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Studies on the role of ammonia in the control of breathing in the Pacific hagfish (Eptatretus stoutii) and rainbow trout (Oncorhynchus mykiss)

University of British Columbia

Ammonia, which is excreted across the gills, is the major nitrogenous waste of fish. It is also a toxicant. My thesis focuses on how and why ammonia influences breathing in fish, using the phylogenetically ancient Pacific hagfish (an agnathan) and the rainbow trout (a teleost) as models. I first characterized the unique breathing mechanism of hagfish, demonstrating a two-phase unidirectional system with a fast suction velar pump for inhalation through the nostril and a much slower force pump for exhalation through gill pouches. High environmental ammonia (HEA) causes an initial hypoventilation, sometimes apnea, and a later sustained hyperventilation. The hypoventilation is independent from responses to O₂ and CO₂ and is mediated by external receptors. The hyperventilation is mediated by increased blood ammonia, detected internally, similar to previous findings on teleosts. In trout, I confirmed an assumption in the literature that ventilation would not affect ammonia excretion. However, I then used chronic internal ammonia loading to upregulate the ammonia transport system (rhesus glycoproteins) in the gills. This removed diffusion limitation so that ammonia excretion became sensitive to ventilation. Hyperventilating trout, therefore, excrete more ammonia. After developing a new less invasive system for the direct measurement of ventilation, I used it to show that HEA hyperventilation is not immediate, but develops gradually, mediated by internal receptors. Indeed, specific application of HEA to the external surface of the gills causes a transient acute hypoventilation, again as in hagfish. Direct application of ammonia to the hindbrain causes hyperventilation by the stimulation of central chemoreceptors, while peripheral chemoreceptors in the gills (neuroepithelial cells) sense increased plasma ammonia. In humans, ammonia buildup in the brain similarly stimulates breathing. I conclude that a role for ammonia in ventilatory control is probably ubiquitous in vertebrates, including the oldest extant representatives (hagfish). Initial hypoventilation is protective against the uptake of a toxicant, while sustained hyperventilation is beneficial at times of internal ammonia loading such post-exercise recovery, and after feeding. This hyperventilation will facilitate not only greater ammonia excretion, but also the greater O₂ uptake needed to recover from exercise and to metabolically process food.

Text

2021-03-16

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/77528

Zoology

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/