Biomedical subjects

A J Gibb

Publications and source records attributed to A J Gibb.

26 records · Page 2Linked to original sources

PubMed1988

The sodium current underlying action potentials in guinea pig hippocampal CA1 neurons.

Neurons were acutely dissociated from the CA1 region of hippocampal slices from guinea pigs. Whole-cell recording techniques were used to record and control membrane potential. When the electrode contained KF, the average resting potential was about -40 mV and action potentials in cells at -80 mV (current-clamped) had an amplitude greater than 100 mV. Cells were voltage-clamped at 22-24 degrees C with electrodes containing CsF. Inward currents generated with depolarizing voltage pulses reversed close to the sodium equilibrium potential and could be completely blocked with tetrodotoxin (1 microM). The amplitude of these sodium currents was maximal at about -20 mV and the amplitude of the tail currents was linear with potential, which indicates that the channels were ohmic. The sodium conductance increased with depolarization in a range from -60 to 0 mV with an average half-maximum at about -40 mV. The decay of the currents was not exponential at potentials more positive than -20 mV. The time to peak and half-decay time of the currents varied with potential and temperature. Half of the channels were inactivated at a potential of -75 mV and inactivation was essentially complete at -40 to -30 mV. Recovery from inactivation was not exponential and the rate varied with potential. At lower temperatures, the amplitude of sodium currents decreased, their time course became longer, and half-maximal inactivation shifted to more negative potentials. In a small fraction of cells studied, sodium currents were much more rapid but the voltage dependence of activation and inactivation was very similar.

Action Potentials↗

PubMed1988

Potassium current activated by depolarization of dissociated neurons from adult guinea pig hippocampus.

Currents were generated by depolarizing pulses in voltage-clamped, dissociated neurons from the CA1 region of adult guinea pig hippocampus in solutions containing 1 microm tetrodotoxin. When the extracellular potassium concentration was 100 mM, the currents reversed at -8.1 +/- 1.6 mV (n = 5), close to the calculated potassium equilibrium potential of -7 mV. The currents were depressed by 30 mM tetraethylammonium in the extracellular solution but were unaffected by 4-aminopyridine at concentrations of 0.5 or 1 mM. It was concluded that the currents were depolarization-activated potassium currents. Instantaneous current-voltage curves were nonlinear but could be fitted by a Goldman-Hodgkin-Katz equation with PNa/PK = 0.04. Conductance-voltage curves could be described by a Boltzmann-type equation: the average maximum conductance was 65.2 +/- 15.7 nS (n = 9) and the potential at which gK was half-maximal was -4.8 +/- 3.9 mV (mean +/- 1 SEM, n = 10). The relationship between the null potential and the extracellular potassium concentration was nonlinear and could be fitted by a Goldman-Hodgkin-Katz equation with PNa/PK = 0.04. The rising phase of potassium currents and the decay of tail currents could be fitted with exponentials with single time constants that varied with membrane potential. Potassium currents inactivated to a steady level with a time constant of approximately 450 ms that did not vary with potential. The currents were depressed by substituting cobalt or cadmium for extracellular calcium but similar effects were not obtained by substituting magnesium for calcium.

4-Aminopyridine↗

PubMed1987

A preparation for patch clamp studies of labelled, identified neurones from guinea pig spinal cord.

Details are described of techniques which allow the isolation of labelled, identified neurones suitable for patch clamp recording from the guinea pig spinal cord. Fluorescent labels, injected into either the hindlimb muscles or the cerebellum, are retrogradely transported to motoneurones or dorsal spinocerebellar tract neurones respectively. Cells are then enzymatically dissociated from spinal cord slices and identified using fluorescence microscopy. Patch clamp or whole cell recordings are then made.

Afferent Pathways↗

PubMed1987

Examination of the mechanisms involved in tetanic fade produced by vecuronium and related analogues in the rat diaphragm.

The effects of vecuronium (Org NC45), Org 7678 and Org 7684 were examined on twitches and tetani recorded from rat isolated diaphragms. Org 7678 and Org 7684 exhibited approximately one tenth of the neuromuscular blocking potency of vecuronium. At concentrations producing equivalent amounts of twitch block, Org 7684 produced significantly less tetanic fade than did vecuronium or Org 7678. In cut muscles both vecuronium and Org 7684 reduced the endplate current (e.p.c.) amplitude (Ie.p.c.), reduced e.p.c. decay time constant (tau e.p.c.), and increased the e.p.c. train rundown. The effects of vecuronium were not voltage-dependent and vecuronium did not change tau noise. The effect of Org 7684 on Ie.p.c. and tau e.p.c. became greater with hyperpolarization, but the effect on e.p.c. train rundown was not voltage-dependent. It is concluded that both vecuronium and Org 7684 produce e.p.c. train rundown and tetanic fade by a prejunctional mechanism. However, whereas postjunctionally vecuronium blocks only the acetylcholine receptor, Org 7684 blocks both the receptor and its associated ion channel.

Animals↗

PubMed1986

Nicotinic antagonists produce differing amounts of tetanic fade in the isolated diaphragm of the rat.

The effects of nicotine antagonists on single twitches, trains of four twitches and tetanic contractions of the isolated diaphragm of the rat were examined. Different drugs were found to produce different amounts of tetanic fade relative to depression of twitch tension. The order of activity from most able, to least able to produce fade was: hexamethonium greater than trimetaphan=atracurium=tubocurarine greater than pancuronium greater than erabutoxin b. The effect of erabutoxin b was distinctive for its almost complete lack of tetanic fade. 3,4-Diaminopyridine increased tetanic fade produced by tubocurarine, atracurium and hexamethonium, but not that produced by erabutoxin b. It is concluded that nicotinic antagonists act at more than one site at the neuromuscular junction. Assuming block of the postjunctional acetylcholine receptor produces tension depression, a second or third site must be involved in producing tetanic fade. The possibility that tetanic fade results from block of the ion channel associated with the postjunctional acetylcholine receptor or from the block of a prejunctional nicotinic receptor is discussed.

4-Aminopyridine↗

PubMed1984

Pre-and post-junctional effects of tubocurarine and other nicotinic antagonists during repetitive stimulation in the rat.

The effects of tubocurarine and trimetaphan have been examined at voltage-clamped rat diaphragm neuromuscular junctions during (a) single and repetitive stimulation of the phrenic nerve in cut muscles and (b) repetitive ionophoretic application of acetylcholine (ACh). Tubocurarine (2.5 X 10(-7)-10(-6)M) produced a concentration-dependent reduction in the amplitude of neurally evoked end-plate currents (e.p.c.s). It also reduced their time constant of decay (tau e.p.c.) in a manner that was independent of membrane potential, and not markedly dependent on the tubocurarine concentration. Likewise the snake alpha-neurotoxin, erabutoxin b, reduced the e.p.c. amplitude and produced a voltage-independent shortening of tau e.p.c. Estimates of mean channel lifetime (tau noise) from ACh-induced e.p.c. fluctuations revealed that (a) tau noise was 46.4 +/- 3.7% shorter than tau e.p.c. measured at the same end-plate. At these same end-plates in the presence of tubocurarine (2.5 X 10(-7)M) tau e.p.c. was 32.6 +/- 1.0% shorter than the control tau e.p.c. but tubocurarine did not change tau noise, (b) trimetaphan (2.5 X 10(-5)-2 X 10(-4)M) produced a concentration-dependent and voltage-dependent reduction of tau e.p.c., and a concentration-dependent reduction of peak e.p.c. amplitude. Trimetaphan (2.5 X 10(-5)M) produced a 50% reduction of tau noise. (a) Both tubocurarine and trimetaphan produced concentration-dependent increases in the run-down of trains of neurally evoked e.p.c.s (50 Hz, 0.4 s). This effect did not vary with membrane potential in tubocurarine, but was voltage dependent when induced by trimetaphan. (b) Erabutoxin b reduced the e.p.c. amplitude but did not produce any increase in the run-down of trains of neurally evoked e.p.c.s. During 50 Hz repetitive ionophoretic application of ACh, tubocurarine (2.5 X 10(-7)M) reduced the amplitude of each current in the train without inducing any run-down of the current amplitudes. This effect was not dependent on the membrane potential. In contrast trimetaphan (2.5 X 10(-5)M) induced a voltage-dependent run-down of trains of ionophoretically evoked e.p.c.s. We conclude that tubocurarine and erabutoxin b reduce the e.p.c. amplitude by blocking the post-junctional ACh receptor. Tubocurarine produces tetanic rundown of e.p.c.s. by a prejunctional mechanism, whereas the effects of trimetaphan during single and repetitive stimulation are at least partly due to block of the open ion channel associated with the ACh receptor.

Acetylcholine↗

PubMed1983

Neuromuscular blocking and autonomic effects of vecuronium and atracurium in the anaesthetized cat.

The effects of vecuronium and atracurium on neuromuscular transmission, on the responses of the heart rate to vagal stimulation and on the responses to preganglionic stimulation of the nictitating membrane were compared in the chloralose-anaesthetized cat. Vecuronium was four times more potent than atracurium as a neuromuscular blocking agent, whereas the two compounds had similar potencies in blocking the effects of stimulation of the cardiac vagus. The vagal/neuromuscular ratios measured at 50% inhibition were 96 for vecuronium and 25 for atracurium. Vecuronium possessed a slightly shorter recovery time than atracurium and shorter duration of action on the soleus muscle. The onset times of the two compounds were not significantly different. Both compounds had longer time-courses of action than suxamethonium. Very large doses of vecuronium decreased the responses of the preganglionic stimulation of the nictitating membrane, suggesting that at high doses the compound possesses ganglion blocking activity. Large doses of atracurium also decrease the nictitating membrane responses and, in some cats, contractions of the nictitating membrane associated with increases in heart rate and arterial pressure were observed.

Anesthesia, General↗

PubMed1983

Neuromuscular and vagal blocking actions of pancuronium bromide, its metabolites, and vecuronium bromide (Org NC45) and its potential metabolites in the anaesthetized cat.

The neuromuscular and cardiac vagus blocking actions of pancuronium, vecuronium (Org NC45) and their respective potential hydroxy metabolites have been studied in the chloralose-anaesthetized cat. Pancuronium was three times more potent as a neuromuscular blocker than its 3-hydroxy derivative, 20 times more potent than the 17-hydroxy derivative and 45 times more potent than the 3,17-dihydroxy derivative. The vagal:neuromuscular block ratios measured at 50% inhibition for these compounds were pancuronium 3.0, 3-hydroxy derivative 6.4, 17-hydroxy derivative 1.1 and 3,17-dihydroxy derivative 0.36 (a value greater than unity indicated greater potency at the neuromuscular junction). Vecuronium was 1.4 times more potent than its 3-hydroxy derivative, 24 times more potent than the 17-hydroxy derivative and 72 times more potent than the 3,17-dihydroxy derivative as a neuromuscular blocker. The vagal:neuromuscular block ratios were vecuronium 79.8, 3-hydroxy derivative 40.4, 17-hydroxy derivative 0.85 and 3,17-dihydroxy derivative 0.15. The 3-hydroxy derivative of vecuronium, the most likely first metabolite of vecuronium, thus possessed only slightly less neuromuscular blocking potency than vecuronium, coupled with a high safety margin between neuromuscular and vagal blocking doses. In addition, the time-course of its action was not different from that of vecuronium. Thus, it is concluded that this potential metabolite is unlikely to give rise to tachycardia in man. It is unlikely that the 17-hydroxy and 3,17-dihydroxy derivatives of vecuronium would be produced in sufficiently great quantities by metabolism from vecuronium to result in either tachycardia or residual neuromuscular blockade.

Animals↗