Adrenergic Inhibition of Carbon Dioxide Excretion by Trout Red Blood Cells In Vitro is Mediated by Activation of Na⁺/H⁺ Exchange

We have used a sensitive new technique to assess the mechanism(s) of adrenergic inhibition of rainbow trout (Oncorhynchus mykiss) red blood cell (RBC) carbon dioxide excretion in vitro. The effect was only apparent using blood acidified to simulate metabolic acidosis. Red blood cell CO₂ excretion was inhibited in a dose-dependent manner by physiologically relevant concentrations of noradrenaline (10– 1000 nmol l⁻¹) or adrenaline (100– 1000 nmol l⁻¹). The β -adrenoceptor antagonist propranolol abolished the inhibitory effect of noradrenaline, whereas the ir-adrenoceptor antagonist phentolamine was without effect. The action of noradrenaline on RBC CO₂ excretion was mimicked by the β-adrenoceptor agonist isoproterenol, but not by the α-adrenoceptor agonist phenylephrine. Therefore, adrenergic inhibition of CO₂ excretion is mediated by RBC β -adrenoceptors, presumably of the β₁ subtype. The Na⁺/H⁺ exchange inhibitor amiloride effectively blocked adrenergic stimulation of Na⁺/H⁺ exchange (as indicated from measurements of pHe and RBC pHi) and entirely prevented the inhibition of CO₂ excretion. Noradrenaline significantly reduced the rate of CO₂ excretion even in the presence of the C1⁻/HCO₃⁻ exchange inhibitor SITS. Therefore, adrenergic inhibition of CO₂ excretion is accomplished via activation of RBC Na⁺/H⁺ exchange rather than by a direct inhibition of CC/ HCO₃⁻ exchange. The observed relationship between CO₂ excretion rates and the RBC transmembrane pH difference (pHe —pHi) and the occurrence of the inhibition only at low pHe provide further evidence of the linkage with RBC Na⁺/H⁺ exchange. We suggest that adrenergic activation of RBC Na⁺/H⁺ exchange impedes CO₂ excretion by causing a rise in intracellular HCO₃⁻levels concurrent with a reduction of intracellular ⁠. The net result is a reduced gradient for HCO₃⁻ entry into the RBC in conjunction with a diminution of the outwardly directed gradient. Thus, the rate of formation of CO₂ from the dehydration of plasma HCO₃⁻ is reduced and, in turn, a portion of this CO₂ is not excreted but recycled through the red blood cell.