Aug. 27, 2002 San Diego, CA -- Most terrestrial amphibians acquire water by absorption across their skin rather than by oral drinking. During periods of rehydration, frogs and toads adopt a posture termed "water absorption response" (WR), thrusting their hindlimbs backwards and pressing the belly surface on to any surface containing water. In many frog and toad species, such as the Bufo breed, a specific area of the belly skin (the seat patch), is used for absorbing water. Even though the seat patch skin only constitutes about ten percent of the total skin area, it is responsible for over 70 percent of the total water uptake by dehydrated toads. The seat patch skin is equipped with an elaborate capillary network. Interrupting capillary perfusion of isolated skin and in living animals results in a washout of salts from the interstitial fluid of the skin, thereby decreasing the osmotic gradient and the osmotic water flow across the skin. Circulation in the skin of this patch seems vital to this process, yet very few studies have investigated rates of blood flow in the seat patch skin while toads were exhibiting water absorption behavior.
A new study has been undertaken to assess blood flow in this patch and investigate how blood flow responds to water exposure. The study was divided into the following three parts:
(1) measuring red cell velocity and water uptake across the seat patch concurrently and related to the expression of water absorption postures by hydrated and dehydrated toads, Bufo woodhousei. This tested the hypotheses that: (a) the increase in red cell velocity, like that in the European common toad Bufo bufo, is greater in dehydrated animals after water exposure, (b) the expression of water absorption behavior is coupled to the increase in red cell velocity and (c) water uptake is correlated with the level of red cell velocity;
(2) using a procedure similar to the above, the researchers tested the hypothesis that the presence of NaCl in the hydration medium enhances red cell velocity and water absorption; and
(3) investigating whether or not: (a) red cell velocity is greater in toads with empty bladders, (b) forced hydration reduces red cell velocity and (c) angiotensin stimulates red cell velocity in toads subject to forced hydration. (Previous studies assert that the greater rate of water uptake by toads with empty bladders resulted from increased blood flow to the skin.)
The authors of "Pelvic Skin Blood Flow and Water Uptake in Toads, Bufo alvarius," are A.L. Viborg and S.D. Hillyard, August Krogh Institute, University of Copenhagen, Denmark, and the University of Nevada, Las Vegas, respectively. They will present their findings at "The Power of Comparative Physiology: Evolution, Integration and Application," a meeting of the American Physiological Society (APS) being held August 24-28, 2002 at the Town & Country Hotel, San Diego, CA. To learn more about the conference, log on to: http://www.the-aps.org/meetings/aps/san_diego/home.htm
Seven Bufo woodhousei and four Bufo punctatus were collected in Clark County, Nevada. Both were selected to compare red cell velocity values with findings from previous studies. The toads were kept in holding terraria containing local desert sand, rocks and pooled tap water. Hydration status was evaluated relative to the standard weight, that is a fully hydrated toad after the urinary bladder has been emptied. Toads are able to store as much as 50 percent of their standard weight as dilute bladder urine, absorption of urine across the bladder wall can offset the osmotic concentration of body fluids that occurs as they lose water by evaporation.
Blood flow in seat patch skin was measured as red cell velocity. During recording of seat patch skin red cell velocity, Bufo woodhousei and Bufo punctatus were placed individually in a plastic box in which a depression had been drilled.
Two experiments were conducted:
* Bufo woodhousei: The standard weight of toads hydrated ad libitum was recorded after the toads had their bladders emptied by inserting a polyethylene cannula into the cloaca combined with gentle abdominal pressure. After resting for 10 minutes to recover from handling stress, seat patch red cell velocity was measured for two minutes with no water in the depression. The toads were dehydrated without access to water for one day, and seat patch red cell velocity in dehydrated toads was measured with no water in the depression and on both deionized water and salt water. In all experiments with Bufo woodhousei, the red cell velocity values recorded when body oscillations or abduction of the hindlimbs occurred, were used to estimate the degree to which blood flow and WR behavior are correlated.
* Bufo punctatus: Three experimental protocols were conducted with this species. The first was identical with that of Bufo woodhousei, to quantify the response of red cell velocity to dehydration and exposure to water or salt water. The second examined the effect of retained water in the urinary bladder on the response of red cell velocity to dehydration and water exposure. The third examined the effect of forced immersion and angiotensin II (AII) on red cell velocity. Forced immersion for a fixed period insures a uniform hydration status that may be greater than that of toads with water ad lib.
The experiments with Bufo woodhousei support the initial hypothesis that the increase in red cell velocity of the pelvic skin of dehydrated toads is a general feature of Bufonids and is associated with the initiation of the water absorption behavior. While there was a small and significant increase in red cell velocity prior to water exposure it is evident that detection of water is the primary stimulus for increased red cell velocity. The seven- to eightfold increase was near maximal within one minute of water exposure and the initiation of WR occurred during or shortly after red cell velocity values had reached a maximal value.
Because the capillary ultrafiltration in amphibians is very much more rapid than mammals, the increase in cutaneous blood flow has been hypothesized to provide a supply of solutes to the subepithelial lymphatic spaces so that the osmotic gradient across the skin is maintained as water is absorbed. It appears that the animals allow some solute accumulation prior to initiating behavior that will increase the cutaneous area available for water absorption. It is noteworthy that red cell velocity reaches maximal values at a dehydration level of about 14 percent and was not further increased at levels as high as 30 percent. Furthermore, the rate of water uptake and seat patch red cell velocity while variable, were not correlated.
It appears that once a maximal level of red cell velocity has been reached, factors other than blood flow regulate water uptake. This is also evident over the duration of the experiments with Bufo woodhousei where the rate of water uptake declines while the increase in red cell velocity remains unchanged. An additional finding is that the increased rate of water uptake from dilute salt solutions does not, as suggested in other studies, result from an increase in cutaneous blood flow and that complete immersion rather than blood flow to the seat patch is more likely the cause.
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