Gopherus agassizii, normally excretes relatively more waste nitrogen as uric acid
than the aquatic turtle, Pseudemys scripta (DANTZLER and SCHMIDT-NIELSEN,
1966). Some tortoises also may alter the relative quantities of these nitrogenous
end-products in response to the state of the reptiles' hydration (KHALIL and HAGGAG, 1955), uric acid predominating when water is restricted.
The urine flow of reptiles exhibits considerable lability (Table 5.4). When water
is administered to the lizard, Trachysaurus rugosus, its urine flow may increase
nearly 50 times as compared to that normally observed, while if the water intake is
restricted, the urine volume may be too small to measure. In reptiles, variations in
urine flow are related to changes in both the GFR (Table 5.5) and reabsorption of
water from the renal tubules. This contrasts with mammals, in which the latter process normally predominates. The possible amplitude of the changes in GFR differs in
various species and may be only 2 or 3-fold in a lizard such as the horned toad,
and more than la-fold in the crocodile (Table 5.5). As indicated by differences in
the inulin or creatine urine/plasma concentration ratios, renal tubular water reabsorption may also vary. In mammals this normally amounts to more than 99% of
the water that is filtered across the glomerulus, but in the horned toad this is only
45% in normally hydrated animals though it may increase to 60% during dehydration (ROBERTS and SCHMIDT-NIELSEN, 1966). In the Australian lizard, Trachysaurus, tubular water reabsorption is only 40% in hydrated individuals, but it may increase to more than 95% when the urine flow is low (SHOEMAKER, LICHT, and
DAWSON, 1966). Thus, the relative importance of the changes in GFR and renal
tubular water reabsorption varies in different species of reptiles and is also
dependent on the physiological circumstances. At high rates of urine flow, increases
in the GFR may playa predominant role in controlling urine volume, but when
the urine volume is small, tubular water reabsorption may be relatively more important. Ideally the two processes probably work in conjunction with each other.
Changes in GFR may be due to an increased rate in filtration across individual glomeruli, or can also result from changes in the numbers of active units (glomerular
intermittency). The latter process has been shown to occur in the snake, Natrix sipedon (LEBRIE and SUTHERLAND, 1962; DANTZLER, 1967a) and the turtle, Pseudemys scripta (DANTZLER and SCHMIDT-NIELSEN, 1966). The site of the tubular water
reabsorption has been examined by 'stop-flow' procedures in Natrix sipedon
(DANTZLER, 1967b) and the results suggest that during antidiuresis, water is reabsorbed from the distal parts of the nephron.
While it is clear that both changes in the GFR and reabsorption of water by
the renal tubule mediate alteration in the urine flow of reptiles, the immediate physiological reasons for such changes are, at the best, speculative. Changes in the GFR
can be envisaged as resulting to some extent from dilution or concentration of
plasma protein, which could, respectively, increase or decrease the forces for ultrafiltration across the glomerulus. Haemodynamic factors, resulting from changes
in the blood pressure, can also conceivably alter glomerular activity. Tubular water
reabsorption can be influenced by the rate at which the filtrate is delivered to the
absorption sites . Water transfer across the renal tubule may occur as an osmotic
accompaniment of solute movement or, as in mammals, result from a change in
the properties of the tubular epithelium, allowing osmotic equilibration between
the two sides to occur more rapidly. We do not know which of these factors occurs
144
than the aquatic turtle, Pseudemys scripta (DANTZLER and SCHMIDT-NIELSEN,
1966). Some tortoises also may alter the relative quantities of these nitrogenous
end-products in response to the state of the reptiles' hydration (KHALIL and HAGGAG, 1955), uric acid predominating when water is restricted.
The urine flow of reptiles exhibits considerable lability (Table 5.4). When water
is administered to the lizard, Trachysaurus rugosus, its urine flow may increase
nearly 50 times as compared to that normally observed, while if the water intake is
restricted, the urine volume may be too small to measure. In reptiles, variations in
urine flow are related to changes in both the GFR (Table 5.5) and reabsorption of
water from the renal tubules. This contrasts with mammals, in which the latter process normally predominates. The possible amplitude of the changes in GFR differs in
various species and may be only 2 or 3-fold in a lizard such as the horned toad,
and more than la-fold in the crocodile (Table 5.5). As indicated by differences in
the inulin or creatine urine/plasma concentration ratios, renal tubular water reabsorption may also vary. In mammals this normally amounts to more than 99% of
the water that is filtered across the glomerulus, but in the horned toad this is only
45% in normally hydrated animals though it may increase to 60% during dehydration (ROBERTS and SCHMIDT-NIELSEN, 1966). In the Australian lizard, Trachysaurus, tubular water reabsorption is only 40% in hydrated individuals, but it may increase to more than 95% when the urine flow is low (SHOEMAKER, LICHT, and
DAWSON, 1966). Thus, the relative importance of the changes in GFR and renal
tubular water reabsorption varies in different species of reptiles and is also
dependent on the physiological circumstances. At high rates of urine flow, increases
in the GFR may playa predominant role in controlling urine volume, but when
the urine volume is small, tubular water reabsorption may be relatively more important. Ideally the two processes probably work in conjunction with each other.
Changes in GFR may be due to an increased rate in filtration across individual glomeruli, or can also result from changes in the numbers of active units (glomerular
intermittency). The latter process has been shown to occur in the snake, Natrix sipedon (LEBRIE and SUTHERLAND, 1962; DANTZLER, 1967a) and the turtle, Pseudemys scripta (DANTZLER and SCHMIDT-NIELSEN, 1966). The site of the tubular water
reabsorption has been examined by 'stop-flow' procedures in Natrix sipedon
(DANTZLER, 1967b) and the results suggest that during antidiuresis, water is reabsorbed from the distal parts of the nephron.
While it is clear that both changes in the GFR and reabsorption of water by
the renal tubule mediate alteration in the urine flow of reptiles, the immediate physiological reasons for such changes are, at the best, speculative. Changes in the GFR
can be envisaged as resulting to some extent from dilution or concentration of
plasma protein, which could, respectively, increase or decrease the forces for ultrafiltration across the glomerulus. Haemodynamic factors, resulting from changes
in the blood pressure, can also conceivably alter glomerular activity. Tubular water
reabsorption can be influenced by the rate at which the filtrate is delivered to the
absorption sites . Water transfer across the renal tubule may occur as an osmotic
accompaniment of solute movement or, as in mammals, result from a change in
the properties of the tubular epithelium, allowing osmotic equilibration between
the two sides to occur more rapidly. We do not know which of these factors occurs
144