Reflex cardiovascular response during injection of capsaicin into skeletal muscle

In an anesthetized dog, injections of capsaicin, a potent stimulator of small myelinated and unmyelinated fibers that originate as free nerve endings, into a neurally intact donor-perfused hindlimb produced a significant (P less than 0.05) increase in mean aortic pressure (20%), heart rate (12%), cardiac output (28%), and respiratory minute volume (97%). Organ blood flows were measured with 25-micrometers radioactive microspheres. During the injection of capsaicin, there was a decrease in renal blood flow (-25%), but liver, spleen, brain, heart, and skeletal muscle flows remained near control values. After section of the afferent neural connection from the donor perfused hindlimb, the responses to the injection of capsaicin were abolished. These reflex cardiovascular and respiratory responses and changes in organ flow caused by stimulation of capsaicin-sensitive receptors in skeletal muscle are similar to those that occur during induced isometric exercise in the hindlimb.