The effect of metabolic fuel availability on thermoregulation and torpor in a marsupial hibernator

Abstract

The physiological signal for torpor initiation appears to be related to fuel availability. Studies on metabolic fuel inhibition in placental heterotherms show that glucose deprivation via the inhibitor 2-deoxy-D-glucose (2DG) initiates a torpor-like state, whereas fatty acid deprivation via mercaptoacetate (MA) does not. As previous studies using inhibitors were limited to quantifying body temperature in placentals, we investigated whether inhibition of glucose or fatty acids for cellular oxidation induces torpor in the marsupial hibernator Cercartetus nanus, and how the response of metabolic rate is related to body temperature. Glucoprivation initiated a torpor-like state in C. nanus, but animals had much higher minimum body temperatures and metabolic rates than those of torpid food-deprived animals and arousal rates were slower. Moreover, 2DG-treated animals were thermoregulating at ambient temperatures of 20 and 12 degrees C, whereas food-deprived torpid animals were thermo-conforming. We suggest that glucoprivation reduces the hypothalamic body temperature set point, but only by about 8 degrees C rather than the approximately 28 degrees C during natural torpor. Reduced fatty acid availability via MA also induced a torpor-like state in some C. nanus, with physiological variables that did not differ from those of torpid food-deprived animals. We conclude that reduced glucose availability forms only part of the physiological trigger for torpor initiation in C. nanus. Reduced fatty acid availability, unlike for placental heterotherms, may be an important cue for torpor initiation in C. nanus, perhaps because marsupials lack functional brown adipose tissue.

Energy availability influences microclimate selection of hibernating bats

“…MA inhibits fatty acid availability by reducing the mitochondrial oxidation of fatty acids (Bauche et Energetics of microclimate selection by bats al., 1983;Dark and Miller, 1998). MA increases food intake in rats on high-fat diets (Scharrer and Langhans, 1986) and affects torpor expression in placental golden-mantled ground squirrels, Spermophilus lateralis (Dark and Miller, 1998), and marsupial eastern pygmy possums, Cercartetus nanus (Westman and Geiser, 2004). In eastern pygmy possums, T b and TMR of individuals injected with MA did not differ significantly from food-deprived individuals (Westman and Geiser, 2004).…”

“…Mercaptoacetate lowers TMR and T b in some eastern pygmy possums, a hibernating marsupial (Westman and Geiser, 2004). In placental mammals, T b is not lowered (Dark and Miller, 1998) or is lowered only slightly (Stamper and Dark, 1997), and the effect of MA on TMR is unknown.…”

“…MA increases food intake in rats on high-fat diets (Scharrer and Langhans, 1986) and affects torpor expression in placental golden-mantled ground squirrels, Spermophilus lateralis (Dark and Miller, 1998), and marsupial eastern pygmy possums, Cercartetus nanus (Westman and Geiser, 2004). In eastern pygmy possums, T b and TMR of individuals injected with MA did not differ significantly from food-deprived individuals (Westman and Geiser, 2004). Dosages of MA used herein are sufficient to cause a substantial increase in food intake in rats (Scharrer and Langhans, 1986), indicating an inhibition of fatty acid oxidation.…”

Termorregulação e depressão metabólica em endotermos.

“…No caso particular de endotermos (aqui subentendidos como aves e mamíferos, somente 1 ), os estados de depressão metabólica ocorrem, na maioria das vezes, em função da demanda energética elevada que esses animais têm, junto a condições ambientais de restrição alimentar e/ou frio intenso (Guppy & Withers, 1999;Westman & Geiser, 2004). Existem, no entanto, outros fatores que podem estar ligados ao desencadeamento de estados hipometabólicos, como privação de água, hipóxia, etc.…”

“…Neste sentido, partimos do estudo geral do modelo de termorregulação, para mostrar como o set-point hipotalâmico pode surgir como um controle, resultado da minimização de um índice que reúna características que se deseje preservar ou minimizar, e que realizaria como conseqüência, aquilo que a literatura caracteriza por entrada em torpor (Westman & Geiser, 2004;Hainsworth, Collins & Wolf, 1977).…”