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Biomedical subjects

M R Clark

Publications and source records attributed to M R Clark.

At least 199 records · Page 11Linked to original sources
PubMed1978

Monovalent cation transport in irreversibly sickled cells.

Using discontinuous density gradients of Stractan II, we have separated sickle cell blood into discrete subpopulations of reticulocytes, mature discoid cells, and irreversibly sickled cells (ISCs). We have measured active and passive fluxes of monovalent cations in mature discoid cells, ISCs, and normal control cells, also separated upon density gradients. These measurements revealed a decreased active cation transport in ISC-rich populations. However, parallel measurements of Na, K-ATPase activity showed normal ouabain-sensitive ATPase activity in ISCs. Passive permeability to external Rb was also normal in ISCs. The observation of depressed pump activity in intact ISCs, contrasted with normal ATPase activity in ISC membranes, suggests the presence of factors in the intact cell which inhibit the active transport of Na and K in ISCs.

Adenosine Triphosphatases↗
PubMed1978

Stimulation of prostaglandin accumulation in preovulatory rat follicles by adenosine 3',5'-monophosphate.

LH stimulates an increase in prostaglandins in vitro in preovulatory follicles from rats pretreated with PMS gonadotropin. The role of cAMP in this action of LH was examined by incubating preovulatory follicles with various substances and determining the resultant prostaglandin (PG) E accumulation by radioimmunoassay. LH (5 microgram/ml) increased PGE accumulation to approximately 4 times the control (181 +/- 23 to 886 +/- 83 pg/follicle). The addition of 20 mM cAMP also stimulated PGE accumulation, and the addition of 20 mM cAMP in the presence of 0.5 mM 1-methyl-3-isobutylxanthine was as effective as LH. Other nucleotides such as ATP, ADP, 3'-AMP, 5'-AMP, cGMP, and O2'-monobutyryl-cAMP did not stimulate PGE accumulation. On the other hand, (Bu)2cAMP, 8-bromo-cAMP, and N6-monobutyryl-cAMP produced an increase in PGE accumulation similar to that observed with LH. In addition, 10 microgram/ml cholera toxin was shown to increase both cAMP and PGE accumulation in preovulatory follicles. These results indicate that the prostaglandin response of follicles is specific for cAMP-like nucleotides or substances capable of increasing intracellular cAMP. The data support the concept that cAMP mediates the effect of LH on PGE accumulation in preovulatory follicles in the rat.

Adenine Nucleotides↗
PubMed1978

Monovalent cation composition and ATP and lipid content of irreversibly sickled cells.

Discontinuous Stractan gradients were used to separate heterogeneous populations of sickle cells into discrete subpopulations containing varying proportions of reticulocytes, mature discoid cells, and irreversibly sickled cells (ISC). The improved homogeneity of these preparations, together with an enhanced yield of ISC, allowed us to distinguish effects of cell maturation from those of irreversible sickling. With these cell preparations we have begun to define physical properties of ISC. We confirmed the marked abnormalities in cation composition of native ISC. Measurements of ATP in ISC did not substantiate prior reports of ATP deficits. Finally, no evidence for substantial loss of membrane lipids during the process of ISC formation could be demonstrated.

Adenosine Triphosphate↗
PubMed1976

Regulation of ovarian steroidogenesis. The disparity between 125I-labelled choriogonadotropin binding cyclic adenosine 3',5'-monophosphate formation and progesterone synthesis in the rat ovary.

The binding of 125I-labelled human choriogonadotropin, formation of cyclic adenosine 3',5'-monophosphate (cyclic AMP), and synthesis of progesterone were examined in ovarian cells from immature rats. Collagenase dispersed ovarian cells were found to respond specifically to lutropin-like activity. The equilibrium dissociation constant (Kd) for the binding of 125I-for the binding of 125I-labelled choriogonadotropin was 1.7-10(-10) M. Progesterone synthesis was increased at least 40-fold and cyclic AMP formation 10-fold in response to maximum hormonal stimulation. The concentration of choriogonadotropin which stimulated progesterone synthesis maximally in Eagle's minimum essential medium--0.1% gelatin (2 ng/ml), resulted in minimal (less than 30% of maximum) increases in cyclic AMP accumulation and hormone binding. Similarly, binding of choriogonadotropin was not saturated at a hormone concentration (50 ng/ml) that stimulated maximal cyclic AMP formation. These results are consistent with the existence of receptor reserve in the ovarian cell. A marked shift in the dose vs. response relationship for progesterone synthesis occurred when fetal calf serum was used to supplement Eagle's minimum essential medium, however. Under these experimental conditions, progesterone synthesis reached a maximum at a hormone concentration of the same order of magnitude as did cyclic AMP formation. It is concluded that the degree of spare receptor effect observed may depend not only on an absolute amount of excess receptor, but also on the readiness of the system to respond in a given fashion.

Animals↗
PubMed1976

Ovarian adenosine 3':5'-cyclic monophosphate-dependent protein kinase(s). Regulation by choriogonadotropin and lutropin in rat ovarian cells.

Choriogonadotropin and lutropin have been found to activate cyclic AMP-dependent protein kinase in ovarian cells isolated by collagenase dispersion from immature rats. The stimulatory effect of gonadotropins was dependent on both hormone concentration and incubation time. Choriogonadotropin at 1 mug/ml fully stimulated the protein kinase activity within 5 min of incubation, and this effect was specific for choriogonadotropin and lutropin-like activity. In addition, protein kinase activity has been characterized with respect to salt sensitivity, cyclic AMP binding, and its responsiveness to gonadotropins and other peptide hormones. Ovarian protein kinase was susceptible to high salt concentrations. The addition of 0.3-1.0 M-NaCl in incubation medium increased the activity ratio with a concomitant decrease in cycle AMP-dependence. The salt effect on protein kinase was observed both from hormone-treated and untreated cells. The hormone-stimulated and unstimulated protein kinase activity was completely stable in the absence of NaCl. No change in the activity ratio was observed when cellular extracts were assayed for protein kinase activity either immediately or after 2 h in the absence of added salt. Gel filtration in the absence of NaCl of cellular extracts prepared from choriogonadotropin-treated and untreated cells showned only a single peak of protein kinase activity that was sensitive to exogenously added cyclic AMP. By contrast, when 0.5 M-NaCl was included in the column buffer, the chromatography of untreated extract showed two peaks of protein kinase activity. The first peak was sensitive to added cyclic AMP, whereas the second peak was insensitive to it. Under identical experimental conditions, protein kinase from gonadotropin-treated cells showed, on gel filtration, only one peak of activity that was totally insensitive to added cyclic AMP. DEAE-cellulose column chromatography of a 20000 g supernatant fraction resulted in a peak of kinase activity that eluted in approx. 0.15 M-NaCl, similar to the similar to the elution of type II protein kinases as described by Corbin et al. (1975) (J. Biol. Chem. 250, 218-225). Choriogonadotropin stimulation produced a decrease in the capacity of protein kinase to bind exogenous cyclic [3H]AMP, with a concomitant increase in the kinase activity ratio. These results are consistent with the notion that cyclic AMP, GENERATED IN SITU Under hormonal stimulation, binds tot he regulatory subunit of protein kinase with subsequent dissociation of the active catalytic subunit from the holoenzyme.

Animals↗
PubMed1976

The role of protein synthesis in the stimulation by LH of prostaglandin accumulation in rat preovulatory follicles in vitro.

Preovulatory follicles isolated from immature rats, treated in vivo with pregnant mare's serum gonadotropin, were incubated in vitro and the accumulation of prostaglandin E measured. The addition of luteinizing hormone (5 mug/ml) increased this accumulation, after a lag period of 3 hours. This delay suggested the involvement of macromolecular synthesis in the mechanism of prostaglandin stimulation by luteinizing hormone. When the synthesis of protein was inhibited by the addition of puromycin (100 muM), the luteinizing hormone stimulation of prostaglandin E in these follicles was completely abolished. This inhibition was not seen with an analogue of puromycin, which does not inhibit protein synthesis, puromycin amino-nucleoside. These data suggest that concomitant protein synthesis is required for the luteinzing hormone stimulation of prostaglandin accumulation in rat follicles.

Animals↗
PubMed1976

Regulation of cyclic adenosine 3', 5' -mono-phosphate dependent protein kinase of rat ovarian cells by luteinizing hormone and human chorionic gonadotropin.

Regulation of cAMP dependent protein kinase activity from rat ovarian cells has been studied in response to luteinizing hormone and human chorionic gonadotropin. Treatment of cells with human chorionic gonadotropin in concentration range of 2.5ng-1000ng/ml resulted in increased accumulation of cAMP,activation of protein kinase followed by the stimulation of progesterone synthesis. A sixfold increase in the activity ratio, defined as the ratio of protein kinase stimulated in situ to that maximally stimulated in vitro by exogenous cAMP, was observed with 1ug/ml of hCG. This concentration of hormone also produced a ten-fold increase in cAMP and a thirty-to forty-fold increase in progesterone synthesis. Protein kinase activation was specific for LH and hCG, as other polypeptide hormones were without any appreciable effect. The stimulation of protein kinase persisted even after the elevated cAMP level began to fall. It appears that the activation of protein kinase is an obligatory early event that mediates an increase in gonadotropin stimulated progesterone synthesis.

Adenosine Triphosphate↗
PubMed1976

Hybrid erythrocytes for membrane studies in sickle cell disease.

A hybrid erythrocyte model for membrane studies in sickle cell disease has been developed. The model consists of normal red cell membranes containing hemoglobin S and sickle cell membranes containing hemoglobin A. In hybrids, complete hemoglobin exchange has been achieved together with restoration of low membrane permeability to potassium. Normal membranes containing HbS sickle upon deoxygenation and assume the characteristic appearance of irreversibly sickled cells (ISC) after prolonged anoxia. It is suggested that the hybrid model will be useful in defining further the process of ISC formation and in studying the influence of sickle hemoglobin upon the function of the surrounding membrane.

Anemia, Sickle Cell↗
PubMed1976

Stabilization of the shape of sickled cells by calcium and A23187.

Evaluation of the role of calcium in irreversible sickling has been approached by treating sickled cells with calcium and the ionophore A23187. A calcium-dependent stabilization of the sickled cell shape was observed after reoxygenation of cells in the presence of ionophore. At low calcium concentrations, this retention of sickled shape was maintained for periods up to 1 hr. However, the morphology of the oxygen-stable sickled cells was like that of deoxygenated sickle cells and significantly different from the characteristic morphology of native irreversibly sickled cells (ISCs). Because the stabilized cells did not fulfill the morphological criterion for ISCs, the shape-stabilizing effect of calcium in this system did not provide additional support for the hypothesis that calcium accumulation was the determining factor in ISC generation.

Anemia, Sickle Cell↗
PubMed1975

Computer simulation of overshoot in saccadic eye movements.

The human horizontal eye movement system produces quick, precise, conjugate eye movements called saccades. These are important in normal vision. For example, reading tasks exclusively utilize saccadic eye movements. The majority of saccades have dynamic overshoot. The amplitude of this overshoot is independent of saccadic amplitude, and is such that it places the image of the stimulus within the retinal region of maximum acuity within a minimum of time. A computer based model of the saccadic mechanisms was used to study the origin of this overshoot. It was discussed that dynamic overshoot cannot be attributed to biomechanism properites of the eye movement mechanism, but must instead be explained by variations in the controlling nervous activity. The form of this neural controller signal is very similar to that required for a time optimal response of an inertial system.

Computers↗
PubMed1975

Closely spaced saccades.

The relationships between saccadic velocity, duration, and magnitude have been used to prove the normalcy of saccades with intersaccadic intervals of less than 200 ms. Pairs of normal saccades with small intersaccadic intervals will have the second saccade larger or smaller and going in the same or the opposite direction than the first saccade. These normal saccades may be horizontal, vertical, or oblique.

Electrooculography↗