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Biomedical subjects

E J Simon

Publications and source records attributed to E J Simon.

At least 163 records · Page 9Linked to original sources
PubMed1975

Kinetics of opiate receptor inactivation by sulfhydryl reagents: evidence for conformational change in presence of sodium ions.

The role of SH groups in opiate-receptor interactions has been further examined. In activation by N-ethylmaleimide of sterospecific opiate binding by rat brain membrane fractions follows pseudo-first order kinetics and exhibits strong temperature dependence. The kinetics indicate that alkylation of a single SH group suffices to block opiate binding. Considerable protection from SH group inactivation is observed when treatment with N-ethylmaleimide is carried out in the presence of an opiate or an antagonist, suggesting close proximity of the SH group to the opiate binding site. The rate of inactivation of receptor binding by N-ethylmaleimide is markedly slower in buffers containing 100 mM NaCl (t1/2 equals 30 plus or minus 1.4 min) than in sodium-free buffers (t1/2 equals 10 plus or minus 1.0 min). Since the rate of alkylation of model SH compounds is unaffected by sodium ions, this protection seems best explained by a conformational change in the receptors that renders the SH groups less accessible to alkylation. The rate of inactivation is not affected by K+, Rb+, or Cs+ and only slightly by Li+. This cation specificity as well as the concentration-response to Na+ are remarkably similar to those previously shown to lead to increased antagonist and decreased agonist binding. We suggest that the same conformational change is involved in the two phenomena.

Alkylation↗
PubMed1975

Properties of centre-hyperpolarizing, red-sensitive bipolar cells in the turtle retina.

1. Responses of centre-hyperpolarizing, red-sensitive bipolar cells were studied by intracellular recording in the retina of the turtle, Pseudemys scripta elegans. The identity of these cells was confirmed by Procion Yellow marking. 2. Circles of light produced hyperpolarizing waves that were graded with intensity and could exceed -30mV in amplitude. The operating intensity range was similar to that of turtle cones. 3. Flashes in the form of an annulus evoked graded depolarizations which could be greater than 10 mV in the dark-adapted state or about 30mV when applied over central backgrounds. 4. Responses proportional to intensity were produced by dim circular stimuli. For radii less than about 200 mum these responses reached peak in approximately 120 msec and were invariant with respect to wave-length or area of illumination. Absolute flash sensitivity varied greatly from cell to cell but in the most sensitive cell encountered was about 460 muV photon(-1) um2. 5. Sensitivity of both bipolar cells and red-sensitive cones was enhanced progressively for enlargements of a circular flash up to 150-200 mum in radius. 6. Increasing the radius of a circle from 200 to 1250 mum caused a decrease of about 75% in bipolar cell sensitivity. This decrease was associated with a marked shortening of the response for all colours. The same enlargement decreased sensitivity of red-sensitive cones by approximately 20% and did not appreciably alter the time course of their response. These effects are attributed to impingement from type I red-sensitive horizontal cells because they have the requisite spatial and spectral properties. 7. Responses of a few bipolar cells were already shortened for 200 mum flashes; this property suggests impingement from type II horizontal cells. 8. For small circles the spectral sensitivity of the bipolar cells considered resembled closely that of red-sensitive cones or horizontal cells. Red backgrounds enhanced the relative sensitivity to green flashes suggesting that these bipolar cells receive input from red-sensitive members of double cones as well as single red-sensitive cones. 9. Steady depolarizing currents injected into bipolar cells decreased the response to either central or annular illumination; hyperpolarizing currents decreased the response to a central flash and increased the response to an annulus.

Animals↗
PubMed1975

Inhibition by levorphanol and related drugs of amino acid transport by isolated membrane vesicles from Escherichia coli.

Levorphanol inhibits the transport of the amino acids proline and lysine by cytoplasmic membrane vesicles derived from Escherichia coli. The degree of inhibition increases with increasing levorphanol concentration and ranges from 26% at 10(-6) M levorphanol to 92% at 10(-3) M levorphanol. The effect is independent of the energy source, since levorphanol inhibits proline uptake to the same extent in the presence of 20 mM d-lactate or 20 mM succinate and in the absence of an exogenous energy source. Levorphanol does not irreversibly alter the ability of membrane vesicles to transport proline, since incubation of membrane vesicles for 15 min in the presence of 0.25 mM levorphanol, a concentration which inhibits proline transport by more than 75%, has no effect on the rate of proline transport by these vesicles once the drug is removed. Both the maximum velocity and the K(m) of proline transport are modified by levorphanol, hence, the type of inhibition produced by levorphanol is mixed. The inhibitor constant (K(i)) for levorphanol inhibition of proline transport is approximately 3 x 10(-4) M. Membrane vesicles incubated in the presence of levorphanol accumulate much less proline at the steady state than do control vesicles. Furthermore, the addition of levorphanol to membrane vesicles preloaded to the steady state with proline produces a marked net efflux of proline. Levorphanol does not block either temperature-induced efflux or exchange of external proline with [(14)C]proline present in the intravesicular pool. Dextrorphan, the enantiomorph of levorphanol, and levallorphan, the N-allyl analogue of levorphanol, inhibit proline and lysine transport in a similar manner. Possible mechanisms of the effects of these drugs on cell membranes are discussed.

Amino Acids↗
PubMed1975

Further properties of stereospecific opiate binding sites in rat brain: on the nature of the sodium effect.

Addition of sodium salts has been reported to enhance stereospecific binding of opiate antagonists while reducing binding of agonists to rat brain homogenate. We have tested, in addition to sodium and potassium, a number of organic cations. Our results support the suggestion that the ability to enhace antagonist binding is not a general characteristic of cations or high ionic strength, but a property of sodium ions. We have shown that the increase in antagonist binding results from an enhancement of binding affinity and not from unmasking of new binding sites. The reduction in etorphine binding in the presence of sodium is due to a decrease in binding affinity. This decrease is largely accounted for by an acceleration in the dissociation rate, while the greater affinity of naltrexone binding appears to be due to an increase in rate of association. Our results are consistent with the hypothesis of a conformational change in opiate binding sites in the presence of sodium, transformed sites exhibiting greater affinity for antagonists and reduced affinity for agonists. Preheating of rat brain P2 fraction at 50 degrees C results in gradual inactivation of stereospecific binding, but an increase in naltrexone binding is consistently observed after heating at 50 degrees C for 2 to 3 minutes, even at optimal concentrations of sodium.

Analgesics, Opioid↗
PubMed1974

Interactions leading to horizontal cell responses in the turtle retina.

1. Small responses to large fields of dim monochromatic lights were recorded intracellularly from luminosity horizontal cells (L-cells), chromaticity horizontal cells (C-cells) and cones in the retinae of turtles, Pseudemys scripta elegans.2. Responses of cones to brief flashes applied over steady backgrounds were studied in order to interpret the corresponding responses of horizontal cells. Steady red or green backgrounds make the responses of red-sensitive cones smaller, faster and often diphasic. Green backgrounds have similar effects on the responses of green-sensitive cones to green flashes, but red backgrounds do not change them appreciably. Responses of double cones have properties intermediate between those of red and green cones.3. L-cells of both type I and type II are hyperpolarized by all visible wave-lengths, and their spectral sensitivity in the linear range resembles that of red cones. Their responses are not invariant with respect to colour, and their sensitivity to green relative to red stimuli increases during red backgrounds. These properties suggest that L-cells are activated mainly by red cones but also receive impingement from the red members of double cones.4. Spectral properties of red/green C-cells resemble those of green cones as modified by the recurrent action of L-cells. They can be explained assuming that red/green C-cells receive their principal impingement from green cones and subsidiary interactions from green/blue C-cells and the green members of double comes.5. The spectral sensitivity of the hyperpolarizing responses of green/blue C-cells is ascribed to impingement from blue cones. Their depolarizing responses have complex properties which suggest that they are brought about by the activity of both L-cells (probably through the blue cones) and red/green C-cells.6. It is concluded that the main properties of the responses of the horizontal cells can be explained by a simple circuit in which each horizontal cell is connected to a corresponding type of cone and the L-cells have a recurrent impingement on all cones. The scheme is modified by additional interactions which operate on the responses of each horizontal cell type.

Animals↗
PubMed1974

Electrical responses of double cones in the turtle retina.

1. Responses to monochromatic flashes were recorded intracellularly from double cones in the retina of the turtle, Pseudemys scripta elegans. Double cones have been identified by intracellular marking with Procion Yellow dye.2. The direct light response of a double cone is a hyperpolarization graded with the intensity of a flash and similar to the response of single cones.3. When flashes were dim, responses were proportional to light intensity but varied in time course as a function of wave-length. They reached peak in about 120 msec for deep red stimuli and about 143 msec for green stimuli.4. When applied over red backgrounds, responses to red flashes became smaller, faster and frequently diphasic, but responses to green remained similar to those recorded from darkness. Green backgrounds made responses to all colours small and fast.5. Linear spectral sensitivity curves have peaks corresponding to the peak sensitivities of single red-sensitive and green-sensitive cones. Red and green backgrounds suppressed red and green sensitivity respectively.6. Large fields of illumination evoked composite responses which included the direct light response, its enhancement from illumination of nearby receptors and the depolarizing effect of luminosity horizontal cell impingement. In the green-sensitive element the depolarizing effect was larger for red than for green flashes, and stimulation with red annuli evoked net depolarizing responses.7. It is concluded that the responses of double cones may be explained by coupling of the responses from red-sensitive and green-sensitive elements each of which has properties otherwise similar to single red-sensitive and green-sensitive cones.

Animals↗
PubMed1973

Stereospecific binding of the potent narcotic analgesic (3H) Etorphine to rat-brain homogenate.

Etorphine, the most potent narcotic analgesic known, was labeled with tritium by catalytic exchange. This drug exhibits stereospecific, saturable binding to rat-brain homogenate. At saturation, the stereospecific binding is 0.1-0.15 pmol/mg of protein. Specific binding is inhibited high salt concentrations, sulfhydryl reagents, and proteolytic enzymes, but is unaffected by phospholipases A and C, sodium azide, sodium fluoride, and prostaglandins E(1) and E(2). Competition for binding of [(3)H]etorphine correlates with agonist and antagonist potencies. The stable, stereospecific binding of an active narcotic analgesic supports the existence of opiate receptors.

Acetamides↗
PubMed1973

Two types of luminosity horizontal cells in the retina of the turtle.

1. Two classes of luminosity horizontal cells have been identified in the retina of the turtle by intracellular recording. These cells differ in the area of retinal surface over which they are sensitive to light. For moderate light intensities the maximum response amplitude of ;type I' cells increases progressively as the radius of a circle of light is enlarged to over 750 mum. Responses of ;type II' cells instead reach maximum amplitude with circular illumination of about 300 mum radius.2. Both types are maximally sensitive to red light and have similar action spectra.3. The cells from which these responses originated were identified by intracellular marking with Procion Yellow dye. Both types were located in the inner nuclear layer. Type I cells had asymmetric, thick, tuberous processes and no apparent discrete somata. Type II cells had thin processes radiating symmetrically from rounded cell bodies.4. The different properties of the two types of cells can be simply explained assuming that the cells of type I are coupled by low-resistance electrical junctions while in cells of type II the junctional resistance is higher.5. Since type I and type II cells differ both morphologically and physiologically, they may have different functions.

Animals↗
PubMed1973

Colour-dependence of cone responses in the turtle retina.

1. Responses to monochromatic lights were recorded intracellularly from red cones, green cones, and luminosity horizontal cells (L-cells) in the retinae of turtles.2. Both types of cones responded to small fields of illumination with graded hyperpolarizations. Red cones were only moderately more sensitive to deep red (680 nm) than to green (550 nm) light while green cones were much more sensitive to the green light than to the red. L-cells produced small responses for flashes of either colour covering small fields.3. Stimulation of large fields with monochromatic lights of moderate or high intensity evoked large L-cell responses and composite responses in cones. These latter include the hyperpolarizing action of the light absorbed by the cone itself (direct response), its enhancement by illumination of the near surround, and the depolarizing effects of L-cell feed-back.4. L-cells respond primarily to the activity of red cones; with sufficient intensity of the light, however, their responses are influenced also by green cones. As a result, if a red and a green light stimulate red cones equally, the L-cell response is larger for the green stimulus.5. Green cones were depolarized by deep red lights of moderate intensity applied over large fields. These depolarizing responses include oscillations which follow closely oscillations in L-cells. Green light applied to the same large fields produced hyperpolarization of green cones.6. Red cones were hyperpolarized by red or green light covering large fields, but the time course of their responses differed for the two colours, reflecting a corresponding difference in L-cell activity.7. Red light in the form of an annulus produced large responses in central L-cells without eliciting direct responses in central green cones. In these conditions green cones developed depolarizing waves which included a large, sharp transient.8. It is concluded from these and other results that the direct response of each cone is modified by two interactions: enhancement only from nearby cones of the same colour and depression controlled (through L-cell feed-back) by cones of all colours. In this way the response of any cone will change as the proportion of responses in cones of different colours changes, this proportion being a function of the wave-length of the light.

Animals↗