The role of spinal cord transmission in the ventilatory response to electrically induced exercise in the anaesthetized dog

1. The ventilatory response to electrically induced ‘exercise’ was studied in six chloralose-anaesthetized dogs. The on-transient and steady-state responses to ‘exercise’ were compared in the same dogs before and after spinal cord transection at T8/9 (dermatome level T6/7) on fifteen occasions. 2. Phasic hind limb ‘exercise’ was induced for periods of 4 min by passing current (2 Hz modulated 50 Hz sine wave) between two needles inserted through the hamstring muscles. The maximum current used was 30 mA. This was below the level previously found to produce an artifactual stimulation of breathing with the cord intact. 3. Cord transection produced no significant change in either the resting values of ventilation (VI) and CO2 production (VCO2) or the ventilatory equivalent for CO2 during ‘exercise’ (△ VI/ △ VCO2). 4. During the steady state of exercise Pa, CO2 was on average significantly lower than at rest with the cord intact (mean △Pa, CO2, - 2·1 mmHg; range - 5·7 to + 1), and higher, though not significantly, with the cord cut (mean Pa, CO2, + 1·2 mmHg; range - 1·5 to + 4·3). However, even in the absence of spinal cord transmission, the ventilatory response to exercise could not be accounted for on the basis of CO2 sensitivity; the △ VI/ △Pa,CO2 obtained with exercise (apparent sensitivity) was significantly greater than that obtained with CO2 inhalation (true sensitivity) both before and after cord section. 5. VI and VCO2 increased more slowly with the cord cut than with the cord intact. This was thought to be due to a slower increase in venous return in the absence of sympathetic innervation of the lower half of the body following cord transection. 6. Similar experiments were performed during muscle paralysis (following gallamine triethiodide). Ventilation was maintained with a respirator controlled by phrenic nerve activity. These experiments showed an increase in ventilation, independent of muscle contraction, which was only present when the cord was intact and which was confined to the on-transient. Only in the absence of spinal cord transmission could there be certainty that the dynamics of the ventilatory response to electrically induced ‘exercise’ was free of artifact. 7. It was concluded that spinal cord transmission is not necessary for the steady-state ventilatory response to electrically induced exercise of the hind limbs. 8. The dog with spinal cord transection provides a suitable model for the study of the chemical control of breathing during electrically induced exercise.