Glycogen synthetase in the sea mussel Mytilus edulis L.—II. Seasonal changes in glycogen content and glycogen synthetase activity in the mantle tissue

Abstract 1. 1. The glycogen content of the mantle tissue reached a maximum in the summer (May–July) with levels of 41.0–53.5% of the dry tissue weight. 2. 2. Seasonal changes in glycogen synthetase activity showed that the I-activity (independent of G6P) increased up to 10-fold in June as compared with December. The measured I-activity of glycogen synthetase was sufficient to account for the accumulation of mantle glycogen in the summer. 3. 3. The I-activity of glycogen synthetase declined rapidly in July of each year. A possible role for the inhibition of glycogen synthetase by high levels of tissue glycogen is suggested. 4. 4. The I-activity in the mantle tissue of mussels on the shore was higher than that for animals starved in the laboratory for 2–3 days. The differences were minimal in early May but increased markedly in late May–July. Starved mussels returned to the shore showed an increase in I-activity of glycogen synthetase. 5. 5. Injection of 30 μmol glucose into the adductor muscle increased the concentration of glucose in the mantle fluid to 2.0–2.5 mM. A similar injection of 60 μ mol glucose resulted in a time-dependent increase in the I-activity of glycogen synthetase. 6. 6. Injection of mussels with mammalian insulin or anti-insulin serum had no effect on the activity of glycogen synthetase. Our results are at variance with those of other workers who have used the mammalian hormone in molluscan studies (see Discussion).