mone which, as we have seen, like tetrapod prolactin but in contrast to the fish 'prolactin', can stimulate the pigeon crop-sac.
The African lungfish lives an aquatic life in regions of Africa that are subjected
to seasonal drought, during which time the rivers and lakes it inhabits dry up . When
this begins to happen the lungfishes dig burrows in the bed of the lake and retreat
into them when the water disappears. While so entombed they surround themselves
(except for the nares) with a cocoon made of secreted mucous and fine particles
of mud, and this limits their evaporative water loss . They aestivate in this manner
until the next wet season, which usually comes in 4 to 6 months, though they can
survive 2 or 3 years and perhaps even longer (see SMITH, 1961). When in this condition they reduce their metabolic rate to a very low level and subsist principally
by catabolizing tissue proteins. The carbohydrate reserves of these fish may even
increase during aestivation which contrasts with the decrease that is observed when
the aquatic fish are fasted (JANSSENS, 1964). Urine formation is too small to measure
during aestivation and urea derived from the protein catabolism is accumulated
in high concentrations in the body. HOMER SMITH found that after prolonged aestivation urea made up as much as 3 % of the entire body weight. When water returns
to the lakes in which the lungfish live, it submerges the burrows and the fish break
out of their cocoons and rise to the surface. According to SMITH and JANSSENS they
then excrete accumulated urea and sodium, most of this being lost in the first two
days.
The role of hormones in the various phases of the life cycle of lungfishes is
largely conjectural. When in their aquatic phase they take up water (the contribution of the gills and skin is not known) at a rate similar to that of other freshwater fishes and excrete this as a dilute urine (Table 7.7). While fishes do not respond to neurohypophysial peptides with an antidiuresis as seen in tetrapods, the
phyletic relationship of the lungfishes and the Amphibia makes it seem possible
that these two groups may both exhibit a 'water balance effect'. However, the injection of vasotocin was not found to produce such an effect in Protopterus (HELLER and BENTLEY, 1965) but instead it results in a diuresis (SAWYER, 1966 a). We
also found that the injection of vasotocin into Protopterus resulted in a net loss
of sodium by the fish and that this is the result of a facilitated urinary loss of this
ion (SAWYER, 1966 a). HELLER and I, in some preliminary experiments, failed to
show any effect of injected aldosterone on the total sodium balance of the lungfishes, but this needs to be explored in more detail, preferably with the aid of isotopes. Hypophysectomized lungfishes appear to live quite happily in fresh water
(MOORHOUSE, 1956; GODET, 1961), but a detailed study of their osmoregulation
has not been made. GODET, however, found that the urine volumes of hypophysectomized Protopterus are reduced, but as the rate of water accumulation is also
less, there is no net change in their water balance. The primary site of such change
in water balance is probably extrarenal rather than renal.
It seems likely that the endocrine glands playa role during the process of aestivation particularly during the period of its onset and during 'awakening'. The alterations in general metabolism probably involve the corticosteroids which facilitate protein catabolism in other vertebrates. The reason for the abolition of urine
formation during aestivation is not known. If Protopterus is hypophysectomized
while in its aestivating condition it appears to survive (GODET, 1961). There are
247
The African lungfish lives an aquatic life in regions of Africa that are subjected
to seasonal drought, during which time the rivers and lakes it inhabits dry up . When
this begins to happen the lungfishes dig burrows in the bed of the lake and retreat
into them when the water disappears. While so entombed they surround themselves
(except for the nares) with a cocoon made of secreted mucous and fine particles
of mud, and this limits their evaporative water loss . They aestivate in this manner
until the next wet season, which usually comes in 4 to 6 months, though they can
survive 2 or 3 years and perhaps even longer (see SMITH, 1961). When in this condition they reduce their metabolic rate to a very low level and subsist principally
by catabolizing tissue proteins. The carbohydrate reserves of these fish may even
increase during aestivation which contrasts with the decrease that is observed when
the aquatic fish are fasted (JANSSENS, 1964). Urine formation is too small to measure
during aestivation and urea derived from the protein catabolism is accumulated
in high concentrations in the body. HOMER SMITH found that after prolonged aestivation urea made up as much as 3 % of the entire body weight. When water returns
to the lakes in which the lungfish live, it submerges the burrows and the fish break
out of their cocoons and rise to the surface. According to SMITH and JANSSENS they
then excrete accumulated urea and sodium, most of this being lost in the first two
days.
The role of hormones in the various phases of the life cycle of lungfishes is
largely conjectural. When in their aquatic phase they take up water (the contribution of the gills and skin is not known) at a rate similar to that of other freshwater fishes and excrete this as a dilute urine (Table 7.7). While fishes do not respond to neurohypophysial peptides with an antidiuresis as seen in tetrapods, the
phyletic relationship of the lungfishes and the Amphibia makes it seem possible
that these two groups may both exhibit a 'water balance effect'. However, the injection of vasotocin was not found to produce such an effect in Protopterus (HELLER and BENTLEY, 1965) but instead it results in a diuresis (SAWYER, 1966 a). We
also found that the injection of vasotocin into Protopterus resulted in a net loss
of sodium by the fish and that this is the result of a facilitated urinary loss of this
ion (SAWYER, 1966 a). HELLER and I, in some preliminary experiments, failed to
show any effect of injected aldosterone on the total sodium balance of the lungfishes, but this needs to be explored in more detail, preferably with the aid of isotopes. Hypophysectomized lungfishes appear to live quite happily in fresh water
(MOORHOUSE, 1956; GODET, 1961), but a detailed study of their osmoregulation
has not been made. GODET, however, found that the urine volumes of hypophysectomized Protopterus are reduced, but as the rate of water accumulation is also
less, there is no net change in their water balance. The primary site of such change
in water balance is probably extrarenal rather than renal.
It seems likely that the endocrine glands playa role during the process of aestivation particularly during the period of its onset and during 'awakening'. The alterations in general metabolism probably involve the corticosteroids which facilitate protein catabolism in other vertebrates. The reason for the abolition of urine
formation during aestivation is not known. If Protopterus is hypophysectomized
while in its aestivating condition it appears to survive (GODET, 1961). There are
247