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Biomedical subjects

P R Adams

Publications and source records attributed to P R Adams.

At least 73 records · Page 4Linked to original sources
PubMed1980

Luteinizing hormone-releasing factor and muscarinic agonists act on the same voltage-sensitive K+-current in bullfrog sympathetic neurones.

The effect of bath-applied luteinizing hormone-releasing factor (LHRF) was recorded in voltage-clamped bullfrog lumbar sympathetic neurones. At a holding potential of -- 30 mV, LHRF induced a steady inward (depolarizing) current and reduced membrane conductance; at -- 60 mV LHRF produced negligible inward current and much less conductance change. The effects of LHRF were accompanied by a reduced amplitude of the slow current relaxations following hyperpolarizing and depolarizing commands, without prominent change in their time-course. This suggests a selective depression of the voltage-sensitive outward K+-current which is suppressed by muscarinic agonists.

Animals↗
PubMed1980

Quinacrine (mepacrine) action at frog end-plate.

1. The effects of quinacrine on end-plate currents (e.p.c.s.), miniature end-plate currents (m.e.p.c.s.) and ionophoretic responses at voltage clamped frog end-plates were investigated. 2. A quinacrine concentration (2 microM) that by itself has little effect on m.p.e.c.s. will considerably attenuate responses to bath applied carbachol. The combined effect of the two drugs causes the m.e.p.c.s. to disappear. 3. The depressant effect of quinacrine on ionophoretic responses to carbachol or acetylcholine is increased in acid solutions and decreased in alkaline solutions, suggesting that quinacrine is active as an acridinium ion. 4. Quinacrine (2-10 microM) causes a use-dependent block of end-plate channels which manifests as an inhibitory effect of an ionophoretic prepulse on the response to a test pulse. The inhibitory interaction decays exponentially with a time constant tau s that depends on the nature of the agonist used for the prepulse, on the quinacrine concentration, and on the membrane potential. 5. Quinacrine (5-20 microM) reduces the amplitude of e.p.c.s. and m.e.p.c.s. It also increases the rate of decay of the e.p.c. or m.e.p.c. tails, which remain exponential. The decay rate constant 1/tau f increases linearly with quinacrine concentration both in the presence and absence of 3 microM-neostigmine. The slope of this linear relation increases slightly with membrane hyperpolarization. 6. These data suggest that quinacrine's main action is a slow, voltage dependent blockade of open end-plate channels, though there are probably additional effects on acetylcholinesterase and channel opening. In accordance with the open channel blocking model, 1/tau s and 1/tau f both increase linearly with quinacrine concentration. However, the slopes of these lines lead to rather different estimates of the forward blocking rate constant (8 X 10(7) and 4 X 10(8) M-1 S-1 respectively). 7. The unblocking rate constant is about 5 S-1 at -80 mV. It is much more voltage dependent than the forward rate constant.

Acetylcholine↗
PubMed1980

End-plate channel opening and the kinetics of quinacrine (mepacrine) block.

1. The effects of quinacrine on the relaxation of the agonist-induced currents in response to a voltage step were investigated at voltage clamped frog end-plates. A fast perfusion technique allowed the application of known concentrations of the agonist acetylcholine (ACh) or carbachol to end-plate viewed with Nomarski optics. 2. In the presence of quinacrine, and in response to a hyperpolarizing voltage jump, an agonist-induced current shows a fast initial relaxational increase and then relaxes slowly back to a new equilibrium level. 3. The slow relaxation can be described by a single exponential with a time constant tau s . tau s gets smaller at increasing quinacrine concentrations (0--2 microM) and the decay rate constant, 1/tau s, increases linearly with quinacrine concentration. Increasing agonist concentration reduces tau s, in a manner dependent on the nature of the agonist. Tau s is markedly lengthened at more hyperpolarized potential, but this voltage effect gets less at higher concentrations of agonist. 4. These data suggest a slow voltage dependent blockage of open end-plate channels by quinacrine. The binding rate constant of quinacrine is estimated as 10(8) M-1 s-1, and the voltage dependent, backward rate constant, as 5 s-1 at -60 mV and 1 s-1 at -140 mV. These values are in fair agreement with those obtained from the analysis presented in the preceding paper. 5. The agonist concentration dependence of the blocking kinetics is compatible with a simple model for channel opening. In this model, independent sequential binding of two agonist molecules leads to an isomerization of the receptor. The intrinsic binding constant of ACh is estimated to be around 20 microM, and for carbachol around 200 microM. Distinct isomerization constants could lead to a maximal activation of 70% of the available channels by ACh, and only 40% by carbachol. 6. An example of a possible interaction in between quinacrine block and desensitization is shown. At the break of an hyperpolarizing jump which has increased quinacrine blockade, a transient increase in the synaptic current is observed with apparently a temporary reduction of the desensitization.

Acetylcholine↗
PubMed1979

Distribution of gaba-receptors and gaba-carriers in the mammalian nervous system.

1. Extrasynaptic GABA-receptors occur on both neurone somata and unmyelinated axons in the mammalian peripheral nervous system. Activation of these receptors leads to depolarization, reduced spike amplitude and slowed conduction, probably mediated through increased Cl- conductance. 2. GABA also depolarizeds preganglionic nerve terminals in the rat superior cervical ganglion and reduces the release of acetylcholine by preganglionic nerve impulses. 3. The Schwann and satellite neuroglial cells surrounding peripheral unmyelinated axons and neurones possess a GABA-carrier promoting net uptake of GABA at external concentrations greater than or equal to 1 microM. 4. The possible significance of extrasynaptic receptors and carriers for GABA is discussed.

Animals↗
PubMed1978

Decamethonium both opens and blocks endplate channels.

Miniature endplate currents, endplate current fluctuations ("membrane noise"), and voltage-jump current relaxations were studied in voltage-clamped frog muscle fibers during decamethonium action. All three types of experiments revealed two kinetic processes controlling the opening of endplate channels, one that reflects agonist action and another that reflects local anesthetic-like blocking activity. The kinetic constants for these two steps were evaluated from measurements of the fast and slow time constants as a function of decamethonium concentration. At -130 mV membrane potential and 13 degrees, the mean open time of decamethonium-activated channels is 2.8 msec. The forward and backward rate constants for channel blocking are 1.7 X 10(7) M-1 sec-1 and 10(3) sec-1. The voltage dependencies of the channel lifetime and of the blocking equilibrium are similar to those seen with pure agonists and local anesthetics, respectively.

Animals↗
PubMed1978

A comparison of current-voltage relations for full and partial agonists.

1. Local conductance changes produced by various bath-applied agonists at frog end-plate membrane were measured using focal recording of extracellular potential in voltage-clamped muscle fibres. The potential difference between a focal micropipette placed on the nerve terminal and another micro-pipette placed on or near inactive membrane was taken as proportional to the agonist-induced current through a small patch of an end-plate membrane. 2. The current-voltage (I--V) relation of active membrane was obtained directly by increasing the membrane potential in a ramp fashion. The change in membrane potential was slow enough for post-synaptic gating processes to reach equilibrium during the ramp. 3. During application of sufficiently low concentrations of full agonists (carbachol, (ACh) and partial agonists (choline and decamethonium) the I--V relation of end-plate membrane showed strong curvature in the range of -60 to -130 mV. The slope of I--V relations increased exponentially with membrane hyperpolarization, an e-fold change in conductance occurring for about 50 mV potential shift. 4. The curvature of the I--V relation of end-plate-membrane activated by the partial agonists choline and decamethonium became less as the agonist concentration was increased, and with high concentrations (choline 15 mM; decamethonium 250 micrometer) the I--V relation became almost straight. 5. When end-plate currents produced by high concentrations of partial agonists were matched by application of equi-active concentrations of carbachol, the carbachol-activated membrane still showed as much curvature in its I--V relation as when low concentrations of carbachol were used. 6. Choline and decamethonium concentrations for which the I--V relation was straight produced much greater depression of miniature end-plate currents than did carbachol concentrations which produced the same membrane current at the holding potential. 7. I--V relations for full agonists at high concentrations were obtained after alpha-bungarotoxin pre-treatment. During application of carbachol (400--500 micrometer) and ACh (30--40 micrometer; after complete inhibition of acetylcholinesterase activity) the I--V relation of end-plate membrane is much less curved than during application of low concentrations. 8. It is concluded that either the voltage sensitivity of agonist-induced end-plate conductance reflects voltage sensitivity of agonist binding, or the partial agonists used can exert a voltage-dependent 'local anaesthetic' action in addition to their agonist activity.

Acetylcholine↗
PubMed1977

Relaxation experiments using bath-applied suberyldicholine.

1. The effect of step changes in membrane potential on the end-plate conductance change produced by bath-applied suberyldicholine was studied in voltage-clamped frog muscle fibres. 2. The suberyldicholine-induced conductance increased exponentially from its previous equilibrium level to a new equilibrium level following a step hyperpolarization. 3. For low suberyldicholine concentrations the time constant of this relaxation was independent of the concentration. 4. For low suberyldicholine concentrations the voltage dependence of equilibrium conductance and relaxation time constants was identical. 5. Bungarotoxin pretreatment did not affect the responses beyond a simple reduction in their amplitude. 6. The conductance evoked by high suberyldicholine concentrations was less voltage-sensitive than that evoked by low concentrations. 7. A new model for explaining noise and relaxation data is proposed. This postulates rate-limiting binding steps followed by a voltage-dependent isomerization.

Animals↗
PubMed1977

Voltage jump analysis of procaine action at frog end-plate.

1. In the absence of procaine the end-plate conductance evoked by suberyldicholine increases exponentially to a new level following a step hyperpolarization. In the presence of procaine the suberyldicholine-evoked conductance first rapidly decreases and then slowly increases following a hyperpolarizing step. The fast relaxation has a time constant of approximately 1 msec, and the slow relaxation a time constant of 10-150 msec.2. The existence and sign of these two relaxations is predicted by a sequential model in which procaine enters and blocks open but not closed end-plate channels. The concentration dependence of the fast and slow relaxation time constants agrees well with the predictions of this model, and allows the apparent dissociation constant for binding of procaine within the open channel to be estimated at about 20 muM at -80 mV membrane potential.3. This apparent binding constant is voltage sensitive. It decreases e-fold for 50 mV hyperpolarization, suggesting that the procaine binding site is electrically half way through the channel.4. Procaine concentrations comparable to the dissociation constant for binding to open channels strongly depress the equilibrium current evoked by low suberyldicholine concentrations. This finding is not in accord with the sequential model.5. A cyclic model in which procaine binds to both closed and open channels explains well the equilibrium observations. The affinity of procaine for closed channels is similar to its affinity for open channels, and is also increased by hyperpolarization. This model also fits well the kinetic observations, if it is assumed that blocked channels open and close much more slowly than unblocked channels.6. The concentration dependence of the relaxation amplitudes disagrees with the predictions of the sequential model, but agrees well with the predictions of the cyclic model.7. No other model appears to explain the various observations as economically as the cyclic channel blocking model. If the model is correct the ;gate' controlling the end-plate channel must be in the inner half of the membrane.

Animals↗
PubMed1976

Dehydrogenation of the phosphonate analogue of glucose 6-phosphate by glucose 6-phosphate dehydrogenase.

6,7 -Dideoxy-alpha-D-gluco-heptose 7-phosphonic acid, the isosteric phosphonate analogue of glucose 6-phosphate, was synthesized in six steps from the readily available precursor benzyl 4,6-O-benzylidene-alpha-D-glucopyranoside. The analogue is a substrate for yeast glucose 6-phosphate dehydrogenase, showing Michaelis-Menten kinetics at pH7.5 and 8.0. At both pH values the Km values of the analogue are 4-5 fold higher and the values approx. 50% lower than those of the natural substrate. The product of enzymic dehydrogenation of the phosphonate analogue at pH8.5 is itself a substrate for gluconate 6-phosphate dehydrogenase.

Deoxy Sugars↗
PubMed1976

Voltage dependence of agonist responses at voltage-clamped frog endplates.

The conductance increment produced at voltage clamped frog endplates by various agonists, applied either iontophoretically or in the bath, increases exponentially with membrane hyperpolarisation, an e-fold change being obtained with shifts of the order of 100 mV. The voltage dependency of this increase is the same for different, but low, agonist concentrations. However, conductance changes evoked by decamethonium increased less with hyperpolarisation than did conductance changes evoked by carbachol or tetramethylammonium. Hyperpolarisation slowed iontophoretic responses to carbachol or decamethonium, and enhanced and prolonged inhibition of carbachol by a brief pulse of decamethonium.

Animals↗
PubMed1976

Response to immunized, parenterally nourished rats to challenge infection with the nematode, Trichinella spiralis.

Experiments were desinged to test the hypothesis that structural and chemical changes in the small intestine associated with total parenteral nutrition (TPN) alter relationships between hosts and enteric parasites. Parenteral feeding of rats immunized to Trichinella spiralis (nematoda) led to a decrease in gut mass. This was reflected by a significant drop in gut wet weight and reductions in mucosal dry weight, DNA, protein, carbohydrate and lipid. These changes were accompanied also by lowered brush border enzyme activity. When challenged orally with 8 X 10(3) T. spiralis larvae and switched immediately to oral feeding, more worms became established in the small intestines of these rats than in rats given similar doses of larvae, but maintained by oral feeding throughout the experiment. Despite the significant difference in initial worm burden, both enterally fed and parenterally fed rats completely expelled their parasites by 7 and 8 days postinfection, respectively. Results indicated that factors involved in preventing establishment of T. spiralis in immunized rats were suppressed by the prolonged absence of food from the GI tract, whereas factors responsible for expelling worms that were able to gain residence in the mucosa appeared to be unaltered or at least functionally reversible.

Animals↗
PubMed1976

A comparison of the time course of excitation and inhibition by iontophoretic decamethonium in frog endplate.

1 The depolarization of frog endplate by a brief iontophoretic application of decamethonium was slower in time course than the inhibition of a long carbachol response produced by the same decamethonium pulse, or than the excitation produced by a brief equipotent carbachol pulse. 2 The delay between the peak inhibition and peak excitation produced by decamethonium, about 50 ms, is too great to be explained by slow receptor activation kinetics, since Katz & Miledi (1973) have shown that the lifetime of decamethonium-activated receptors is only 0.25 milliseconds. 3 If doses of carbachol and decamethonium are adjusted to give response amplitudes in the ratio corresponding to the ratio of their presumed maximum responses, then there is little difference in the time courses of the responses. 4 This observation, together with the finding that increasing the dose applied slows the decamethonium response much more than the carbachol response, suggests that a decamethonium response contains contributions from a much wider area of receptive membrane than does a carbachol response of equal amplitude. 5 Simulation shows that these geometrical effects are sufficient to account for the rapidity of inhibition compared to excitation without postulating slow receptor kinetics. 6 It is pointed out that similar effects may account for certain results obtained in iontophoretic studies of desensitization.

Animals↗
PubMed1976

Drug blockade of open end-plate channels.

1. The actions of amylobarbitone, thiopentone, methohexitone and methyprylone at voltage-clamped frog end-plates were studied. 2. In the presence of barbiturates the conductance change evoked by an iontophoretic carbachol application was reduced by a prepulse of carbachol. The extra inhibition evoked by a prepulse disappeared exponentially with a time constant of 150-200 ms. 3. Barbiturates produce an increased rate of decay of nerve evoked endplate currents. Tne concentration and voltage dependence of the barbtiruate e.p.c. decay rates tally with the hypothesis that the increased rate of decay is due to block of active receptor-channel complexes by barbiturates with a rate constant of 10(6) M-1S-1. 4. Conductance changes produced by bath applied agonists were depressed by thiopentone, the effect becoming greater the higher the agonist concentration. This effect, and also the observation that the concentration of thiopentone required to depress the bath agonist response is much greater than the apparent dissociation constant for binding to active receptor-channel complexes calculated from kinetic measurements, suggest that the selectivity for binding to open receptor-channel complexes is very high. 5. Methyprylone, which is structurally similar to the barbiturates, is only a weak antagonist and shows no interpulse interaction. It was predicted that methyprylone should produce fast and slow components in the e.p.c. decay, and this prediction was verified. 6. In the presence of barbiturates large iontophoretic carbachol applications produce conductance changes which show fast and slow components. Under these conditions the effects of carbachol prepulses become complex. However the effects are qualitatively consistent with the notion that different components of the response are contributed by channels located at various distances from the iontophoretic pipette tip. 7. All the data agree with a model in which the channel has three stages: closed, open and blocked. Only open channels can block, and blocked channels can only open.

Acetylcholine↗
PubMed1975

A study of desensitization using voltage clamp.

The onset and offset of desensitization produced by carbachol and hexyl and heptyl TMA at voltage-clamped frog endplates were studied. Desensitization onsets exponentially with a rate constant which is proportional to the agonist concentration. The proportionality constant varies with the agonist used. The plateau current finally attained falls steeply to low values with increasing agonist concentration, and with further increase in concentration varies relatively little. The offset of desensitization is sometimes accompanied by the appearance of a second transient increase in current. It is proposed that desensitization is due to channel blockade by the agonist molecule itself.

Animals↗