Acid-base and electrolyte status in carp (Cyprinus carpio) exposed to low environmental pH

Carp (Cyprinus carpio) were exposed to environmental water pH (pHw) step changes from 7.4 to 5.1, 5.1 to 4.0 and 4.0 to 3.5 pH, PCO2, PO2 and lactate in dorsal aortic blood, [Na+], [K+] and [Cl-] in dorsal aortic plasma, base loss, and ammonia excretion were determined as a function of time after each pHw step change. At pHw 5.1 the measured blood acid-base and electrolyte parameters remained essentially unchanged; the base loss, however, was increased by a factor of 2. When pHw was lowered to 4.0 an additional severe increase in the 'net base loss', expressed as the difference between base loss and ammonia excretion, resulted in progressive reduction of arterial pH and [HCO3-]. The electrolyte status was also severely disturbed by progressively falling plasma [Na+] and [Cl-], which is attributed to failure of the active H+/Na+ and HCO3-/Cl- exchange mechanisms in the gills. At pHw 4.0 the acid-exposure syndrome is characterized by acid-base and electrolyte disturbances apparently not related to hypoxia. However, at pHw 3.5, tissue hypoxia, due to disturbances of gill gas exchange and to Bohr and Root effects, appears to be an additional important factor aggravating the disturbances of acid-base and electrolyte status.