Myocardial inotrophy of CO2 in water- and air-breathing vertebrates.

A negative-inotropic effect of CO2 on myocardial contractility presumably occurs because increasing H+ concentration competes with Ca2+ at cellular membranes and proteins. Since air-breathing vertebrates have higher blood and tissue CO2 concentration than water breathers the question was raised whether the cardiac cell has a modified sensitivity to CO2 correlated with the evolutionary transition of vertebrates from water breathers to air breathers. The water-breathing fish, Salmo gairdneri, and the air-breathing turtle, Pseudemys scripta, were selected as experimental animals, since their total CO2 concentration differs markedly (3.0 and 16.0 mmol.kg-1). Electrically paced isometric ventricular strips from both species were subjected to a stepwise increase in PCO2 from 25 to 114 Torr (pH0 7.80 to 7.0; HCO3- 30 mM). Trout were additionally exposed to the same pH0 changes at 5 mM HCO3- by a stepwise increase in PCO2 (4.5-12 Torr). At each increase in PCO2 the turtle heart showed a lesser negative inotropic effect than trout. The present findings offer direct evidence that the negative inotropic effect of CO2 on heart muscle is inversely proportional to the in vivo levels of tissue CO2 concentration. The results obtained are discussed in relation to phylogenetical and ecological aspects of acid-base balance.