1. Liver, kidney, brain, skeletal muscle, and cardiac muscle from one newborn and three adult long-snouted dolphins (Stenella plagiodon) were obtained for enzyme studies.

2. All of the dolphin tissues exhibited cytochrome oxidase, succinic dehydrogenase, and malic dehydrogenase activity. Considerable differences in the enzyme activities of the various tissues were noted, with cardiac muscle exhibiting the highest respiratory enzyme activity. The enzyme activities of dolphin tissues were lower than those of the corresponding rat tissues.

3. All of the dolphin tissues exhibited adenosine triphosphatase activity which was accelerated by magnesium and manganese but, in contrast to rat tissues, was only slightly activated by calcium.

4. Measurements of the distribution of acid-soluble phosphorus in dolphin tissues indicated that glycolysis in all of the tissues examined proceeded through the Emden-Meyerhof phosphorylation scheme.

5. The average glycogen content of dolphin skeletal muscle was 0.98 per cent as compared with 0.16 to 0.20 per cent for rat skeletal muscle. The high glycogen content of dolphin skeletal muscle indicates a ready source of substrate for glycolysis even during submergence when the blood supply may be differentially shunted to other organs.

6. Measurements of the organ weights of dolphins showed that the lungs occupy over three times and the liver one-half as much of the total body weight as do these organs in the rat. The heart and the thyroid gland of the dolphin are also larger in proportion to the total body weight than in the rat while the relative weights of the other tissues in the two species are about the same.

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