Reversible temperature-dependent differences in brown adipose tissue respiration during torpor in a mammalian hibernator

Abstract

Although seasonal modifications of brown adipose tissue (BAT) in hibernators are well documented, we know little about functional regulation of BAT in different phases of hibernation. In the 13-lined ground squirrel, liver mitochondrial respiration is suppressed by up to 70% during torpor. This suppression is reversed during arousal and interbout euthermia (IBE), and corresponds with patterns of maximal activities of electron transport system (ETS) enzymes. Uncoupling of BAT mitochondria is controlled by free fatty acid release stimulated by sympathetic activation of adipocytes, so we hypothesized that further regulation at the level of the ETS would be of little advantage. As predicted, maximal ETS enzyme activities of isolated BAT mitochondria did not differ between torpor and IBE. In contrast to this pattern, respiration rates of mitochondria isolated from torpid individuals were suppressed by ~60% compared with rates from IBE individuals when measured at 37°C. At 10°C, however, mitochondrial respiration rates tended to be greater in torpor than IBE. As a result, the temperature sensitivity (Q₁₀) of mitochondrial respiration was significantly lower in torpor (~1.4) than IBE (~2.4), perhaps facilitating energy savings during entrance into torpor and thermogenesis at low body temperatures. Despite the observed differences in isolated mitochondria, norepinephrine-stimulated respiration rates of isolated BAT adipocytes did not differ between torpor and IBE, perhaps because the adipocyte isolation requires lengthy incubation at 37°C, potentially reversing any changes that occur in torpor. Such changes may include remodeling of BAT mitochondrial membrane phospholipids, which could change in situ enzyme activities and temperature sensitivities.

Keywords: Q10; electron transport system; hibernation; mitochondria; uncoupled thermogenesis.

Fig. 1.

Oxygen consumption of isolated brown adipose tissue adipocytes from 1 interbout euthermia (IBE) animal measured at 10°C. Arrows indicate times at which the following additions were made: cells were added and a basal respiration rate was recorded; norepinephrine (NE) was added to achieve the NE-stimulated rate.

Fig. 2.

Oxygen consumption of isolated brown adipose tissue mitochondria from 1 IBE animal measured at 10°C using octanoyl carnitine as a fuel. Arrows indicate times at which the following additions were made: Mito, isolated mitochondria; OC, octanoyl carnitine and malate; GDP, guanosine 5′-diphosphate; ADP, adenosine 5′-diphosphate; Oligo, oligomycin.

Fig. 3.

Uncoupled respiration rates from isolated brown adipose tissue mitochondria using pyruvate (A) and octanoyl carnitine (B) as fuel. Respiration rates of isolated mitochondria were standardized to protein concentration. Values represent means + SE and n = 4 for all groups. Statistical significance between torpor and IBE is denoted with an asterisk (P < 0.05).

Fig. 4.

Maximal enzyme activity of electron transport system complexes using homogenized, isolated, brown adipose tissue mitochondria assayed at 10°C (A) and 37°C (B) and standardized to protein content. Values are means + SE except where sample size <3, in which case error bars represent range. Sample sizes for 10°C activities are 6 (torpor) and 2 (IBE) for all 5 complexes. Sample sizes for 37°C activities are 9 (torpor) and 7 (IBE) for complexes I–III and V. Sample sizes for complex IV 37°C rates are 5 (torpor) and 2 (IBE). No significant differences were observed between hibernation states for any of the complexes (P > 0.05).

Fig. 5.

Interaction plots of mitochondrial respiration rates from isolated brown adipose tissue mitochondria at 10°C and 37°C using pyruvate (A) and octanoyl carnitine (B) as fuel. Points connected by a line represent data from the same individual.

Fig. 6.

Basal (A) and norepinephrine (NE; B)-stimulated oxygen consumption rates of isolated brown adipocytes. Rates are standardized to cell concentration. Values represent means + SE. There were no significant differences between torpor (n = 5) and IBE (n = 4).

Fig. 7.

Response of oxygen consumption rate to norepinephrine (NE) stimulation in isolated brown adipocytes. The fold change in respiration rate from the basal rate to the NE-stimulated rate was calculated for each individual. Values represent means + SE. There were no significant differences between torpor (n = 5) and IBE (n = 4).