THE problem of the enlargement of the kidneys of rats fed on diets rich in protein has, in recent years, been receiving considerable attention notably by Addis and the Mackays [1926, 1, 2] who have shown that the hypertrophy is proportional to the protein consumption and have elaborated a formula which expresses this relationship in terms of protein consumed and kidney weight per unit of surface area. The cause of this enlargement, however, stillremains obscure. The excretion of urea naturally suggests itself as a causal factor. The feeding of urea, however, in amounts equivalent to what would be excreted on a high protein diet has been shown to produce a much smaller effect on the kidneys [Osborne et al., 1926; Mackay, Mackay and Addis, 1931]. The effect of feeding proteins of different origin has however scarcely been investigated. Osborne et al. [1926] found that caseinogen, gliadin and beef residues all produced an enlargement of the kidneys but no actual numerical data on the basis worked out by the Mackays and Addis [1927; 1928] were elaborated. The object of the following experiments was an attempt to find out the cause of this hypertrophy. Two possibilities suggest themselves. Do the kidneys in the rat form a depot for reserve protein, or is the hypertrophy a response to an increase in physiological function? It is possible in this connection that the kidneys are associated in some way with the intermediary metabolism of protein quite apart from the mere excretion of nitrogenous end-products. EXPERiMENTAL. A series of experiments was carried out to compare the following proteins and protein derivatives, and in addition the effect of an acid and alkaline dietcaseinogen, liver, gelatin, wheat gluten, glycine, glutamic acid, urea, NaH2PO4 and NaHCO3. The experimental procedure adopted in these experiments was practically the same as that employed by the Mackays [1927]. Standard albino rats 30 days old and obtained from the same source as far as each particular series of experiments is concerned were employed throughout. They were kept in cages open to the front with not more than 6 rats to a cage. The sexes were kept apart. The room in which they were kept was well lit and open to the south and was maintained fairly constant in temperature day and night. Water and food were allowed ad lib., and the food consumption was measured each day. The rats were weighed about every third day. The duration of the experiments varied from 30 to 39 days in the different series. At the end of the experimental
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