A comparative analysis of putative oxygen-sensing cells in the fish gill

Onset and early development of hypoxic ventilatory responses and branchial neuroepithelial cells in Xenopus laevis

Onset and ontogeny of the O₂ chemoreceptive control of ventilation was investigated in Xenopus laevis. The density and size of branchial serotonin-immunoreactive neuroepithelial cells (5-HT-IR NECs) were also determined using confocal immunofluorescent microscopy. Larvae started gill ventilation at 3 days post-fertilization (dpf), and, at this early stage, acute hypoxic exposure produced an increase in frequency from 28 ± 4 to 60 ± 2 beats x min⁻¹. Concurrent with the onset of ventilatory responses, 5-HT-IR NECs appeared in the gill filament bud. Lung ventilation began at 5 dpf and exhibited a 3-fold increase in frequency during acute hypoxia. At 10 dpf, gill ventilatory sensitivity to hypoxia increased, as did NEC density, from 15 ± 1 (5 dpf) to 29 ± 2 (10 dpf) cells x mm of filament⁻¹. Unlike ventilation frequency, gill ventilation amplitude and lung expired volume were unaltered by acute hypoxia. Chronic exposure to moderate hypoxia, at a P(O₂) of 110 mmHg, attenuated acute responses to moderate hypoxia at 10 and 14 dpf but had no effect at more severe hypoxia or at other stages. Chronic hypoxia also stimulated 5-HT-IR NECs growth at 21 dpf. Collectively, larvae at 5 dpf exhibited strong O₂-driven gill and lung ventilatory responses, and between 10 and 21 dpf, the early hypoxic responses can be shaped by the ambient P(O₂).

Control of respiration in fish, amphibians and reptiles

Fish and amphibians utilise a suction/force pump to ventilate gills or lungs, with the respiratory muscles innervated by cranial nerves, while reptiles have a thoracic, aspiratory pump innervated by spinal nerves. However, fish can recruit a hypobranchial pump for active jaw occlusion during hypoxia, using feeding muscles innervated by anterior spinal nerves. This same pump is used to ventilate the air-breathing organ in air-breathing fishes. Some reptiles retain a buccal force pump for use during hypoxia or exercise. All vertebrates have respiratory rhythm generators (RRG) located in the brainstem. In cyclostomes and possibly jawed fishes, this may comprise elements of the trigeminal nucleus, though in the latter group RRG neurons have been located in the reticular formation. In air-breathing fishes and amphibians, there may be separate RRG for gill and lung ventilation. There is some evidence for multiple RRG in reptiles. Both amphibians and reptiles show episodic breathing patterns that may be centrally generated, though they do respond to changes in oxygen supply. Fish and larval amphibians have chemoreceptors sensitive to oxygen partial pressure located on the gills. Hypoxia induces increased ventilation and a reflex bradycardia and may trigger aquatic surface respiration or air-breathing, though these latter activities also respond to behavioural cues. Adult amphibians and reptiles have peripheral chemoreceptors located on the carotid arteries and central chemoreceptors sensitive to blood carbon dioxide levels. Lung perfusion may be regulated by cardiac shunting and lung ventilation stimulates lung stretch receptors.

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.

Nervous control of the gills

The fish gill is a highly complex organ that performs a wide variety of physiological processes and receives extensive nervous innervation from both afferent (sensory) and efferent (motor) fibres. Innervation from the latter source includes autonomic nerve fibres of spinal (sympathetic) and cranial (parasympathetic) origin whose primary role is to induce vasomotor changes within the respiratory or nonrespiratory pathways of the gill vasculature. Autonomic control of the gill occurs by nerve fibres identified as adrenergic, cholinergic, and more recent evidence indicates that nonadrenergic-noncholinergic (NANC) nerve fibres, such as those that express amines, peptides, or nitric oxide, may also play an important role. The distribution and physiological function of NANC nerve fibres, however, is less clear. This review primarily discusses histochemical studies that have characterized the nervous innervation and autonomic control of the gill vasculature. In addition, supporting evidence from recent studies for the efferent control, or modulation, of other homeostatic processes in the gill is examined.

New developments on gill innervation: insights from a model vertebrate

The fish gill is a highly specialized and complex organ that performs a variety of important physiological functions. In this article, we briefly review the innervation of important structures of the branchial region, such as the gill filaments, respiratory lamellae and pseudobranch, and discuss the physiological significance of this innervation within the context of homeostatic functions of the gill, such as oxygen sensing and ion regulation. Studies in zebrafish utilizing techniques of confocal microscopy and immunolabelling, with specific antibodies against neuronal markers, have recently led to the characterization of innervation patterns in the gills not attained with traditional techniques of histochemistry and electron microscopy. We will discuss the association of putative sensory nerve fibres with O2-chemoreceptive neuroepithelial cells and the implications of dual sensory pathways for cardiorespiratory and vascular control. In addition, the idea of the neural control of ion regulation in the gill based on the apparent innervation of mitochondria-rich cells, and the role of innervation in the pseudobranch, will be presented.