The Effect of Dehydration on the In Vivo Acid-base Status Of The Blood in the Toad Bufo Viridis

In a careful study of the acid-base relationships in Bufo marinus, Boutilier et al. (1979) also looked at the effects of dehydration. They found that arterial pH was maintained at a rather constant level by the ventilatory regulation of in the arterial blood and by some acidification of the urine.

During a recent study on the acid-base status of the blood in the euryhaline toad Bufo viridis (Katz, 1979, 1980), I have looked also at the effect of dehydration. The results of this study are reported here, and will be discussed in relation to the results of Boutilier et al. (1979).

The observations were made using a permanent cannula (Clay-Adams polyethylene tubes), which was implanted into the iliac artery under MS-22 anaesthetic (Katz, 1980). Blood pH, and were determined using a Radiometer PHM73 pH/blood gas monitor through BMS 3, mk 2, blood microsystem and Radiometer electrodes. Anaerobic blood samples for analysis were taken from unrestrained animals. HCO₃⁻was calculated using the Henderson–Hasselbach equation and the proper constants taken from Severinghaus, Stupfel & Bradley (1956). The toads were dehydrated either in the air, under moderate conditions for 2–3 days, or by placing them in hypertonic solutions (1000 m-osmol) for 6–7 h until they lost c. 15% weight. Chloride was determined with a Radiometer CMT 10 chloride titrator.

Table 1 summarizes the results of the in vivo determinations made on blood of the toads before and after dehydration of about 15% of their initial body weight (the bladder was emptied by catheterization before the start of the dehydration). It can be seen that there were some common features in both methods of dehydration such as the changes in the pH and of the blood (Table 1, ‘I’ and ‘II’). However, there is clearly a difference in the changes between the two groups. There was only a 0·12 decrease in the pH of the blood, and 30% increase in the arterial in the slowly dehydrated group (‘I’). A much greater acidification (a 0·45 decrease in the PH), was found on the other hand in the toads which were dehydrated rapidly (‘II ‘), accompanied with nearly 40% increase in the arterial ⁠. Moreover, in the later group there was also a marked and significant decrease in the concentration of HCO₃⁻ in the blood. Table 2 compares the in vitro buffering capacity of the whole blood of control hydrated toads to that of dehydrated toads, and shows that it was practically the same under both conditions.

The acid-base parameters of the blood (pH, and HC0₃⁻) in Bufo viridis under control conditions (Table 1), compare quite closely to the results reported by Boutilier et al. (1979) on the blood of Bufo marinus under control (hydrated) conditions. It seems then that these two species differ in the way their acid-base status respond to dehydration. While Boutilier et al. (1979) observed in Bufo marinus that both arterial pH and were regulated fairly well within a narrow range, in Bufo viridis dehydration resulted in a characteristic respiratory acidosis. This was accompanied in B. viridis with the appearance of considerable amount of non-carbonic acid in the blood of toads which were dehydrated in hypertonic solutions (Table 1, ‘II’).

The respiratory acidosis which was developed in Bufo viridis upon dehydration could result from a failure to eliminate CO₂ either through the skin or through the lungs, or both. The first possibility, namely an important change in the permeability of the skin to CO₂, does not seem possible. Firstly, it was observed (Katz, 1980) that the arterial and other acid-base parameters were maintained at rather constant values whether the toads were kept under control conditions (free access to tap water) when their skin appears rather dry, or when they were partially or fully immersed in tap water. Secondly, dryness of the skin was excluded in the present study in the conditions of dehydration in hypertonic solution (Table 1, ‘II’). These seem to show that the role of the skin in regulating the arterial in the toad is only of minor importance, and supports a similar conclusion by Jackson & Braun (1979) in the bullfrog. The arterial will be determined then by the efficiency of the ventilatory respiration in eliminating the excess CO₂. This has been found quite efficient in Bufo marinus in normoxic atmosphering conditions (Boutilier et al. 1979) and will need a careful study in Bufo viridis.

There was a marked difference in the acid-base picture of B. viridis in the two methods of dehydration in this study (Table 1), reflected in the metabolic acidosis which was observed in the second group (‘II’). Toads which are dehydrated slowly in the air are usually quiet and gather with very little activity, while those dehydrated in hypertonic solutions were observed rather active. This may explain, at least in part, the excess acid found in the blood of those toads (Table 1, ‘II’). On the other hand the rapidity of dehydration in the hypertonic solutions should be considered in respect to the regulatory function of the kidneys; the renal mechanisms could be of adaptational type, requiring time and therefore not compensating for the rapid and large changes seen in the second method (Yoshimura et al. 1961). It should await, however, a quantitative assessment of the renal contribution to the acid-base status in the toad under variable environmental conditions, before the whole picture in B. viridis can be drawn. This species, which is extremely euryhaline, may be more resistant to changes in its ‘milieu intérieur’, and allow for a considerable variation also in the acid-base status of its blood (Katz, 1979).