Energy and Calcium Metabolism

Under normal activity levels, energy production in the amphibian is produced primarily by aerobic metabolic pathways, while outbursts of unusual activity are sustained by anaerobic metabolic pathways (Gatten et aI., 1992; Pough et aI., 1992). Amphibians that make explosive escape attempts, such as ranids, utilize anaerobiosis for this activity and quickly fatigue with the buildup of lactate (any salt derived from lactic acid) in the muscle tissues. Thus those amphibians with a low aerobic scope may suddenly collapse during physical restraint or during capture attempts. The sustainability of this anaerobic activity is brief, often lasting less than 2 minutes. Other amphibians, such as bufonids, have a high aerobic scope and can put up a continuing struggle with minimal signs of fatigue.

There is a strong correlation between an amphibian's hunting techniques and its aerobic scope. Sit and wait predators (e.g., ranids, ceratophryne leptodactylids) have a low aerobic scope while active foragers (e.g., bufonids) have a high aerobic scope. There may be marked differences in the aerobic scope between age classes of a given species, with young specimens having a lower aerobic scope (Taigen & Pough, 1981).

Recovery from fatigue varies among species, but the rate of oxygen consumption appears to be one of the main limiting factors (Gatten et aI., 1992). The clinician should avoid exhausting the critically ill amphibian patient through excessive handling. Slightly elevated levels of atmospheric oxygen for several minutes prior to and immediately following handling is recommended for the critical patient. Bubbling oxygen through water used to moisten the amphibian patient may help speed recovery. Typically, amphibians return to resting values of oxygen consumption within an hour of exercise-induced exhaustion (Gatten et aI., 1992).

The majority of the energy for nonsustainable exercise in amphibians occurs via glycolysis. As glycogen is metabolized, the concentration of lactate in the muscles rises whereas succinate, pyruvate, and other metabolites are relatively unaffected. Total-body lactate can take several hours to clear from an amphibian's system. The metabolism of lactate has been poorly studied in amphibians, but what little is known indicates that it is very different from lactate metabolism in mammals. In one experiment to determine the fate of lactate, specimens of the American toad, Bufo americanus, were exercised and treated with radioactively labeled lactate and glucose (Withers et al. 1988). Whereas mammals subjected to similar experiments oxidized the majority of the lactate (up to 950/0 ), the toads oxidized less than 100/0 of the la beled lactate. Excretion of lactate is limited in amphibians.

The lactate level in the urine of amphibians is extremely low, if present at all, and the gills or skin play seemingly insignificant roles in eliminating lactate from the aquatic amphibians studied.

Although increased levels of blood lactate do not appear to directly correlate with muscle fatigue in amphibians, it is known that lactate plays a role in the development of fatigue in amphibians, probably due to its contribution to the level of free hydrogen ions in the muscle. The hydrogen ions contribute to the development of acidified inorganic phosphorus ions, such as H₂P0₄, and appear to be the proximate cause of fatigue in amphibians and mammals (Gatten et aI., 1992).

The scarce amount of information on lactate and acid-base metabolism in amphibians has been scarcely addressed compared to the volume of material devoted to mammals. The pH of amphibian blood can vary widely in normal specimens, and although compensatory mechanisms appear similar in amphibians and mammals, the exact nature and contribution of the different mechanisms are not well understood in amphibians (Boutilier et. aI, 1992; Shoemaker et. aI, 1992). Given that the fate of endogenous lactates is poorly understood in amphibians, as are the compensatory mechanisms for dealing with metabolic acidosis or alkalosis, the clinical use of sodium lactate (found in lactated Ringer's solution) is not advisable in amphibians.

Calcium Metabolism. Calcium homeostasis is regulated through the interplay of four hormones. The activity of calcitonin and parathormone has been documented in anurans and salamanders, yet vitamin D activity has only been

documented in anurans and prolactin activity has only been documented in salamanders. Calcitonin is the hypocalcemic hormone that increases bone calcium deposition. Amphibian calcitonin appears to be significantly different from bovine calcitonin (Boschwitz & Bern, 1971) and salmon calcitonin (Bentley, 1983), and calcitonin may even vary between species, since the black-spotted frog, Rana nigromaculata, did not respond to one source of frogderived calcitonin (Oguro & Sasayama, 1985). Parathormone elevates blood calcium by mobilizing calcium from the bones. Vitamin D elevates plasma calcium levels in adult anurans and increases calcium uptake in larval anurans, while prolactin promotes hypercalcemia in adult salamanders. Calcium is an important regulatory ion, for it is involved in many metabolic pathways throughout the body including activity of muscles and nerves.

Calcium is absorbed from the environment either across the skin or through the gastrointestinal tract, while it is lost to the environment through urine, feces, and the skin. In addition to bone and the endolymphatic sacs, the skin can be a significant storage site of calcium, with up to 30% of the total body calcium stored in the skin of some anurans (Baldwin & Bentley, 1980).

Hypocalcemia is a serious metabolic derangement and can affect the striated and smooth musculature of the amphibian. Tetany may develop in hypocalcemic striated muscle while smooth muscle dysfunction may result in lack of peristalsis and associate gastrointestinal bloating. The permeability of the skin to calcium allows treatment of hypocalcemia via baths of calcium ion solutions such as calcium gluconate. Effects of hypercalcemia are not clinically defined at present.