Organ weights and tyrosine aminotransferase levels in the rat during adaptation to periodic fasting

The present study was undertaken to determine how rats adapt to periodic fasting. In 2 experiments rats were trained to an '8 + 16' schedule (8 hr fed, 16 hr fasted) and then shifted to an '8 + 40' schedule (8 hr fed, 40 hr 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 suggest 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.