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The classical approach to the study of the nitrogen metabolism of tissues has been through observations of urinary nitrogen changes. Efforts at more direct measurement have led to studies of NPN fractions in the blood of normal (Harrison and Long, 1940), nephrectomized (Bondy and Engel, 1947), and eviscerated (Frame and Russell,1946) animals under a variety of conditions. New approaches which have recently been introduced are the analyses of whole tissues before and after experimental manipulation of the animal (White and Dougherty, 1947) and the use of isotopes to calculate breakdown and synthesis rates(Sprinson and Rittenberg, 1949). Direct in vitro observation of the nitrogen metabolism of tissues has been discouraged by the finding that during incubation there is always a net loss of nitrogen from the tissue slices. However, if the loss of nitrogen during incubation is a constant, reproducible quantity
Daniel L. Kline (Thu,) studied this question.