AJP - Regulatory, Integrative and Comparative Physiology, Vol 234, Issue 3 155-R157, Copyright © 1978 by American Physiological Society
Myocardial inotrophy of CO2 in water- and air-breathing vertebrates
O. Poupa, H. Gesser and K. Johansen
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.
