PERIPHERAL O2 DIFFUSION DOES NOT LIMIT MUSCLE O2 UPTAKE ON-KINETICS

Abstract

385 To test the hypothesis that muscle O2 uptake (˙VO2) on-kinetics is at least in part limited by peripheral O2 diffusion, we determined this kinetics in: 1) normoxia (C); 2) hyperoxia (H); 3) hyperoxia accompanied by administration of a drug (RSR13, Allos Therapeutics) which right-shifts the HbO2-dissociation curve (H+RSR13). The study was conducted on isolated canine gastrocnemius muscles (n=5) during transitions from rest to 3-min of electrically stimulated isometric tetanic contractions(200 ms trains; 50 Hz; 1 contraction/2 s; 60-70% peak ˙VO2). In all conditions, prior to and during contractions, muscle blood flow (˙Q) was kept constant at a level measured at steady state during contractions in preliminary trials with spontaneous adjustment of ˙Q. Adenosine was infused intra-arterially to prevent inordinate pressure increases with the elevated ˙Q. ˙Q was measured continuously, arterial (CaO2) and popliteal venous (CvO2) O2 concentrations were determined at rest and at 5-7 intervals during contractions, and ˙VO2 was calculated as ˙Q · (CaO2 - CvO2). PO2 at 50% HbO2 saturation (P50) was calculated for each condition. Mean capillary PO2 (P[horizontal bar over]c[horizontal bar over]a[horizontal bar over]pO2) during contractions was estimated by numerical integration. P50 was higher in H+RSR13 (41±1 Torr, [horizontal bar over]x±SE) than in C and in H (31±1). P[horizontal bar over]c[horizontal bar over]a[horizontal bar over]pO2 was higher in H(97±9 Torr) vs. C (53±3), and in H+RSR13 (197±39) vs. H. Time to reach 63% of the difference between baseline and steady state˙VO2 during contractions was 24.7±2.7 s in C, 26.3±0.8 in H, 24.7±1.1 in H+RSR13 (NS). Enhancement of peripheral O2 diffusion (by increased P[horizontal bar over]c[horizontal bar over]a[horizontal bar over]pO2 at constant O2 delivery) did not affect muscle ˙VO2 on-kinetics.