Number Of Occupied Receptors Research Articles

Epidermal growth factor receptors in rat ovarian tissue

Epidermal growth factor is known to have mitogenic effects in the ovary. To determine whether this effect is receptor-mediated, and whether the receptor number changes during the estrus cycle, in this study we have evaluated the total and occupied EGF receptor (EGFr) concentrations and receptor binding characteristics in adult rat ovaries collected at diestrus and early estrus. These represent stages before and after the pituitary LH/FSH surges of the estrus cycle. The 100 000 g plasma membrane fractions were isolated and EGFr concentrations evaluated with and without 1 mM EDTA treatment at pH 4.5 in order to release receptor-bound ligand and therefore assess the unoccupied and total receptor concentrations. EGF receptor affinity and number were analyzed by Scatchard analysis. No statistically significant difference between unoccupied and total receptor concentration in the diestrus phase ovaries was found. The percent-age of occupied receptors (total-unoccupied) in diestrus membranes was 7.1%. In early estrus ovaries, a significant increase ( P < 0.05) was observed in total receptor concentration when compared with the number of unoccupied receptors. The percentage of occupied receptors in early estrus membranes was 35.7%. Comparison between the ovaries from the two phases indicated an almost twofold increase in total EGF receptor number in early estrus membranes when compared with diestrus membranes. Furthermore, the percentage of occupancy between the two studied groups indicate a significant increase ( P < 0.05) in the number of occupied receptors in early estrus when compared to diestrus. Thus, we have demonstrated that not only EGF receptor concentrations are modulated through the rat estrus cycle, but also the concentration of EGF-like substances bound to such receptors.

Aging Affects Epidermal Growth Factor Receptor Phosphorylation and Traffic Kinetics

With increasing donor age, cultured human fibroblasts express fewer epidermal growth factor receptors and display decreased mitogenic responsiveness to epidermal growth factor. To determine age-associated differences in epidermal growth factor receptor phosphorylation and traffic kinetics, we studied in fibroblasts derived from donors of different ages autophosphorylation of the receptor after ligand binding and trafficking of the receptor–ligand complexes. We now report an age-associated delay in the rate of receptor phosphorylation after epidermal growth factor stimulation. Furthermore, receptor/ligand trafficking is affected by aging. There is an age-associated decrease and delay in the number of occupied receptors that are transported intracellularly and in their rate of clearance from the plasma membrane. Our data show that aging affects receptor/ligand activation and processing and suggest that the decreased cellular mitogenic response with aging may be, at least in part, the result of decrements in receptor activation and processing.

125I-thrombin binds to clustered receptors on noncoated regions of mouse embryo cell surfaces.

We used electron microscope autoradiography (EMAR) to visualize the interaction of 125I-thrombin with its surface receptors on mouse embryo (ME) cells. Autoradiographic grains were spaced over the surface of cells in a periodic nonrandom pattern, indicating 125I-thrombin association with clusters of thrombin receptors. The grain spacing varied slightly from cell to cell, indicating subpopulations of cells with different numbers of thrombin receptors. The average distance between grains on ME cells after binding 125I-thrombin (125 ng/ml) at 37 degrees C was 1.65 +/- 0.49 microns. The average distance between grains on prefixed cells and cells incubated with 125I-thrombin at 4 degrees C was not significantly different from that observed at 37 degrees C. This indicates that thrombin receptors are clustered before thrombin binding and that the thrombin receptor aggregates do not redistribute into large aggregates on the surface of cells subsequent to thrombin binding. The number of grains per cluster also does not change under these three binding conditions. Thus, the number of occupied receptors in each cluster appears to be constant. On the basis of the average grain number and spacing, we estimate that each cluster is approximately 400 nm in diameter containing approximately 550 thrombin-binding sites. These receptor-clusters are not associated with specialized structures or coated regions of the membrane. Additionally, grains observed within cells were not found associated with coated vesicles. Therefore, neither the clustering patterns nor internalization of 125I-thrombin are characteristic of molecules which bind to receptors and are internalized by receptor-mediated endocytosis.

Epilepsy and schizophrenia: A neurochemical bridge

This paper reviews some of the evidence in the literature that suggests neurochemical processes by which the regulation of seizure threshold and the onset of schizophrenic-like symptoms are interrelated. For those patients who experience the alternation of seizures and acute psychoses the following working hypothesis is presented: The central dopaminergic synapse is described as a malfunctioning regulatory circuit. Insufficient feedback inhibition or lifting of the setpoint leads to an increased number of occupied receptors. This might cause schizophrenic-like symptoms. Insufficient release of feedback control or lowering of the setpoint leads to a decreased number of occupied receptors. This might increase seizure susceptibility. The neurochemical arguments in support of this hypothesis will be discussed in detail. The localization of the dopaminergic synapses involved in the development of schizophrenic-like symptoms is subject to speculation. The meso-limbic dopaminergic system has to be taken into consideration.

Existence of receptors bound to endogenous estradiol in breast cancers of premenopausal and postmenopausal women

The amounts of free and occupied estrogen receptors were determined in cytosols and cellular (cytoplasmic + nuclear) extracts of 23 human breast cancers. In vivo undersaturation of the receptors was observed in all tumors. A positive correlation was found between the degree of saturation and the concentration of circulating estrogens in premenopausal women. The saturation of the sites was higher in cellular extracts than in cytosols, pointing to the existence of receptor-estradiol complexes bound to nuclei. Our results suggest that the absolute number of occupied receptors depends upon the level of available cytoplasmic receptors as well as upon the level of tissue estradiol.