Physiological Control of Water Exchange in Anurans

Research on water exchange in frogs has historically assumed that blood
osmotic potential drives water exchange between a frog and its
environment, but here we show that the “seat patch” (the primary site of
water exchange in many anurans), or other sites of cutaneous water uptake,
act as an anatomic “compartment” with a water potential controlled
separately from water potential of the blood, and the water potential of
that compartment can be the driver of water exchange between the animal
and its environment.  We studied six frog species (Xenopus
laevis, Rana pipiens, R. catesbeiana, Bufo boreas, Pseudacris
cadaverina and P. regilla) differing in
ecological relationships to environmental water. We inferred
the water potentials of seat patchesfrom water exchanges
by frogs in sucrose solutions ranging in water potential from 0 to 1000
-kPa.  Terrestrial and arboreal species had seat patch water
potentials that were more negative than the water potentials of more
aquatic species, and their seat patch water potentials were similar to the
water potential of their blood, but the water potentials of venters of the
more aquatic species were different from (and less negative than) the
water potentials of their blood.  These findings indicate that
there are physiological mechanisms among frog species that can be used to
control water potential at the sites of cutaneous water
uptake, and that some frogs may be able to adjust the hydric
conductance of their skin when they are absorbing water from very dilute
solutions.  Largely unexplored mechanisms involving aquaporins
are likely responsible for adjustments in hydric conductance, which in
turn, allow control of water potential at sites of cutaneous
water uptake among species differing in ecological habit and the
observed disequilibrium between sites of cutaneous water
uptake and blood water potential in more aquatic species.