading to membrane depolarization and activation of calcium signaling. While it has been formally established that this cascade of events leads to increased aldosterone production, the mechanisms leading to increased cell proliferation still need to be elucidated.

Increase in Sodium Conductance Decreases Firing Rate and Gain in Model Neurons

We studied the effects of increased sodium conductance on firing rate and gain in two populations of conductance-based, single-compartment model neurons. The first population consisted of 1000 model neurons with differing values of seven voltage-dependent conductances. In many of these models, increasing the sodium conductance threefold unexpectedly reduced the firing rate and divisively scaled the gain at high input current. In the second population, consisting of 1000 simplified model neurons, we found that enhanced sodium conductance changed the frequency-current (FI) curve in two computationally distinct ways, depending on the firing rate. In these models, increased sodium conductance produced a subtractive shift in the FI curve at low firing rates because the additional sodium conductance allowed the neuron to respond more strongly to equivalent input current. In contrast, at high input current, the increase in sodium conductance resulted in a divisive change in the gain because the increased conductance produced a proportionally larger activation of the delayed rectifier potassium conductance. The control and sodium-enhanced FI curves intersect at a point that delimits two regions in which the same biophysical manipulation produces two fundamentally different changes to the model neuron's computational properties. This suggests a potentially difficult problem for homeostatic regulation of intrinsic excitability.

Integrating Genetics and Genomics in Primary Aldosteronism

Primary aldosteronism (PA) is the most common form of secondary hypertension, with an estimated prevalence of ≈10% in referred patients and 4% in primary care1,2 but as high as 20% in patients with resistant hypertension.3,4 It is characterized by hypertension with low plasma renin and elevated aldosterone that is often seen with hypokalemia. PA occurs as a result of a dysregulation of the normal mechanisms controlling adrenal aldosterone production. The 2 major causes are aldosterone-producing adenomas (APAs) and bilateral adrenal hyperplasia (BAH), also called idiopathic hyperaldosteronism. The early detection of PA has an enormous impact on clinical outcome and survival, given the major cardiovascular adverse effects of aldosterone excess, which are independent of blood pressure,5,6 and predict outcome after surgical treatment.7,8 Patients with PA have a significantly higher risk of nonfatal myocardial infarction, atrial fibrillation, and stroke compared with age-, sex-, and blood pressure–matched essential hypertensives.5 Furthermore, changes in cardiac structure and function9 and partially reversible renal dysfunction10 have been described, whereas the increased occurrence of metabolic abnormalities in patients with PA is still a matter of debate.11,12 Although the 2008 guidelines for the management of PA have been pivotal for homogenizing screening procedures and treatment among specialized centers,13 there remain a few critical issues related to diagnosis, subtype differentiation, and treatment of nonsurgically correctable forms. Reliable diagnostic and prognostic biomarkers are lacking for more sensitive and specific screening, as well as new therapeutic avenues, because medical and/or surgical treatment of PA leads to normotension in only a minority of patients. This may come from a better understanding of the pathogenic mechanisms of the disease, in particular, identification of the genetic and molecular determinants leading to the development of APA and …

Primary Aldosteronism Takes (KCNJ)Five!

Primary aldosteronism (PA) is the most common form of endocrine hypertension, due to autonomous aldosterone production from the adrenal cortex. Patients with PA typically present with hypertension, high plasma aldosterone levels associated with low plasma renin activity, and varying degrees of hypokalemia and metabolic alkalosis (1). The prevalence of PA increases with the severity of hypertension and is currently estimated around 10% in referred patients and 4% in primary care (2, 3) but as high as 20% in patients with resistant hypertension (4, 5). The two main causes of PA are aldosterone-producing adenoma (APA) and bilateral adrenal hyperplasia (BAH), also called idiopathic hyperaldosteronism. Efficient and timely screening for PA is of major importance, given the severe cardiovascular outcome of aldosterone excess that is independent of blood pressure levels (6, 7). Yet, the pathogenic mechanisms leading to aldosterone hypersecretion and cell proliferation are largely unknown. Recently, a few recurrent somatic mutations of the potassium inwardly-rectifying channel, subfamily J, member 5 (KCNJ5) gene, coding for the potassium channel Kir3.4, have been implicated as a cause of APA, whereas an inherited mutation was identified in a family with a Mendelian form of early severe hypertension that features massive adrenal hyperplasia and is referred to as familial hyperaldosteronism type 3 (FH3) (8, 9). These mutations all lie near or within the selectivity filter of the Kir3.4 channel; they result in a loss of channel selectivity, with increased sodium conductance leading to membrane depolarization. These changes are presumed to be responsible for constitutive aldosterone secretion and cell proliferation by promoting opening of membrane voltage-dependent calcium channels that is followed by activation of the calcium signaling pathway, the main trigger for aldosterone production in adrenal zona glomerulosa cells. In this issue of Endocrinology, Oki et al. (10) now formally establish a causal relationship between KCNJ5 mutations and hyperaldosteronism. They demonstrate that the inherited KCNJ5 T158A mutation produces a marked stimulation in aldosterone biosynthesis that is dependent on membrane depolarization followed by calcium influx into adrenal cortical carcinoma cells (10). The paper by Oki et al. (10) indeed tackles a central issue that had not previously been addressed, i.e. the causal link between KCNJ5 mutations, membrane depolarization, aldosterone overproduction, and cell proliferation. By transiently infecting adrenocortical HAC15 cells with a lentivirus expressing wild-type KCNJ5 (potassium inwardly-rectifying channel, subfamily J, member 5) or mutated KCNJ5 T158A, the authors show that expression of channels harboring the T158A mutation potentiated basal aldosterone production, which was further stimulated by angiotensin II and the protein kinase A activator forskolin. Using different fluorescent dyes, they confirmed the enhanced sodium influx through the mutated Kir3.4 channel, leading to membrane depolarization and increased intracellular calcium concentrations. KCNJ5 T158A-transduced cells presented significantly increased expression of CYP11B2, the gene coding for aldosterone synthase, which ensures the last three enzymatic steps of aldosterone biosynthesis. Expression of CYP11B1, coding for 11 -hydroxylase, was also increased, as was basal and stimulated cortisol production. KCNJ5 T158A also induced significant production of 18-oxocortisol, a hybrid steroid largely produced in affected members of the original family with FH3 (11). CYP11B2 expression and al-

Elucidation of the Molecular Basis of Selective Recognition Uncovers the Interaction Site for the Core Domain of Scorpion α-Toxins on Sodium Channels

Neurotoxin receptor site-3 at voltage-gated Na(+) channels is recognized by various peptide toxin inhibitors of channel inactivation. Despite extensive studies of the effects of these toxins, their mode of interaction with the channel remained to be described at the molecular level. To identify channel constituents that interact with the toxins, we exploited the opposing preferences of LqhαIT and Lqh2 scorpion α-toxins for insect and mammalian brain Na(+) channels. Construction of the DIV/S1-S2, DIV/S3-S4, DI/S5-SS1, and DI/SS2-S6 external loops of the rat brain rNa(v)1.2a channel (highly sensitive to Lqh2) in the background of the Drosophila DmNa(v)1 channel (highly sensitive to LqhαIT), and examination of toxin activity on the channel chimera expressed in Xenopus oocytes revealed a substantial decrease in LqhαIT effect, whereas Lqh2 was as effective as at rNa(v)1.2a. Further substitutions of individual loops and specific residues followed by examination of gain or loss in Lqh2 and LqhαIT activities highlighted the importance of DI/S5-S6 (pore module) and the C-terminal region of DIV/S3 (gating module) of rNa(v)1.2a for Lqh2 action and selectivity. In contrast, a single substitution of Glu-1613 to Asp at DIV/S3-S4 converted rNa(v)1.2a to high sensitivity toward LqhαIT. Comparison of depolarization-driven dissociation of Lqh2 and mutant derivatives off their binding site at rNa(v)1.2a mutant channels has suggested that the toxin core domain interacts with the gating module of DIV. These results constitute the first step in better understanding of the way scorpion α-toxins interact with voltage-gated Na(+)-channels at the molecular level.

Mutations in KCNJ5 Gene Cause Hyperaldosteronism

### K+ Channel Mutations in Adrenal Aldosterone-Producing Adenomas and Hereditary Hypertension Choi et al Science . 2011;331:768–772 A new report in Science finds that a few somatic KCNJ5 mutations explain a subset of aldosterone-producing adenomas, the most frequent secondary cause of arterial hypertension. Primary aldosteronism (PAL) is the most common secondary form of arterial hypertension, with an estimated prevalence between 6% and 10% and as high as 20% in patients with resistant hypertension.1,2 In the majority of cases, it is the result of either an adrenal aldosterone producing adenoma (APA) or a bilateral adrenal hyperplasia (also known as idiopathic hyperaldosteronism).3 However, the molecular mechanisms underlying aldosterone hypersecretion and nodulation of the adrenal cortex are still largely unknown. In this article,4 Lifton's group opens a completely new area by establishing the concept that recurrent somatic mutations at the KCNJ5 gene coding for the potassium channel Kir3.4 could account for a substantial proportion of APAs but also that a germinal KCNJ5 mutation can cause a very rare autosomal dominant and early-onset form of PAL, characterized by bilateral massive adrenal hyperplasia and referred to as FH-3.5,6 Using whole exome sequencing on four APA blood DNA pairs from unrelated subjects, the authors have identified somatic mutations of the KCNJ5 gene in two …

SK3 polyglutamine tract polymorphism and acute oxaliplatin neurotoxicity: A translational study

21076 Background: Acute neurotoxicity of l-oxaliplatin (l-OHP), characterized by cold-induced paresthesias, muscle cramps and tightness, seems electrophysiologically related to an abnormal nerve hyperexcitability (NHE) resembling neuromyotonia, an autoimmune disease caused by autoantibodies directed against voltage-gated potassium channels (VGKC). This l-OHP-related channelopathy seems to be reversible, but there is no agreement about the channel involved, most investigators favouring voltage-gated sodium channels. SK3 VGKC are mainly located in the peripheral nervous system and are characterized by an highly polymorphic CAG motif. We hypothesized that SK3 potassium channel dysfunction might be responsible for nerve hyperexcitability in patients treated with l-OHP. Methods: Pts eligible for an l-OHP-containing regimen were enrolled. Detailed neurological examination, nerve conduction studies (NCS) and needle electromyography (EMG) were performed before and after l-OHP administration. Furthermore, a venous blood sample was obtained from each pt in order to perform genetic analysis. Genomic DNA was extracted from pts’ leukocytes and PCR-mediated amplification of the second polyglutamine CAG tract of SK3 gene was carried out; PCR products were analyzed on polyacrylamide gel stained with Ethidium Bromide. Results: We evaluated 20 pts (14M, 6F); mean age was 59 yrs. According to neurophysiologic data it was possible to divide pts into 3 groups: G0 (no symptoms or signs of NHE), 7 pts; G1 (cold induced paresthesias-mild NHE), 8 pts; G2 (muscle cramps-severe NHE), 5 pts. Genetic analysis showed different alleles ranging from 125 to 225 bp. In particular, 75% of pts carrying a 175 bp allele experienced severe acute toxicicy (G2 group), and 15% mild toxicity (G1 group). The 175 bp allele was found only in 1/7 G0 pts (15%); in these pts the alleles distribution was diffusely scattered. Conclusions: Acute and reversible NHE observed in some pts treated with l-OHP might be produced either by an increased sodium conductance or by a decreased activity of VGKC. The high percentage of the 175 bp allele, that we found in the G2 group, induces us to think that the polymorphism at CAG locus of SK3 may be responsible for l-OHP toxicity. Sequence analysis of PCR gene products is ongoing. No significant financial relationships to disclose.

Long QT syndrome and anaesthesia

Long QT syndrome (LQTS) is an arrhythmogenic cardiovascular disorder resulting from mutations in cardiac ion channels. LQTS is characterized by prolonged ventricular repolarization and frequently manifests itself as QT interval prolongation on the electrocardiogram (ECG). The age at presentation varies from in utero to adulthood. The majority of symptomatic events are related to physical activity and emotional stress. Although LQTS is characterized by recurrent syncope, cardiac arrest, and seizure-like episodes, only about 60% of patients are symptomatic at the time of diagnosis. 3 The clinical features of LQTS result from a peculiar episodic ventricular tachyarrhythmia called ‘torsade de pointes’. ‘Twisting of the points’ describes the typical sinusoidal twisting of the QRS axis around the isoelectric line of the ECG. Usually torsade de pointes start with a premature ventricular depolarization, followed by a compensatory pause. The next sinus beat often has a markedly prolonged QT interval and abnormal T wave. This is followed by a ventricular tachycardia that is characterized by variation in the QRS morphology, and a constantly changing R-R interval (Fig. 1). The ‘short-long-short’ cycle length sequence heralding torsade de pointes is a hallmark of LQTS. Commonly, the episode of torsade de pointes is self-terminating, producing a syncopal episode or pseudoseizure, secondary to the abrupt decrease in cerebral blood flow. The majority of episodes of sudden death in LQTS result from ventricular fibrillation triggered by torsade de pointes, although the mechanism of this deterioration is unknown. Traditionally, LQTS has been classified as either familial (inherited) or acquired. However, it is likely that many patients with previously labelled acquired LQTS carry a silent mutation in one of the genes responsible for congenital LQTS. 22 119 The evidence for this hypothesis has been gradually emerging over the past few years. It is important for anaesthetists to be aware of this concept, as it means that a much higher proportion of the general population may be affected by asymptomatic mutations in genes encoding cardiac ion channels than was thought previously. The prevalence of LQTS in developed countries may be as high as 1 per 1100‐3000 of the population. 32 119 About 30% of congenital LQTS carriers have an apparently normal phenotype, and thus a normal QT interval, and remain undiagnosed until an initiating event. 105 Fatal arrhythmias associated with primary electrical disease of the heart such as the Brugada and LQTS, probably account for 19% of sudden deaths in children between 1 and 13 yr of age, and 30% of sudden deaths that occur between 14 and 21 yr of age. 10 Furthermore, there is a strong association between prolonged corrected QT interval (QTc) in the first week of life and risk of sudden infant death syndrome. 86

Regulation of ion and fluid transport across the distal pulmonary epithelia: new insights

this issue of the American Journal of Physiology-Lung Cellular and Molecular Physiology features nine articles that provide new insights into the mechanisms and the cellular pathways that regulate ion transport across the distal pulmonary epithelia in both the adult and fetal lung. Although much has

Horizons in Pain Research. San Diego, California, May 19-21, 1995.

Horizons in Pain Research. San Diego, California, May 19-21, 1995.

Structural and Electrophysiological Analysis of Annexin V Mutants: Mutagenesis of Human Annexin V, an in Vitro Voltage-gated Calcium Channel, Provides Information about the Structural Features of the Ion Pathway, the Voltage Sensor and the Ion Selectivity Filter

Annexin V binds to phospholipids in a calcium-dependent manner and exhibits calcium channel activity in vitro. We prepared a variety of mutants yielding information about the structure-function relationship of the ion channel activity. All mutants were characterized by X-ray crystallography, electron microscopy and electrophysiological measurements. Their structures are insignificantly changed whereas their electrophysiological properties are drastically different. Glu95, located in the central hydrophilic pore of the molecule, is crucial for the ion selectivity filter as its exchange leads to reduced calcium and increased sodium conductance. The removal of Glu17, located on the protein surface and far from the ion conduction pathway, leads to the appearance of a second conductance level of 9 pS in addition to the conductance level of about 30 pS in the wild-type molecule. This was also the case for Glu78, which is part of a weak calcium binding site. The exchange of Glu17 and Glu78 produced a mutant retaining only the smaller conductance level. We conclude that these two residues influence the angle between the two halves of the molecule, which determines the diameter of the ion conduction pathway, thereby leading to the occurrence of a second conductance level.

Voltage interaction between sarcolemma and junctional SR membrane

A computer model, based on the electric field model for propagation in cardiac tissue, was developed to examine the possible interactions between the cell surface and the junctional sarcoplasmic reticulum (J-SR) at the cell diads (regions where the SR membrane comes in close proximity to the cell membrane). The surface sarcolemmal membrane (SL) of the cell was modeled as 5 sections (units 1–5) of excitable membrane, conforming to a Hodgkin-Huxley kinetic model as modified by D. Noble for purkinje fibers. The J-SR was modeled as a tubular ring (1 μm-wide) around the perimeter of the cell, composed of two parallel membranes: one membrane (unit 6) apposed to the SL and the other membrane (unit 7) further from the SL. These two concentric rings of excitable membrane were modeled as a torus inside the cylindrical cell. The seven units were connected by a resistive network. The equivalent circuit for this model was solved numerically on an IBM-AT compatible computer. For stimulation, a 2 mA/cm 2 current pulse was applied to unit 1 for 0.5 msec, and the voltages of all the units were monitored. Under our standard conditions, an action potential (AP) initiated at unit 1 propagated immediately to unit 2, then to units 4 and 5. Unit 3, the surface membrane directly opposed to the J-SR, fired after a slight delay. There was a further delay from the time unit 3 fired until J-SR units 6 and 7 fired. The total delay from the first surface unit of the cell to the J-SR units under standard conditions was 1.13 msec. We believe the transfer of excitation from the surface of the cell to the SR occurs due to current flow through the J-SR units 6 and 7 when surface unit 3 is firing. Since the gap between unit 6 and 3 is very narrow (e.g., ca 150 Å), the shunt resistance for current flowing between these membranes is very high, thus current is forced to flow through units 6 and 7. This current hyperpolarizes unit 6, but depolarizes unit 7 to threshold causing it to fire an AP, which then propagates to unit 6. Increased sodium conductance in the SR facilitates propagation, as does a decrease in the thickness of the gap between the surface and J-SR membranes. An increase in the surface area of J-SR unit 7 slowed propagation. Thus any organelle which is physically located in very close proximity to the surface membrane will “hear” the electrical activity that occurs in the surface membrane. This model could lead to new insights into the calcium release process in muscle during excitation-contraction coupling, and other signal transduction processes.

Role of Na+ and Ca++ in guinea pig trachealis contraction induced by cooling.

The role of extracellular Na+ and Ca++ in cooling-induced contraction (CIC) was studied in guinea pig trachealis muscle. Tracheal preparations were tested in Krebs-Ringer bicarbonate (KRB) solution and then in either: (1) a low Na+ sucrose KRB, (2) KRB plus amiloride, a sodium channel blocker, or (3) KRB plus nifedipine, a calcium channel blocker. When the trachealis muscle was cooled from 37 to 20 degrees C in KRB, the value of the isometric tension increased on average by 25% of the maximal tension induced by acetylcholine; however, when the tissue was tested in the low Na+ sucrose KRB or the KRB containing amiloride, CIC was totally prevented. In the presence of nifedipine the mean value of CIC was only 10% of the acetylcholine-induced contraction, which was also observed not to be affected by extracellular Na+ or Ca++; whereas the KCl-induced contraction was affected by Ca++ but not by Na+. The results suggest that rapid cooling of the smooth muscle cells causes an increase in sodium conductance and a decrease in the activity of the Na+-K+ pump, resulting in membrane depolarization. During depolarization a significant quantity of Ca++ enters the cell, contributing to the magnitude of CIC.

Electrophysiological characterization of functionally distinct 5-hydroxytryptamine receptors on guinea-pig submucous plexus

Intracellular recordings were made from neurons of the guinea-pig submucous plexus and the actions of 5-hydroxytryptamine on the postsynaptic membrane and on evoked synaptic potentials were examined. 5-Hydroxytryptamine produced two types of direct postsynaptic responses: (1) A depolarization associated with a fall in input resistance was observed in all cells. Voltage-clamp and ion substitutions showed that this depolarization resulted primarily from an inward sodium current. This response could be as brief as 30 ms; it showed desensitization and was selectively abolished by 0.2–2 μM ICS 205–930. (2) A depolarization (or inward current) associated with a decreased conductance was observed in about 50% of neurons, usually after the first response was blocked by ICS 205–930. This response was due to a decreased potassium conductance; the minimum time course of this response was 8–10 s. It did not show desensitization and was not sensitive to blockade by currently available antagonists of 5-hydroxytryptamine, nicotinic and/or muscarinic receptors. Higher concentrations of 5-hydroxytryptamine were required to produce the sodium conductance increase than the potassium conductance decrease; 2-methyl-5-hydroxytryptamine was equally effective in producing these responses. 5-Hydroxytryptamine also caused a barrage of “spontaneous” nicotinic excitatory post-synaptic potentials which were sensitive to tetrodotoxin. This response desensitized, was blocked by ICS 205–930 and is presumed to reflect exitation of other cholinergic cell bodies in the plexus by the sodium conductance increase mechansim described. The evoked nicotinic excitatory postsynaptic potential and the adrenergic inhibitory postsynaptic potential were decreased by 5-hydroxytryptamine; a portion of this inhibition showed desensitization and was blocked by ICS 205–930 as well as by the muscarinic receptor antagonists, atropine and pirenzepine. The ICS 205–930-insensitive portion of this inhibition could not be attributed to activation of 5-hydroxytryptamine-1 or 5-hydroxytryptamine-2 receptors. Thus, the following conclusions are drawn: 5-hydroxytryptamine excites submucous plexus neurons by activating two distinct 5-hydroxytryptamine receptors. Activation of the 5-hydroxytryptamine-3 receptor (sensitive to ICS 205–930) produces a depolarization mediated by an increased sodium conductance. The same effect occurring in other cholinergic cell bodies initiates action potentials which are responsible for the 5-hydroxytryptamine-induced release of acetylcholine. Acetylcholine so released can, in turn, activate inhibitory presynaptic muscarinic receptors located on other cholinergic and adrenergic fibers and this contributes to inhibition of evoked transmitter release. Activation of another 5-hydroxytryptamine receptor, which belongs to neither 5-hydroxytryptamine-1, nor 5-hydroxytryptamine-2, nor 5-hydroxytryptamine-3 receptor types, produces a slow postsynaptic depolarization due to a decreased potassium conductance, the physiological consequences of which are a prolonged neuronal excitability.

The effects of two types of pyrethroid on rat skeletal muscle

The mechanical and electrophysiological effects of two synthetic pyrethroids, cismethrin (Type I) and deltamethrin (Type II) have been studied in whole rat gastrocnemius muscle and in isolated perfused rat diaphragm. In both directly or indirectly stimulated preparations deltamethrin but not cismethrin had a positive inotropic effect which in the case of the whole animal could be reversed by increasing the stimulation frequency. In the isolated diaphragm the deltamethrin effect could be prevented by TTX suggesting it is primarily the result of an increased sodium conductance. Deltamethrin also reversed a partial curare blockade in the diaphragm. Electrophysiological studies showed that the inotropic effect of deltamethrin is accompanied by repetitive muscle action potentials. In addition deltamethrin significantly increased muscle fibre input resistance, a novel finding which may underlie the curare reversal effect. It is concluded that the effectiveness of deltamethrin compared with cismethrin in causing repetitive firing in the muscle membrane is attributable to the greater duration of the after-depolarization produced by deltamethrin.

Cyclic GMP is involved in the excitation of invertebrate photoreceptors.

The hyperpolarizing receptor potential in vertebrate rod photoreceptors appears to be mediated by the second messenger, cyclic GMP. Injection of cGMP into rods or application of cGMP to inside-out membrane patches activates a conductance resembling that produced by light. Light produces a rapid reduction of cGMP in living rods, leading to closure of sodium channels and membrane hyperpolarization. In most invertebrate photoreceptors the response to light is depolarizing. We have investigated whether cGMP is involved in controlling the increase in sodium conductance that underlies this depolarization. We show here that injection of cGMP into Limulus photoreceptors produces a depolarization that mimics the receptor potential. We also show that the cGMP concentration of the squid retina increases rapidly during exposure to light. These results support the hypothesis that cGMP mediates the light-induced depolarization in invertebrate photoreceptors and suggests that vertebrate and invertebrate phototransduction may be more similar than previously thought.