PubMed Health⌕ Search

Biomedical subjects

M L Michaelis

Publications and source records attributed to M L Michaelis.

54 records · Page 3Linked to original sources
PubMed1982

An ion exchange chromatographic method for the determination of synaptosomal calcium uptake.

A simple, rapid method for determining depolarization-induced 45Ca influx into synaptosomes is described. Synaptosomes which had been depolarized in the presence of 45Ca were applied to a small column of Chelex-100 resin to separate free Ca2+ from that taken up by the tissue. Approximately 70% of the synaptosomal protein applied to the column was recovered in the initial eluate. The magnitude of 45Ca uptake was dependent on the amount of Ca2+ in the incubation medium and on the KCI concentration. Calcium influx reached a plateau after 90 sec of incubation at 24 degrees C. The Na+ channel activator veratridine also produced a substantial influx of 45Ca, and this effect was blocked by tetrodotoxin. Thus, this ion exchange procedure makes it possible to measure depolarization-induced Ca2+ influx in synaptosomes without subjecting them to high vacuum or centrifugation pressures or to EGTA-containing solutions.

Calcium↗
PubMed1982

Differential effects of metal ligands on synaptic membrane glutamate binding and uptake systems.

The high affinity, Na+-independent L-[3H]glutamate binding process in synaptic membranes and in the purified binding protein was shown to be inhibited to an almost equal extent by the metal ligands NaN3, KCN, and o-phenanthroline, and by 2,4,5-trihydroxyphenylalanine (6-OH DOPA). The high affinity, Na+-dependent glutamate transport activity in these membranes was almost totally insensitive to NaN3, o-phenanthroline, KCN, and 6-OH DOPA. These agents, especially 6-OH DOPA, may be useful tools in achieving a discrimination between putative physiologic receptors and uptake carrier sites for L-glutamate in synaptic membranes. The sensitivity of the glutamate binding sites to the effects of the metal ligands may be correlated to the presence of an iron-sulfur center in the purified glutamate binding protein. some of the characteristics of this metallic center were explored by optical and paramagnetic resonance spectroscopic techniques and are described in this study.

Animals↗
PubMed1981

Effects of 2-chloroadenosine on electric potentials in brain synaptic membrane vesicles.

Isolated synaptic plasma membrane vesicles developed an internal negative membrane potential (delta psi) following loading with potassium succinate and incubation in NaCl, sodium succinate, or Tris succinate media. Membrane delta psi was monitored by measuring triphenyl[3H]methylphosphonium ion ([3H]TPMP+) accumulation by these vesicles. Estimates of delta psi ranged from --6.9 mV for vesicles incubated in sodium succinate to --28 mV for membranes incubated in NaCl. Intravesicular TPMP+ accumulation was strongly dependent on the K+ diffusion potential and was enhanced by the K+ ionophore valinomycin and by the adenosine analog 2-chloroadenosine (2-Cl-Ado). The stimulation of TPMP+ influx by 2-Cl-Ado was dependent on the concentration of this agent, independent of Cl- fluxes, and sensitive to inhibition by the methylxanthine theophylline. The increase of delta psi of the synaptic membrane vesicles caused by 2-Cl-Ado paralleled the hyperpolarization of neurons produced by adenosine and 2-Cl-Ado in physiological systems.

2-Chloroadenosine↗
PubMed1981

Biochemical-molecular characteristics of the brain synaptic membrane glutamate receptor.

The studies described above have provided evidence for the presence of a high affinity, glutamate binding glycoprotein which has some of the pharmacologic characteristics of the recognition site of neuronal glutamate receptors. In addition, evidence was presented for the distinct molecular nature of this glutamate binding protein as compared to that of the kainic acid binding macromolecule in synaptic membranes and also, for the differential characteristics of the glutamate binding sites as compared to the glutamate transport carriers. Finally, information was presented which is suggestive of the presence and functional integrity of glutamate receptor.ion channel complexes in synaptic membrane preparations.

Animals↗
PubMed1980

Chronic ethanol intake and synaptosomal glutamate binding activity.

The studies presented above add further support to the notion that one of ethanol's most important sites of activity may be the plasma membrane of cells. In addition, they provide an example of how ethanol's effects on these membranes may affect the function of protein molecules associated with nerve cell membranes such as the receptive sites for the putative excitatory neurotransmitter L-glutamic acid. The result of chronic ethanol administration is an apparently enhanced sensitivity to glutamate action in the CNS as measured by either the increases in L-glutamate binding activity or the increased Ca2+ mobilization as a result of glutamate interaction with the synaptosomal membranes. Both of these processes exhibit an enhancement of the maximum response with little or no change in the KD for binding of L-glutamic acid to its receptor site. Finally, it was demonstrated by means of in vivo experiments that there is an apparent sensitization to glutamate during the induction of ethanol dependence, and that this enhanced sensitivity may be causally linked to some of the signs of the post withdrawal CNS hyperexcitability.

Alcoholism↗
PubMed1980

Ethanol effects on synaptic glutamate receptors and on liposomal membrane structure.

Exposure of synaptic plasma membranes to 50 mM ethanol in vitro brought about a 3.5 degrees C decrease in the transition temperature of the high affinity glutamate binding process in these membranes. Ethanol had no effect on the energy of activation of glutamate binding below the transition temperature but decreased the energy of activation above the transition temperature. Electron paramagnetic resonance (EPR) studies of lipid organization of egg lecithin and bovine brain phospholipid liposomes indicated that ethanol at low concentrations (0.04--2 mM) caused small increases in the rigidity of the membrane near the surface. At higher concentrations (0.04--2 M) ethanol brought about increasing fluidization of both the surface and inner areas of the bilayer. Even at 4 mM concentration ethanol enhanced the ordered to fluid state transition of liposome membranes as shown by a 5.2 degrees C and 1.9 degrees C decrease in the transition temperatures of the membrane determined with the cholestane EPR probe.

Animals↗
PubMed1968

Applying "group" contingencies to the classroom study behavior of preschool children.

A group of 12 children were enrolled in a preschool class. During the first experimental stage they participated in special events contingent on token earning. Tokens were acquired by engaging in a variety of study behaviors. After a level of study behavior was established under this contingency, the special events were provided noncontingently. Study behavior declined throughout the noncontingent stage. Reestablishing the original contingencies produced an immediate return to the initial level of study behavior. Noncontingent special events reduced the amount of independent study, group participation, and cooperative study. The study behavior of each child was altered in the same direction, though differences in the magnitude of effects from child to child were observed.

Journal Article↗
PubMedDate not supplied

Age-dependent alterations in synaptic membrane systems for Ca2+ regulation.

The effects of aging on two neuronal plasma membrane Ca2+ regulating systems have been examined using synaptic membranes isolated from the brains of adult (5-7-month-old) and aged (23-25-month-old) Fisher 344 rats. The kinetic characteristics of the Na+-dependent Ca2+ transport system were found to be altered in the aged animals. The affinity of the transport carrier for Ca2+ was decreased in membranes from aged animals, with very little change in the maximal transport capacity of the system. The activity of the synaptic membrane Ca2+-activated, Mg2+-dependent ATPase was also altered in membranes from aged animals. In this system, however, the Vmax for Ca2+ activation of the enzymatic activity was lower in the aged animals, while there was no change in the K0.5 for Ca2+ activation. The magnitude of the alterations was small, but the differences were consistent. Even small changes in the effectiveness of the synaptic plasma membrane systems which participate in the maintenance of low intraterminal Ca2+ could progressively affect the integrity of synaptic transmission and lead eventually to neuronal cell death.

Aging↗
PubMedDate not supplied

Molecular mechanisms underlying age-dependent alterations in calcium homeostasis: the need for more information and new tools.

The picture that emerges from the numerous studies on altered calcium regulation in the nervous system as a function of aging does not permit the formulation of a scheme that accounts for available data. The solution to this dilemma requires a more complete understanding of the calcium regulating systems and new tools with which to probe the molecular events responsible for physiological and behavioral alterations in the aging organism.

Aging↗
PubMedDate not supplied

Differential effects of ethanol on two synaptic membrane Ca2+ transport systems.

The Na+-Ca2+ exchange activity in synaptic plasma membranes is inhibited by very low concentrations of ethanol (less than 25 mM). The high affinity Mg2+- and ATP-dependent Ca2+ transport in highly purified synaptic membranes is much less sensitive to inhibition by ethanol, with no statistically significant inhibition observed until an ethanol concentration of nearly 800 mM was used. Manipulations of the lipid environment designed to increase membrane fluidity enhanced the activity of the Na+-Ca2+ exchange system but inhibited the ATP-dependent Ca2+ pump. The differential responses of the two synaptic plasma membrane Ca2+ transporting systems to such modifications of membrane structure suggest that these two ion transport processes differ in the extent to which their activity is dependent on the lipid microenvironment in which they reside. Thus, the effects of ethanol on the Na+-Ca2+ antiporter may represent a fairly selective inhibitory process.

Adenosine Triphosphate↗
PubMedDate not supplied

Age-related decrease in brain synaptic membrane Ca2+-ATPase in F344/BNF1 rats.

We used Fisher 344/Brown Norway hybrid rats (F344/BNF1) to determine whether previously reported decreases in brain synaptic plasma membrane (SPM) Ca2+-ATPase activity in inbred F344 rats also occurred in the hybrids. Plasma membrane Ca2+-ATPase (PMCA) activity in SPMs from F344/BNF1 rat brains showed a progressive age-dependent decrease in Vmax from 60.9 +/- 3.7 nmol Pi/mg/min (n = 6) in 5-month rats to 32.4 +/- 3.6 nmol Pi/mg/min (n = 6) in 34-month animals, with no change in K (act) for Ca2+. Immunoreactive PMCA in SPMs also decreased by approximately 20% at 34 months, and the calmodulin (CaM) bound to membranes following extraction with EDTA also declined progressively with age. The effectiveness of CaM in stimulating PMCA activity was significantly lower when CaM was purified from the brains of old vs. young F344 rats and when CaM from 5-month rats was oxidized in vitro. These results indicate: 1) that PMCA activity in SPMs from longer lived F344/BNF1 hybrids also decreases with age; 2) that part of the reduction in PMCA activity is due to loss of PMCA from the membranes; and 3) that age-related structural changes in CaM may decrease its interaction with proteins in SPMs.

Aging↗
PubMedDate not supplied

Perineurial cell ensheathement of muscle fibers: a new syndrome of fatigable muscle weakness mimicking myasthenia gravis.

Many newly discovered pathological and physiological variants of neuromuscular junction function have been identified. We report a case of a myasthenia gravis-like syndrome with onset of ptosis at age six years and eventual evaluation for peripheral weakness twenty years later. Subsequent muscle biopsy showed a previously unreported finding of perineurial cell ensheathement of the muscle fibers. We suggest that the ensheathement of the muscle cells by perineurial cells may alter the microenvironment interfering with neuromuscular transmission.

Adult↗