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PubMed1989

Carbachol inhibits atrial contractility in the presence of potassium channel blocking agents.

To elucidate the role of potassium channel activation in muscarinic inhibition of atrial contractility, we studied the influence of K+ channel blockers on the effects of the muscarinic agonist carbachol in isolated guinea pig auricles. We tested BaCl2, tetraethylammoniumchloride (TEA), and 9-aminotetrahydroacridine (THA), which block K+ channels, for their ability to antagonize the effects of carbachol on atrial contractility and functional refractory period. Due to inhibition of K+ outward currents, BaCl2, TEA, and THA markedly blocked the carbachol-induced shortening of refractory period and, to a lesser extent, antagonized its negative inotropic action. BaCl2, TEA, and THA shifted the concentration-response curve of the negative inotropic action of carbachol to the right; the most pronounced effect was obtained with TEA (DR = 30 at 3 x 10(-3) M). The maximum negative inotropic effect of carbachol, however, was only slightly reduced by the K+ channel blockers, and carbachol clearly inhibited atrial contractility even in the absence of any shortening of refractory period. These results suggest the existence of an additional cholinergic, negative inotropic mechanism, distinctly different from activation of atrial K+ channels.

Animals↗
PubMed1987

Variation of membrane properties in hair cells isolated from the turtle cochlea.

1. Hair cells were enzymatically isolated from identified regions of the turtle basilar papilla and studied with the patch-electrode technique. The experimental aim was to relate the resonance properties seen during current injection to the membrane currents measured in the same cell under whole-cell voltage clamp. 2. Solitary hair cells had resting potentials of about -50 mV, and produced a damped oscillation in membrane potential at the onset and termination of a small current step; the resonant frequency varied from 9 to 350 Hz between cells, and was correlated with the region of papilla from which a cell had been isolated. The inferred frequency map was consistent with the tonotopic arrangement described previously in the intact papilla. 3. Depolarizations from the resting potential under voltage clamp activated a large net outward current with a steep voltage dependence, and the steady-state current-voltage relationship was strongly rectified about the resting potential. Input resistances tended to be smaller in cells with higher resonant frequencies, although there was no concurrent variation in membrane area as inferred from the cell capacitance. 4. The kinetics of the outward current evoked by a small depolarizing step depended upon the resonant frequency, fo, of the hair cell, and were slower in low-frequency cells. On repolarization to the resting potential the current decayed exponentially with a time constant that changed from 150 ms in the lowest-frequency cell to less than 1 ms in the highest-frequency one. The time constant was approximately proportional to 1/f0(2). 5. Following repolarization to different membrane potentials, the tail current was found to reverse around -80 mV, indicating that the outward current was due mainly to K+. 6. The outward current was abolished by extracellular application of 25 mM-tetraethylammonium chloride (TEA), or on exchange of Cs+ for K+ in the intracellular medium filling the recording electrode, each experiment supporting the contention that K+ is the major current carrier. Such treatments also removed the oscillations in membrane potential evoked by imposed current steps. 7. Addition of TEA or intracellular perfusion with Cs+ also revealed a fast inward current with an ionic sensitivity consistent with its being carried by Ca2+. Like the K+ current, the Ca2+ current was activated by small depolarization from the resting potential, and over this voltage range it was about five to ten times smaller than the K+ current. Its activation was more rapid than the fastest outward currents in high-frequency cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1988

Use-dependent reduction and facilitation of Ca2+ current in guinea-pig myocytes.

1. Action potentials, calcium currents (iCa) and cell contraction have been recorded from single guinea-pig myocytes during periods of stimulation from rest. Voltage clamp was carried out using a single microelectrode. Cell contraction was measured optically. All experiments were performed at 18-22 degrees C. 2. An inverse relationship was observed between cell contraction and action potential duration or iCa. Mixed trains of action potentials and voltage clamp pulses preserved this relationship. Long voltage clamp pulses induced negative 'staircases' of iCa and positive 'staircases' of cell contraction. A facilitation of iCa was observed during repetitive stimulation with clamp pulses of 100 ms duration or less and was accompanied by a decrease in cell contraction. 3. The voltage dependence of inward current staircases was found to depend on Ca2+ entry rather than membrane voltage for long voltage clamp pulses and was not affected by 30 mM-TEA or 50 microM-TTX. Current reduction was greatest at 0 mV (P less than 0.05) when iCa was largest. Changes in cell contraction during pulse trains showed a similar voltage dependence. The time constant of current staircases was only mildly voltage dependent. 4. Interference with normal cellular mechanisms for Ca2+ uptake and release by strontium, 1-5 mM-caffeine and 1 microM-ryanodine increased current staircases and could abolish iCa facilitation with short clamp pulses. 5. Variations in the level of Ca2+-dependent inactivation of iCa can explain many features of the changes in iCa during stimulation after rest. Long clamp pulses (or action potentials) may increase cell Ca2+ loading and inhibit iCa. Short clamp pulses reduce available Ca2+ for cell contraction and this may reflect a lowered myoplasmic Ca2+ level which allows facilitation of iCa.

Action Potentials↗
PubMed1989

Voltage-activated membrane currents in rat cerebellar granule neurones.

1. Voltage-activated currents have been recorded from cerebellar granule neurones in explant cultures from young rats (1-9 days old). Cells were examined with whole-cell patch-clamp methods. Depolarizing pulses from a pre-pulse potential of -100 mV evoked a rapidly activated transient inward current, and an outward current which decayed in two phases. The ionic dependence, kinetics and pharmacological properties of these currents have been studied. 2. Peak inward Na+ currents in cells from 7-day-old rats were in the range 350-450 pA. No evidence was found for the presence of calcium currents. Thus, inward current was unchanged in zero Ca2+, 1 mM-EGTA solution. No inward current was obtained in medium containing 10 mM-Ba2+ and tetrodotoxin (TTX). Supplementing the pipette (i.e. intracellular) solution with Mg-ATP did not reveal any Ca2+ current. 3. Depolarizing steps (from -100 mV) in TTX-containing solution gave an early transient outward current and a late outward current. The transient current resembled IA described in other cells, and reversed close to EK in both normal and elevated potassium concentrations, indicating that K+ is the predominant charge carrier. Depolarizing steps from -50 mV failed to give a transient outward current, and gave only a slowly rising current which resembled the late potassium current, IK. 4. Inactivation of the transient current was examined by applying test depolarizations from increasingly negative pre-pulse potentials (-50 to -120 mV): half-inactivation occurred at -72 mV. Transient outward currents decayed exponentially with time constants, tau, of 7.3-25.3 ms at 0 mV. The time course of removal of inactivation in cells held at -50 mV, and given increasingly long pre-pulses to -100 mV, was exponential with tau = 35 ms. 5. Both transient and late outward currents were reversibly abolished by addition to the bathing medium of 10 mM-Ba2+ or 1 mM-quinine. Outward K+ current was not dependent on external calcium. Tetraethylammonium (20 mM) selectively reduced the late outward current; the peak transient current was reduced by less than 20%. 4-Aminopyridine (2 mM) showed little selectivity between transient and late outward currents. 6. It is concluded that cerebellar granule cells from young rats possess voltage-activated inward Na+ current as well as two types of K+ current, IA and IK. In terms of neuronal functioning, the properties of the transient outward current may confer a role in regulating excitability and in repolarization, but a definitive statement will require knowledge of the cellular location and relative densities of channels in granule cells in vivo.

4-Aminopyridine↗
PubMed1984

Electrophysiology of dentate gyrus granule cells.

The orthodromic synaptic responses, membrane properties, and responses of dentate gyrus granule cells (DGCs) to several convulsant agents were studied in the in vitro hippocampal slice preparation. Orthodromic stimulation via the perforant pathway (PP) evoked excitatory-inhibitory postsynaptic potentials (EPSP-IPSP) sequences in 27 of 34 DGCs studied. In the majority, only one action potential could be evoked by supramaximal orthodromic stimulation. In recordings from DGC somata, overshooting spikes could be evoked either orthodromically or by current injections. Small-amplitude, fast transients were seen in 5 of 34 DGCs. The current/voltage (I-V) characteristic of most DGCs was linear throughout a range of membrane potentials between 15 and 20 mV negative and 5 and 15 mV positive to the resting potential. At the extremes of this range nonohmic behavior was noted. Exposure of slices to agents that block IPSPs, such as penicillin, bicuculline, picrotoxin, and media containing low Cl- concentrations, eliminated PP-evoked hyperpolarizations in DGCs and prolonged the repolarizing phase of the PP EPSP. In contrast to findings in hippocampal pyramidal cells and neocortical neurons, blockade of IPSPs did not lead to the development of orthodromically evoked slow depolarizations and burst discharges. After slices were exposed to 5 mM tetraethylammonium, current pulses evoked slow spikes, which were resistant to tetrodotoxin and presumably mediated by Ca2+. Spontaneous burst discharges or bursts evoked by brief depolarizing pulses did not occur under these conditions. Substitution of Ba2+ for Ca2+ in the perfusion solution resulted in development of spontaneous slow membrane depolarizations and burst discharges in DGCs. Burst discharges could be directly evoked and spikes were prolonged and resistant to tetrodotoxin (TTX). After hyperpolarizations lasting 200-1,000 ms, associated with a conductance increase and presumably due to a Ca2+-activated K+ conductance, followed directly evoked spike trains in 5 of 20 DGCs. These data suggest that Ca2+ conductances may be evoked in DGCs under certain circumstances but are not prominent during activation of DGCs under standard in vitro recording conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

5-HT modulation of hyperpolarization-activated inward current and calcium-dependent outward current in a crustacean motor neuron.

1. Serotonergic modulation of a hyperpolarization-activated inward current, Ih, and a calcium-dependent outward current, Io(Ca), was examined in the dorsal gastric (DG) motor neuron, with the use of intracellular recording techniques in an isolated preparation of the crab stomatogastric ganglion (STG). 2. Hyperpolarization of the membrane from rest with maintained current pulses resulted in a slow time-dependent relaxation back toward rest and a depolarizing overshoot after termination of the current pulse. In voltage clamp, hyperpolarizing commands negative to approximately -70 mV caused a slowly developing inward current, Ih, which showed no inactivation. Repolarization back to the holding potential of -50 mV revealed a slow inward tail current. 3. The reversal potential for Ih was approximately -35 mV. Raising extracellular K+ concentration ([K+]o) from 11 to 22 mM enhanced, whereas decreasing extracellular Na+ concentration ([Na+]o) reduced the amplitude of Ih. These results indicate that Ih in DG is carried by both K+ and Na+ ions. 4. Bath application of serotonin (5-HT; 10 microM) caused a marked increase in the amplitude of Ih through its active voltage ranges. 5. The time course of activation of Ih was well fitted by a single exponential function and strongly voltage dependent. 5-HT increased the rate of activation of Ih. 5-HT also slowed the rate of deactivation of the Ih tail on repolarization to -50 mV. 6. The activation curve for the conductance (Gh) underlying Ih was obtained by analyzing tail currents. 5-HT shifted the half activation for Gh from approximately -105 mV in control to -95 mV, resulting in an increase in the amplitude of Gh active at rest. 7. Two to 4 mM Cs+ abolished Ih, whereas barium (200 microM to 2 mM) had only weak suppressing effects on Ih. Concomitantly, Cs+ also blocked the 5-HT-induced inward current and conductance increase seen at voltages negative to rest. In current clamp, Cs+ caused DG to hyperpolarize 3-4 mV from rest, suggesting that Ih is partially active at rest and contributes to the resting membrane potential. 8. Depolarizing voltage commands from a holding potential of -50 mV resulted in a total outward current (Io) with an initial transient component and a sustained steady-state component. Application of 5-HT reduced both the transient and sustained components of Io. 9. Io was reduced by 10-20 mM tetraethylammonium (TEA), suggesting that it is primarily a K+ current.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1989

Presynaptic calcium currents and facilitation of serotonin release at synapses between cultured leech neurones.

1. The role of presynaptic Ca2+ entry in facilitation of transmitter release has been analysed by voltage-clamp measurements at synapses formed in culture by Retzius and P neurones isolated from the central nervous system (CNS) of the leech. The transmitter released by Retzius cells is serotonin. 2. Synaptic transmission persisted in solutions containing raised concentrations of divalent cations, reduced concentrations of Na+, and tetraethylammonium (TEA+) and 4-AP (to block K+ currents). Ca2+ and Sr2+ were more effective in promoting transmitter release than Ba2+, as assessed by the postsynaptic potentials in P cells. The degree and time course of facilitation in Ca2+- and Sr2+-containing solutions were similar to those observed for synapses bathed in normal L-15 medium. 3. Transmitter release depended upon the amplitude and the duration of presynaptic depolarization and inward Ca2+ current. Peak Ca2+ currents and postsynaptic potentials occurred with depolarizing steps to +15 mV. Frequent or prolonged pulses depressed the postsynaptic potentials. 4. Pairs of depolarizing pulses that caused facilitation were accompanied by identical inward Ca2+ currents. These results indicate that the mechanism responsible for facilitated serotonin release must occur following Ca2+ entry and that residual Ca2+ plays a role.

4-Aminopyridine↗
PubMed1992

Calcium and potassium currents in leg motoneurons during postembryonic development in the hawkmoth Manduca sexta.

During insect metamorphosis the nervous system is reorganized to accommodate changes in behavior. In the hawkmoth, Manduca sexta, many identified larval motoneurons persist to innervate new adult muscles, while undergoing changes in dendritic morphology and synaptic connections. The thoracic leg motoneurons, for example, innervate different sets of muscles in the larva and adult and participate in distinct types of locomotor behavior in the two stages of life. To determine whether changes in the biophysical properties of these motoneurons accompany the structural and functional modifications that have been described, we used the whole-cell voltage-clamp technique to compare the Ca2+ and K+ currents expressed by leg motoneurons isolated from the larval, pupal and adult stages of Manduca. After 24 h in culture, the somata of leg motoneurons isolated from all three stages expressed voltage-sensitive Ca2+ currents that could be blocked by Cd2+, Co2+ or Ni2+. The currents were larger with Ba2+ as the charge carrier. The Ca2+ current density was significantly lower in these motoneurons during the early pupal stage than in either the larva or adult. Similar experiments revealed both transient and sustained K+ currents in the leg motoneurons that could be blocked with Cs+. There was a significant decrease in the density of the transient, inactivating outward current in leg motoneurons isolated from the early pupal stage. Thus, the levels of some types of ionic currents are modulated during metamorphosis. These changes may be important for the developmental or behavioral changes that accompany metamorphosis.

Animals↗
PubMedDate not supplied

Effects of oral sodium picosulphate Picolax on urea and electrolytes.

AIM: To investigate the effects of sodium picosulphate (Picolax) oral bowel preparation on levels of serum urea, electrolytes and glucose in patients undergoing bowel preparation for barium enema. METHOD: A prospective, non-randomised, pre-test and post-test trial was conducted with 144 patients aged 34-87 years, who had agreed to undergo a barium enema. Changes in serum urea, sodium, potassium, magnesium and glucose following Picolax treatment were investigated. RESULTS: There were statistically significant post-Picolax reductions in serum concentrations of urea (mean difference 0.556 (95 per cent confidence intervals (CI) 0.321-0.791) mmol/L), sodium (mean difference 1.299 (0.799-1.799) mmol/L) and potassium (mean difference 0.163 (0.0853-0.241) mmol/L). Similar findings were observed for sodium and urea when the sample was stratified according to age: under 60 years (n=56) or 60 years and over (n=88). However, there was no significant change in potassium in patients aged under 60 years. CONCLUSION: In normal circumstances, use of Picolax oral bowel preparation results in a statistically, but not clinically, significant reduction in concentration of serum urea, sodium and potassium.

Administration, Oral↗
PubMedDate not supplied

[Effect of ions on determination of 3',5'-AMP by the method of saturation].

The effects of La2+, Pb2+, Ba2+, Zn2+, Ca2+, Mg2+, Mn2+, NH+4 in concentrations of 10(-7), 10(-5), 10(-3) M were studied as applied to the protein binding of cyclic 3' : 5' = AMP. It is shown that Ba2+, La2+, Pb2+ increase the binding, while Zn2+ has no effect, and Mn2+, NH+4 decrease the binding of 3' : 5' = AMP. The effects of Ca2+ and Cd2+ were dependent on the concentration. The maximal binding in the presence of Ca2+ and Cd2+ was observed at a concentration of 10(-5) M and 10(-3) M, respectively. It is suggested that Mg2+ and Ca2+ take part in the regulation of cyclic AMP binding with proteinkinase in vivo.

Barium↗
PubMed1989

Direct inhibition of inositol-1,4,5-trisphosphate-induced Ca2+ release from brain microsomes by K+ channel blockers.

Tetraethylammonium and 9-tetraethylammonium have previously been reported to inhibit inositol-1,4,5-trisphosphate (IP3)-induced Ca2+ release from brain microsomes, purportedly by blocking potassium channels [Biochem. J. 258:617-620 (1988)]. The effects of these and other K+ channel blockers have been studied here in greater detail using a spectrophotometric assay for Ca2+ movements into and out of canine brain microsomes. IP3-induced Ca2+ release was inhibited by substitution of K+ in the medium with nominally impermeant cations or by addition of most of the K+ channel blockers tested. Nevertheless, addition of valinomycin to the medium (to provide an alternative pathway for counter-ion K+ movements) failed to alleviate the inhibition of IP3-induced Ca2+ release caused by K+ channel blockers. To determine whether these substances act by inhibition of IP3 binding or by direct interaction with the Ca2+ channel of the internal store that promotes IP3-induced Ca2+ release, their effect on [3H]IP3 binding was investigated. None of the K+ channel blockers tested inhibited [3H]IP3 binding. Nearly all the K+ channel blockers appear to interact directly with a Ca2+ channel of the intracellular stores or perhaps interfere with its coupling to the IP3 receptor. Because of their multiplicity of actions, these substances cannot be presumed to be either selective K+ channel blockers or selective inhibitors of IP3-induced Ca2+ release from internal stores. Three of them were even found to partially inhibit valinomycin-stimulated 86Rb uptake into liposomes.

4-Aminopyridine↗
PubMed1995

Effect of endoscopic variceal sclerotherapy on esophageal motor functions and gastroesophageal reflux.

UNLABELLED: Sclerotherapy results in significant local complications, both immediate and delayed. This study was designed to examine the esophageal pathophysiology underlying these complications. METHODS: We prospectively evaluated esophageal transit, motility abnormalities and gastroesophageal reflux (GER) with barium studies and esophageal functional scintigraphy in 24 patients (20 men, 4 women; mean age 33 +/- 12.4 yr) before sclerotherapy (Phase I), after two sessions (Phase II), following variceal eradication (Phase III) and 4 wk later (Phase IV). RESULTS: Varices were obliterated after 5.6 +/- 1.9 sessions of intravariceal sclerotherapy performed weekly with 1% polidocanol (17.3 ml per session). There was no baseline Phase I dysmotility or reflux. Phase II studies recorded a marked delay of esophageal global and segmental (mid and distal) transit time in 98.2% of patients by scintigraphy and 90% by barium studies. Incoordinate contractions and aperistalsis were observed in 0, 66.7%, 58.3% and 33.8% of patients from Phases I-IV studies, respectively. Barium studies revealed tertiary waves and reverse peristalsis in 0, 50%, and 75% of patients from Phases I-III; strictures were observed in 0, 1, and 3 patients during Phases I-III. GER was detected scintigraphically in 0, 58.3%, 25% and 16.6% during Phases I-IV sequentially. In contrast, barium studies grossly underestimated GER (0, 5% and 15% at phases I-III). CONCLUSION: There was strong concordance between esophageal symptoms, transit, motility abnormalities and GER (p < 0.05). Variceal eradication (Phases III and IV) was associated with a gradual recovery of esophageal symptoms, ulcers and all abnormal scintigraphic parameters. Sclerosant-induced chemical esophagitis in association with peptic esophagitis due to gross reflux following sclerotherapy possibly can explain the symptoms in most patients.

Adult↗
PubMed1994

NH4+ transport pathways in cells of medullary thick ascending limb of rat kidney. NH4+ conductance and K+/NH4+(H+) antiport.

To characterize NH4+ transport in the renal medullary thick ascending limb (MTAL), cell pH was monitored with the use of 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein in suspensions of rat MTAL tubules in CO2-free media. Abrupt exposure to NH4Cl led to initial cell alkalinization (NH3 entry) followed by cell acidification due to NH4+ influx. The latter had 1 microM amiloride- and barium-sensitive components; the barium effect was unchanged when K+ conductances were completely blocked by quinidine, as assessed with the use of the cell membrane potential-sensitive fluorescent probe 3,3'-dipropylthiadicarbocyanine. NH4+ entry-induced depolarization was abolished by 1 microM amiloride, but it was unaffected by barium. In NH(4+)-free media, barium, verapamil, and raising of the extracellular K+ concentration alkalinized MTAL cells; imposing an outward directed K+ gradient in Na(+)-free medium induced cellular acidification, which was abolished by barium and verapamil but not by other K+ channel and Na+/H+ antiport inhibitors (quinidine and 2 mM amiloride). As measured with a K(+)-selective extracellular electrode, a component of K+ efflux (in presence of furosemide, ouabain, and quinidine) was stimulated by decreasing the extracellular pH from 7.4 to 7.0 and inhibited by barium and verapamil. It was also demonstrated that the K+/H+ antiporter transports NH4+ better than H+ at physiological NH4+ and H+ concentrations. These results demonstrate the presence in MTAL cells of two novel NH4+ transport pathways, amiloride-sensitive NH4+ conductance and barium- and verapamil-sensitive K+/NH4+(H+) antiport.

Amiloride↗
PubMedDate not supplied

A new preoperative immunochemotherapy for the treatment of locally advanced esophageal cancer.

A promising preoperative immunochemotherapy regimen for locally advanced esophageal cancer is herein described. A 67-year-old man suffering from severe dysphagia was diagnosed with unresectable esophageal cancer at initial examination because of a tumor of 11 cm in length and suspicion of trachea invasion. Neoadjuvant immunochemotherapy was undertaken for the down-staging. Interleukin-2 (IL-2) (3.5 x 10(5) Japan reference units), nedaplatin (7 mg/m2) and 5-FU (300 mg/m2) were administered intravenously daily for 5 days a week for three weeks. The gross findings of a barium esophagogram and esophagoscopy revealed significant tumor regression in both size and shape. The patient underwent an esophagectomy through a laparotomy followed by a right thoracotomy. The surgical specimens were serially sectioned and examined microscopically. All of the surgical margins were clear (upper and lower margins as well as the adventitia), and there was no evidence of lymph node metastasis. The surgical specimen revealed neoplastic squamous ghost cells surrounding significant lymphocyte infiltration. This appears to be a unique feature of this particular neoadjuvant immunochemotherapy.

Aged↗