Respiratory control in the lungfish, Neoceratodus forsteri (krefft)

Abstract

Abstract-1. Respiratory control has been studied in the lungfish, Neoceratodus forsteri by measuring ventilation (V e ), oxygen uptake (VO 2 ), per cent O 2 extraction from water, breathing rates of branchial and aerial respiration and changes in blood gas and pulmonary gas composition during exposure to hypoxia and hypercarbia.2. Hypoxic water represents a strong stimulus for compensatory increase in both branchial and aerial respiration. Water ventilation increases by a factor of 3 or 4 primarily as a result of increased depth of breathing.3. The ventilation perfusion ratio decreased during hypoxia because of a marked increase in cardiac output. Hypoxia also increased the fraction of total blood flow perfusing the lung. Injection of nitrogen into the lung evoked no compensatory changes.4. It is concluded that the chemoreceptors eliciting the compensatory changes are located on the external side facing the ambient water or in the efferent branchial blood vessels.5. Elevated pCO 2 in the ambient water depressed the branchial respiration but stimulated aerial respiration.6. It is suggested that the primary regulatory effect of the response to increased ambient pCO 2 is to prevent CO 2 from entering the animal, while the secondary stimulation of air breathing is caused by hypoxic stimulation of chemoreceptors located in the efferent branchial vessels.

Physiological responses to prolonged aquatic hypoxia in the Queensland lungfish Neoceratodus forsteri

“…This pattern is consistent with previous studies of N. forsteri (Johansen et al, 1967;Fritsche et al, 1993) and parallels the immediate response to hypoxia observed in other facultative air breathers such as the bowfin, Amia calva (Johansen et al, 1970). Johansen et al (1967) showed that although N. forsteri increases branchial ventilation in response to hypoxia, both oxygen extraction and total oxygen uptake actually decrease during the same period. The plateau in branchial ventilation rate and subsequent air breathing by fish in this study are consistent with that conclusion.…”

“…Consecutive air breathing has been reported in N. forsteri previously, at similar levels of hypoxia to those used during this investigation (Johansen et al, 1967;Fritsche et al, 1993). However; this is the first time that air breathing has been monitored on a time scale of days rather than minutes following exposure to hypoxic conditions.…”

“…Increased whole blood oxygen affinity favours greater uptake of oxygen during each air breath (Weber et al, 1979). In addition, loss of chemically bound oxygen across the gills to the water is minimised by having blood with a higher affinity for oxygen (Johansen et al, 1967). In N. forsteri, this combination of frequent air breathing and increased whole blood oxygen affinity appears able to preserve oxygen delivery even at very low levels of Pw O2 .…”

“…When subject to acute hypoxia, N. forsteri responds first by increasing the frequency and stroke volume of branchial ventilation, and then by commencing aerial breathing (Johansen et al, 1967;Fritsche et al, 1993). However, previous studies have assessed individuals exposed to hypoxia for periods of only a few hours.…”

Air-breathing adaptation in a marine Devonian lungfish

“…Their adaptations have also been interpreted as for buoyancy control (Graham 1997), to allow the expulsion of excessive carbon dioxide (Thomson 1969), or to supply the heart with an additional source of oxygen (Farmer 1999). However, in many extant fishes including Neoceratodus, activity is a stronger stimulus for air breathing than aquatic hypoxia ( Johansen et al 1967). The question remains as to whether such adaptations were environmentally or metabolically induced.…”