Amplitude Of Depolarization Research Articles

Halothane Inhibition of Recombinant Cardiac L-type Ca2+ Channels Expressed in HEK-293 Cells

Background. Volatile anesthetics depress cardiac contractility, which involves inhibition of cardiac L-type calcium channels. To explore the role of voltage-dependent inactivation, the authors analyzed halothane effects on recombinant cardiac L-type calcium channels (alpha1Cbeta2a and alpha1Cbeta2aalpha2/delta1), which differ by the alpha2/delta1 subunit and consequently voltage-dependent inactivation.Methods. HEK-293 cells were transiently cotransfected with complementary DNAs encoding alpha1C tagged with green fluorescent protein and beta2a, with and without alpha2/delta1. Halothane effects on macroscopic barium currents were recorded using patch clamp methodology from cells expressing alpha1Cbeta2a and alpha1Cbeta2aalpha2/delta1 as identified by fluorescence microscopy.Results. Halothane inhibited peak current (I(peak)) and enhanced apparent inactivation (reported by end pulse current amplitude of 300-ms depolarizations [I300]) in a concentration-dependent manner in both channel types. alpha2/delta1 coexpression shifted relations leftward as reported by the 50% inhibitory concentration of I(peak) and I300/I(peak)for alpha1Cbeta2a (1.8 and 14.5 mm, respectively) and alpha1Cbeta2aalpha2/delta1 (0.74 and 1.36 mm, respectively). Halothane reduced transmembrane charge transfer primarily through I(peak) depression and not by enhancement of macroscopic inactivation for both channels.Conclusions. The results indicate that phenotypic features arising from alpha2/delta1 coexpression play a key role in halothane inhibition of cardiac L-type calcium channels. These features included marked effects on I(peak) inhibition, which is the principal determinant of charge transfer reductions. I(peak) depression arises primarily from transitions to nonactivatable states at resting membrane potentials. The findings point to the importance of halothane interactions with states present at resting membrane potential and discount the role of inactivation apparent in current time courses in determining transmembrane charge transfer.

N‐Benzyl‐p‐toluene sulphonamide allows the recording of trains of intracellular action potentials from nervestimulated intact fast‐twitch skeletal muscle of the rat

In skeletal muscle, the intracellular recording of trains of action potentials is difficult owing to the movement of the muscle upon stimulation. A potential tool for the removal of muscle movement is the cross-bridge cycle blocker, N-benzyl-p-toluene sulphonamide (BTS), although the effects of BTS on the passive and active membrane properties of intact muscle fibres are not known. Rat extensor digitorum longus (EDL) muscle was used to show that 50 mum BTS reduced tetanic force to approximately 10% of control force, without markedly altering muscle excitability. Incubation with BTS did not alter intracellular K+ content or Na+-K+ pump activity, but produced minor decreases in intracellular Na+ content (7%), resting 22Na+ influx (14%) and excitation-induced 22Na+ influx (29%). Despite these alterations to Na+ fluxes, BTS did not impair muscle excitability, since membrane conductance, resting membrane potential (RMP), rheobase current and the amplitude, overshoot and maximum rate of depolarization of the action potential were all unaltered. However, BTS did induce a small (8%) decrease in the maximum rate of repolarization of the action potential and an increase in the refractory period. The minor effects of BTS on muscle membrane properties did not compromise the ability of the muscle to propagate action potentials, even during tetanic stimulation. In conclusion, BTS can be used successfully to reduce contractility, allowing the intracellular recording of action potentials during both twitch and tetanic contraction of nerve-stimulated muscle, thus making it an excellent tool for the study of electrophysiology in fast-twitch skeletal muscle.

Motoneurones "Learn" and "Forget" Physical Activity

In spite of our knowledge of activity related adaptations in supraspinal neurones and skeletal muscles, very little is known concerning adaptations in alpha-motoneurones to alterations in chronic activity levels. Recent evidence shows that the biophysical properties of alpha-motoneurones are plastic and adapt to both increases and decreases in chronic activation. The nature of the adaptations--in resting membrane potential, spike threshold, afterhyper-polarization amplitude,and rate of depolarization during spike generation--point to involvement of density, type, location, and/or metabolic modulation of ion conductance channels in the motoneuronal membrane. These changes will have significant effects on how motoneurones respond when activated during the generation of movements, and on the effort required to sustain activation during prolonged exercise. Since the adaptations most likely involve structural changes in the motoneurones and changes in protein synthesis, and change the output response of the cells to input, they are considered to be learning responses. Future research directions for examining this issue are outlined.

An electrophysiological study of excitatory purinergic neuromuscular transmission in longitudinal smooth muscle of chicken anterior mesenteric artery

1. The object of the present study was to clarify the neurotransmitters controlling membrane responses to electrical field stimulation (EFS) in the longitudinal smooth muscle cells of the chicken anterior mesenteric artery. 2. EFS (5 pulses at 20 Hz) evoked a depolarization of amplitude 19.7+/-2.1 mV, total duration 29.6+/-3.1 s and latency 413.0+/-67.8 ms. This depolarization was tetrodotoxin (TTX)-sensitive and its amplitude was partially decreased by atropine (0.5 microM); however, its duration was shortened by further addition of prazosin (10 microM). 3. Atropine/prazosin-resistant component was blocked by the nonspecific purinergic antagonist, suramin, in a dose-dependent manner, indicating that this component is mediated by the neurotransmitter adenosine 5'-triphosphate (ATP). 4. Neither desensitization nor blocking of P2X receptor with its putative receptor agonist alpha,beta-methylene ATP (alpha,beta-MeATP, 1 microM) and its antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic (PPADS, up to 50 microM), had significant effect on the purinergic depolarization. In contrast, either desensitization or blocking of P2Y receptor with its putative agonist 2-methylthioATP (2-MeSATP, 1 microM) and its antagonist Cibacron blue F3GA (CBF3GA, 10 microM) abolished the purinergic depolarization, indicating that this response is mediated through P2Y but not P2X receptor. 5. The purinergic depolarization was inhibited by pertussis toxin (PTX, 600 ng ml(-1)). Furthermore, it was significantly inhibited by a phospholipase C (PLC) inhibitor, U-73122 (10 microM), indicating that the receptors involved in mediating the purinergic depolarization are linked to a PTX-sensitive G-protein, which is involved in a PLC-mediated signaling pathway. 6. Data of the present study suggest that the EFS-induced excitatory membrane response occurring in the longitudinal smooth muscle of the chicken anterior mesenteric artery is mainly purinergic in nature and is mediated via P2Y purinoceptors.

Polychlorinated biphenyl (PCB) alters acid-sensitivity of cultured neurons derived from the medulla oblongata

Polychlorinated biphenyls (PCBs) are known as environmental pollutants that may cause adverse health problems. However, little is known about the effects of PCBs on acid-sensitive neurons of the medulla oblongata, which regulate respiration. Therefore, the present study was designed to examine whether PCB alters acid-sensitivity of cultured neurons derived from the rat medulla oblongata. When extracellular pH was shifted from 7.4 to 7.0, acid-sensitive neurons showed depolarization, which was measured by voltage-sensitive fluorescent dye. Exposure to PCB (Aroclor 1254) decreased the amplitude of depolarization in low pH and increased the resting membrane potential in a dose-dependent manner. Taken together, our results indicate that PCB potentially influences acid-sensitivity through alteration of the membrane potential of acid-sensitive neurons, which could affect the regulation of respiration.

Potentiation of spinal N-methyl-D-aspartate-mediated nociceptive transmission by cocaine-regulated and amphetamine-regulated transcript peptide in rats

The present study examined the effects of cocaine-regulated and amphetamine-regulated transcript peptide (CARTp) fragment 55-102, on N-methyl-D-aspartate (NMDA)-mediated nociceptive transmission in vivo and in vitro. In-vivo experiments were conducted in Sprague-Dawley rats to evaluate the effects of CARTp on thermal hyperalgesia induced by NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Intrathecal NMDA (1, 2, 4 nmol) or AMPA (0.5, 1, 2 nmol) dose-dependently decreased the tail-flick latency. Intrathecal CARTp was without effect on the tail-flick latency. Interestingly, it significantly enhanced NMDA-induced, but not AMPA-induced, nociceptive effects. The in-vitro effects of CARTp on NMDA-induced or AMPA-induced depolarizations in substantia gelatinosa neurons were studied in rat spinal cord slices. CARTp (100, 300 nM), which caused no significant change of membrane potentials, increased the amplitude of NMDA-induced depolarizations in substantia gelatinosa neurons with little effect on AMPA-induced depolarizations. The present study demonstrates that exogenously applied CARTp selectively facilitates NMDA receptor-mediated nociceptive transmission.

Effects of inhibitors of nonselective cation channels on the acetylcholine-induced depolarization of circular smooth muscle from the guinea-pig stomach antrum

In circular smooth muscle bundles isolated from the guinea-pig stomach antrum, the effects of quinidine, Ni2+, flufenamic acid, niflumic acid, La3+, SKF-96365 and 4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) on acetylcholine (ACh)-induced depolarization were investigated. Recording membrane potentials from smooth muscle cells with intracellular microelectrodes revealed that ACh (1 microM) depolarized the membrane by 5-8 mV and increased the amplitude and frequency of slow potentials. These effects were inhibited by atropine. Quinidine (10 microM) increased the amplitude of ACh-induced depolarization, with no alteration to the properties of slow potentials. Ni2+ (50 microM) transiently (5-10 min) depolarized the membrane by about 5 mV, with an associated increase in frequency and amplitude of slow potentials. In the stabilized condition with Ni2+, the amplitude of ACh-induced depolarization remained unchanged. Flufenamic acid (10 microM) inhibited the generation of slow potentials, with no change in either the amplitude of ACh-induced depolarization or of the amplitude and frequency of slow potentials generated during ACh stimulation. A high concentration of flufenamic acid (100 microM) depolarized the membrane and increased the amplitude of ACh-induced depolarization. Niflumic acid (10 microM) hyperpolarized the membrane and increased the amplitude and frequency of slow potentials and also the amplitude of ACh-induced depolarization. DIDS (100 microM) hyperpolarized the membrane and inhibited the amplitude and frequency of slow potentials, with no alteration to the amplitude of ACh-induced depolarization. SKF-96365 (3-50 microM) depolarized the membrane in a concentration-dependent manner, but did not change the level of ACh-induced depolarization. La3+ (50 microM) did not alter the properties of the slow potentials or the ACh-induced responses. These results provide evidence that ACh-induced depolarization is not inhibited by chemicals known to inhibit non-selective cation channels. We suggest that muscarinic receptor-mediated signal transduction may be different in smooth muscle and interstitial cells.

Modulatory action of metabotropic glutamate receptor (mGluR) 5 on mGluR1 function in striatal cholinergic interneurons

Within basal ganglia, group I metabotropic glutamate receptor subtypes (mGluR1 and 5) frequently co-localize in the same neuron. However, little is known about how these receptors functionally interact. We addressed this issue by means of electrophysiological recordings of striatal cholinergic interneurons, a neuronal subtype that co-express both group I mGluRs. The group I non-selective agonist 3,5-DHPG induced a membrane depolarization/inward current that was prevented by co-application of LY 367385, a selective mGluR1 antagonist, and SIB 1757 or MPEP, blockers of mGluR5 subtype. The reversal potential for the response to 3,5-DHPG was close to the equilibrium potential for potassium channels. Repeated bath or focal applications of 3,5-DHPG induced a progressive decline in the amplitude of the membrane depolarization, suggesting that group I mGluRs undergo receptor desensitization. Interestingly, in the presence of the mGluR5 blocker, SIB 1757, this event was not observed, whereas it occurred in LY 367385. PKC blockers chelerythrine and calphostin C mimicked the inhibitory effect of SIB 1757. In a subset of interneurons, in MPEP or SIB 1757, 3,5-DHPG induced a 0.5–1 Hz oscillatory response, that was prevented by L-type Ca 2+ channel blockers, and by the tyrosine kinase inhibitors genistein and lavendustin. Together, these data suggest that mGluR5 modulates mGluR1 activity to shape cell excitability.

Electrotonic Coupling in the Anterior Pituitary of a Teleost Fish

The anterior pituitary of teleost fish contains a variety of endocrine cells, which, under control from the hypothalamus, release trophic hormones and thereby play a major role in reproduction, social behavior, and growth. In fish, hypothalamic fibers directly innervate the pituitary. The hypothalamic hormones released from these fibers bind to membrane receptors on pituitary cells, triggering action potentials, a rise in cytosolic calcium, and exocytosis. It is unclear whether these activities are confined to the stimulated cell or propagate to adjacent cells. We addressed this issue using whole cell and perforated patch-clamp techniques in a novel, hypothalamo-pituitary slice preparation from the tilapia fish (Oreochromis niloticus). Pituitary cells at rest generated occasional spontaneous spikes and sharp depolarizations of lower amplitude. The latter probably represented spikes in neighboring, electrotonically coupled cells. The presence of electrotonic communication, probably mediated by gap junctions, was also supported by the finding that Lucifer Yellow diffuses between cells. To quantify this connectivity, we performed simultaneous recording from pairs of adjacent cells. Thirty-three percent of the cells exhibited strong reciprocal coupling. Coupling coefficients ranged between 0.18 and 0.31, and coupling resistances ranged between 16 and 39 GOhm. The electrical junctions were effective low pass filters, attenuating action potentials much more than low frequency waveforms. We conclude that electrical activities of anterior pituitary cells in teleost fish are synchronized by coupling through gap junctions. Regulation of this coupling may play a critical role in determining complex patterns of pituitary hormone secretion.

Origin and Spread of Periinfarct Depolarization in Rats Detected by ECoG and MEG

The knowledge about the origin and the spatio-temporal pattern of propagation of spreading depolarization (SD) after focal ischemic brain infarction is limited. Using the information of the simultaneously recorded ECoG and MEG it seems possible to describe the localization of underlying neurophysiological processes. We investigated in 8 rats 32 periinfarct depolarizations by simultaneously recorded ECoG and MEG. The ECoG was recorded by a grid of 4×4 electrodes with a spatial distance of 1.25mm between adjacent electrodes. The MEG was recorded by a 16-channel Micro-SQUID system built at the Biomagnetic Center (1st order asymmetric gradiometers, 6.7mm pick-up-coil diameter, 30mm baselength, covering an area of 3.2×3.2cm2). Infarction was initiated via occlusion of the right middle cerebral artery. From the ECoG the moment and location of the first detected negative deflection, the sequence of the involvement of cortical regions and the spatial distribution of the amplitude of depolarization were determined. From MEG the temporal pattern of main intracortical current was estimated. The frequency of SD was 5/hr (=12±6min). Negative deflection of electric potential could be detected at first over rostro-medial regions. The location of the maximal depolarization amplitude varied intra- and interindividually. In most cases SD waves propagated from rostro-medial to caudal regions. In 26 of 32 electric SD, magnetic field changes were detected. Often (n=12) long-lasting magnetic field changes started before electric changes. The mean duration of these changes varied considerably (205±163s). We suppose that the origin of periinfarct depolarization is frequently located in frontal cortical regions and that subcortical depolarization may contribute to the MEG signal.

A cholinergic synaptically triggered event participates in the generation of persistent activity necessary for eye fixation.

An exciting topic regarding integrative properties of the nervous system is how transient motor commands or brief sensory stimuli are able to evoke persistent neuronal changes, mainly as a sustained, tonic action potential firing. A persisting firing seems to be necessary for postural maintenance after a previous movement. We have studied in vitro and in vivo the generation of the persistent neuronal activity responsible for eye fixation after spontaneous eye movements. Rat sagittal brainstem slices were used for the intracellular recording of prepositus hypoglossi (PH) neurons and their synaptic activation from nearby paramedian pontine reticular formation (PPRF) neurons. Single electrical pulses applied to the PPRF showed a monosynaptic glutamatergic projection on PH neurons, acting on AMPA-kainate receptors. Train stimulation of the PPRF area evoked a sustained depolarization of PH neurons exceeding (by hundreds of milliseconds) stimulus duration. Both duration and amplitude of this sustained depolarization were linearly related to train frequency. The train-evoked sustained depolarization was the result of interaction between glutamatergic excitatory burst neurons and cholinergic mesopontine reticular fibers projecting onto PH neurons, because it was prevented by slice superfusion with cholinergic antagonists and mimicked by cholinergic agonists. As expected, microinjections of cholinergic antagonists in the PH nucleus of alert behaving cats evoked a gaze-holding deficit consisting of a re-centering drift of the eye after each saccade. These findings suggest that a slow, cholinergic, synaptically triggered event participates in the generation of persistent activity characteristic of PH neurons carrying eye position signals.

Primary afferents: Experimental ulcers enhance P2x receptor function in rat gastric sensory neurons

Gastric ulceration evokes hyperexcitability in gastric sensory neurons and contributes, in part, to the development of gastric hyperalgesia. P2X receptors have been implicated to play a role in nociception. We hypothesized that gastric ulcers enhance P2X receptor-mediated responses in gastric sensory neurons. The rat stomach was surgically exposed and a retrograde tracer (DiI) was injected into multiple sites in the stomach wall. Ulcers were produced by a single injection (0.1ml; 60%) of acetic acid (HAc) into the gastric lumen. Saline injection (SAL) served as control. Nodose (NG) or dorsal root ganglion (DRG) cells were harvested and cultured 7 days later for whole-cell recordings. In current-clamp mode, the percentage of α,β-metATP- or ATP-responsive neurons was increased in DRG cells following gastric ulceration (SAL vs. HAc- metATP: 28.1 vs. 71.9 %; ATP: 40 vs. 63.6 %). α,β-metATP induced-depolarization amplitude was also increased after gastric ulceration (SAL vs. HAc- DRG: 15.9+4.5 vs. 25.8+2 mV; NG: 25.9+1.5 vs. 42.2+3 mV), with greater number of action potentials. Gastric ulceration also enhanced the ATP-evoked depolarization amplitude, to a similar degree. Almost all α,β-metATP-sensitive neurons were responsive to capsaicin after gastric ulceration. In voltage-clamp mode, currents triggered by application of α,β-metATP could be differentiated as fast or slow based on activation and desensitization kinetics. Both current components were inhibited by the P2X receptor blocker PPADS. Gastric ulceration increased the current density of both current components without altering the EC50 (SAL vs. HAc- DRG: Ifast- 3.9+0.4 vs. 8.7+1.9 pA/pF, Islow- 7.7+2.4 vs. 14.2+2 pA/pF; NG: Ifast- 18.9+5 vs. 72.9+9 pA/pF, Islow- 63.9+7.1 vs. 103.8+6.3 pA/pF). The slowly desensitizing current decayed significantly slower after gastric ulceration. These data demonstrate that gastric ulcers significantly sensitize peripheral afferent neurons by changing the expression and/or properties of voltage-, heat- and P2X receptors, which may contribute to the development of dyspeptic symptoms.

Sensitivity limits for voltage control of P2Y receptor-evoked Ca2+ mobilization in the rat megakaryocyte.

G-protein-coupled receptor signalling has been suggested to be voltage dependent in a number of cell types; however, the limits of sensitivity of this potentially important phenomenon are unknown. Using the non-excitable rat megakaryocyte as a model system, we now show that P2Y receptor-evoked Ca2+ mobilization is controlled by membrane voltage in a graded and bipolar manner without evidence for a discrete threshold potential. Throughout the range of potentials studied, the peak increase in intracellular Ca2+ concentration ([Ca2+]i) in response to depolarization was always larger than the maximal reduction in [Ca2+]i following an equivalent amplitude hyperpolarization. Significant [Ca2+]i increases were observed in response to small amplitude (< 5 mV, 5 s duration) or short duration (25 ms, 135 mV) depolarizations. Individual cardiac action potential waveforms were also able to repeatedly potentiate P2Y receptor-evoked Ca2+ release and the response to trains of normally paced stimuli fused to generate prolonged [Ca2+]i increases. Furthermore, elevation of the temperature to physiological levels (36 degrees C) resulted in a more sustained depolarization-evoked Ca2+ increase compared with more transient or oscillatory responses at 20-24 degrees C. The ability of signalling via a G-protein-coupled receptor to be potentiated by action potential waveforms and small amplitude depolarizations has broad implications in excitable and non-excitable tissues.

Intracardiac (Superior Vena Cava/Right Atrial) ECGs Using Saline Solution as the Conductive Medium for the Proper Positioning of the Shiley Hemodialysis Catheter: Is It Not Time To Forgo the Postinsertion Chest Radiograph?

Hemodialysis (HD) is often administered in critical care areas to patients with chronic renal failure as a continuation of the HD they are receiving on an ambulatory basis, and to patients who develop such a need for the first time or may require HD only transiently. The double-lumen Shiley central venous catheter (SCVC), inserted via the brachiocephalic veins, is often employed for HD, and it is customary to obtain a chest radiograph to ensure proper positioning of the tip of the SCVC within the superior vena cava (SVC) or high right atrium (RA). This practice is implemented to evaluate for complications stemming from the insertion of the SCVC and subsequent mishaps due to low positioning of the tip of the catheter in the RA or right ventricle. Intracardiac ECGs obtained via a saline solution-filled SCVC as the conductive medium can be easily recorded serially and periprocedurally to ensure proper positioning of the tip of the SCVC in the SVC or high RA based on the evaluation of the appearance and amplitude of atrial depolarization, thus rendering chest radiographs redundant.

Activity of the basolateral K+ channels is coupled to the Na+-K+-ATPase in the cortical collecting duct.

Microelectrode and patch-clamp techniques were used in the isolated cortical collecting duct to study the effects of stimulating Na+-K+-ATPase by raising bath K+ (Fujii Y and Katz AI. Am J Physiol Renal Fluid Electrolyte Physiol 257: F595-F601, 1989 and Muto S, Asano Y, Seldin D, and Giebisch. Am J Physiol Renal Physiol 276: F143-F158, 1999) on the transepithelial (VT) and basolateral membrane (VB) voltages and basolateral K+ channel activity. Increasing bath K+ from 2.5 to 8.5 mM resulted in an initial hyperpolarization of both VT and VB followed by a delayed depolarization. The effects of raising bath K+ on VT and VB were attenuated by decreasing luminal Na+ from 146.8 to 14.0 mM and were abolished by removal of luminal Na+, whereas those were magnified in desoxycorticosterone acetate (DOCA)-treated rabbits. Increasing bath K+ also led to a significant reduction of the intracellular Na+ and Ca2+ concentrations. The transepithelial conductance (GT) or fractional apical membrane resistance (fRA) were unaltered during the initial hyperpolarization phase, whereas, in the late depolarization phase, there were an increase in GT and a decrease in fRA, both of which were attenuated in the presence of low luminal Na+ (14.0 mM). In tubules from DOCA-treated animals, bath Ba2+ not only caused a significantly larger initial hyperpolarization of VT and VB but also blunted the late depolarization by high bath K+. Nomega-nitro-l-arginine methyl ester (l-NAME) partially mimicked the effect of Ba2+ and decreased the amplitude of the late depolarization. Patch-clamp experiments showed that raising bath K+ from 2.5 to 8.5 mM resulted in an increased activity of the basolateral K+ channel, which was absent in the presence of l-NAME. We conclude that stimulation of Na+-K+-ATPase increases the basolateral K+ conductance and that this effect involves suppression of nitric oxide-dependent inhibition of K+ channels.

Polysynaptic slow depolarization and spiking activity elicited after induction of long-term potentiation in rat auditory cortex

Polysynaptic activity was recorded in supragranular pyramidal neurons before and after the induction of long-term potentiation (LTP) in slices obtained from rat auditory cortex. LTP was induced by tetanic stimulation of layer IV. In the pyramidal neurons exhibiting LTP, repetitive stimulation at 50 Hz with 15 pulses triggered a slow 15–35 mV depolarization lasting 0.5–2 s with two to five spike discharges. There was no such response before the induction of LTP or in the neurons that did not exhibit LTP. Slow depolarization with spike discharges was blocked by an NMDA receptor antagonist but not by a metabotropic glutamate receptor antagonist. The reversal potential of the slow depolarization was approximately −7 mV and the membrane resistance decreased during slow depolarization, suggesting that the slow depolarization was produced by polysynaptic excitatory post-synaptic potentials. LTP was also induced by low frequency stimulation paired with a depolarizing current injection. In the pyramidal neurons exhibiting LTP after the paired stimulation, the slow depolarization amplitude was small and repetitive stimulation did not trigger spike discharges. Tetanic stimulation is expected to induce LTP in the polysynaptic neural circuits connecting many pyramidal neurons. The present findings suggest that polysynaptic activity can be generated in the potentiated neural circuits. Such activity might serve to read out the memory stored in polysynaptic neural circuits in the cerebral cortex.

Obligatory role of diastolic voltage oscillations in sino-atrial node discharge

The role of diastolic voltage oscillations in the initiation and maintenance of pacemaker discharge was studied in guinea pig-isolated sino-atrial (SA) node by means of a microelecrode technique. When [K +] o is suitably increased, the maximum diastolic potential decreases and all action potentials (APs) assume the characteristics of dominant pacemakers (slow responses with U-shaped diastolic depolarization). Subsequently, as the slope and amplitude of diastolic depolarization (DD) decreases, the threshold is missed, unmasking the fused oscillatory potentials V os and ThV os. As high [K +] o perfusion continues, the oscillatory potentials become separated, V os following the AP and ThV os appearing later on, when DD enters a less negative voltage range (oscillatory zone). ThV os grow in amplitude and attain the threshold, thereby insuring a slow discharge. If [K +] o is further increased, the smaller ThV os miss the threshold and SA node becomes quiescent. On reducing high [K +] o, ThV os re-appear, increase in size and initiate spontaneous discharge. As they occur progressively earlier during DD, ThV os eventually fuse with V os: at that stage, DD appears to continue directly into the upstroke (U-shaped DD) and the oscillations are no longer seen. During recovery in Tyrode solution, size and slope of V os and of ThV os further increase and cause a faster discharge. When APs assume a subsidiary configuration, their DD (no longer U-shaped) abruptly terminates into the upstroke. In high [K +] o, increasing [Ca 2+] o or applying a fast drive increase the size and slope of V os and of ThV os, which in turn restore or accelerate discharge. In contrast, low [Ca 2+] o abolishes V os and ThV os and causes SA node arrest. Low [Ni 2+] (35.5 μM) increases the rate whereas high [Ni 2+] (0.73 mM) stops the SA node. Ryanodine eliminates V os and ThV os and markedly slows or stops discharge. Thus, ThV os and V os are separate voltage oscillations that play an obligatory role in the initiation and maintenance of SA node discharge, V os by steepening early DD and ThV os by attaining the threshold in the dominant pacemaker range, either by gradually increasing during late DD at slow rates or by fusing with V os at fast rates. Both V os and ThV os are Ca 2+ dependent, but apparently in different ways.

Subthreshold membrane potential oscillation mediates the excitatory effect of norepinephrine in chronically compressed dorsal root ganglion neurons in the rat

Injured dorsal root ganglion (DRG) neurons often develop adrenergic sensitivity. To investigate the mechanisms of this phenomenon, the effects of norepinephrine (NE) on membrane potential of large- and medium-sized A-type neurons from chronically compressed DRG were recorded electrophysiologically in vitro. NE induced a depolarization in both control (26/36) and injured (56/62) neurons, whereas the incidence and amplitude of NE-induced depolarization in the injured neurons were significantly higher than that in controls. Following NE-induced depolarization, a subthreshold membrane potential oscillation (SMPO) was triggered or enhanced that initiated or increased repetitive firing in a fraction of injured neurons (15/56). After the SMPO was selectively abolished by application of tetrodotoxin (TTX), NE-induced depolarization failed to produce repetitive firing, even with a greater depolarization. Application of Rp-cAMPS (500 μM), a selective inhibitor of protein kinase A (PKA), decreased both SMPO and repetitive firing evoked by NE application or by intracellular current injection. Conversely, Sp-cAMPS (500 μM), a PKA activator, had a facilitating effect on both the SMPO and the repetitive firing. These results strongly suggest that a PKA mediated triggering and enhancement of SMPO may be responsible for the excitatory effects of NE on sensory neurons in neuropathic rats.

Effects of pethidine on cardiac electrophysiological properties

To investigate the effects of pethidine on electrophysiological properties of the isolated ventricular myocytes and the underlying mechanism. Langendorff was applied to perfuse rat heart model and whole-cell current clamp and voltage clamp techniques were used. Pethidine decreased heart rate (HR) in a concentration dependent manner and caused severe atrioventricular block (AVB) at >or=250 micromol/L. Pethidine reduced action potential amplitude and maximal rate of depolarization, prolonged action potential duration. Pethidine at 100 micromol/L decreased sodium currents (I(Na)), transient outward potassium currents (I(to)), delayed rectifier potassium currents (I(k)) and L-type calcium currents (I(Ca.L)) to (60.7+/-6.9)%, (55.4+/-5.6)%, (65.1+/-8.0)% and (67.4+/-10.1)% of control levels,respectively. These effects could be recovered by washout. Naloxone, an opioid receptor antagonist, could not abolish the effects of pethidine on ionic currents. Pethidine decreased HR and induced AVB, which may be related to the inhibition of I(Na), I(to), I(k) and I(Ca-L) of heart. The depression of cardiac currents is not mediated by opioid receptor.

5-HT decreases contractile and electrical activities in lymphatic vessels of the guinea-pig mesentery: role of 5-HT 7-receptors.

1 Constriction measurements and intracellular microelectrode recordings were performed in vitro on lymphatic vessels isolated from the guinea-pig mesentery to investigate whether 5-hydroxytryptamine (5-HT) affected lymphatic pumping and smooth muscle membrane potential. 2 5-HT decreased in a concentration-dependent manner the frequency of constrictions induced by intraluminal vessel perfusion. In nonperfused vessels, 5-HT hyperpolarized the lymphatic smooth muscle membrane potential and decreased the frequency and amplitude of spontaneous transient depolarizations (STDs). 3 The actions of 5-HT were significantly reversed by the 5-HT(7) receptor antagonist (2R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrrolidine (SB269970, 0.5 micro M) and by the 5-HT(1/2/5/7) receptor antagonists methysergide (0.5 micro M), and were mimicked by the 5-HT(1/7)-receptor agonist, 5-CT. 4 The 5-HT(4)-receptor antagonists 1-methyl-1H-indole-3-carboxylic acid [1-2-[(methyl sulfonyl) amino] ethyl-4-piperidinyl] methyl ester (GR113808, 1 micro M) and (1-piperidinyl) ethyl 1H-indole 3-carboxylate (SB203186, 1 micro M) did not significantly affect the 5-HT-induced responses. The 5-HT(4)-receptor agonist 1-(4-amino-5-chloro-2-methoxy-phenyl)-3-[1-(2-methylsulfonylamino) ethyl-4-piperidinyl]-1-propanone hydrochloride (RS67506) decreased the constriction frequency, albeit only at 50 micro M and without affecting the smooth muscle membrane potential. 5 Responses to 5-HT were attenuated by the nitric oxide synthase inhibitor N(G)-nitro L-arginine (100 micro M), whereas indomethacin (10 micro M) and tetrodotoxin (1 micro M) were without effects. 6 5-HT-induced responses were inhibited by the ATP-sensitive K(+) channel blocker, glibenclamide (10 micro M) and the cAMP-dependent protein kinase inhibitor N-[2-(p-bromociannamylamino)-ethyl]-5-isoquinolinesulfonamide-dichloride (H89, 10 micro M) blocked the hyperpolarization. 7 These results suggest that 5-HT modulates the rate of lymphatic vessel pumping by eliciting K(ATP) channel-mediated smooth muscle hyperpolarization and decrease in STD activity, which appear to be mediated by activation of 5-HT(7) receptors coupled to cAMP production.