Biomedical subjects
Publications and source records attributed to L A Davis.
The administrative role includes follow-up with others to ensure that work has been accomplished. Effective follow-up is both a complex skill and an art that involves many related management activities. The author presents ten principles that cover preparation for follow-up, identification of when and what to follow up, and specific techniques for following up.
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1. Cat hind limb peripheral nerves were fitted with cuff recording electrodes, and their distal portions were later cut and ligated to prevent regeneration. The compound action potential amplitude initially declined with a time constant between 1 and 2 months and then remained relatively unchanged for periods of more than a year. Similar but smaller changes were observed in the conduction velocity of the nerves which also stabilized after a few months. 2. In nerves that were cut and resutured to their distal stumps or sutured directly to nearby muscles, a recovery was observed. The time course was well fitted by an initial exponential decay with a similar time constant to that above, followed by an exponential recovery with a longer time constant (3-4 months). Nerve conduction, muscle potentials and twitch tension often recovered to control values, even when the amplitude of the nerve compound action potential remained depressed. 3. Thus, nerve fibres survive axotomy for long periods of time and continue to conduct action potentials, even if unable to regenerate to appropriate end-organs. When regeneration is permitted, a fraction of nerve fibres may reinnervate nearly all end-organs. The diameter and conduction velocity of these nerve fibres presumably increase toward control values, while other fibres remain subnormal in these parameters. 4. Factors in the design of cuff electrodes which determine the amplitude of compound action potentials are described in an Appendix.
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The effect of phosphate supplementation on bone remodeling was assessed in six mature, healthy beagle dogs. The phosphate supplement was given in divided doses orally, daily for 12 weeks in the form of a neutral potassium phosphate preparation. The dose averaged 108 mg P/kg per day, which is double the normal canine phosphorus intake. Bone remodeling was assessed by measurement, at sacrifice, of areas of cortical bone containing different color-coded tetracyclines which had been continuously administered during 12-week control and treatment periods; remodeling was assessed kinetically during the control and treatment periods by replicate studies employing 47Ca intravenously. Both techniques demonstrated that the principal effect of phosphate supplementation was a significant stimulation of bone formation. Within cortical bone, formation was doubled, from an average of 2.7% to 5.3% per year. The major location of new bone deposits was endosteal. Whole skeletal mineral accretion, measured kinetically, increased 45% above an average control value of 0.154 g/day. These studies suggest that, in the adult dog, "normal" plasma phosphate levels are suboptimal for new bone formation. Even with this short duration of administration, phosphate produced microscopic calcification of the renal parenchyma. However, there was no biochemical evidence of renal functional impairment.
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Abscisic acid was measured in developing cotton fruit (Gossypium hirsutum) by means of gas-liquid chromatography. High levels of abscisic acid occurred in correlation with abortion and abscission of young fruit, with low germination of immature seed, and with senescence and dehiscence of mature fruit. Declining or low levels of abscisic acid occurred in correlation with the period of most rapid fruit growth and with high germination of immature and mature seed. Young fruit of cultivar Acala 4-42 contained about twice as much abscisic acid as young fruit of cultivar Acala SJ-1, and this difference is correlated with a higher rate of young fruit abscission in Acala 4-42. Young fruit abscising late in the fruiting season contained about twice as much abscisic acid as young fruit abscising early in the fruiting season.
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This paper describes a new method for qualitative and quantitative assay of abscisic acid and other acidic plant hormones, such as indoleacetic acid and the gibberellins, by the gas-liquid chromatography of their trimethylsilyl derivatives. Interfering substances in plant extracts were largely removed by preliminary column chromatography with carbon-celite and elution of the abscisic acid with 60% acetone, permitting direct determination of abscisic acid by gas-liquid chromatography using a flame ionization detector. (A level of 0.65 mg/kg fr wt was found.) This method enables measurement of amounts of abscisic acid as low as 0.025 mug. In impure samples collected by gas-liquid chromatography the abscisic acid recovered could be measured quantitatively by use of its ultraviolet absorption maximum at 260 mmu.
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