Neal J. Smatresk

7 Afferent Inputs Associated with Cardioventilatory Control in Fish

Publisher Summary This chapter describes the afferent inputs associated with cardioventilatory control in fish. The basic respiratory rhythm and rhythmic contractions of the heart result from the actions of endogenous rhythm generators and do not require afferent feedback for their initiation or maintenance in fish. Mechanoreceptors sensitive to displacement of the pharynx, pharyngeal pads, gill arches, gill rakers and filaments, and air-breathing organs have been identified in fishes. As in other vertebrates, these mechanoreceptors appear to be simple free nerve endings located in connective tissue or muscle. There is direct and indirect evidence that indicates that some fishes may possess intracardiac receptors homologous to mammalian atrial and ventricular stretch receptors. The internally oriented chemoreceptors respond to the mixed venous blood somewhere between the ventral aorta and the afferent filamental artery. Nociceptors with characteristics similar to juxtapulmonary receptors in mammalian lungs have been demonstrated in the gills of dogfish.

Effects of central and peripheral chemoreceptor stimulation on ventilation in the marine toad, Bufo marinus

The contributions of central and peripheral chemoreceptors to respiratory control in lightly anesthetized Bufo marinus, were assessed by measuring the ventilatory responses to unidirectional ventilation (UDV) of the lungs at several concentrations of CO2 or O2, during intracranial perfusion (ICP) with hypercapnic acidic (5% CO2, pH 7.2) or hypocapnic alkaline (0% CO2, pH 8.3) mock CSF solutions. Peripheral chemoreceptor stimulation alone (hypoxia or hypercapnia during ICP with hypocapnic alkaline CSF) significantly increased breathing frequency and amplitude. ICP with hypercapnic acidic CSF further stimulated ventilation, primarily by significantly increasing the number of breaths/bout of breathing and decreasing the non-ventilatory time at all levels of peripheral ventilatory drive. When peripheral and central chemoreceptor stimulation was low toads were apneic. Stimulation of either central or peripheral chemoreceptors was sufficient to reinitiate breathing. Responses to ICP were greatest when perfusion was directed to the ventral medullary surface (VMS). These results suggest that the initiation of breathing and overall levels of breathing are functions of the combined afferent input from peripheral chemoreceptors and central CO2/pH sensitive chemoreceptors, located near the VMS. Stimulation of central chemoreceptors, however, produced longer duration bouts of rhythmic breathing than did peripheral chemoreceptor stimulation.

Evidence for two oxygen-sensitive chemoreceptor loci in channel catfish, Ictalurus punctatus

Sodium cyanide (NaCN) was used as a chemical probe to help localize externally and internally oriented oxygen-Sensitive chemoreceptors and to identify the reflex effects they control in anesthetized, spontaneously breathing channel catfish (Ictalurus punctatus). Fish responded rapidly to NaCN (500μg) given in the inspired water flow (external NaCN) with transient bradycardia and a more prolonged stimulation of gill ventilation frequency (fg) and opercularpressure amplitude (POP). Internal injections of NaCN (50 μg), given via the dorsal aorta, stimulated fg and POP after about a 42-s latency but had no effect on heart rate (fb). Injections of NaCN (50 μg) into the ventral aorta significantly reduced the latency of response, stimulating POP and fg in 9–16s. Cardiovascular and ventilatory variables returned topreinjection levels within 30 min. These results suggest that cardiovascular and ventilatory reflex responses to hypoxia in channel catfish are mediated by both externally and internally oriented O₂-sensitive chemoreceptors. Externally oriented chemoreceptors appear to monitor the O2 tension of the inspired water and elicit bradycardia and hyperpnea when stimulated by aquatic hypoxia or external NaCN. Internal O2-sensitive chemoreceptors appear to respond to changes in blood O2 stimulus levels, in orjust downstream from the gills, and reflexively increase gill ventilation when stimulated by hypoxemia.

Branchial chemoreceptors mediate ventilatory responses to hypercapnic acidosis in channel catfish.

The effects of hyperoxic hypercapnia on cardiovascular and ventilatory variables and blood gas and acid/base parameters were examined in conscious and anesthetized spontaneously breathing (ASB) channel catfish, Ictalurus punctatus. These separate experiments were designed to determine: (1) if channel catfish show a ventilatory response to hypercapnic acidosis when blood O(2) content is maintained in conscious animals; and (2) whether branchial receptors innervated by cranial nerves IX and X mediate this response. The combination of high O(2) and CO(2) tensions allowed the cardioventilatory effects of hypercapnic acidosis to be assessed independently of Root or Bohr mediated changes in blood O(2) content. In the absence of significant changes in dorsal or ventral aorta O(2) content, hyperoxic hypercapnia significantly stimulated ventilation, relative to hyperoxic exposure. Hypercapnic acidosis, however, had no significant effects on blood pressure or heart rate. Branchial denervation in ASB fish abolished the ventilatory response to hypercapnic acidosis. The results indicate that hypercapnic acidosis independently stimulates ventilation in channel catfish. This response is mediated by CO(2)/pH-sensitive branchial receptors innervated by cranial nerves IX and X.

The ecological role of caudal lamellae loss in the larval damselfly, Ischnura posita (Hagen) (Odonata: Zygoptera)

SummaryDamselfly larvae may autotomize and regenerate any of their 3 caudal lamellae. At least one missing or regenerating lamella was evident in 50.1% of field collected Ischnura posita larvae. Lamellae loss during molting is very infrequent (1 out of 117 recorded molts). Laboratory trials indicate that conspecifics remove lamellae and that this process is density dependent. The percentage of larvae losing lamellae during 24 h trials ranged from 73.5 at the highest density tested to 17.3 at the lowest density. I. posita larvae are cannibalistic. The presence of lamellae reduces an individuals chance of being cannibalized. More than twice as many final instar lamellae-less larvae were cannibalized during 24 h trials than analogous individuals having 3 lamellae at experimental initiation. Costs are also associated with lamellae autotomy. 1) Although individuals without lamellae can swim they are more reluctant to release from a wooden stalk and swim when threatened (9% release) than are larvae with lamellae (29% release). Since swimming is part of their repertoire of anti-predator behaviors this behavioral shift should be detrimental. 2) Caudal lamellae function in O2 uptake. Trials were conducted with larvae having and not having lamellae in an experimental horizontal oxygen gradient system. Relative to larvae without lamellae, those with lamellae preferred deeper depths at PO2 values greater than 70 torr. Many lamellae-less larvae distributed themselves at the water surface throughout the range of PO2 values tested. Differential depth distribution between larvae with and without lamellae is highly significant (P < 0.01).

The ventilatory responses of the caecilian Typhlonectes natans to hypoxia and hypercapnia.

Typhlonectes natans empty their lungs in a single extended exhalation and subsequently fill their lungs by using a series of 10–20 inspiratory buccal oscillations. These animals always use this breathing pattern, which effectively separates inspiratory and expiratory airflows, unlike most urodele and anuran amphibians that may use one to many buccal oscillations for lung inflation and typically mix expired and inspired gases. Aquatic hypoxia had no significant effect on the breathing pattern or mechanics in these animals. Aerial hypoxia stimulated ventilatory frequency and increased the number of inspiratory oscillations but had little effect on inspiratory and expiratory tidal volume. Aquatic hypercapnia elicited a large significant increase in air‐breathing frequency and minute ventilation compared to the small stimulation of minute ventilation seen during aerial hypercapnia. Some animals responded to aquatic hypercapnia with a series of three or four closely spaced breaths separated by long nonventilatory periods. Overall, T. natans showed little capacity to modulate expiratory or inspiratory tidal volumes and depended heavily on changing air‐breathing frequency to meet hypoxic and hypercapnic challenges. These responses are different from those of anurans or urodeles studied to date, which modulate both the number of ventilatory oscillations in lung‐inflation cycles and the degree of lung inflation when challenged with peripheral or central chemoreceptor stimulation.