Ammonia as a stimulant to ventilation in rainbow trout Oncorhynchus mykiss

Ammonia is the third most important respiratory gas in ammoniotelic fish after oxygen and carbon dioxide. We here investigated the effects of elevated plasma ammonia on ventilation in freshwater rainbow trout. Intact trout fitted with indwelling dorsal aortic catheters were given injections (over 5 min) of Cortland saline, isotonic high ammonia solutions (NH(4)HCO(3), (NH(4))(2)SO(4), NH(4)OH at pH 8.0, and NH(4)OH at pH 9.0), and other solutions as controls for acid-base effects, while ventilatory rate (VR) and buccal pressure amplitude (DeltaP(buccal)) were recorded. All high ammonia solutions resulted in immediate elevations of plasma Tamm(a), Pa(NH3), and [NH(4)(+)](a), and increases in ventilatory DeltaP(buccal) and VR to different degrees. However, while Pa(O2) remained constant, in every case there was a confounding change in one or more components of acid-base status (decreases in pH(a) or increases in [HCO(3)(-)](a) or Pa(CO2) in different treatments), although the ventilatory responses to ammonia injections were generally larger than could be explained by changes in acid-base status. Therefore a series was performed in which normal blood perfusion of the gills was replaced by ventral aortic perfusion with either Cortland saline or Cortland saline plus high ammonia in which pH, [HCO(3)(-)], P(CO2), and P(O2) remained unchanged. Although ventilation was depressed in these anaesthetized, spontaneously ventilating preparations, perfusion with high ammonia saline increased DeltaP(buccal). In a final series, trout were infused for 24h with Cortland saline, isotonic NH(4)HCO(3), or isotonic (NH(4))(2)SO(4) solutions. The two ammonia solutions both caused persistent elevations in VR and DeltaP(buccal), together with similar large increases in plasma Tamm(a), Pa(NH3), and [NH(4)(+)](a). As there was no changes in Pa(O2), pH(a), Pa(CO2), or [HCO(3)(-)](a) in the (NH(4))(2)SO(4) infusion series, this, together with the ventral aortic perfusion experiment, provides the most convincing evidence that ammonia stimulates ventilation. We suggest several circumstances (post-feeding, post-exercise) where the role of ammonia as a ventilatory stimulant may have adaptive benefits for O(2) uptake, and propose that ammonia-induced hyperventilation may also facilitate ammonia excretion in rainbow trout.