Decreased uptake of exogenous substrates following graded muscle stimulation.

An exercising in vivo canine hindlimb model was used to assess the pattern of exogenous substrate delivery and uptake as a function of contraction frequency. The limb was stimulated to contract at 0, 4, 8, 12 8, 4, and 0 Hz for 10-min periods. Blood flow was proportional to stimulatory frequency. Oxygen consumption increased to a maximum at 12 Hz and subsequently declined. Glucose, free fatty acid, and lactate uptakes decreased despite increased deliveries and maintained arterial concentrations at 12 Hz. At this point there was a net release of glycerol and lactate from the muscle. Microvascular shunting was not the cause of decreased exogenous substrate uptake at 12 Hz. A switch from exogenous to endogenous muscle substrates occurred with the onset of decreased substrate uptake. The mechanism that facilitates this switch from exogenous to endogenous substrates remains unknown.