at far higher rates than has so far been demonstrated in terrestrial species (Table
5.8). The propensity of terrestrial lizards to form nasal gland secretions seems to
be rather variable, even within the same species. A lizard, Iguana iguana,('Fred')
kept in SCHMIDT-NIELSEN'S laboratory at Duke University regularly secreted large
amounts of salt from his nasal gland, while others of this species, which were
brought into the laboratory, often failed to perform in this way. I have observed
similar differences in the behaviour of individual chuckawallas, Sauromalus obesus.
It is not clear why such variation occurs but it may reflect the previous environmental conditions under which the animals lived. The state of development of such
'salt' glands may vary; green turtles kept in sea-water for several months have orbital glands 40% larger than those that are maintained in fresh water (HOLMES and
McBEAN, 1964).
The precise mechanism that controls secretion from salt glands in reptiles is
unknown but by analogy may be similar to that in birds. Osmotic stimuli and cholinergic drugs have a similar action in both groups of vertebrates, so that secretion
is possibly activated (as in birds) by changes in plasma concentration stimulating
a parasympathetic nerve supply to the glands. In birds the adrenocortical steroid
hormones influence such secretions and this may also be so in reptiles. HOLMES
and McBEAN (1964) found that injections of amphenone, which reduces the rate
of formation of corticosteroids in mammals and reptiles (PHILLIPS, CHESTER
JONES, and BELLAMY, 1962a), reduced extrarenal salt excretion in the green turtle.
That the action of this drug is indeed mediated in this way was indicated by the
ability of corticosterone to overcome its effect and enhance salt excretion. Amphenone also reduced excretion of sodium relative to that of potassium, implying that
the corticosteroids enhance sodium more than potassium levels in the nasal secretions of this reptile. TEMPLETON and his collaborators (1968) have shown that adrenalectomy in the desert iguana, Dipsosaurus dorsalis, results in copious secretion
of fluid from the nasal gland, but in contrast to that normally seen in this species.
it has a high Na/K ratio. The injection of aldosterone into such lizards resulted
in a decrease of this ratio towards normal levels. Thus in Dipsosaurus aldosterone
affects the composition of the salt gland secretion in a manner which parallels its
action on the mammalian kidney viz: sodium retention and potassium excretion.
It is apparent that the effects of corticosteroids on the green turtle and the desert
lizard are not consistent with each other. However, it will be recalled that the usual
primary pattern of sodium relative to potassium excretion in the salt glands of
the two species are opposite to each other, and this could influence the pattern of
regulation.
e) Role of Adrenocortical Tissue
Reptiles, like all vertebrates, possess adrenocortical tissue (see CHESTER JONES,
1957) but its physiological role is inferred more often by analogy with other groups
of vertebrates, than from direct experimental evidence. Adrenalectomy is a difficult
procedure in reptiles and results in death within a few days in lizards like Trachysaurus rugosus and Dipsosaurus dorsalis (BENTLEY, 1959b; TEMPLETON et al., 1968).
In Trachysaurus this operation results in an elevated plasma potassium level, as
154
5.8). The propensity of terrestrial lizards to form nasal gland secretions seems to
be rather variable, even within the same species. A lizard, Iguana iguana,('Fred')
kept in SCHMIDT-NIELSEN'S laboratory at Duke University regularly secreted large
amounts of salt from his nasal gland, while others of this species, which were
brought into the laboratory, often failed to perform in this way. I have observed
similar differences in the behaviour of individual chuckawallas, Sauromalus obesus.
It is not clear why such variation occurs but it may reflect the previous environmental conditions under which the animals lived. The state of development of such
'salt' glands may vary; green turtles kept in sea-water for several months have orbital glands 40% larger than those that are maintained in fresh water (HOLMES and
McBEAN, 1964).
The precise mechanism that controls secretion from salt glands in reptiles is
unknown but by analogy may be similar to that in birds. Osmotic stimuli and cholinergic drugs have a similar action in both groups of vertebrates, so that secretion
is possibly activated (as in birds) by changes in plasma concentration stimulating
a parasympathetic nerve supply to the glands. In birds the adrenocortical steroid
hormones influence such secretions and this may also be so in reptiles. HOLMES
and McBEAN (1964) found that injections of amphenone, which reduces the rate
of formation of corticosteroids in mammals and reptiles (PHILLIPS, CHESTER
JONES, and BELLAMY, 1962a), reduced extrarenal salt excretion in the green turtle.
That the action of this drug is indeed mediated in this way was indicated by the
ability of corticosterone to overcome its effect and enhance salt excretion. Amphenone also reduced excretion of sodium relative to that of potassium, implying that
the corticosteroids enhance sodium more than potassium levels in the nasal secretions of this reptile. TEMPLETON and his collaborators (1968) have shown that adrenalectomy in the desert iguana, Dipsosaurus dorsalis, results in copious secretion
of fluid from the nasal gland, but in contrast to that normally seen in this species.
it has a high Na/K ratio. The injection of aldosterone into such lizards resulted
in a decrease of this ratio towards normal levels. Thus in Dipsosaurus aldosterone
affects the composition of the salt gland secretion in a manner which parallels its
action on the mammalian kidney viz: sodium retention and potassium excretion.
It is apparent that the effects of corticosteroids on the green turtle and the desert
lizard are not consistent with each other. However, it will be recalled that the usual
primary pattern of sodium relative to potassium excretion in the salt glands of
the two species are opposite to each other, and this could influence the pattern of
regulation.
e) Role of Adrenocortical Tissue
Reptiles, like all vertebrates, possess adrenocortical tissue (see CHESTER JONES,
1957) but its physiological role is inferred more often by analogy with other groups
of vertebrates, than from direct experimental evidence. Adrenalectomy is a difficult
procedure in reptiles and results in death within a few days in lizards like Trachysaurus rugosus and Dipsosaurus dorsalis (BENTLEY, 1959b; TEMPLETON et al., 1968).
In Trachysaurus this operation results in an elevated plasma potassium level, as
154