Spinal opiate modulation of cardiovascular reflexes in the exercising dog.

The possible role of spinal opiate receptors on hind limb muscle afferents, participating in arterial blood pressure and heart rate responses to ischemic exercise, were investigated following lumbosacral intrathecal injection of morphine and naloxone in chronically instrumented dogs. Morphine markedly attenuated these responses. Naloxone had no effect alone but blocked the morphine responses. Thus, spinal opiate receptors may modulate ascending afferent information mediating cardiovascular reflexes from ischemic muscle.