Rh protein expression in branchial neuroepithelial cells, and the role of ammonia in ventilatory control in fish

Abstract

Bill Milsom has made seminal contributions to our understanding of ventilatory control in a wide range of vertebrates. Teleosts are particularly interesting, because they produce a 3rd, potentially toxic respiratory gas (ammonia) in large amounts. Fish are well known to hyperventilate under high environmental ammonia (HEA), but only recently has the potential role of ammonia in normal ventilatory control been investigated. It is now clear that ammonia can act directly as a ventilatory stimulant in trout, independent of its effects on acid-base balance. Even in ureotelic dogfish sharks, acute elevations in ammonia cause increases in ventilation. Peripherally, the detection of elevated ammonia resides in gill arches I and II in trout, and in vitro, neuroepithelial cells (NECs) from these arches are sensitive to ammonia, responding with elevations in intracellular Ca(2+) ([Ca(2+)]i). Centrally, hyperventilatory responses to ammonia correlate more closely with concentrations of ammonia in the brain than in plasma or CSF. After chronic HEA exposure, ventilatory responsiveness to ammonia is lost, associated with both an attenuation of the [Ca(2+)]i response in NECs, and the absence of elevation in brain ammonia concentration. Chronic exposure to HEA also causes increases in the mRNA expression of several Rh proteins (ammonia-conductive channels) in both brain and gills. "Single cell" PCR techniques have been used to isolate the individual responses of NECs versus other gill cell types. We suggest several circumstances (post-feeding, post-exercise) where the role of ammonia as a ventilatory stimulant may have adaptive benefits for O2 uptake in fish.

Physiological and molecular responses of the spiny dogfish shark (Squalus acanthias) to high environmental ammonia: scavenging for nitrogen

“…For example, are the molecular responses identified in the present study a response to internal or external ammonia elevation? A recent study of ventilatory control in S. acanthias found that the hyperventilatory response to HEA was mediated by internal rather than external ammonia levels (Zhang et al, 2014). …”

Does ammonia trigger hyperventilation in the elasmobranch, Squalus acanthias suckleyi?

“…This more chronic response led to an elevation in breathing amplitude rather than breathing frequency, and occurred without any change in pH a , P aO2 , P aCO2 , or [HCO 3 − ] a . Whether this ventilatory control by ammonia in elasmobranchs is peripheral by branchial NECs or central by sensors responding to brain [T Amm ] elevation, or by both as in teleosts (Zhang et al, 2014), remains to be examined. Wood et al (2010).…”

“…Peripheral control in trout involves NECs and their associated afferent nerves located on the 1st and 2nd gill arch, with receptors on the 2nd gill arch detecting only internal ammonia (slow hyperventilatory response after high environmental ammonia exposure), whereas those on the 1st arch ammonia (faster response) may additionally sense external ammonia to some degree (Zhang et al, 2011(Zhang et al, , 2014. Central control of hyperventilation seemed to be dependent on brain [T Amm ] rather than plasma [T Amm ] or cerebrospinal fluid [T Amm ].…”

“…In this study it was ventilatory stroke volume (large changes) rather that ventilation frequency (small changes) which was primarily increased, and the authors suggested that hyperventilation was an adaptive response to facilitate ammonia excretion, as well as to help elevate oxygen uptake at times of increased oxidative demands, such as after exercise or after a meal. Later studies demonstrated that ammonia's respiratory role as a ventilatory stimulant was probably mediated both peripherally, involving branchial NECs (Zhang et al, 2011(Zhang et al, , 2014, as well as centrally through the brain as hyperventilatory responses to ammonia correlate more closely with concentrations of ammonia in the brain than in plasma or CSF (Zhang et al, 2013), and that the response appeared to be primarily to internal rather than external ammonia (Zhang et al, 2011(Zhang et al, , 2014.…”

The sensing of respiratory gases in fish: Mechanisms and signalling pathways

“…Thus, in addition to being the putative peripheral O 2 chemoreceptors in fish, there is emerging evidence that the NECs also serve as CO 2 sensors giving them a bimodal sensory function resembling that of the carotid body Type 1 cells. In addition, there is accruing evidence that fish NECs are the mediators of the hyperventilatory responses associated with HEA (Zhang et al, 2011(Zhang et al, , 2014. Recent developments related to specific features of O 2 , CO 2 and NH 3 sensing will be discussed in greater detail below.…”