Time Course of Changes in Enzyme Activities and Blood Respiratory Properties of Killifish during Long-Term Acclimation to Hypoxia

Evidence is presented for quantitative enzymatic changes in fishes in response to the oxygen content of their environment. Fundulus heteroclitus significantly increased the specific activities of lactate dehydrogenase, malate dehydrogenase, and glucose-phosphate isomerase in liver tissue during the first 28 days of acclimation to hypoxia (1.5-2.5 ppm dissolved oxygen). However, the activities of these enzymes returned to control levels after 35 days of hypoxic existence. Unlike liver tissue, white skeletal muscle demonstrated no consistent pattern during hypoxic acclimation. Blood hematocrit increased and red blood cell organic phosphate levels decreased and remained so during the entire acclimation period (35 days). These adaptations facilitate the uptake of oxygen at the gills in an oxygen-poor environment. Serum lactate concentration increased during the first 3 wk of hypoxia. However, this parameter returned to control levels by 28 days of acclimation. These observations, together with behavioral observations, are consistent with an improved delivery and utilization of oxygen after long-term acclimation in a hypoxic environment. The enzymatic changes are thus a short-term response to hypoxic stress.