Ruby-throated hummingbirds suppress ATP demand to save energy during torpor

Ruby-throated hummingbirds are heterothermic and enter torpor at night to save energy. We recently investigated the ATP supply pathway through mitochondria as a mechanism for whole animal metabolic rate suppression observed in torpor, but found, unlike heterothermic mammals, mitochondrial function is not actively suppressed in torpid hummingbirds. In this study, we tested the hypothesis that the inhibition or downregulation of major ATP-consuming enzymes (SERCA and Na⁺/K⁺ ATPase) is involved in whole animal metabolic rate suppression in torpid hummingbirds. Specific SERCA activity in the pectoralis muscle did not change between torpor and normothermia, but the phosphorylation of the "uncoupling" protein sarcolipin (SLN) increased by 14% in torpor. This indicates a downregulation of ATP demand and reduced heat generation as a possible mechanism of metabolic suppression. Na⁺/K⁺ ATPase-specific enzyme activity was 43% lower in torpor than in normothermia, although the phosphorylation state of the enzyme was unchanged. Taken together, we suggest that hummingbirds suppress pathways of ATP demand during torpor as a mechanism to drive metabolic rate suppression in concert with passive metabolic suppression initiated by adjusting thermoregulation and allowing core body temperature to decrease.NEW & NOTEWORTHY The cellular mechanisms that underlie hummingbird torpor are only beginning to be studied. We are the first to report two ATP-demand pathways downregulated in torpid ruby-throated hummingbirds. We characterized SERCA and Na⁺/K⁺ ATPase activity in normothermia and torpor. SERCA activity was 10-13× higher than in songbirds. Sarcolipin was more phosphorylated in torpor, reducing heat production, and Na⁺/K⁺ ATPase activity fell 43%. These shifts lower ATP demand and conserve energy during torpor.