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 (NH4HCO3, (NH4)2SO4, NH4OH at pH 8.0, and NH4OH at pH 9.0), and other solutions as controls for acid–base effects, while ventilatory rate (VR) and buccal pressure amplitude (ΔPbuccal) were recorded. All high ammonia solutions resulted in immediate elevations of plasma Tamma, PaNH3PaNH3, and [NH4+]a, and increases in ventilatory ΔPbuccal and VR to different degrees. However, while PaO2PaO2 remained constant, in every case there was a confounding change in one or more components of acid–base status (decreases in pHa or increases in [HCO3−]a or PaCO2PaCO2 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, [HCO3−], PCO2PCO2, and PO2PO2 remained unchanged. Although ventilation was depressed in these anaesthetized, spontaneously ventilating preparations, perfusion with high ammonia saline increased ΔPbuccal. In a final series, trout were infused for 24 h with Cortland saline, isotonic NH4HCO3, or isotonic (NH4)2SO4 solutions. The two ammonia solutions both caused persistent elevations in VR and ΔPbuccal, together with similar large increases in plasma Tamma, PaNH3PaNH3, and [NH4+]a. As there was no changes in PaO2PaO2, pHa, PaCO2PaCO2, or [HCO3−]a in the (NH4)2SO4 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 O2 uptake, and propose that ammonia-induced hyperventilation may also facilitate ammonia excretion in rainbow trout.