2. Salt Exchange
a) Skin
The relative impermeability of the integument considerably restricts the movements of salts across the body surface of reptiles . However, precise measurements
of the movements of ions like sodium and potassium are lacking. When the caiman
is placed in distilled water it loses sodium across its skin, the net loss amounting
to about 1 ,u-equiv/cm
2h
(BENTLEY and SCHMIDT-NIELSEN, 1965). This is about
half the rate of loss in frogs and salamanders kept under similar conditions. Unlike
the skin of the amphibians, no electrical p. d. could be recorded across the caiman
integument in vitro when it was bathed on both sides with physiological saline,
though a small p.d.(4 mY, outside negative) was seen when the outside media
was diluted. The latter can be accounted for by different rates of diffusion of sodium
and chloride, an observation that at least confirms the permeability of the skin to
ions. When the caiman is placed in 3.3 % sodium chloride solution (= to sea-water)
it gains large amounts of sodium chloride but this occurs as a result of drinking,
little measurable uptake occurring across the skin. I have also measured cutaneous
sodium loss in the softshell turtle when it is bathed in distilled water and it loses
this ion at about the same rate as the caiman. When the diamondback terrapin, Malaclemys centrata, is kept in sea-water, circumstantial evidence obtained from
sodium levels in the blood and urine indicates that a small accumulation of sodium
is taking place through the skin (BENTLEY, et al., 1967a). In normal physiological
circumstances cutaneous exchanges of solutes are probably only of minor significance, but as apparent from the above experiments, the information is fragmentary.
Species like the caiman and softshell turtle have a more permeable integument than
most reptiles so they must be considered atypical, while the conditions (bathed
in distilled water) for such measurements are hardly physiological. By utilizing labelled isotopes of sodium and potassium it should be possible to measure movements of electrolytes across the skin of a wider range of reptiles, under conditions
that can be more properly considered physiological.
b) Kidney
Excretion and conservation of electrolytes occur during the process of urine formation. If excess water is accumulated, as a result of feeding, drinking or cutaneous
uptake, it will be excreted by the kidney with a simultaneous, and unavoidable,
loss of some salt. The ability of reptiles to form a hypoosmotic urine by
reabsorption of such salts from the glomerular filtrate minimizes such losses. When
the diamondback terrapin is fasted and maintained in fresh water, the urine concentration is about 60 m-osmole/l, while that of the sodium present is less than
1 m-equiv/I (BENTLEY et al., 1967a). I have found the concentration of the urine
of the softshell turtle, kept under the same conditions, to be similar, while Pseudemys scripta forms a urine with a higher sodium (about 5 m-equiv/l) level. Terrestrial
reptiles do not appear to perform as well as these aquatic species. I have observed
sodium concentrations as low as 15 m-equiv/l in the lizard, Trachysaurus rugosus,
but ROBERTS and SCHMIDT-NIELSEN (1965) found that in three species of lizards
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a) Skin
The relative impermeability of the integument considerably restricts the movements of salts across the body surface of reptiles . However, precise measurements
of the movements of ions like sodium and potassium are lacking. When the caiman
is placed in distilled water it loses sodium across its skin, the net loss amounting
to about 1 ,u-equiv/cm
2h
(BENTLEY and SCHMIDT-NIELSEN, 1965). This is about
half the rate of loss in frogs and salamanders kept under similar conditions. Unlike
the skin of the amphibians, no electrical p. d. could be recorded across the caiman
integument in vitro when it was bathed on both sides with physiological saline,
though a small p.d.(4 mY, outside negative) was seen when the outside media
was diluted. The latter can be accounted for by different rates of diffusion of sodium
and chloride, an observation that at least confirms the permeability of the skin to
ions. When the caiman is placed in 3.3 % sodium chloride solution (= to sea-water)
it gains large amounts of sodium chloride but this occurs as a result of drinking,
little measurable uptake occurring across the skin. I have also measured cutaneous
sodium loss in the softshell turtle when it is bathed in distilled water and it loses
this ion at about the same rate as the caiman. When the diamondback terrapin, Malaclemys centrata, is kept in sea-water, circumstantial evidence obtained from
sodium levels in the blood and urine indicates that a small accumulation of sodium
is taking place through the skin (BENTLEY, et al., 1967a). In normal physiological
circumstances cutaneous exchanges of solutes are probably only of minor significance, but as apparent from the above experiments, the information is fragmentary.
Species like the caiman and softshell turtle have a more permeable integument than
most reptiles so they must be considered atypical, while the conditions (bathed
in distilled water) for such measurements are hardly physiological. By utilizing labelled isotopes of sodium and potassium it should be possible to measure movements of electrolytes across the skin of a wider range of reptiles, under conditions
that can be more properly considered physiological.
b) Kidney
Excretion and conservation of electrolytes occur during the process of urine formation. If excess water is accumulated, as a result of feeding, drinking or cutaneous
uptake, it will be excreted by the kidney with a simultaneous, and unavoidable,
loss of some salt. The ability of reptiles to form a hypoosmotic urine by
reabsorption of such salts from the glomerular filtrate minimizes such losses. When
the diamondback terrapin is fasted and maintained in fresh water, the urine concentration is about 60 m-osmole/l, while that of the sodium present is less than
1 m-equiv/I (BENTLEY et al., 1967a). I have found the concentration of the urine
of the softshell turtle, kept under the same conditions, to be similar, while Pseudemys scripta forms a urine with a higher sodium (about 5 m-equiv/l) level. Terrestrial
reptiles do not appear to perform as well as these aquatic species. I have observed
sodium concentrations as low as 15 m-equiv/l in the lizard, Trachysaurus rugosus,
but ROBERTS and SCHMIDT-NIELSEN (1965) found that in three species of lizards
149