METABOLIC AND WORK CAPACITY OF SKELETAL-MUSCLE OF PFK-DEFICIENT DOGS STUDIED IN-SITU

Mechanical and metabolic relationships of muscle lacking phosphofructo kinase (PFKD) activity were compared with muscle having normal phospho fructokinase (NORM) activity by using the gastrocnemius-plantaris musc le group with isolated circulation in situ. Muscle contractile propert ies were similar in both groups. Initial power output (W) during repet itive tetanic (200 ms, 50 impulses/s) isotonic contractions was simila r in both groups; however, W declined significantly more (30-80%) in P FKD than in NORM muscle over time, with a constant O-2 uptake (VO2)/W. Despite similar O-2 and substrate delivery, PFKD muscle had a lower V O2 (42-55%), less glucose uptake, similar free fatty acid uptake, and lactic acid uptake, rather than output, during contractions. Muscle ve nous H+ concentration, strong ion difference, and PCO2 increased durin g contractions, the magnitude of change being smaller in PFKD muscle. Elevating arterial lactate concentration before contractions in PFKD m uscle resulted in significant improvements in W and VO2 without alteri ng the acid-base exchange at the muscle. Increasing O-2 delivery by in creasing arterial O-2 concentration in PFKD dogs did not improve W or VO2., We conclude that, despite no inherent mechanical or contractile differences, PFKD muscle has a severely limited oxidative capacity and exaggerated fatigue and blood flow responses to contractions due to l imited substrate metabolism resulting from the inability to utilize gl ycogen and/or glucose.