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Biomedical subjects

B K Davis

Publications and source records attributed to B K Davis.

At least 37 records · Page 2Linked to original sources
PubMed1991

Kinetics of rapid RNA evolution in vitro.

A rapidly acquired partial resistance to the replicase antagonist, ethidium bromide (EB), seen by Spiegelman and coresearchers in Q beta RNA variants competitively replicating under defined conditions in vitro, reflected existence of a pool of mutant RNA molecules, preadapted to EB, and their cross-propagation from the pre-EB optimum species, MDV-1, and from other kindred variants, some of which remained undetected, according to this quantitative analysis of midivariant RNA replication kinetics. DNAlike features of their evolution, such as the cloning of variants from an MDV-1 subtype and a compliance with the fundamental theorem of natural selection, resulted from the suppression, both real and apparent, of intrinsic RNA heterogeneity through sampling and detection methods, and also by the ascendency of self-propagation over cross-propagation with advancement of a superior variant. The deficit in mean polymer fitness, compared with optimum levels, determines the lower limit of this heterogeneity. Stability conditions for frequency equilibrium and strategies for counteracting viral drug resistance have been considered.

Biological Evolution↗
PubMed1991

Variations in polymer fitness at elevated mutation rates.

Two series expressions were obtained that give the first and second order rates of change in population fitness during competitive replication at elevated mutation rates. At their zero-error limit, the respective power series reduces to the second (Fisher's theorem) and third moments of the fitness distribution. The first series maximized the variation in mean polymer fitness, for a given amount of population covariance. From experimental results reported by Spiegelman's group on evolution in vitro among Q beta RNA variants, it was demonstrated: (i) terms in the (second) series fall-off at a rate broadly equal to the replicase error (epsilon congruent to 10(-4); (ii) the rate of change in mean RNA fitness (polymer formation rate constant) corresponds to the variance in fitness; and (iii) agreement exists between second order rate changes in fitness and the third moment (skewness) regression line, extending over 20 successive replication reactions. The impact of these findings on the standard model of evolution has been discussed.

Biological Evolution↗
PubMed1991

The behavioral effects of bilateral middle cerebral artery hemorrhagic ischemia in rat.

After learning position discrimination in a T-maze water escape task, rats had either a 2 mm section of the middle cerebral artery removed bilaterally (bMCA) or they received a sham operation. Beginning on the day of surgery either total brain gangliosides (50 mg kg-1) or saline were administered daily for five days. Of the several measures of neurological function that were tested, only a temporary deficit in grasping with the front paws was observed in bMCA damaged rats. Ganglioside treatment normalized this practical function. Memory of the preoperative habit was not influenced by bMCA damage, but acquisition of a reversal of this habit was compromised. Ganglioside treatment did not influence this deficit. Acquisition of a spatial alternation strategy was influenced by neither the bMCA lesion nor the ganglioside treatment. The preservation that accompanies bMCA interruption might serve as a useful model of the functional declines that accompany stroke and frontal lobe damage.

Animals↗
PubMed1991

Photo-activation of resin cements through porcelain veneers.

The purpose of this study was to determine the effects of porcelain opacity, chemical catalyst, and exposure time on polymerization of light-activated resin-composite cements. Samples of microfill and hybrid composites, with and without catalyst (i.e., dual-cure and visible-light-activated), were polymerized by exposure to visible light through porcelain discs of different opacities. Microhardness testing (KHN) was used to compare degree of cure for each material at various exposure times. Porcelain opacity did not significantly affect hardness. However, the results indicated that a chemical catalyst and prolonged curing times might be essential for clinical success.

Analysis of Variance↗
PubMed1984

Adenosquamous carcinoma of the prostate.

An unusual adenosquamous carcinoma originating in the prostate of a 70-year-old man is described. Four years prior to the development of this lesion, a diagnosis of adenocarcinoma had been established by needle biopsy and the patient had been treated with radiation. Light microscopic examination revealed a pattern of adenocarcinoma predominant in the periurethral region and squamous cell carcinoma at the periphery. The latter neoplastic component involved the bladder and rectum and extended to the pelvic wall. Immunoperoxidase staining for prostatic acid phosphatase occurred in both cellular components, and electron microscopic examination confirmed the squamous differentiation of tumor cells.

Adenocarcinoma↗
PubMed1983

Diagnosis of Legionnaires' disease. An update of laboratory methods with new emphasis on isolation by culture.

A prospective clinical study of 40 cases of legionnaires' disease combined with serial laboratory examinations enabled us to present an update as well as new recommendations concerning the use of diagnostic tests for legionnaires' disease. Transtracheal aspirate specimens are the optimal specimen for recovery of Legionella pneumophila by culture as well as the most sensitive method for early diagnosis. In addition, with recent improvements in culture media and methodology, L pneumophila can now be readily isolated from sputum. Examination of respiratory specimens by direct immunofluorescence (DFA) is useful, but the sensitivity is much less than that of culture. The yield from DFA examination directly correlates with the number of L pneumophila recoverable by cultural methods; thus, the DFA test result may be negative in an early or mild case of legionnaires' disease. Antibody titers were elevated in 27% of cases within one week of onset of pneumonia and may, therefore, be useful in early diagnosis in selected patients.

Antibodies, Bacterial↗
PubMed1983

Binding by glycoproteins of seminal plasma membrane vesicles accelerates decapacitation in rabbit spermatozoa.

Fertilizing capacity among uterine-capacitated rabbit sperm cells declined exponentially during incubation with membrane vesicles from seminal plasma. In suspensions containing an average of 0.42 mg vesicle protein/10(6) sperm, decapacitation occurred with a half-time of 23 min (ki (native vesicles) = 1.78 +/- 0.14 h-1). Exposing these membrane vesicles to pronase retarded decapacitation, prolonging its half-time to 51 min (ki (pronase-digested vesicles) = 0.81 +/- 0.06 h-1). Cholesterol-bearing liposomes suppressed sperm-fertilizing capacity at a comparable rate. In suspensions containing an average of 0.52 mg lipid/10(6) sperm, decapacitation had a half-time also of 51 min (ki (liposomes) = 0.82 +/- 0.14 h-1). These lower inhibition rates accompanied diminished rates of vesicle uptake by spermatozoa. Membrane vesicles labeled with phosphatidyl[14C]choline rapidly bound to epididymal sperm cells, displaying a half-time of 2.3 min (ka (native vesicles) = 18.0 +/- 0.35 h-1). Following pronase treatment, this interval increased to 17 min (ka (pronase-digested vesicles) = 2.48 +/- 0.37 h-1). Liposome binding data yielded a half-time of 28 min (ka (liposomes) = 1.47 +/- 0.17 h-1). Postbinding decapacitation half-times for these vesicles, given by the difference between binding and decapacitation intervals, appear broadly alike: native vesicles, 21 min, pronase-digested vesicles, 34 min, and liposomes, 23 min. During this interval, a vesicle antifusigen (cholesterol) apparently transfers to the sperm plasma membrane inhibiting the acrosome reaction. The lipid bilayer in these membrane vesicles withstood proteolytic attack, as seen by electron microscopy. Pronase acted principally to hydrolyze vesicle glycoproteins, which evidently bind to the sperm surface during decapacitation.

Animals↗
PubMed1981

Timing of fertilization in mammals: sperm cholesterol/phospholipid ratio as a determinant of the capacitation interval.

A survey of species differences in the duration of capacitation, T, has revealed that they closely correlate with sperm cholesterol/phospholipid mole ratios, R : T = 8R - 1 (r2 = 0.97, in which r is Pearson's correlation coefficient). Because uterine cells displayed low relative cholesterol concentrations, spermatozoa evidently experience a negative external cholesterol gradient (positive phospholipid gradient) during capacitation. A decrease in sperm R-value is suggested, therefore, to accompany capacitation. The idea received strong support from a kinetic analysis of capacitation intervals, based on the rate of cholesterol efflux from sperm cells in utero. Lipid-binding serum proteins in uterine fluid are attributed with removing a sterol barrier to the Ca2+-facilitated membrane fusion that initiates the acrosome reaction. Tight cell junctions prevent permeation of the male generative tract by these proteins (capacitation factors). Furthermore, seminal plasma contains a decapacitation factor, identified as a membrane vesicle (cholesterol donor) component of this fluid, that reverses capacitation. Initiation of the sperm acrosome reaction among mammals could be the first fusion process found to be physiologically modulated through the membrane bilayer cholesterol level.

Acrosome↗
PubMed1980

Studies on the mechanism of capacitation: albumin-mediated changes in plasma membrane lipids during in vitro incubation of rat sperm cells.

Plasma membrane isolated from rat sperm cells after incubation in vitro had a significantly lower cholesterol/phospholipid mole ratio when the medium contained serum albumin. Transfer of albumin-bound phospholipids to the membrane can largely account for this effect. The result is broadly consistent with a previously proposed model for albumin-induced destabilization of sperm membrane (capacitation) and its reversal by seminal plasma membrane vesicles. Albumin also decreased sialic acid and, more specifically, ganglioside levels, presumably by promoting release of sperm neuraminidase. Cholesteryl ester comprised up to 0.5 mol/mol of cholesterol in these plasma membrane preparations.

Adenosine Triphosphatases↗
PubMed1980

Interaction of lipids with the plasma membrane of sperm cells. I. The antifertilization action of cholesterol.

The inhibitory action of cholesterol-containing suspensions on fertilizing capacity in uterine-capacitated rabbit sperm cells showed a direct dependence on the concentration of sterol. Dispersion with synthetic phosphatidylcholine as a nonsonicated suspension or as liposomes did not alter this antifertilization effect. Esterification of the sterol, however, caused a complete loss of inhibitory activity. Cholesterol inhibited induction of the acrosome reaction among epididymal rat spermatozoa incubated under chemically defined conditions. Other agents with a negative effect on the acrosome reaction were seminal plasma membrane vesicles and palmitic acid. Egg lecithin-liposomes and bovine serum albumin, especially after being delipidated, facilitated it. These results corroborate the viewpoint that changes in the lipid bilayer of sperm plasma membrane significantly influence fertilizing capacity among mammalian spermatozoa.

Animals↗
PubMed1980

Interaction of lipids with the plasma membrane of sperm cells. II. Evidence of a membrane thermotropic transition.

Cholesterol and phosphatidylcholine uptake from dipalmitoyl-phosphatidylcholine liposomes by rabbit spermatozoa showed a complex dependence on temperature in these experiments. At 5 degrees and 20 degrees C, the rate of lipid uptake correlated with temperature. However, from 20 degrees to 37 degrees C uptake did not evidently increased. The results in interpreted as evidence of a thermotropic transition in the sperm plasma membrane. Data are presented showing incorporation of these lipids, especially that of cholesterol, into sperm plasma membrane.

Animals↗
PubMed1979

Studies on the mechanism of capacitation. II. Evidence for lipid transfer between plasma membrane of rat sperm and serum albumin during capacitation in vitro.

1. Evidence has been provided for the transfer of phosphatidyl[14C]choline and [3H]cholesterol between bovine serum albumin and cauda epididymal rat spermatozoa in Krebs-Ringer bicarbonate medium, which can promote sperm capacitation. 2. An analysis of the lipid composition in both albumin and spermatozoa revealed that phospholipid levels decreased in the protein and increased by roughly comparable amounts in sperm cells during incubation in vitro. 3. Cholesterol (free + ester) increased in albumin and decreased in spermatozoa. Changes in the amount of esterified cholesterol were solely responsible for the increase associated with albumin, whereas whole sperm cell extracts showed a significant decline in free cholesterol. 4. The composition of albumin-bound fatty acids did not alter appreciably as a result of incubation with spermatozoa. 5. Rates of [14C]palmitic acid utilization by spermatozoa suggest that lipid synthesis accounted for less than 5% of the changes observed under the conditions of this study. 6. These results are interpreted as broadly supporting our previous proposal that lipid exchange between albumin and sperm cells is implicated in sperm capacitation in vitro. Specifically, the results are compatible with the idea that a decreased cholesterol/phospholipid ratio in the sperm plasma membrane facilitates this transformation.

Animals↗