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Biomedical subjects

W Ulbricht

Publications and source records attributed to W Ulbricht.

At least 37 records · Page 2Linked to original sources
PubMed1978

Rates of block by procaine and benzocaine and the procaine-benzocaine interaction at the node of Ranvier.

1. Action potentials and their maximum rates of rise, VA, were measured in single myelinated nerve fibres of the frog, Rana esculenta at room temperature. 2. On applying 1 mM procaine (pH 7.2) at 20 Hz stimulus frequency, half of the final VA reduction was reached at ton = 0.27 s; on applying 0.5 mM benzocaine (pH 7,2) at 50 HZ, ton was 0.12 s. Increasing the stimulus frequency between 2 and 50 HZ increased the rate of block by procaine but not by benzocaine. 3. Recovery in Ringer solution (pH 7.2) from 30-s treatment with 1 mM procaine (pH 7.2), the equieffective 0.15 mM procaine (pH 8.9) and from 0.5 mM benzocaine (pH 7.2) was 54%, 31% and 70%, respectively, within 1 s. 4. Changing between alkaline Ringer solution (pH 8.9) and 1 mM procaine (pH 7.2) led to transitory excessive block. Changing between 1 mM procaine (pH 7.2) and acid Ringer solution (pH 6.0) and washing out 10 mM procaine (pH 5.5) with neutral Ringer solution also led to a non-monotonic change in VA. 5. If hyperpolarizing pulses (30 ms, 20 mV) preceded the stimuli, changing the frequency of the pulse pairs led to a gradual moderate relief of block in procaine, turning off prepulses (at 10 HZ) to a gradual increase of block. In benzocaine changing from 1 to 10 HZ had no effect but turning off prepulses led to a prompt large increase of block. In procaine + benzocaine the membrane responded much as in benzocaine alone. At 1 HZ (prepulses) VA in 0.4 mM procaine was smaller than in 0.4 mM procaine + 0.3 mM benzocaine. 6. These phenomena can be explained on the assumption of voltage-dependent binding of benzocaine and procaine to a common receptor. The rate of block appears to be limited by access to the receptor, more in the case of benzocaine than of procaine.

Action Potentials↗
PubMed1976

Block of potassium channels of the nodal membrane by 4-aminopyridine and its partial removal on depolarization.

1. Voltage clamp experiments were done on single myelinated nerve fibres of the frog, Rana esculenta. 2. 53 muM 4-aminopyridine (4-AP) reduced IK to about one-fifth if tested with infrequent (1/min) and short (10 ms) depolarizing pulses; the onset time constant under these circumstances was ca. 160 s (14-15 degrees C). After prolonged treatment the effect was virtually irreversible. 3. At equilibrium with 4-AP, increasing the frequency of short pulses removed part of the block, the block removal accelerating with increasing pulse duration and frequency. 4. In 53 muM 4-AP unblocking of K channels during long (0.8 s) depolarizing pulses proceeded with a time constant, taur, of ca. 0.2 s. Restoration of block at the resting potential proceeded with a much larger time constant, tau'r, of ca. 1 min. 5. The stationary fraction, rinfinity, of K channels conducting in 53 muM 4-AP was 0.66, 0.41, and 0.24 at V = 120, 50, and 0 mV, respectively. 6. In a series of experiments with [4-AP] varying between 13.3 and 848 muM, taur decreased from 0.25 to 0.10 s (V = 130 mV, ca. 17 degrees C) while rinfinity followed the empirical relation 1/rinfinity = 1 + ct + cv exp(-0.77 EF/RT) with E = V - 70 mV. ct and cv are dimensionless quantities that increase with [4-AP] and reflect the voltage-independent and voltage-dependent component, respectively, of block. 7. Block of K channels and partial removal are also observed with inward IK at raised [K+]O. Removal proceeds on depolarization even if IK is additionally but temporarily suppressed by tetraethylammonium. Hence neither direction nor amplitude of IK but only the pulse potential seems to determine the extent of block for a given [4-AP].

Animals↗
PubMed1976

Saxitoxin and procaine act independently on separate sites of the sodium channel.

1. Voltage clamp experiments were done on single myelinated nerve fibres of the frog, Rana esculenta. 2. The time course of procaine action (1.0 mM at pH 7.2) was obtained from changes in INa on changing solutions during repetitive (1 HZ) depolarizing pulses of constant amplitude following hyperpolarizing prepulses. The mean half times of onset and offset of procaine block were 3.7 and 28 s, respectively. In the presence of 1.4 nM saxitoxin (STX) the corresponding times were virtually the same, 3.1 and 27 s. 3. Similarly, the time course of partial relief from procaine block that is obtained by increasing the frequency of the prepulse-test pulse pairs from 1-10 HZ was unaffected in the presence of STX. 4. Comparison of the equilibrium effects of procaine concentrations ranging from 0.03-1.0 mM suggest a one-to-one drug-receptor reaction. The fraction of Na channels blocked at equilibrium with 1.0 mM procaine, 1.4 nM STX, and 1.0 mM procaine + 1.4 nM STX was 0.81, 0.49, and 0.90, respectively. This result and the kinetic behaviour fully agree with the idea of two separate and independent receptors for procaine and STX.

Animals↗
PubMed1975

The rates of saxitoxin action and of saxitoxin-tetrodotoxin interaction at the node of Ranvier.

Voltage clamp experiments were done on single nodes of Ranvier of Rana esculenta. Equilibrium effects were obtained from INa-V curves, the rates of action from changes in INa on changing solutions during repetitive depolarizing pulses. 2. Saxitoxin (STX) exclusively and reversibly blocked Na channels, the effect being fully described by a one-to-one reaction between STX and a receptor at the channel with an equilibrium dissociation constant, Ks, of 1.4 nM; the mean offset rate constant k2s, was 1.76 X 10(-2) sec-1 (16 +/- 1 degree C;pH 7.2), 1.7 times the value for tetrodotoxin (TTX). 3. At pH 5.6, K2S WAs increased by a factor of 1.33 while the equilibrium STX effects was decreased in a way suggesting competition between STX and protons. 4. After pretreatment of nodes with 3.1 nM TTX the extra block on adding 9.0 nM STX as well as its relief on taking out STX of the TTX-STX mixture revealed transients in the time course of receptor occupation. 5. These non-monotonic time courses are incompatible with the idea of two independent blocking sites (for STX and TTX) per channel but could be quantitatively fitted by analog-computed curves assuming competition between STX and TTX for the same site.

Animals↗
PubMed1975

The reaction between tetrodotoxin and membrane sites at the node of Ranvier: its kinetics and dependence on pH.

Voltage clamp experiments were done on single nodes of Ranvier to study the inhibition of the sodium permeability by tetrodotoxin (TTX). Equilibrium results could be excellently fitted on the assumption that a sodium channel is blocked when one toxin molecule binds to it, the equilibrium dissociation constant, KT of this reaction being 3.6 nm at 20 degrees C. Onset and offset of block could be quantitatively interpreted to be determined by the rates of the TTX-channel reaction whose average constants, at room temperature, were 3 times 10-6M-minus 1s-minus 1 for the association (k1) and 1.4 times 10-minus 2 s-minus 1 for the dissociation (k2). The dependence of the constants on temperature could be described by Arrhenius plots yielding activation energies of 29.3, 85.5 and 41.0 (57.3) kJ/mol for KT, k2 and k1 (K1 derived from onset alone), respectively. At low pH the relative TTX effect was clearly less than at neutral pH. These results could be explained by a model involving the competition of TTX and protons for the same receptor to which protons bind as a function of membrane potential.

Animals↗
PubMed1975

The influence of pH on equilibrium effects of tetrodotoxin on myelinated nerve fibres of Rana esculenta.

1. The experiments were done on single nodes of Ranvier of Rana esculenta. The effects of tetrodotoxin and H ions were determined either by the reduction of the maximum rate of rise, VA, of action potentials evoked with threshold stimuli or in the voltage clamp by the decrease of the peak Na permeability, PNa. 2. With the tetrodotoxin sample used throughout the investigation the equilibrium dissociation constant, KT, of the toxin-receptor reaction at neutral pH was determined to be 2-8 nM. Between 1-55 and 15-5 nM tetrodotoxin the normalized value, A, of VA, was found to be related to the normalized toxin concentration cT = [TTX]/2-8 nM by the empirical equation log [(1-A)/A] = 1-22 log cT-0-573. 3. On increasing the pH (up to 8-8) the effect of tetrodotoxin diminished as revealed by an increase in A. The apparent reduction of cT (as calculated from A) suggests that the toxin is active only in its cationic forms. 4. Weakly acid tetrodotoxin solutions (7-3 less than pH less than or equal to 5-5) reduced A to a lesser degree than did neutral toxin solutions in spite of the inherent depressing effect of acid pH on A (A = 0-5 at about pH 5-5). In more acid toxin solutions A decreased again and at pH 4-6 it was about equal to the value in toxin-free solution. 5. When, after equilibrium in an acid toxin solution, the perfusate was suddenly changed to neutral Ringer solution A jumped to a higher value A' as measured 1 sec after the switch. Since the blocking effect of hydrogen ions subsided within a fraction of a second while the time constant of the toxin washout is of the order of 1 min, A' reflects the number of Na channels blocked by tetrodotoxin at acid pH. 6. In acid toxin-free solution the peak PNa as obtained in voltage clamp experiments was reduced by a voltage-dependent factor (cH + 1)-1 with CH = [H+]/KH(E) and KH(E) = 2-04 muM exp (0-34 EF/RT). Adding tetrodotoxin resulted in another reduction by a constant factor p'T. 7. Experiments employing various combinations of toxin concentration (3-1-93 nM) and pH values (7-3-5-2) confirm the decreased toxin effect at low pH. Moreover, p'T was smaller (the additional toxin effect larger) when the membrane had been kept depolarized and thus cH reduced during equilibration. This suggests that tetrodotoxin cations and H ions compete for the same blocking site. A quantitative fit, however, requires additional assumptions.

Action Potentials↗
PubMed1975

The influence of pH on the rate of tetrodotoxin action on myelinated nerve fibres.

1. The experiments were done on single myelinated nerve fibres of Rana esculenta. The rates of toxin effect were studied either by measuring the maximum rate of rise, VA, of repetitively evoked action potentials or by measuring Na currents during periodic impulses in the voltage clamp. 2. VA measurements showed that in alkaline solutions (pH up to 8-8) the offset rate was unchanged while the onset was slowed in quantitative agreement with an assumed decrease in the active cationic form of tetrodotoxin. 3. Both VA measurements and those in the voltage clamp revealed a decrease in T'off, the offset time constant and in increase in the onset time constant, T'on, as the pH was lowered. 4. For tetrodotoxin concentrations, [TTX], up to 400 nM and pH values down to 5-3 the simple relation T'on/T'off = p'R held, where p'T is the constant factor by which the Na permeability was reduced at equilibrium with a given [TTX]. 5. The agreement between kinetic and equilibrium results was also valid when, at constant [TTX] and pH. p'T was modified by the holding potential during equilibration. 6. No unequivocal explanation of the results can be given but some of their features resemble acid catalysis.

Action Potentials↗
PubMed1973

The rate of action of tetrodotoxin on myelinated nerve fibres of Xenopus laevis and Rana esculenta.

1. The experiments were done on single Ranvier nodes of Xenopus laevis (voltage clamp) and Rana esculenta (action potentials). Rate and size of the effect of tetrodotoxin were determined by the reversible reduction of either the sodium inward current (Xenopus) or of V(A), the maximum rate of rise of the action potential (Rana).2. The results of tetrodotoxin block at equilibrium could be excellently fitted by assuming a one-to-one reaction between toxin molecules and sodium channels of the Xenopus membrane with an equilibrium dissociation constant K = 3.60 nM at room temperature. V(A) was not linearly related to the fraction of unblocked sodium channels and 10.9 nM tetrodotoxin was necessary on the average to reduce V(A) to 50% in Rana motor fibres; in sensory fibres a lower concentration sufficed.3. Onset and offset of the tetrodotoxin effect on Xenopus nodes could be quantitatively interpreted as being determined by the rates of the tetrodotoxin channel reaction. Experiments with 3.1 and 15.5 nM tetrodotoxin at room temperature yielded an association rate constant, k(1), of 2.94 x 10(6)M(-1) sec(-1) and a dissociation rate constant, k(2), of 1.42 x 10(-2) sec(-1). In these experiments the equilibrium dissociation constant, K, was 3.31 nM. If determined solely from the onset in the two tetrodotoxin concentrations, k(1) = 3.25 x 10(6)M(-1) sec(-1) and K = k(2)/k(1) = 4.08 nM was calculated.4. In Rana fibres the onset and offset of V(A) reduction by 15.5 and 31 nM tetrodotoxin was evaluated using the equilibrium effects of intermediary tetrodotoxin concentrations for calibration. The average results at room temperature were k(1) = 4 x 10(6)M(-1) sec(-1), k(2) = 1.4 x 10(-2) sec(-1) and K = 3.4 nM.5. The very short latency with which V(A) started to decline when tetrodotoxin was suddenly applied proved that the toxin had ready access to the membrane.6. The temperature dependence of k(1), k(2) and K in the Xenopus experiments could be described by Arrhenius plots yielding activation energies, E(a), of 9.8, 20.5 and 7.0 kcal/mole, respectively, corresponding to Q(10) values of 1.82, 3.42 and 1.53 (between 12 and 22 degrees C). For k(1), determined from onset alone, E(a) = 13.7 kcal/mole (Q(10) = 2.25) was obtained. Although in Rana the temperature dependence of the rate constants could not be determined directly, the Q(10) for k(2) must have been of the order of 3.7. The results suggest that the rate of the toxin action on the nodal membrane of Xenopus and Rana is limited by the tetrodotoxin-sodium site reaction.

Action Potentials↗