Acid-base regulation and ion transfers in the carp (Cyprinus carpio): pH compensation during graded long- and short-term environmental hypercapnia, and the effect of bicarbonate infusion

ABSTRACT To study both temporal and quantitative effects of hypercapnia on the extent of pH compensation in the arterial blood, specimens of carp (Cyprinus carpio) were exposed to a of about 7·5mmHg (1 mmHg = 133·3Pa) (1% CO2) in the environmental water for several weeks, and a second group of animals was subjected to an environmental of about 37 mmHg (5 % CO2) for up to 96 h. A third series of experiments was designed to test the possibility that infusion of bicarbonate would increase the extent of plasma pH compensation. Dorsal aortic plasma pH, and [HCO3−], as well as net transfer of HCO3−-equivalent ions, NH4+, Cl− and Na+, between fish and ambient water, were monitored throughout the experiments. Exposure to environmental of 7·5 mmHg resulted in the expected respiratory acidosis with the associated drop in plasma pH, and subsequent compensatory plasma [HCO3−] increase. The compensatory increase of plasma bicarbonate during long-term hypercapnia continued during 19 days of exposure with plasma bicarbonate finally elevated from 13·0 mmol l−1 during control conditions to 25·9 mmol l−1 in hypercapnia, an increase equivalent to 80% plasma pH compensation. Exposure to 5 % hypercapnia elicited much larger acid-base effects, which were compensated to a much lesser extent. Plasma pH recovered to only about 45 % of the pH depression expected at constant bicarbonate concentration. At the end of the 96-h exposure period, plasma [HCO3−] was elevated by a factor of 2·5 to about 28·2mmol l−1. The observed increase in plasma bicarbonate concentration during 5 % hypercapnic exposure was attributable to net gain of bicarbonate equivalent ions from ( or release of H+-equivalent ions to) the environmental water. Quantitatively, the gain of 15·6 mmol kg−1 was considerably larger than the amount required for compensation of the extracellular space, suggesting that acid—base relevant ions were transferred for compensation of the intracellular body compartments. The uptake of bicarbonate-equivalent ions from the water was accompanied by a net release of Cl− and, to a smaller extent, by a net uptake of Na+, suggesting a 75 % contribution of the Cl−/HCO3− exchange mechanism. Infusion of bicarbonate after 48 h of exposure to 7·5 mmHg had only a transient effect on further pH compensation. The infused bicarbonate was lost to the ambient water, and pre-infusion levels of bicarbonate were reattained within 24 h. Repetition of the infusion did not result in a notable improvement of the acid-base status. These observations are consistent with the idea of a ‘threshold’ of the bicarbonate retaining and resorbing structures of the fish.