Temperature and sex dependent effects on cardiac mitochondrial metabolism in Atlantic cod (Gadus morhua L.)

To test the hypothesis that impaired mitochondrial respiration limits cardiac performance at warm temperatures, and examine if any effect(s) are sex-related, the consequences of high temperature on cardiac mitochondrial oxidative function were examined in 10 degrees C acclimated, sexually immature, male and female Atlantic cod. Active (State 3) and uncoupled (States 2 and 4) respiration were measured in isolated ventricular mitochondria at 10, 16, 20, and 24 degrees C using saturating concentrations of malate and pyruvate, but at a submaximal (physiological) level of ADP (200 mu M). In addition, citrate synthase (CS) activity was measured at these temperatures, and mitochondrial respiration and the efficiency of oxidative phosphorylation (P:O ratio) were determined at [ADP] ranging from 25-200 mu M at 10 and 20 degrees C. Cardiac morphometrics and mitochondrial respiration at 10 degrees C, and the thermal sensitivity of CS activity (Q(10)=1.51), were all similar between the sexes. State 3 respiration at 200 mu M ADP increased gradually in mitochondria from females between 10 and 24 degrees C (Q(10)=1.48), but plateaued in males above 16 degrees C, and this resulted in lower values in males vs. females at 20 and 24 degrees C. At 10 degrees C, State 4 was similar to 10% of State 3 values in both sexes [i.e. a respiratory control ratio (RCR) of similar to 10] and P:O ratios were approximately 1.5. Between 20 and 24 degrees C, State 4 increased more than State 3 (by similar to 70 vs. 14%, respectively), and this decreased RCR to similar to 7.5. The P:O ratio was not affected by temperature at 200 mu M ADP. However, (1) the sensitivity of State 3 respiration to increasing [ADP] (from 25 to 200 mu M) was reduced at 20 vs. 10 degrees C in both sexes (K-m values 105 +/- 7 vs. 68 +/- 10 mu M, respectively); and (2) mitochondria from females had lower P:O values at 25 vs. 100 mu M ADP at 20 degrees C, whereas males showed a similar effect at 10 degrees C but a much more pronounced effect at 20 degrees C (P:O 1.05 at 25 mu M ADP vs. 1.78 at 100 mu M ADP). In summary, our results demonstrate several sex-related differences in ventricular mitochondrial function in Atlantic cod, and suggest that myocardial oxidative function and possibly phosphorylation efficiency may be limited at temperatures of 20 degrees C or above, particularly in males. These observations could partially explain why cardiac function in Atlantic cod plateaus just below this species' critical thermal maximum (similar to 22 degrees C) and may contribute to yet unidentified sex differences in thermal tolerance and swimming performance. (C) 2014 Elsevier Ltd. All rights reserved.