Abstract
The red cell concentrations of ATP and GTP were investigated in Tilapia nilotica in normal and hypoxic waters. Reduction of water oxygen content diminished the cellular concentrations of both organophosphates. The concentration of ATP fell by about 70% of its normoxic values (2.4 ± 2.6 µ mol Pi 100 ml) at an O2 concentration of 4.5 mg l−1 in the water; further reductions of aquatic oxygen levels were not associated with significant changes in cellular ATP. Normal cellular levels of GTP (25.8 ± 2.7 µ mol Pi: 100 ml RBC) decreased progressively with advanced hypoxia especially at critical aquatic O2 concentrations. The ratio of the concentrations of ATP:GTP (2.4:1) in normoxic water decreased with hypoxia till it approached the equivalence in water of less than 5.5 mg l−1 O2. It is suggested that ATP is more important in modulation of blood oxygen affinity during mild or transient hypoxia and that GTP exerts its regulatory action over a wider range of aquatic O2 concentrations.
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