Resistance and Venous Oxygen Dynamics during Sinusoidal Exercise of Dog Skeletal Muscle

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Abstract
We observed the response of vascular resistance and venous oxygen (O2) saturation to sinusoidally modulated continuous exercise of the isolated dog calf. Stimulus frequency was varied sinusoidally between 0.5 Hz and 1 Hz with modulation frequencies ranging from 0.005 Hz to 0.05 Hz. Venous O2 responses were corrected for dispersion due to vascular and catheter transit. As modulation frequency was increased, the amplitude of the response of both resistance and venous O2 to changing stimulation frequency decreased. The dynamics of the resistance response depended on flow rate. At high constant flow (56 ±8 ml/100 g min-1) the resistance response followed higher modulation frequencies without loss of amplitude (Bode corner frequency = 0.016 ± 0.003 Hz) than it did at low constant flow (25 ± 7 ml/100 g min-1) (Bode corner frequency = 0.0049 ± 0.0006 Hz). The dynamics of the venous Osaturation response were not significantly altered by flow. In the low constant-flow range, vascular resistance and venous O2 saturation had similar dynamics in response to sinusoidal modulation of exercise rate. This finding indicates that oxidative metabolism may be linked to the resistance changes at low flow. At high constant flow, however, the vascular resistance response followed higher modulation frequencies without loss of amplitude than did venous O2 saturation in three of four dogs. Given certain assumptions, it is doubtful that changes in oxidative metabolism are linked to changes in vascular resistance at high constant flow. Vascular resistance, under the conditions of this experiment, appeared to be controlled by at least two mechanisms, the slower of which may be related to oxidative metabolism.