UBC Theses and Dissertations
Adenosine receptor brockade and hypoxia tolerance in rainbow trout and Pacific hagfish Bernier, Nicholas J.
The physiological properties of adenosine may be essential in the control of energy metabolism for the survival of animals exposed to oxygen shortages. Accordingly, in this thesis the hypothesis that adenosine modulates the response of rainbow trout and Pacific hagfish to acute hypoxic exposure was tested. Three different experimental series were conducted to investigate the possible roles of adenosine in hypoxia tolerance. Rainbow trout were exposed to either a Pw02 of 30 torr for 6 h (SERIES I), or to 25 torr for 1 h (SERIES II). Pacific hagfish were exposed either to a Pw0 of 10 or 30 torr for 1 h (SERIES III). In all three series, the role of adenosine was assessed by comparing the response of animals treated with non-specific adenosine receptor blockers to control shams under hypoxic and normoxic conditions. Three different areas were investigated for adenosine's actions in these experiments: 1) the recruitment of anaerobic metabolism; 2) the stress response; and 3) the role of erythrocytes for oxygen capacitance. Relative to hypoxic sham fish, increases in plasma [lactate] with hypoxic exposure were greater in the animals injected with the adenosine receptor (AR) blocker theophylline in all three series. This response to AR blockade was also associated with a more rapid and pronounced metabolic acidosis in SERIES I & II. In hagfish, plasma [lactate] increased following exposure to a Pw0 of 10 torr but not 30 torr, and plasma acidosis was only observed in the animals exposed to a Pw0 of 10 torr and treated with theophylline. In SERIES II, only the tissues from the hypoxic theophylline treated trout had significant increases in tissue [lactate] when compared to the normoxic groups. Decreases in creatine charge were observed in the heart and red muscle, but not white muscle, of theophylline treated fish. The glycogen content of the heart also decreased following theophylline treatment. The tissue metabolites of hypoxic trout treated with enprofylline, an AR blocker with very weak affinity, were similar to the hypoxia sham fish, and the increase in plasma lactate was intermediate to the hypoxic theophylline and sham groups. Both AR blockers had no measurable effects on normoxic controls. These findings indicate that AR blockade results in a more rapid and pronounced recruitment of anaerobic metabolism following acute hypoxic exposure in rainbow trout and Pacific hagfish. In SERIES I, plasma [Cortisol] increased after 10 min of hypoxic exposure and remained elevated in the theophylline group. An increase in plasma [Cortisol] was observed after 30 min of hypoxia but was transient in the hypoxic sham trout. In SERIES II, after 10 min of hypoxic exposure, the plasma [adrenaline] in the theophylline and enprofylline treatments were respectively 16 and 4 fold higher than in the hypoxic sham treatment. This difference, although not as pronounced, was maintained after 60 min of acute hypoxia between the theophylline treatment and the two other hypoxic groups. In hagfish, whereas plasma [adrenaline] did not change following exposure to a Pw0 of 10 torr in the hypoxic sham group, the [adrenaline] increased 3.8 fold within 10 min in the theophylline group and returned to control levels by 60 min. AR blockade with methylxanthines had no effect on the concentrations of plasma Cortisol and catecholamines in the normoxic animals. These results indicate that adenosine receptor blockade modulates the primary stress response of hypoxic rainbow trout and Pacific hagfish. In rainbow trout, serial blood sampling in SERIES I resulted in a greater [Hb] decrease in the hypoxic group treated with theophylline than in the hypoxic sham group. In SERIES II, an increased in [Hb] was observed in the hypoxic sham group but not in the hypoxic groups treated with the AR blockers. In the hagfish experiment, AR blockade had no effect on the relative decrease in [Hb] in all the treatments. These results indicate that in hypoxic rainbow trout, AR blockade may prevent splenic release of rbc by abolishing the stimulatory effects of catecholamines on this tissue. Unlike rainbow trout, Pacific hagfish may not increase their [Hb] under acute hypoxic conditions. Results from these experiments show marked differences between rainbow trout and Pacific hagfish in their response to hypoxia, and in the strategy that each utilizes to resist such conditions. However, results also indicate that adenosine has an important protective role in both species, and that the actions of adenosine in fish, as in other vertebrates, may have a common tendency to reduce energy expenditure, while improving oxygen delivery. Specifically, adenosine may reduce the extent to which anaerobic metabolism is recruited upon acute hypoxic exposure, modulate the circulating levels of the primary stress hormones catecholamines and Cortisol, and play a role in maintaining the oxygen carrying capacity of the trout by modulating the splenic contribution of rbc.