Enhanced Hypoxia Tolerance Research Articles

Calorie restriction in the rotifer Brachionus plicatilis enhances hypoxia tolerance in association with the increased mRNA levels of glycolytic enzymes

The rotifer Brachionus plicatilis shows a typical sigmoid growth curve, where calorie restriction (CR) and hypoxia are thought to be introduced at high population density in the stationary phase. CR may induce a shift from aerobic to anaerobic metabolism in this stationary phase, possibly contributing to an increased hypoxia tolerance. This study was undertaken to investigate the effect of CR on hypoxia tolerance at the molecular level. When rotifers were cultured under CR (fed every second day) or fed ad libitum (AL), and subsequently exposed to hypoxia, those in the CR group had a higher survival rate than their AL counterparts. We then cloned cDNAs encoding three glycolytic enzymes, glyceraldehyde 3-phosphate dehydrogenase (GAPDH), enolase (ENO), and phosphoglycerate mutase (PGM) and compared their accumulated mRNA levels between CR and AL rotifers at ages of 1–8 days by quantitative real-time PCR. The CR group showed significantly higher mRNA levels of GAPDH and ENO than their AL counterparts. Furthermore, rotifers in the stationary phase showed higher mRNA levels of these enzymes than those in the exponential growth phase. These results suggest that CR induces anaerobic metabolism, which possibly contributes to population stability under hypoxia in the stationary phase.

Hemoglobin Genotype Has Minimal Influence on the Physiological Response of Juvenile Atlantic Cod (Gadus morhua) to Environmental Challenges

Hemoglobin (Hb) polymorphism in cod is associated with temperature-related differences in biogeographical distribution, and several authors have suggested that functional characteristics of the various hemoglobin isoforms (HbIs) directly influence phenotypic traits such as growth rate. However, no study has directly examined whether Hb genotype translates into physiological differences at the whole animal level. Thus, we generated a family of juvenile Atlantic cod consisting of all three main Hb genotypes (HbI-1/1, HbI-2/2, and HbI-1/2) by crossing a single pair of heterozygous parents, and we compared their metabolic and cortisol responses to an acute thermal challenge (10 degrees C to their critical thermal maximum [CTM] or 22 degrees C, respectively) and tolerance of graded hypoxia. There were no differences in routine metabolism (at 10 degrees C), maximum metabolic rate, metabolic scope, CTM (overall mean 22.9 degrees +/- 0.2 degrees), or resting and poststress plasma cortisol levels among Hb genotypes. Further, although the HbI-1/1 fish grew more (by 15%-30% during the first 9 mo) when reared at 10 degrees +/- 1 degrees C and had a slightly enhanced hypoxia tolerance at 10 degrees C (e.g., the critical O(2) levels for HbI-1/1, HbI-2/2, and HbI-1/2 cod were 35.56% +/- 1.24% , 40.56% +/- 1.99%, and 40.20% +/- 1.19% air saturation, respectively), these results are contradictory to expectations based on HbI functional properties. Thus, our findings (1) do not support previous assumptions that growth rate differences among cod Hb genotypes result from a more efficient use of the oxygen supply-that is, reduced standard metabolic rates and/or increased metabolic capacity-and (2) suggest that in juvenile cod, there is no selective advantage to having a particular Hb genotype with regards to the capacity to withstand ecologically relevant environmental challenges.

Does the ability to metabolically downregulate alter the hypoxia tolerance of fishes?: a comparative study using cunner (T. adspersus) and Greenland cod (G. ogac)

In this study, the metabolic response and tolerance of cunner (a temperate wrasse species capable of metabolic depression) to graded hypoxia (water O(2) saturation 100-10%) was measured at two temperatures ( approximately 1 and 8 degrees C), and compared with that of the Greenland cod (Gadus ogac). Cunner had significantly lower oxygen consumption (MO(2)) values at both 1 (21.2+/-2.4 mg O(2) kg(-0.83) hr(-1)) and 8 degrees C (31.6+/-1.5 mg O(2) kg(-0.83) hr(-1)) as compared with the Greenland cod (64.0+/-4.3 and 92.5+/-6.3 mg O(2) kg(-0.83) hr(-1), respectively) when measured under normoxia. In addition, this species was considerably more hypoxia tolerant than Greenland cod, as evidenced by an ability to tolerate lower water O(2) saturations and their significantly lower critical oxygen saturation (S(crit)) values (16.9 and 21.4% vs. 40.4 and 54.8% water O(2) saturation at 1 and 8 degrees C, respectively). Surprisingly, the normoxic Q(10) value for cunner MO(2) (1.77) was not indicative of a hypometabolic state at 1 degrees C (based on previous experiments on this species). However, 1 degrees C cunner immediately decreased their MO(2) to a new steady state (44% below normoxic levels) when water O(2) saturation was lowered, and this resulted in a Q(10) of approximately 4.5 at oxygen levels between 80 and 20% saturation. This study provides the first data to suggest that fish capable of metabolic depression have an enhanced hypoxia tolerance, and that the cunner's hypometabolic state at cold temperatures is plastic and dependent on other environmental parameters.