Los ritmos diarios en la composición de la orina son menos marcados en las hembras que en los machos. El patrón de eliminación urinaria, mayor durante la noche, se cree debido a que las chinchillas son animales nocturnos. La presión osmótica de la orina (1500 mOsmoles/l) es hipertónica respecto al plasma (300 mOsmoles/l aprox), aunque es menor que la encontrada en la rata por ejemplo. Sorprende que la orina de la chinchilla no sea más hipertónica dado el árido hábitat del que procede. Puede que se explique por el hecho de que la chinchilla utiliza los chorros de orina como mecanismo defensivo, lo que le obligarÃa a disponer de una cantidad copiosa de orina. Los estudios anatómicos comparativos de los riñones de los roedores indican que la capacidad de concentración urinaria está relacionada con el tamaño de la papila. El riñón de la chinchilla muestra una papila larga y voluminosa similar a la del jerbo o la rata canguro, que son dos roedores capaces de producir una orina muchÃsimo más hipertónica que el plasma. Es posible que la discrepancia entre la estructura renal y la capacidad de concentrar la orina del riñón de la chinchilla se deba a la selección artificial con fines comerciales, que ha podido producir numerosas alteraciones de su fisiologÃa. Es probable que el anión más frecuente en la orina sea el bicarbonato, ya que el fosfato se ha detectado en pocas ocasiones y siempre en bajas concentraciones. La excreción de calcio es alta, especialmente durante el dÃa. Al contario que en la rata la concentración de potasio es superior a la de sodio. No se cree que las dietas ricas en potasio sean la causa de esta elevada concentración de potasio en la orina. Es posible que los clásicos mecanismos de excreción renal del potasio no sean completamente aplicables a las chinchillas y demás histricomorfos. Las concentraciones de magnesio en la orina a menudo superan las de sodio. Se cree que la chinchilla tiene una extraordinaria capacidad intestinal para la absorción de magnesio. Sin embargo se deconocen los mecanismos que expliquen esta alta capacidad de absorción y excreción.
La concentración urinaria máxima con privación
de agua o en respuesta a la administración de hormona antidiurética, es menor en la
chinchilla que en la rata albina y no se observa reducción en la
concentración urinaria de sodio bajo privación de comida en la chinchilla. Se está
analizando la posibilidad de una sensibilidad alterada a la hormona antidiurética o a
los mineralocorticoides. Un autor ha comentado que, comparada con otros roedores, C. laniger se parece a las especies desertÃcolas en su
capacidad para extraer agua útil de soluciones concentradas de NaCl.
En la práctica clÃnica hemos observado gran variabilidad en el color de la orina. Desde orinas de color amarillento muy poco acusado a orinas de color rojizo intenso. Los cristales de calcio son frecuentes pero no están siempre presentes y la cantidad en que aparecen es muy variable. En las siguientes imágenes vemos la orina de 6 chinchillas mantenidas en la misma jaula y con la misma alimentación. Se puede apreciar el distinto color y el diferente grado de presencia de cristales de carbonato de calcio.
Están a nuestro alcance otros datos referentes a la funcionalidad renal que hemos podido hallar citados fundamentalmente en estudios urológicos referidos a otras especies animales, especialmente la rata. De momento os ofrecemos estos datos en inglés. Según los vayamos traduciendo los iremos poniendo en español.
4 This explicit model analysis of solute and water
exchanges within the medulla confirms the general
patterns that have been predicted based on anatomy
and permeability data while pointing to some discrepancies,
especially concerning the micropuncture data on
urea absorption by short nephrons and the supposed
profiles along the IMCD. A more serious problem
pointing, in this author’s opinion, to some fundamental
lack in the present formulation, is that this model and
all previous models of this system predict a lower
instead of steeper inner medullary gradient if permeabilities
along the inner medullary LDL are raised,
whereas these permeabilities are uniformly found to be
higher in species that concentrate better than in those
that do not. In a recent study aimed specifically at this
question, Layton et al. (21) confirmed, in chinchilla
kidneys, the high urea permeability of LDL under
conditions resembling those in vitro as closely as possible.
This issue thus remains central to the question of
the inner medullary concentrating mechanism.
5 Water absorption from the descending limbs of Henle’s
loop is of primary importance for the concentrating
ability of the mammalian kidney, because ≈90% of
filtered water is reabsorbed before it reaches the cortical
collecting duct. Reports in the literature describe
water- and solute-permeable descending limbs in hamsters, rats, chinchillas, and
rabbits. The descending limb of Henle’s
loop has been found in all species to be water permeable
and moderately permeable to NaCl and urea. The
adjacent ascending thin limb (ATL) of Henle’s loop is
characterized by extremely low water permeability and
very high chloride permeability.
Chou et al. showed that the osmotic water permeability
of the descending limb in chinchillas is not
uniformly high along its length. Instead, the distal 20%
of the long-loop descending thin limb (DTL) was shown
to have a relatively low water permeability (50 µm/s).
Chou et al. also showed the NaCl and urea permeabilities
to be nonuniform in the chinchilla descending
limb, attaining a maximum in the innermost segment
of 98.4 and 47.6 µm/s, respectively.
In vitro perfusion studies of thin descending limbs in
hamsters, rats, and chinchillas have demonstrated
a water-permeable descending limb. Low-tomoderate
solute permeabilities have been measured
for several species in DTL, e.g., sodium permeability in
hamsters and NaCl permeability in chinchillas. In addition, little or no Na+,K+-ATPase activity
has been recorded in thin descending limbs in the inner
medulla, e.g., in rats. The measurements from
perfusion studies and the data from immunolocalization
suggest that mathematical models of the urineconcentrating
mechanism might investigate a distribution
of water- and solute-transporting segments of DTL
on the basis of the distribution of AQP1 and ClC channels observed. Our observations, as well as those
of others, show transitions from AQP1-positive to
ClC-positive segments with no significant colocalization
of solute and water channels. Hence, mathematical
models might test the effect of water-permeable
segments of DTL juxtaposed and in series with solutepermeable
segments of DTL on solute concentration
gradients in the inner medulla and on the kidney’s
ability to concentrate urine.
6 High-Urea Permeability Parameter Values
To further explore the impact of high urea permeability
of LDL (Pu
LDL), we also used a second parameter
set, which was based
mainly on permeability measurements in Henle’s loops
of chinchilla (although some of the values are from the
rat literature, because there is not a complete set of
measurements for chinchilla). The chinchilla has been
reported to concentrate its urine as high as 7,600
mosM. We will call this the “high-Pu†parameter
set. Table 4 shows the values that are different from
the baseline set. Here, we did not adopt the high value
of LDL salt permeability used by Layton et al.
Here, we present the results of several key simulations
demonstrating the effect of IM metabolic osmole
production (glyocolytic conversion of glucose to lactate)
in the flat medullary model described above. Using the
baseline parameter set, we show that conversion
of 15% of the glucose entering the medulla suffices
to engender a sizeable IM osmotic gradient, mainly by
amplifying the IM recycling of NaCl. We also show that
this simulated osmotic gradient is essentially unaffected
by raising the urea permeability of the thin
descending limbs even to values several times higher
than those reported in the microperfusion literature.
Then, using a set of parameters corresponding more
closely to the chinchilla kidney, which has an even
higher value of Pu
LDL than the rat, we show that urea
can accumulate to levels closer to observed values and
yet still be independent of the lactate effect on NaCl
recycling.
High Pu
LDL:
We explored the role of Pu
LDL in this model using both
the baseline parameter set of Table 2 (based on measurements
for the rat kidney and also chosen to facilitate
comparison with earlier 3-D models) and a parameter
set based on values reported for the chinchilla kidney [as reported in Layton et al.],
which has an an even higher Pu
LDL than the rat.
Subsequent measurement of tubular permeabilities
by in vitro microperfusion was in direct conflict
with these predictions; e.g., Pu
LDL was found to be
low in the rabbit, which does not develop a highly
concentrated urine, but quite high in species with wellconcentrated
urine, such as the chinchilla and the
rat .
7 A second lesson for the mammalian system is that
prebend enlargements are likely to play a significant
role in the outer medullary and inner medullary concentrating
mechanisms. Although this role has been
previously hypothesized, this study of the quail
medullary cone indicates, in the context of a specific
physiological setting, that the PBEs can significantly
increase concentrating capability. Prebend segments
have been identified in the long loops of Henle of the
chinchilla; these segments have transport properties
(low water permeability and high solute permeabilities)
similar to thin ALs in chinchilla. PBEs have
been reported in long loops in rats. Short loops
of Henle in rat have recently been found to have a long
prebend segment that appears to be water impermeable
but urea permeable. This segment, which is
coextensive with the inner part of the inner stripe of
the outer medulla, has been hypothesized to play a role
in urea cycling, but it may also enhance concentrating
capacity by removing the osmotic load that
would otherwise be presented to the inner stripe by DL
fluid.
8 Structural identification of DTL-type and ATL-type
segments of single thin limbs of Henle’s loop in the inner
medulla. Rat inner medullary DTL were differentiated
from ATL on the basis of structural characteristics
when viewed with a compound microscope equipped
with Nomarski DIC optics. The cells of the DTL with
their nuclei appearing to bulge into the lumen create a
distinct luminal outline. The cells of the ATL
appear flatter than those of the DTL and exhibit
dominating, round nuclei. A distinct luminal
outline is absent in the ATL. These structural
characteristics closely resemble those previously reported
for the chinchilla thin limbs of the inner medulla. When first beginning to tease out thin limbs, we
made certain that the cells we were defining as ‘‘DTLtype’’
based on the work of Chou and Knepper were
seen at the beginning of the DTL (following the straight
portion of the proximal tubule) and that the cells we
were defining as ‘‘ATL-type’’ were seen at the end of the
ATL where it joined the thick segment of the ascending
limb. In this way, we thought that we could be absolutely
certain that segments with these cell types truly
represented either DTL or ATL. The reason for this is
that we were interested in the papillary thin limbs
(those from juxtamedullary nephrons), and it was
rarely possible to tease out a whole DTL or ATL.
The presence of a large number of mixed-type thin
limbs in Munich-Wistar rat kidneys, along with the
presence of mixed-type thin limbs in the kidneys of
other species, suggests that they may serve a common
and important function in mammals. At present, we
can only speculate on their possible significance. In
chinchillas and rats (and, we assume, other mammalian
species), the water permeability of the DTL is
markedly higher than the water permeability of the
ATL.
9 Chou y Knepper encontraron que en la chinchilla el grado de permeabilidad hÃdrica disminuÃa significativamente en segmentos cercanos al extremo de la papila comparándolos con segmentos de la región externa de la medula interna, y anotaron el rango de permeabilidades de todos los niveles de la medula interna de la chinchilla, incluida la región externa. En la rata, al menos,  la baja permeabilidad hÃdrica en segmentos de los niveles inferiores de la médula interna es debida aparentemente a la ausencia de expresión de AQP1.
10 The second IM
segment, which we call LDL3 and which corresponds to the
AQP1-null segment of the DTL, was assumed to be impermeable
to NaCl but to have a water permeability of 400 µm/s in
the pipe mode, based on micropuncture experiments in rat
DTLs from the deep the IM, and to have no water permeability
in the SS mode, consistent with our finding of no AQP1
expression and with micropuncture experiments in chinchilla
indicating a water permeability of 50 µm/s in DTLs from the
deep IM.
In the SS mode, we assumed that all IM loop segments have
at least a moderate permeability to urea and that the urea
permeabilities of LDL3, the prebend segment, and the ATL are
very large. The LDL2S, ATLS, and LDL2 were assigned a
permeability of 13 x 10-5 cm/s. The urea permeabilities
of the LDL3, the prebend segment, and the ATL were suggested
to us by the high permeabilities measured in the long
loops of Henle of chinchilla. The permeability of the prebend segment and ATL was taken to be 150 x 10-5 cm/s,
lower than the measured value of 170 x 10-5 cm/s in chinchilla
ATL, but about an order of magnitude larger than
reported values in rat of 14–23 x 10-5 cm/s. The
permeability for LDL3 was taken to be 100 x 10-5 cm/s,
about twice the value of 48 x 10-5 cm/s reported in chinchilla
for the lower DTL. Our value for urea permeability in
LDL3 supports the effective function of the SS mode and may
not be unreasonable, because results from microperfusion studies
of DTLs may have been skewed by tubules that spanned
more than one functional segment.
The pipe mode concentrates more effectively when loop of
Henle urea permeabilities (in units of 10-5 cm/s) are sufficiently
small (≈1–5), whereas the SS mode concentrates more
effectively when the loop urea permeabilities are sufficiently
large (more than or equal to ≈100); intermediate values (e.g.,
≈10–50) significantly decrease concentrating capability in
both modes. Because perfused tubule studies
have indicated that chinchilla loops are highly permeable to
urea, whereas our preliminary data suggest that rat
loops are nearly impermeable to urea, both the low and high urea permeability
limits (i.e., both the pipe and SS mode) could be exploited
in vivo, depending on species. On the other hand, definitive
evidence that the loops are only moderately urea permeable
when animals are in an antidiuretic state would cast doubt on
both modes.
Unlike most other recent model studies,
we have not considered our loop of Henle urea permeability
choices to be bound by average values reported in perfused
tubule experiments; rather, in conformity with
our own recent experiments and unpublished preliminary
observations, we
have assumed low loop of Henle urea permeabilities in AQP1-
null loop segments, or, alternatively, we have assumed high
urea permeabilities in those segments, similar to values reported
in chinchilla. Urea permeabilities obtained in perfused
tubule measurements in the rat, hamster, and rabbit have
exhibited a large range of variability, and the
measurements were obtained under experimental circumstances
that may not have been representative of normal
function in antidiuretic animals.
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