Phosphorylase activity and glycogenolysis in the working turtle heart

¹ Department of Zoology, Cornell University, Ithaca, New York

Measurements of phosphorylase a and t activities were made on perfused aerobic and anaerobic hearts from Pseudemys scripta and Emydoidea blandingi. Aerobically a/t ratio averaged 25% and was independent of arterial pressure (Pa) and work rate; anaerobically, the ratio ranged from 50 to 75% and was dependent on Pa but not on work rate. Phosphorylase t activity in O2 or N2 ranged from 15 to 45 µmoles Pi/g min (assayed at 37.5 C). Phosphorylase isolated from turtle hearts by starch column chromatography yielded the following Km values: Kglym = 1.0 mg/ml; Kampm = 0.25 mm; Kpim = 7.2 mm. Direct tissue analyses for (Pi), (AMP), and (ATP) permitted estimate of phosphorylase activity in working hearts. Tissue (AMP) was approximately equal to KAMPM aerobically and 8 x KAMPM anaerobically, permitting high phosphorylase b activity in vivo. It is estimated that the greatest limitation on rate of glycogenolysis is attributable to (Pi). Anaerobic rate of glycogen breakdown could be accounted for by phosphorylase t activity, (Pi), (AMP), and (ATP). Aerobic glycogenolysis rate is smaller by at least an order of magnitude than estimated rate computed on same basis.

Key Words: glycogenolysis, rate of, in cardiac muscle • adenine • nucleotide and phosphorylase activity • inorganic phosphate and phosphorylase activity • cardiac muscle metabolism

Submitted on August 21, 1963