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Biomedical subjects

D Barker

Publications and source records attributed to D Barker.

At least 145 records · Page 8Linked to original sources
PubMed1978

Repression of biotin biosynthesis in Escherichia coli during growth on biotin vitamers.

A strain of Escherichia coli in which the lacZ gene was fused to the bioA promoter was constructed. Colonies of this strain formed Lac(+) colonies on low-biotin agar (1.6 to 4.1 nM) and Lac(-) colonies on high-biotin agar (41 nM). This lac-bio fusion strain was used to study the question of whether cells growing on the biotin vitamers d-biotin-d-sulfoxide (BDS) and dethiobiotin (DTB) generate enough biotin to give maximal repression of beta-galactosidase synthesis. Repression by high concentrations (400 nM) of BDS was almost maximal (about 96%), whereas DTB repression reached a saturation level of about 80% with increasing DTB concentrations. The levels of repression obtained with both vitamers were sufficient to cause the colonies to appear Lac(-). When the lac-bio fusion was transduced into lines carrying mutations (bis) that prevent reduction of BDS to biotin, the transductants were not repressed by added BDS. Repression by BDS is unlikely to result from accumulation of extracellular biotin-related substances because (i) washed bis(+) cells were not detectably derepressed when transferred into medium containing BDS and (ii) washed bis cells were not detectably repressed when transferred into medium in which bis(+) cells had grown. Lactose agar plates containing high concentrations of DTB or BDS comprise an efficient selective medium for bioB or bis mutants and were used to isolate spontaneous mutations of these genes. This method should be adaptable to the selection of mutations in any biosynthetic pathway subject to end-product repression.

Biotin↗
PubMed1978

Plants as tools.

Explore the source record for details and available documents.

Child↗
PubMed1978

Protection and safety in the X-ray department.

It should be obvious from the foregoing that the greatest protection to the patient and to the radiographers is a constant awareness of the potential hazards from ionising radiation, combined with a regular check on all procedures and all equipment.

Accident Prevention↗
PubMed1977

Types of intra- and extrafusal muscle fibre innervated by dynamic skeleto-fusimotor axons in cat peroneus brevis and tenuissimus muscles, as determined by the glycogen-depletion method.

1. The types of intra- and extrafusal muscle fibre innervated by dynamic skeleto-fusimotor (beta) axons were determined by using a modification of the glycogen-depletion method of Edström & Kugelberg (1968) combined with histochemical tests for various enzyme reactions. A single beta axon was prepared in each of the experiments, which were carried out on six peroneus brevis and two tenuissimus muscles. 2. The intrafusal distribution of dynamic beta axons is almost exclusively restricted to bag1 fibres. The bags fibre was depleted in each of twenty-four beta-innervated spindle poles; the only fibres of a different type depleted intrafusally were a bag2 fibre in one pole and a long chain in another. 3. Depletion in the bag1 fibres was usually restricted to one zone in one pole, generally in a mid-polar location. 4. The extrafusal muscle fibres depleted by dynamic beta axons belong to the slow oxidative type as defined by Ariano, Armstrong & Edgerton (1973). The number of such fibres in each motor unit could not be accurately determined, but is almost certainly small. 5. The slow oxidative muscle fibres innervated by dynamic beta axons were not depleted over their entire length. Since there is no reason to assume that they are not twitch fibres, it would seem that the localized depletions result from the conditions required to obtain glycogen depletion, i.e. long periods of motor stimulation applied during the occlusion of the muscle's blood supply. Under similar experimental conditions depletion of glycogen was also restricted to portions of fibres in fast oxidative-glycolytic motor units, but extended over most of the length of the fibres in fast glycolytic units.

Action Potentials↗
PubMed1977

Effect of changes in Pco2 on intraocular tension.

Elevated levels of inspired CO2 and blood Pco2 resulted in moderate elevation of IOT. A marked rapid decrease in IOT to a level less than baseline value was noted when inspired CO2 was suddenly decreased to ambient levels. Decrease in IOT was more pronounced than the decrease in Pco2 and increase in blood pH. Changes in IOT appeared related to the rate of change of Pco2 rather than the actual level of Pco2. Increased ventilatory excursions with constant inspired CO2 levels did not cause any elevation of IOT, but a minimal compensatory drop in IOT below resting values occurred when increased ventilatory excursions were discontinued. It is postulated that the changes in IOT noted are the result of sudden changes in aqueous production or ocular blood volume.

Carbon Dioxide↗
PubMed1976

Distribution of fusimotor axons to intrafusal muscle fibres in cat tenuissimus spindles as determined by the glycogen-depletion method.

1. The distribution of fusimotor axons to bag1, bag2 and chain muscle fibres in cat tenuissimus spindles has been studied using a modification of the glycogen-depletion technique of Edstrrom & Kugelberg (1968). Single fusimotor axons were stimulated intermittently at 40-100/sec for long periods (30-90 sec) during blood occlusion. Portions of muscle containing the activated spindles were quick-frozen, fixed in absolute ethanol during freeze-substitution, and then embedded in paraffin wax. Serial transverse sections were stained for glycogen using the periodic acid-Schiff method, and examined for depletion. 2. Dynamic gamma axons (i.e. those that increase the dynamic index of primary-ending responses to ramp stretches of large amplitude) depleted bag1 fibres almost exclusively. 3. Static gamma axons (i.e. those that reduce or abolish the dynamic index) depleted both bag and chain fibres. Bag1 and bag2 fibres were depleted about equally. 4. A single static gamma axon may activate both bag and chain fibres in one spindle (the most common pattern), chain fibres only in another, and bag fibres only in a third spindle. 5. Static gamma axons with conduction velocities less than 25 m/sec also had a non-selective distribution, but no depletion was observed in bag2 fibres. 6. The zones of depletion produced by dynamic gamma axons were distributed more or less equally in the intra- and extracapsular parts of spindle poles, whereas those produced by static gamma axons were mainly intracapsular. 7. The results are compared with the glycogen-depletion studies of Brown & Butler (1973, 1975) and our own study of the distribution of static gamma axons to spindles in which all other motor axons had degenerated (Barker, Emonet-Dénand, Laporte, Proske & Stacey, 1973). The implications of the finding that both static gamma and dynamic gamma axons activate bag1 fibres are discussed.

Animals↗
PubMed1973

Morphological identification and intrafusal distribution of the endings of static fusimotor axons in the cat.

1. Tenuissimus muscles of the cat were prepared in which the motor innervation was reduced to a single gamma axon by cutting all the other motor axons and allowing them to degenerate during a period of 7-12 days. The function of the surviving gamma axon was then determined, and the distribution of its endings ascertained in teased, silver preparations.2. In the ten muscles successfully prepared the function of the surviving gamma axon was static and the motor innervation distributed to the spindles consisted of trail endings. The conduction velocities of the axons ranged from 33 to 48 m/sec.3. A detailed histological analysis was made of thirty spindles innervated by six of the surviving static axons.4. The six static axons distributed trail endings to both bag and chain muscle fibres in the poles of thirty spindles with about twice the frequency of supplying them to poles in which the distribution was restricted exclusively to one type of muscle fibre or the other.5. The density of trail innervation supplied to the bag fibres, in terms of the mean number of terminals per fibre, was typically from one and a half to twice that supplied to the chain fibres. On the other hand, whereas the number of bag fibres supplied with trail endings in a spindle pole was seldom more than one, the number of chain fibres innervated was usually two in a range of one to four.6. The possible effects that partial denervation might have had on the spindles are discussed, but it is concluded that they are unlikely to have affected the results.

Action Potentials↗