Dynamics of oxygen uptake following exercise onset in rat skeletal muscle

Technical limitations have precluded measurement of the (V) over dot o(2) profile within contracting muscle (m(V) over dot o(2)) and hence it is not known to what extent (V) over dot o(2) dynamics measured across limbs in humans or muscles in the dog are-influenced by transit delays between the muscle microvasculature and venous effluent. Measurements of capillary red blood cell flux and microvascular Po-2 (Po(2)m) were combined to resolve the time course of m(V) over dot o(2) across the rest-stimulation transient (1 Hz, twitch contractions). m(V) over dot o(2) began to rise at the onset of contractions in a close to monoexponential fashion (time constant, J = 23.2 +/- 1.0 sec) and reached it's steady-state value at 4.5-fold above baseline. Using computer simulation in healthy and disease conditions (diabetes and chronic heart failure), our findings suggest that: (1) m(V) over dot o(2) increases essentially immediately ( < 2 sec) following exercise onset; (2) within healthy muscle the J blood flow (thus O-2 delivery, J (Q) over dot o(2)m) is faster than Jm(V) over dot o(2) such that oxygen delivery is not limiting, and 3) a faster Po(2)m fall to a Po(2)m value below steady-state values within muscle from diseased animals is consistent with a relatively sluggish (Q) over dot o(2)m response compared to that of m(V) over dot o(2). (C) 2002 Published by Elsevier Science B.V.