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Biomedical subjects

H Schoemaker

Publications and source records attributed to H Schoemaker.

At least 73 records · Page 4Linked to original sources
PubMed1984

Temperature-dependent modulation of [3H]nitrendipine binding by the calcium channel antagonists verapamil and diltiazem in rat brain synaptosomes.

Binding of the dihydropyridine calcium channel antagonist [3H]nitrendipine to an intact rat brain mitochondrial-synaptosomal fraction (P2) was specific, saturable, temperature-dependent and of high affinity (Kd = 115-467 pM). The effects of the calcium channel antagonists verapamil and diltiazem on [3H]nitrendipine binding and their temperature dependence were investigated. At 0 and 25 degrees C, verapamil inhibited [3H]nitrendipine binding incompletely in a manner consistent with an allosteric modulation and nearly independent of the incubation temperature. The effects of diltiazem, however, were found to be highly temperature-dependent. At 25 and 37 degrees C, 10 microM diltiazem enhanced [3H]nitrendipine binding to values of 140 and 200% of control, respectively. At 0 degrees C, 10 microM diltiazem inhibited [3H]nitrendipine binding to a value of 68% of control. Analysis of saturation isotherms at steady state demonstrated that at all temperatures studied the effects of verapamil and diltiazem on [3H]nitrendipine binding were due to alterations in the ligand dissociation constant (Kd). At 25 degrees C, these alterations were mediated by changes in the rate of ligand-receptor complex dissociation. Competition studies of verapamil and diltiazem at 25 and 0 degrees C indicate that the effects of these two drugs on [3H]nitrendipine binding are mutually exclusive. We conclude that the binding of [3H]nitrendipine is allosterically modulated by spacially related binding sites for verapamil and diltiazem.

Animals↗
PubMed1983

Effect of chronic ethanol consumption on central and peripheral type benzodiazepine binding sites in the mouse brain.

Following chronic exposure of C57/BL6 mice to ethanol the binding of [3H]Ro5-4864 and [3H]propyl-beta-carboline-3-carboxylate to benzodiazepine binding sites in the brain was studied. Peripheral-type benzodiazepine binding sites were measured using the probe [3H]Ro5-4864. Chronic ethanol treatment resulted in a significant increase in [3H]Ro5-4864 binding due to a 43% increase in receptor density. The affinity of [3H]Ro5-4864 for the receptor was not significantly affected. The binding of [3H]propyl-beta-carboline-3-carboxylate to central-type benzodiazepine receptors was not affected by chronic ethanol treatment.

Alcoholism↗
PubMed1983

Dopamine uptake by rat striatal synaptosomes: time- and temperature-dependent decay and protection by dithiothreitol and dopamine.

The uptake of [3H]dopamine (DA) into rat striatal synaptosomes in the presence of a monoamine oxidase inhibitor was studied using a filtration technique. After a 10-min preincubation period, a fast initial uptake of [3H]DA was seen. Uptake reached a maximum after 4 min of incubation. If incubation was continued for more than 7 min, a gradual decrease in synaptosomal [3H]DA levels was found. Uptake was dependent on preincubation time; initial uptake velocity and maximal uptake decreased irreversibly with increasing preincubation periods. Moreover, the capacity of the synaptosomes to retain the [3H]DA during longer incubation times was progressively affected. The decrease in initial uptake activity was due to a decrease in the Vmax of the transport system. Dithiothreitol (2.8 mM) protected synaptosomal uptake activity against deterioration at 37 degrees C. Also, DA itself (10(-7)M) stabilized the uptake mechanism if added to the suspension before preincubation was started. Since [3H]DA uptake observed after loading the synaptosomes with labeled DA was similar to the uptake seen if the synaptosomes were not previously loaded with DA, it was concluded that under these conditions synaptosomal DA is completely exchangeable with exogenous substrate. Prolonged storage of the synaptosomes at 0 degree C also resulted in a time-dependent decrease in uptake activity (t1/2 = 116 min). The addition of unlabeled DA or dithiothreitol to the suspension did not affect instability at 0 degree C.

Animals↗
PubMed1983

Specific high-affinity binding sites for [3H]Ro 5-4864 in rat brain and kidney.

The binding of the novel ligand [3H]Ro 5-4864 to membrane preparations of rat kidney and brain was studied. [3H]Ro 5-4864 binds with high affinity (Kd = 0.6 nM) to a single saturable population of benzodiazepine recognition sites on renal membranes. Binding is rapidly reversible and, based on its pharmacological spectrum, takes place at the peripheral-type, Ro 5-4864-sensitive receptor. Specific high-affinity (Kd = 1.1 nM) [3H] Ro 5-4864 binding to the peripheral-type benzodiazepine binding site can also be demonstrated using rat brain membranes. [3H] Ro 5-4864 lacks stereospecificity with regard to chiral activity in position 3. A comparison of benzodiazepine inhibitory potency and structural features reveals that whereas a 4'-substitution assures specificity for the peripheral-type receptor, an N-methyl moiety is essential for optimal activity. [3H]Ro 5-4864 binding to brain membranes is temperature sensitive and is not modulated by barbiturates, convulsants, gamma-aminobutyric acid and chloride anions. The pyrazolopyridine derivative tracazolate inhibits [3H] Ro 5-4864 binding. The regional and subcellular distribution of binding is distinctly different from that previously demonstrated for [3H]benzodiazepine binding in the brain. The olfactory bulb shows the highest binding density, whereas the cerebral cortical, striatal and hippocampal areas are lowest among those areas studied. In the brain, [3H]Ro 5-4864 binding was found to sediment with the nuclear fraction. In conclusion, the present study shows that [3H]Ro 5-4864 is a selective ligand of the peripheral-type benzodiazepine binding site that can unequivocally be demonstrated in the kidney as well as the brain. The physiological significance of these findings, however, remain to be established.

Animals↗
PubMed1983

beta-Endorphin and its metabolites stimulate motility of the dog small intestine.

Previous studies have demonstrated that beta-endorphin and enkephalins are released into the systemic circulation by the pituitary and adrenal medulla, respectively. To determine whether the small intestine could be a target for circulating beta-endorphin, segments of small intestine were removed from anesthetized dogs and perfused with Krebs-bicarbonate buffer containing beta-endorphin (1 microgram/ml), while motility was recorded and venous effluent collected in 1-min fractions (23 ml). beta-Endorphin significantly (P less than .002) increased motility of intestinal segments. High-performance liquid chromatographic analysis of the venous effluent identified, among others, several alpha- and gamma-type endorphins. Several of the identified peptide fragments were then perfused through intestinal segments to determine their motility effects. alpha-Endorphin, gamma-endorphin, des-tyrosine-alpha-endorphin and des-tyrosine-gamma-endorphin, significantly increased motility at doses of 1 microgram/ml. These responses were characterized by an increase in phasic contractions of constant amplitude and frequency. To determine regional specificity and site of beta-endorphin metabolism during perfusion, we studied time course processing of beta-endorphin in mucosal and muscularis homogenates in vitro. The mucosa was much more enzymatically active than the muscularis and produced 3-fold more gamma-endorphin than alpha-endorphin, whereas the reverse was found in the muscularis. These studies demonstrate that the small intestine can metabolize beta-endorphin into a number of active fragments which increase motility and suggest a regional specificity of enzymatic processing of beta-endorphin in the dog intestine.

Animals↗
PubMed1983

Radioimmunometric assay for a monoclonal antibody-defined tumor marker, CA 19-9.

We describe a solid-phase radioimmunometric sandwich assay for a new tumor marker defined by a monoclonal antibody (19-9). This antibody reacts with a carbohydrate antigenic determinant (CA 19-9) found at low concentrations in sera from healthy individuals but frequently increased in sera from patients with adenocarcinomas. The assay is sensitive and simple to perform. It requires duplicate 100-microL samples and may be performed in 6 h. The concentration of CA 19-9 in samples is determined by reference to a standard curve, which is essentially linear from 0 to 120 arbitrary units/mL. The average CV is approximately 10% in the range of 5.8 to 120 units/mL. The minimum detectable dose is 1.4 units/mL and analytical recovery of CA 19-9 is 97.6 to 100.6%. The average concentration of CA 19-9 in sera from 1020 healthy individuals was 8.4 (SD 7.4) units/mL; only 0.6% of such sera had concentrations greater than 37 units/mL. The assay has high specificity (98.5%), even among patients with benign diseases, and has high sensitivity (up to 79%) for patients with gastrointestinal adenocarcinomas, especially those of the pancreas.

Adenocarcinoma↗
PubMed1982

Interaction between dopamine influx and efflux in rat striatal synaptosomes.

Exchange release has previously been proposed as the mechanism by which dopamine is transported over the synaptosomal membrane. An increase in carrier-mediated dopamine release should therefore result in an enhanced rate of synaptosomal dopamine uptake. In order to test this hypothesis, rat striatal synaptosomes were incubated with 14C-dopamine until equilibrium. Then, the 14C-dopamine concentration in the medium was reduced in order to elicit various rates of net dopamine efflux, while influx was monitored using tracer amounts of 3H-dopamine. Dopamine release was concentration dependent and saturable and thus may be carrier-mediated. In contrast to that expected for a mechanism of exchange release, dopamine influx was depressed as compared to equilibrium conditions. Furthermore, nomifensine, a specific inhibitor of dopamine influx, did not attenuate dopamine efflux. It is concluded that dopamine transport over the synaptosomal membrane may not be via a mechanism of strict exchange release.

Animals↗
PubMed1982

Identification of beta-endorphin-6(16-17) as the principal metabolite of des-tyrosin-gamma-endorphin (DTgammaE) in vitro and assessment of its activity in neurotransmitter receptor binding assays.

Des-tyrosine-gamma-endorphin (beta-endorphin-(2-17); DTgamma E) lacks direct in vitro activity at dopaminergic receptors, but does inhibit in vivo [3H]spiperone binding in various rat brain areas. The principal objective of these studies was to test the hypothesis that DTgammaE may exert its selective, neuroleptic-like activity through an active metabolite. Accordingly, DTgammaE was incubated at 37 degrees C in a whole rat brain homogenate of neutral pH after which samples were prepared for HPLC analysis. The major, heat-stable metabolite of DTgammaE was identified as the clinically active, beta-endorphin related fragment, beta-endorphine-(6-17). The beta-endorphin sequences 4-17, 5-17, l0-17, 12-17 and 2-16 were also present but in minor amounts. Identical results were obtained studying DTgammaE metabolism using rat striatal tissue slices. Neurotransmitter receptor binding experiments showed that beta-endorphin-(6-17) was inactive at central dopaminergic, serotonergic, muscarinic, benzodiazepine and opiate receptors measured in vitro. Thus, like DTgammaE, beta-endorphin-(6-17) differs from classical neuroleptics in that it does not inhibit in vitro [3H]spiperone binding in the corpus striatum, frontal cortex or mesolimbic areas of the rat brain. It may be that DTgammaE and beta-endorphine-(66-17) exert their selective neuroleptic-like activity through an indirect inhibition of central dopaminergic activity, possibly in combination with an in vivo antagonism of the postsynaptic dopamine receptor.

Animals↗