EFFECT OF WATER ALKALINITY ON GILL CO2 EXCHANGE AND INTERNAL PCO(2) IN AQUATIC ANIMALS

In addition to metabolic CO2 production and gill ventilatory flow rate , expired water PCO2 is very dependent on water acid-base balance in a complex way. This is particularly true in carbonated waters at low am bient PCO2 and high pH, where CO2 excreted in the gill water may be bu ffered by carbonate ions, leading to an increased CO2 capacitance coef ficient. The higher the carbonate alkalinity (CA) and the lower the in spired PCO2 (i.e., the higher the inspired water pH), the stronger the carbonate buffering and the smaller the increase of PCO2 in the gill water during respiratory CO2 exchanges. As a consequence, as shown by a number of reported data, increasing the CA leads to blood hypocapnia and respiratory alkalosis at constant low, but not at high, inspired PCO2. In the low range of inspired PCO2, internal PCO2 becomes very se nsitive to even small changes of water PCO2, which ma)i explain at lea st in part the large variability oi reported blood PCO2 values in gill breathers. Water CA also influences the amplitude of respiratory acid -base disturbances caused by changes of the gill ventilatory flow rate . Carbonate buffering of excreted CO2 and thus dependence of blood PCO 2 On water alkalinity requires catalysis of CO2 hydration by carbonic anhydrase, that must, be available from the water side oi the gill epi thelium. COMP BIOCHEM PHYSIOL 119A;1:131-136, 1998. (C) 1998 Elsevier Science Inc.