Can lactate-evoked cardiovascular responses be used to identify muscle ergoreceptors?

The increase in blood pressure and heart rate which accompanies muscular exercise is in part a reflex mediated by afferent nerve fibres in the group III and IV (small myelinated and unmyelinated) range. It has been reported that perfusion of lactate ions into hindlimb muscles is an effective stimulus for these reflex responses. To investigate this hypothesis further, and to test adequacy of the controls used, a solution containing 15 mM lactic acid was perfused through a hind limb of urethane-anaesthetised rats, the leg's circulation being isolated from the rest of the body. During lactate perfusion, increases were seen in arterial blood pressure and heart rate. Denervation of the entire leg abolished the responses. To locate the receptors involved in the reflex, selective denervations of skin or muscle were performed. Clear responses were never seen when the leg was skinned or denervated by section of cutaneous nerves. On the other hand, responses to lactate perfusion were still seen following section of all nerves supplying the muscles of the leg, leaving the cutaneous innervation largely intact. It is concluded that perfusion of a hindlimb with lactate solutions is not an adequate technique to identify ergoreceptors in muscle.