Cold-induced metabolic depression in cunner (Tautogolabrus adspersus): A multifaceted cellular event

Abstract

Metabolic depression and dormancy (i.e., stopping/greatly reducing activity and feeding) are strategies used by many animals to survive winter conditions characterized by food shortages and cold temperatures. However, controversy exists on whether the reduced metabolism of some fishes at cold temperatures is due to dormancy alone, or also involves active metabolic depression. Thus, we acclimated winter-dormant cunner [Tautogolabrus adspersus, a north temperate wrasse which in Newfoundland is at the northern limit of its distribution] and winter-active Atlantic salmon (Salmo salar) to winter (0°C; 8h light: 16h dark) and summer (10°C; 16h light: 8 h dark) conditions, and measured the thermal sensitivity of ATP-producing and O2-consuming processes in isolated liver mitochondria and hepatocytes when exposed in vitro to temperatures from 20 to 0°C and 10 to 0°C, respectively. We found that: 1) liver mitochondrial State 3 respiration and hepatocyte O2 consumption in cunner were only ~ one-third and two-thirds of that measured in salmon, respectively, at all measurement temperatures; 2) cunner mitochondria also have proton conductance and leak respiration (State 4) values that are only approximately one-third of those in salmon; 3) the mitochondria of cunner show a dramatic reduction in respiratory control ratio (from ~ 8 to 3), and a much greater drop in State 3 respiration, between 10 and 5°C (Q10 values in 10- and 0°C-acclimated fish of 14.5 and 141.2, respectively), as compared with salmon (3.9 and 9.6, respectively); and 4) lowering temperature from 5 to 0°C resulted in ~ 40 and 30% reductions in hepatocyte O2 consumption due to non-mitochondrial respiration and Na+-K+-ATPase activity, respectively, in cunner, but not in salmon. Collectively, these results highlight the intrinsic capacity for metabolic depression in hepatocytes and mitochondria of cunner, and clearly suggest that several cellular processes play a role in the reduced metabolic rates exhibited by some fishes at cold temperatures.

Fig 1

State 3 (A, B) and State 4 (C, D) respiration of isolated liver mitochondria from 0°C and 10°C acclimated cunner (A, C) and salmon (B, D), measured at 0°C, 5°C, 10°C and 20°C. Letters indicate significant differences within a species amongst assay temperatures, whereas an asterisk (*) indicates a significant difference between species at a particular assay temperature (P < 0.05); as determined by a 3-way repeated measures ANOVA, followed by 2-way ANOVAs and Newman-Keuls post-hoc tests. There were no significant acclimation effects. Q₁₀ values calculated over the ranges 0–5°C, 5–10°C and 10–20°C are reported. Q₁₀ values and letters in bold refer to the 10°C acclimated groups. Values are means ± s.e.m., N = 6–9.

Fig 2

P:O ratio (A, B) and respiratory control ratio (RCR; C, D) values for isolated liver mitochondria from 0 and 10°C acclimated cunner (A, C) and salmon (B, D), measured at 0°C, 5°C, 10°C and 20°C. Letters indicate significant differences within a species amongst assay temperatures, whereas an asterisk (*) indicates a significant difference between species at a particular assay temperature (P < 0.05). There were no significant acclimation effects or significant differences between species at a particular assay temperature; as determined by a 3-way repeated measures ANOVA, followed by 2-way ANOVAs and Newman-Keuls post-hoc tests. Values without a letter in common are significantly different within a species between assay temperatures. Q₁₀ values and letters in bold refer to the 10°C acclimated groups. Values are means ± s.e.m., N = 7–9.

Fig 3. Kinetics of proton leak.

Parallel measurements of O₂ consumption (i.e., succinate oxidation) and membrane potential of isolated liver mitochondria from 0 and 10°C acclimated cunner (A) and salmon (B), measured at 5°C and 20°C. Maximal proton leak rates are indicated by the top right-most points. There were no significant acclimation effects. Values are means ± s.e.m., N = 7–8.

Fig 4

Total O2 consumption of isolated hepatocytes (A, B) and the percentage of O₂ consumed by non-mitochondrial activities (C, D) and by Na⁺-K⁺-ATPase activity (E, F) of isolated hepatocytes from 0 and 10°C acclimated cunner (A, C) and salmon (B, D), measured at 0°C, 5°C and 10°C. Letters indicate significant differences within a species between assay temperatures whereas an asterisk (*) and a plus sign (+) indicate a significant difference between species at a particular assay temperature at P < 0.05 and 0.05 < P < 0.1; as determined by a 3-way repeated measures ANOVA, followed by 2-way ANOVAs and Newman-Keuls post-hoc tests. There were no significant acclimation effects. Q₁₀ values and letters in bold refer to 10°C acclimated groups. Values are means ± s.e.m., N = 7–8.