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Organ Weights and Tyrosine Aminotransferase Levels in the Rat during Adaptation to Periodic Fasting¹

McArdle Laboratory for Cancer Research and the Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706

The present study was undertaken to determine how rats adapt to periodic fasting. In two experiments rats were trained to an "8 + 16" schedule (8 hours fed, 16 hours fasted) and then shifted to an "8 + 40" schedule (8 hours fed, 40 hours fasted). Stomach, liver, epididymal and perirenal fat pads, kidney, testes, and body weights were measured to observe adaptation to the "8 + 40" schedule. Three types of tissue weight control were observed. First, organ growth may be halted and tightly regulated with body size (liver, kidney, and stomach); second, organ weight may be reduced as energy stores are used (epididymal and perirenal fat pads); and third, organ weight may be maintained at or near control levels (testes). In addition, the activity of the liver enzyme tyrosine aminotransferase (TAT) was monitored, and the appearance of a "secondary fasting adapted" peak of TAT activity was used as a measure of adaptation to the "8 + 40" regimen. The secondary peak was observed after 18 days on the "8 + 40" regimen. Results from measurement of stomach dry weight contents coupled with much previous work suggests that the secondary peak is hormonally induced. The "8 + 40" regimen is proposed as an excellent system for the study of the interrelationships between plasma tyrosine, TAT, tyrosine hydroxylase, norepinephrine, and their roles in sleep-wake and controlled feeding cycles.

KEY WORDS: • controlled feeding • tyrosine aminotransferase • adaptation • organ weights

¹ Financial support was provided in part by grants CA 07175 and T01-CA-5002 from the National Cancer Institute.

² To whom requests for reprints should be sent.

Manuscript received 19 June 1974.