RELATIONSHIPS BETWEEN IN-VIVO AND IN-VITRO MEASUREMENTS OF METABOLISMIN YOUNG AND OLD HUMAN CALF MUSCLES

This study compared in vivo measurements of muscle metabolism in human s with magnetic resonance spectroscopy (MRS) and in vitro analysis of biopsies. Healthy subjects [4 young males, 28.2 +/- 6.8 (SD) yr, and 6 older subjects (5 males, 1 female), 66 +/- 6.0 yr] performed a maxima l cycle ergometer test, and MRS measurements of the calf muscles and n eedle biopsies of the lateral gastrocnemius were performed. Biopsies w ere analyzed for fiber type and citrate synthase (CS) activity. MRS me asurements of inorganic phosphate (P(i)), phosphocreatine (PCr), ATP, and pH were made using a 1.8-T 78-cm clear-bore magnet-and-spectromete r system. Two or three 5-min bouts of plantar flexion were performed a gainst variable resistance to deplete PCr levels to 50% of resting val ues (mean end pH 6.99). PCr values during recovery were fit to an expo nential curve, and the rate constant (PCr(rate)) was calculated. PCr(r ate) was used as an index of oxidative metabolism. Older subjects had lower peak O2 uptake (VO2peak) values (19.2 +/- 5.6 vs. 49.5 +/- 8.1 m l O2 . min-1 . kg-1), CS activities (16 +/- 2.8 vs. 25 +/- 2.6 mmol . kg wet wt-1 . min-1), and PCr(rate), values (25.3 +/- 8.5 vs. 37.5 +/- 5.3 mmol PCr . kg wet wt-1 . min-1) than young subjects. PCr(rate) co rrelated with CS activity, and both PCr(rate) and CS activity correlat ed with Vo2peak (P < 0.05). No correlations were found between percent fiber type and PCr(rate), CS activity, and Vo2peak. These results sup port studies that showed decreases in muscle metabolism with age in he althy humans and show a good correlation between in vivo and in vitro measurements of oxidative metabolism.