A pH-stat method is described for determining the initial rate of efflux of H+ equivalents from trout erythrocytes after stimulation with catecholamines. This measure of transport capacity of the Na+/H+ exchanger was sensitive to medium pH with a peak at 7.24 ± 0.09 pH units. After washing blood into a trout saline the exchange capacity recovered to a maximal value after 2 h and remained stable for at least 24 h at 5°C. Internal Na+ concentration reached a low, stable value also within 2–3 h of washing. The antiporter was comparatively unstable when incubated at room temperature. The concentrations for half-maximal responses to the agonists adrenaline, noradrenaline and isoproterenol were 3.0 ± 1.1 × 10−7, 2.5 ± 3.0 × 10−8 and 1.5 ± 0.6 × 10−8mol l−1, respectively.

The variation of antiporter capacity with time of the year was followed over a 24month period. Capacity was highest during early summer at 200–250 mmol h−1 l−1 packed cell volume (pcv), but declined from November to March to a low of approximately 100 mmol h−1 l−1 pcv. Exchange capacity increased rapidly during the early spring back to values characteristic of early summer. It appears that both temperature and photoperiod are involved in regulating the spring increase.

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