Adaptive tolerance of fish myocardium to hypercapnic acidosis.

Isometric, electrically paced strips of cardiac ventricle from two species of fish (plaice, Pleuronectes platessa; cod, Gadus morrhua) with different tolerance to hypoxia were compared with respect to effects of hypercapnic acidosis. Acidosis was induced by altering the equilibrating gas mixture for the muscle strip chamber from 3% CO2 in 97% 02 to 15% CO2 in 85% O2. The pH was varied further by changing the NaHCO3 content of the Cortland-Ringer solution used in the muscle chamber. After onset of acidosis with the highest buffer value of the Cortland-Ringer solution (35.7 mM NaHCO3), the force decay was similar for the initial 10 min of exposure to high Pco2. Subsequently the cod heart continued to lose force at the same rate, whereas the plaice heart regained a cardiac contractile force that after 40 min even exceeded prehypercapnic values. When buffer values were varied by changing the bicarbonate content of the Cortland-Ringer solution in steps from 0.0 to 35.7 mM NaHCO3, the cod heart showed steep force decays at all buffer values during hypercapnic acidosis. The plaice heart showed a similar decline at low buffer values but at a bicarbonate concentration above 23.8 mM NaHCO3, the initial force decline was reversed and prehypercapnic force restored.