Glycogen Metabolizing Enzymes Activities in Cardiac Muscle Compared with Skeletal Muscle

Abstract

The main energy source available to cardiac muscle is different from that available to skeletal one (Gibbs 1978). It is known that glycogen is not normally utilized by aerobic heart. However, the significance of glycogen metabolism in cardiac muscle has not been clearly elucidated. This study has been designed to assess glycogen content together with its anabolic and catabolic enzymes in cardiac and skeletal muscles with special emphasis on lower activities of glycogen metabolizing enzymes, relatively higher activity of cAMP-dependent protein kinase (protein kinase A) and possibility of ischemia-induced direct activation of phosphorylase in cardiac muscle. Materials and MethodsFed adult male rabbits were anesthetized with pentobarbital intravenously. The preparation of muscle extracts was similar to the method of Burchell, Cohen and Cohen (1976). Skeletal muscle (M. vastus lateralis) and cardiac left ventricular muscle were rapidly removed, washed by saline solution, finely minced and homogenized with 5 volume of chilled 4.0 mM EDTA-0.5 mM phenylmethylsulfonyl fluoride-0.05 mM p-tosyl-l-lysine chloromethyl ketone (pH 7.4). The homogenate was centrifuged at 12,000 g for 20 min and the supernatant was used in the present experiments. In part, for control experiment to ischemic condition, both muscles were quickly removed and frozen in liquid nitrogen. Each muscle was then ground into powder in a mortar with a pestle. The enzymes were extracted as above. Muscle phosphorylase b was prepared and glycogen synthase, phosphorylase kinase and phosphorylase were assayed as previously described (Hashimoto, Mizuta, Tsutou, Nakamura and Yamamura 1983;Mizuta, Hashimoto, Tsutou, Eishi, Takemura, Nirisawa and Namamura 1984;Tsutou, Nakamura, Negami, Mizuta, Hashimoto and Yamamura 1985). Branching enzyme,debranching enzyme, glycogen and protein concentration were assayed according to the report of Moruzzi, Bergamini and Bergamini (1981). The assay of protein kinase A was the same as the report of Kemp, Graves, Benjamini and Krebs (1977). Student's t-test was used for analyses of the data. Results and DiscussionThere are many reports on glycogen metabolism in skeletal and cardiac muscles. However, there is no comparative study of activities of glycogen metabolizing enzymes including branching and debranching enzymes. The comparison of them in both muscles was summarized in Table 1. It was shown that the activities of enzymes except for protein kinase A in cardiac muscle were lower than those in skeletal one. This suggests that cardiac muscle does not utilize glycogen efficiently as main energy source in ordinary state. Kemptide, which was used in the present study, has been known to be the most specific substrate of protein kinase A (Kemp et al. 1977). Though there were no statistical differences, the total and active form activities of protein kinase A in cardiac muscle, compared with those in skeletal one, showed a slightly higher tendency. Fig. 1 Active form ratio (%) of glycogen synthase (G.S.), phosphorylase kinase (Ph. K.) a...