398
KJELL JOHANSEN
Experiments on juvenile specimens of the African lungfish have produced data in conflict with those cited above (Jesse et al., 1967). Deoxygenated water elicited an increase of both branchial and air breathing
in the juveniles. However, these specimens survived on water breathing
alone for several days, which contrasts markedly with other observations
on adult fish. In addition, the experimental procedure evoked hypoxia
simultaneously in water and in the air phase and consequently could
stimulate the fish via the lung, as well as the gills. The interesting possibility emerges from these studies that control of breathing changes
markedly during the ontogenetic development of the African lungfish.
A logical and simple experiment used to test the importance of air
breathing in fish is to deprive them of access to air. Depending on the
temperature and gas composition of the external water, such experiments
have produced variable results, but they show that the degree of internal
oxygenation clearly influences the breathing efforts. The attempts to
breathe air increase in all air-breathing fishes, while branchial breathing
appears to increase only in species normally dependent on branchial
breathing for an important part of their oxygen uptake.
Experiments in which the fish have surfaced into atmospheres of controlled gas composition, while the conditions in the water have remained
stable, have been more conclusive. Only in a few experiments have internal blood gas tensions been correlated with such procedures. In the
African lungfish, introduction of hypoxic gas into the lung causes a
prompt and sharp increase in the rate of air breathing. Simultaneous
measurements of arterial O2 tension show that air breathing is stimulated
at much higher values than those recorded when the fish practices undisturbed breathing in normal air (Johansen and Lenfant, 1968). This
implies that the chemosensitive areas which trigger the response to inhalation of hypoxic gas are more sensitive to the rate of change than to
the actual level of oxygen tension in the hypoxic condition. Experiments
on Electrophorus support such a contention by showing remarkably
prompt responses to changes in composition of inhaled air. Figure 17a
illustrates how the rate of air breathing was changed after a single breath
of air following hypoxic breathing. This implies a location of the chemoreceptors in the buccal mucosa or in the blood path in intimate contact
with the gas in the mouth. A location in the systemic arteries is ruled out
because of the complete shunt of blood from the gas exchange organ to
the systemic veins. The resulting changes in systemic arterial blood are
much too slow to represent an important feedback stimulus. Figures 17b
and 18 show that inhalation of oxygen-enriched atmospheres causes a
depression of air breathing and is important by suggesting the removal
of a tonic P,,-dependent stimulus. This indicates that normal spontaneous
KJELL JOHANSEN
Experiments on juvenile specimens of the African lungfish have produced data in conflict with those cited above (Jesse et al., 1967). Deoxygenated water elicited an increase of both branchial and air breathing
in the juveniles. However, these specimens survived on water breathing
alone for several days, which contrasts markedly with other observations
on adult fish. In addition, the experimental procedure evoked hypoxia
simultaneously in water and in the air phase and consequently could
stimulate the fish via the lung, as well as the gills. The interesting possibility emerges from these studies that control of breathing changes
markedly during the ontogenetic development of the African lungfish.
A logical and simple experiment used to test the importance of air
breathing in fish is to deprive them of access to air. Depending on the
temperature and gas composition of the external water, such experiments
have produced variable results, but they show that the degree of internal
oxygenation clearly influences the breathing efforts. The attempts to
breathe air increase in all air-breathing fishes, while branchial breathing
appears to increase only in species normally dependent on branchial
breathing for an important part of their oxygen uptake.
Experiments in which the fish have surfaced into atmospheres of controlled gas composition, while the conditions in the water have remained
stable, have been more conclusive. Only in a few experiments have internal blood gas tensions been correlated with such procedures. In the
African lungfish, introduction of hypoxic gas into the lung causes a
prompt and sharp increase in the rate of air breathing. Simultaneous
measurements of arterial O2 tension show that air breathing is stimulated
at much higher values than those recorded when the fish practices undisturbed breathing in normal air (Johansen and Lenfant, 1968). This
implies that the chemosensitive areas which trigger the response to inhalation of hypoxic gas are more sensitive to the rate of change than to
the actual level of oxygen tension in the hypoxic condition. Experiments
on Electrophorus support such a contention by showing remarkably
prompt responses to changes in composition of inhaled air. Figure 17a
illustrates how the rate of air breathing was changed after a single breath
of air following hypoxic breathing. This implies a location of the chemoreceptors in the buccal mucosa or in the blood path in intimate contact
with the gas in the mouth. A location in the systemic arteries is ruled out
because of the complete shunt of blood from the gas exchange organ to
the systemic veins. The resulting changes in systemic arterial blood are
much too slow to represent an important feedback stimulus. Figures 17b
and 18 show that inhalation of oxygen-enriched atmospheres causes a
depression of air breathing and is important by suggesting the removal
of a tonic P,,-dependent stimulus. This indicates that normal spontaneous