Phosphofructokinase activity in human skeletal muscle: effects of euglycaemic hyperinsulinaemia and fasting

Euglycaemic (approximately 5.5 mmol l-1) hyperinsulinaemic (60 mU [m2]-1 min-1) clamps were performed for 2 h after a 10 h and after a 72 h fast on man. Biopsies were obtained from the quadriceps femoris muscle before and after each clamp and analysed in vitro for phosphofructokinase (PFK) activity under optimal (pH 8.2) and regulatory conditions (pH 7.0 and low substrate concentrations). Insulin stimulated, and fasting inhibited basal and insulin stimulated muscle glycolysis in vivo. However, PFK activity, measured in vitro, was not altered by any of the treatments (at pH 8.2: basal [10 h] = 102 +/- 5 mmol kg-1 dry wt min-1, clamp [10 h] = 104 +/- 6; basal [72 h] = 107 +/- 6, clamp [72 h] = 110 +/- 4 (at pH 7.0: basal [10 h] = 10.7 +/- 0.9, clamp [10 h] = 10.7 +/- 1.7; basal [72 h] = 11.7 +/- 2.4, clamp [72 h] = 9.7 +/- 1.6). Similarly, the activity ratio (7.0/8.2) was also not significantly altered by any of the treatments. Euglycaemic hyperinsulinaemia did not increase and fasting did not decrease the contents of activating hexose phosphates in muscle. The activity ratio increases after administration of epinephrine in rabbit muscle and the increase is thought to be mediated through a combination of increased hexose phosphates and adenine nucleotides (cyclic AMP) (Mansour TE. J Biol Chem 1972; 247: 6059-66). We confirmed that 2 h of epinephrine infusion also increased the activity ratio and hexose phosphates (but not fructose 2,6-P2) in human muscle.(ABSTRACT TRUNCATED AT 250 WORDS)