More than a decade has passed since the thought was presented that there is an inverse relationship between dietary phosphate and caries susceptibility.' This was based in part on animal studies,2 in which diets varying from high to low in phosphate content resulted in teeth varying from low to high in carbonate concentration, respectively. It was suggested that the low-carbonate teeth were less soluble in dilute acids and therefore more resistant to at least one type of caries, whereas high-carbonate teeth were more acid soluble and less caries-resistant. By 19523 this concept led to the first trial experiments on a series of cotton rats in which low-carbonate and high-carbonate teeth were produced by feeding high-phosphate and low-phosphate diets, respectively, prior to being placed on a cariogenic diet.4 Two diets with greatly differing Ca/P ratios were selected in an attempt to highlight the effect on tooth carbonate composition. These experiments demonstrated that a high phosphate diet is cariostatic. The present report is an extension of these findings. EXPERIMENTAL METHODSCotton rat colony.-The original breeding animals were obtained from the Public Health Institute of Lansing, Michigan. All animals in our stock colony were kept on a mixture of Pratt's pellets and Bill's stock diet.5The conditions that proved successful in the breeding of these rats were as follows: a regimen of 12 hours of light and 12 hours of darkness, temperature-controlled at 780 F., a light-proof compartment available to the animals within the cage into which they can retire when disturbed, no bedding material, and daily handling as an aid in taming. Prior to the introduction of this last condition, the rats would injure themselves by jumping about wildly upon slight provocation. Downloaded from Animal regimen.-The experiments utilized 189 cotton rats which were distributed among the various experimental groups in such a way as to provide comparable hereditary backgrounds and comparable ratios of males to females. The experimental work extended over a 5-year interval from 1949 to 1954. In view of this extended collection of data and certain changing background influences, the results have been evaluated separately for the 1949-52 period and for the 1953-54 period. Between 1949 and 1952 the animals were wild and could be handled only with difficulty, whereas during 1953 and 1954 the animals were relatively tame and easier to handle. This treatment of the data permitted the best comparative evaluation of the experimental findings in a study conducted over such a lengthy period.At 16 days of age-the normal weaning time for this species-the cotton rats were weaned and placed on either the low-phosphate-high-calcium diet (402) or the highphosphate-low-calcium diet (403) for 4 weeks. Both these diets have a low ability to initiate and maintain carious lesions in the cotton rat through oral environmental influences. During the following period of 12-14 weeks, the corresponding cariogenic D and E diets, which contained similar calcium-to-pho...
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Adaptation of the Murphy and Jachan method for serum thyroxine for routine use in a clinical chemistry laboratory is described. Details in the original paper have been amplified, and operations such as mixing, resin addition, and temperature control modified in the direction of greater precision and simplicity. A comparison study was made of the performance of the method in our laboratory with the reported performance in the laboratory of the original authors. In agreement with Murphy and coworkers and Cassidy et al, it was found that conditions could be established whereby the method was both reproducible and capable of yielding significant diagnostic information in the presence of iodinated radiographic compounds.
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