The conversion of plasma HCO₃^- to CO₂ by rainbow trout red blood cells in vitro: adrenergic inhibition and the influence of oxygenation status

This study employed a recently developed radioisotopic assay (Wood and Perry 1991) to examine the inhibition, induced by catecholamines, of the conversion of plasma HCO₃^- to CO₂ in acidotic trout blood, and the influence of oxygenation status on the response. Blood was incubated in vitro at P_CO2= 2 torr, and 10^-6 M noradrenaline was employed as the adrenergic stimulus. In particular we investigated whether the inhibition of plasma HCO₃^- conversion could be explained by a limited supply of H⁺s for the intracellular HCO₃^- dehydration reaction because of competition by the adrenergically activated Na/H⁺ exchanger. Hypoxia (P_O2= 15 torr) was employed as a tool to intensify this competition. Hypoxia raised RBC pHi, pHe, and plasma total CO₂ concentration (C_CO2) by the Haldane effect, and increased the magnitude of Na⁺/H⁺ activation, expressed as the change in the transmembrane pH gradient (pHe-pHi). However hypoxia did not alter the inhibition of the conversion of plasma HCO₃^- to CO₂ caused by noradrenaline. Hypoxia itself stimulated the RBC-mediated conversion of plasma HCO₃^- to CO₂ by about 20% in the presence or absence of noradrenaline. The conversion rate was strongly correlated with pHe, pHe-pHi, and plasma C_CO2 in these experiments, but not with pHi. We conclude that adrenergically mediated inhibition in the conversion of plasma HCO₃^- to CO₂ by trout RBCs is not due to competitive limitation on intracellular H⁺s, but rather to changes in the electrochemical gradient for HCO₃^- entry and/or to CO₂ recycling from plasma to RBC. The deoxygenated condition helps to promote CO₂ excretion at the level of the RBC.