Influence of initial respiratory status on the short-and long-term activity of the trout red blood cell β-adrenergic Na⁺/H⁺ exchanger

Abstract

The effects of ambient O₂ partial pressure and CO₂ partial pressure on the intensity of rainbow trout (Oncorhynchus mykiss) red blood cell β-adrenergic Na⁺/H⁺ exchange were investigated. This was accomplished in vitro by continuously monitoring whole blood extracellular pH, partial pressures of O₂ and CO₂ and by measuring red blood cell water content and Na⁺ concentration before and 30 min after the addition of a catecholamine mixture (final nominal concentrations: 250 nmol·l^-1 adrenaline and 20 nmol·l^-1 noradrenaline). The experiments were performed under six different initial conditions combining two ambient partial pressures of CO₂ (1.50 and 6.75 torr) and three ambient partial pressures of O₂ (15, 30 and 150 torr). The activation of red blood cell Na⁺/H⁺ exchange (as indicated by marked reductions of whole blood pH) was followed by transient reductions in blood partial pressures of CO₂ and O₂ (2 min) resulting from the shift of the CO₂/HCO₃ ^- equilibrium within the cell and the subsequent binding of O₂ to the haemoglobin. The initial reduction in blood CO₂ partial pressure was followed by a rise reflecting the titration of plasma HCO₃ ^- by extruded H⁺. At low partial pressure of CO₂ (1.50 torr) there was a pronounced stimulatory effect of hypoxia on the initial intensity of the extracellular acidification (5 min), whereas at high CO₂ partial pressure (6.75 torr) hypoxia actually lowered the extent of the initial acidification. In all cases, Na⁺/H⁺ exchange activation was accompanied by increases in cell water content and red blood cell Na⁺ levles when measured 30 min after addition of catecholamines. Both hypercapnia and hypoxia increased the magnitude of these changes although the largest changes in cell water content and Na⁺ levels were observed under hypercapnic conditions. Thus, the long-term activity (as determined by measuring cell water and Na⁺ levels) of the Na⁺/H⁺ exchanger was enhanced both by hypercapnia and hypoxia regardless of the initial CO₂ partial pressure. The initial activity (5 min), on the other hand, although stimulated by hypercapnia was influenced by hypoxia in opposing directions depending upon the initial CO₂ partial pressure of the blood.