Fasting elicits immune modulation and leukocyte redistribution in bullfrogs (Lithobates catesbeianus)

An integrated range of behavioral, physiological, and biochemical adjustments reduces metabolic expenditure and energy utilization during fasting to maintain homeostasis. In order to cope with these adjustments, several vertebrates can modulate immune function and corticosterone secretion while fasting to save energy. However, in ectothermic animals that can fast for longer periods due to their low metabolic rate, the underlying corticosterone and immune modulation is still not well understood. This study aimed to investigate corticosterone and immune modulation during fasting in the bullfrog (Lithobates catesbeianus). Bullfrogs were divided into two treatments: a 'control group', fed twice a week with fish feed for 31 days; and a 'fasting group', which remained fasting for 31 days. On the 31st day of experiment, blood was collected to measure corticosterone plasma levels and immune function (neutrophil/lymphocyte ratio, plasma bacterial killing ability - BKA, and hemagglutination) variables. Fasting did not affect corticosterone plasma levels but increased neutrophil/lymphocyte ratio and hemagglutination, while BKA was decreased. In this way, fasting modulates the innate immune function in male bullfrogs by increasing the NL ratio and hemagglutination activity while reducing BKA, without affecting plasma CORT levels or fat body mass content. Thus, 31 days of fasting induces blood immune cell redistribution and an immunomodulatory response without significant activation of the HPI axis or depletion of energy stores.