Effects of ambient PO2 on hemoglobin-oxygen affinity and red cell ATP concentrations in a benthic fish, Pleuronectes platessa.

Previous studies have demonstrated that acclimation of mammals and fish to hypoxic conditions elicits opposite responses in red cell organic phosphate levels (an increase and a decrease, respectively). This investigation focuses on the "reverse" response - when the benthic fish Pleuronectes platessa, which occurs locally under hypoxic conditions, is subjected to higher oxygen tensions. Compared to the hypoxic (PO2 =30 mm Hg) situation the ATP:Hb ratio in increased (1.1 to 1.8) and oxygen affinity is increased (i.e. P50 changed from 18 to 30 mm, at pH 7.7 and 15 degrees C) by acclimation to Po2 of 150 mm Hg. Under hyperoxic conditions (Po2 =300 mm Hg) the ATP:hemoglobin ratio and blood P50 showed a surprising decrease and returned to values comparable to those in hypoxic fish (to 1.3 and 18 mm Hg, respectively). When "stripped" of organic and inorganic ions hemoglobin solutions prepared from fish acclimated at 30 and at 150 mm Hg have the same P50 and Bohr effect values, showing that the intra-erythrocytic factor rather than changes in the hemoglobin molecules, are involved. The pure hemoglobin shows high sensitivity to ATP at physiological concentrations. There is evidence that apart from the direct helcotropic effects, ATP decreases oxygen affinity in the red cells through modification of the Donnan distribution of protons across the red cell membrane. Fish acclimated to 30 and 150 mm Hg oxygen tensions showed the same oxygen consumption rates at the same oxygen tensions.