Sensitivity of CO2 excretion to blood flow changes in trout is determined by carbonic anhydrase availability

Abstract

The blood transit time through the gills of rainbow trout ( Oncorhynchus mykiss) was modified by manipulation of cardiac output (V˙b). The experiments tested the hypothesis that efficiency of CO2 excretion is sensitive to changes in blood flow owing to chemical equilibrium limitations. An extracorporeal blood shunt was used to continuously monitor blood gases in fish in which V˙b was elevated (by 13.3 ± 2.4 ml · min−1 · kg−1) by intravascular saline injection or reduced (by 10.8 ± 1.8 ml · min−1 · kg−1) by removal of plasma. The arterial partial pressure of CO2(PaCO2 ; an index of CO2 excretion efficiency) was increased with elevated V˙b and was decreased with reduced V˙b such that the changes in PaCO2 exhibited a significant positive sigmoidal relationship with the changes in V˙b ( r 2 =0.75; P< 0.05). In contrast, there was no significant relationship between changes in the arterial partial pressure of O2(PaO2 ; an index of O2 uptake efficiency) and changes in V˙b ( r 2 = 0.07; P > 0.05). The intravenous administration of carbonic anhydrase (CA; 10 mg/kg) before vascular volume loading eliminated the increase in PaCO2 with increased V˙b that was observed in control fish.